Purification of Water

 Consumption of unsafe water continues to be one of the major causes of the diarrhoeal disease deaths occurring annually, mostly in children. Various processes of the water purification are developed in order to overcome the problem of water pollution.

In the drinking water treatment, the water has to be collected from their original sources such as rivers and reservoirs and purifed. After the purification, that water should be healthy for human consumption and free of harmful microorganisms and organic and inorganic pollutants. 

Drinking water treatment involves various steps according to the quality of raw water. Water from different sources and places has different characteristics. Inorganic compounds such as cadmium, chromium, copper, lead, mercury, arsenic, etc., natural and synthetic organic compounds and living organisms such as bacteria, algae, viruses, etc. may be present in water contributing to turbidity, particles, color, taste, odor, etc. 

Common contaminants in tap water fall into these categories:

• Suspended solid particles

• Colloids

• Dissolved inorganic salts

• Dissolved organic compounds

• Micro-organisms

• Dissolved gasses

The appropriate method of water purification can be developed according to the composition of the water to be treated. The water purification is generally carried out by the processes such as aeration, sedimentation, coagulation, flocculation, filtration, disinfection etc.   

General steps in purification of drinking water includes Aeration, Sedimentation, Filtration, Disinfection.

1. Aeration

This is an optional treatment. Raw water is first collected in large aeraton tank and the aerated by bubbling compressed air through perforated pipes. Aeration removes bad odors and volatile organics (e.g. solvents), carbon dioxide, some taste and odour causing compounds. It also removes metal such as iron, manganese by precipitating then as their respective hydroxides.

In groundwater, iron is usually present as dissolved ferrous compounds. Manganese is usually present as dissolved manganous compounds. Aeration converts them to insoluble hydroxides and which is removed by filtration. 

Aeration processes introduce oxygen into water and removes gases and volatile compounds. For oxygen transfer, aerators are designed so that water flows in a thin film to achieve efficient aeration.              

                  

                     

  2. Sedimentation/Settling

Water may have large sized organic materials such as leaves, and gravels which have run off from the soil. Sedimentation is done so that the suspended particles settle down depending on their size and weight and conditions of the stored water. Sedimentation can be done in a settling tank. 

 Aerated water is placed in settling tank and stored. During storage about 90% of suspended solids settle down within 24 hrs and the water becomes clear. Certain heavier toxic chemicals also settle down during storage.  Pathogenic bacteria gradually die and bacterial count decreases by 90% in about 5-7 days of storage. Sedimentation provides partial reduction of microorganisms in water but does not sterilize the polluted water. During storage organic matter present in water is oxidized by microorganisms.

        Sedimentation tanks are usually rectangular with inlet and outlet at opposite ends of the tank. The inlet distributes the incoming flow as evenly as possible across the tank width and the outlet collect the clarified water. Sedimentation tanks require cleaning occasionally for good performance.

 -Assisted sedimentation- 

Simple sedimentation can reduce turbidity and solids in suspension but take time so the process is enhanced by  physical processes such as flocculation  or chemical processes such as coagulation. These treatments are used to remove very light suspended solids that do not settle by themselves during storage. A precipitate, or floc, which entraps these impurities is formed and settles down with time. Coagulation and flocculation are used to remove colour, turbidity, algae and other microorganisms. Flocculation is gentle mechanical agitation which allows suspended particles to form aggregates/flocs and gradually settle down.  The rate of sedimentation is also enhanced by adding alum, iron, salts, colloid silicates which act as coagulants.  The suspended materials and microorganisms are entrapped by coagulants and settle down rapidly as flocs. This procedure is called coagulation. The floc is separated from the treated water by sedimentation and/or filtration. Sedimentation provides partial reduction of microorganisms in water due to their settling down on bottom but does not sterilize the polluted water.   

The quantity of the coagulants to be added are determined by raw water quality. Water from storage tank is placed in coagulation tank and then some precipitating agents such as alum (aluminium sulphate), lime, iron salts (ferric sulphate) etc. are added in water and mixed.

 

4. Filtration

 After sedimentation the water is further purified by filtration. There are two types of sand filters which are used in water purification such as slow sand filter and rapid sand filter:

(i) Slow Sand Filter: In slow sand filtration plants the rate of filtration of water is slow; hence the plant requires a considerable area. This plant consists of a concrete floor containing drainage tubes (for collection of filtered water). The tile is covered with rock, gravel, coarse sand and then 2 to 1 feet of fine sand. Water is passed through this plant. Water passes slowly through the filter and collected by drain pipes at the bottom which later on is pumped into a reservoir. The capacity of slow sand filter plant is to filter about 5 million of water per acre per day.

    If water is turbid, slow sand filters are clogged soon. Therefore, turbid water, which is to be filtered, should be clarified first by sedimentation, thereafter, passed through slow sand filters. 

    

          

 

Water purification is done  by straining  and action of microorganisms. In the surface of layers of fine sand, a colloidal material, consisting of bacteria, algae and protozoa, is attached. This mucilaginous material called Scmutzdecke or “dirt layer”, closes the pores between the sand grains and makes filtration more effective. Sand grains have positive charges and bacterial cell walls have negative charge. Therefore, bacteria are adsorbed on the surface of sand. Protozoa ingest bacteria. Due to intense microbial interactions, organic contents of water is reduced.

Through slow sand filter plant, the pathogenic microorganisms such as Giardia and its cysts which are not removed by any other methods can be filtered from water. 

When filtration efficiency of the plant is reduced, due to deposition of thick mucilaginous material, the plant is subjected to cleaning. It is cleaned by forcing cleaned water backward i.e. back washing through the beds of gravels and sands without disturbing the fine sand.

(ii) Rapid Sand Filter

Similar to slow sand filter, the rapid sand filter is also constructed. This plant consists of layers of sand, gravel and rock. The water is allowed to pass through rapid sand filter plant. This plant depends on physical trapping of fine particles. The pores of the plants are soon clogged. It is cleaned by forcing cleaned water backward i.e. back washing through the beds of gravels and sands without disturbing the fine sand.

About 99% bacteria are removed by this plant. But it does not remove Giardia lamblia cysts, Cryptospordium oocysts and viruses which are removed through slow sand filter. Therefore, water collected after filtration needs further treatment. 

Rapid sand filter plant operates about 50 times faster than slow sand filter plant, and can deliver about 150 to 200 million gallons of water per acre per day. It requires less land area, less cost and less maintenance.

  

4. Disinfection: 

Disinfection is the final step of water purification. Some of the bacteria pass through filter even after filtration and must be killed before consumption of water. Therefore, disinfection of public water supply needs to be done. 

The filtered water is finally purified by using disinfectants which kills pathogenic as well as other microorganism in water.  Several disinfection methods are used in water treatment. 

    Disinfection with chlorine is the most widely used method for large water supplies but is less common in small supplies. Solutions of sodium hypochlorite were used but in recent years, chlorination of public water supply has become popular. 

Chlorination involves the release of chlorine gas in water which gets readily mixed up with water. The amount of chlorine required ie, chlorine demand of a water body, depends on organic matter and number of microorganisms present in water, and duration of time to act upon. High concentration of chlorine quickly acts upon microorganisms and vice-versa. 

    The amount of chlorine required for disinfection is called chlorine demand. Water is chlorinated to contain about 0.1 to 0.2 ppm of residual chlorine which reaches to this concentration after 20 minutes of its addition. However, if the concentration of chlorine exceeds its demand, peculiar odour and tastes are experienced. 

    The mechanism of action of chlorine on microorganisms is by the formation of highly reactive nascent oxygen. After reacting with water, chlorine is converted into hypochlorous acid which in turn quickly releases nascent oxygen. The nascent oxygen soon oxidises the cellular components of microorganisms as well as organic matter.  

    Chlorination is performed by Breakpoint chlorination method. Break point is the point till where chlorine is added to water such that the demand for chlorine is fully met. When chlorine is added first to water, nascent oxygen liberated oxidises the organic compounds present in the water. At the break point, free ‘available’ chlorine is detected in water. This is "residual chlorine". The organic compounds present are oxidized and the chlorine demand of the water body is fully satisfied at the break point.  

    Breakpoint chlorination requires a dose of around 10 mg/l chlorine and leave a resultant free available chlorine residual in the range 0.1 to 0.2 ppm. The actual dose depends on water quality and has to be determined for each water. It is recommended that the contact time should be at least 30 minutes.  

    Chlorination can be achieved by using liquefied chlorine gas, sodium hypochlorite solution or calcium hypochlorite granules. Chlorine gas is very reactive and highly toxic and must be carefully stored and handled. It is used for treatment of large public supplies but not recommended for treatment of small water supplies. 

Advantages

· It oxidises completely organic compounds, ammonia &other reducing compounds 

· It removes colour in water, due to organic matter. 

· It destroys completely all the disease-producing bacteria 

· It removes both odour and taste from water, due to organic matter 

· It provides residual protection against recontamination 

· Ease-of-use and acceptability 

· Scalability and low cost 

Disadvantages

· Relatively low protection against microbial spores, protozoa and viruses. 

· Lower disinfection effectiveness in turbid waters. 

· Potential taste and odor objections. 

· Potential long-term effects of chlorination by-products. 

    If action of chlorine prolongs in water containing high amount of organic matter, chloramines, are formed. Change in odour and taste of water is due to the formation of chlorophenols. In the presence of high organic matter, chlorine reacts with it and produces different halomethanes which are a group of potential carcinogenic compounds. 

*Super chlorination is a water treatment process in which the addition of excess amounts of chlorine to a water supply to ensure disinfection within a short contact time. Super chlorination, also known as hyper chlorination, temporarily increases the free chlorine residual in a water distribution system.

Super  chlorination is the addition of large doses of chlorine to the water followed by de -chlorination, which is the removal of excess of chlorine after disinfection
This method is applicable to heavily polluted waters whose quality fluctuates greatly.

A high free chlorine residual (i.e. above 5 mg/L) is effective against most bacteria (including Legionella).

Superchlorination should be undertaken :

• While installing a new water infrastructure
• for remediation of affected infrastructure following a detection of Legionella or other microbial hazard of high risk to patients or residents
• based on the complexity of the plumbing infrastructure, in areas where biofilm growth is suspected (e.g. low flow pipe sections), on a scheduled basis (e.g. every six months).

Superchlorination is most commonly used when water has very high bacteria content and generally comes from river sources or where some form of pollution has occurred. It is also an important part of swimming pool maintenance because it keeps chlorine content at the right level to effectively kill off bacteria and other contaminants.

*Shock chlorination is an effective and safe way to remove bacteria from a domestic well and the cold water part of a household water supply system. The chlorine should be present in a concentration that is lethal to bacteria and disinfection should take place long enough to ensure that all bacteria are killed. Shock chlorination is the most widely recommended means of treating bacterial contamination in home water systems such as wells, springs, and cisterns. 

Care should be taken to ensure that the dose of chlorine is adequate and that bleach and harmful chemicals are removed from the system before it is used for drinking water supply again.

Shock chlorination is recommended:

•          upon completion of a new well or when an unused well is returned to service

     ·         if annual water test results indicate the presence of bacteria
     ·         if a well system is opened for any installation, repair or maintenance
    ·      whenever the well is surrounded by flood waters (standing water around or covering the well casing)
     ·         if well water becomes muddy or cloudy after a rain

 Ozone

     One common method of disinfecting wastewater is ozonation (ozone disinfection). Ozone is an unstable gas that can destroy bacteria and viruses. It is formed when oxygen molecules (O₂) collide with oxygen atoms (O) to produce ozone (O₃). 

Ozone is generated by an electrical discharge through dry air or pure oxygen and is generated onsite because it decomposes to elemental oxygen in a short amount of time. After generation, ozone is fed into a contact chamber containing the wastewater to be disinfected.   

· Ozone is more effective than chlorine in destroying viruses and bacteria.  

· The wastewater needs to be in contact with ozone for just a short time (approximately 10 to 30 minutes).  

· Ozone decomposes rapidly, and therefore, it leaves no harmful residual that would need to be removed from the wastewater after treatment.  

· Ozone is generated onsite, and thus, there are fewer safety problems associated with shipping and handling. 

However,

· Low dosages may not effectively inactivate some viruses, spores, and cysts. 

· Ozone is very reactive and corrosive, and extremely irritating/possibly toxic, so need proper care and equipment.   

· Ozonation is not economical - the cost of treatment is relatively high. 

· There is no residual activity to indicate the efficacy of ozone disinfection.

  Ultraviolet irradiation (UV) 

UV is the preferred method for disinfection of small supplies with small distribution networks or retention time. Chlorination may be more suitable for larger operations in which it is necessary to maintain a residual disinfectant during storage and distribution. 

    UV light damages the deoxyribonucleic and ribonucleic acids (DNA and RNA) and prevents the reproduction of microorganisms. Microorganisms are thereby inactivated. In general, viruses are most resistant to UV disinfection compared to protozoan cysts (e.g., Cryptosporidium) and bacteria. 

       UV disinfection efficiency is particularly affected by water quality and flow rate. The water to be disinfected must be of good quality and particularly low in colour and turbidity. 

UV devices can be scaled to fit any size or type of drinking water treatment need, from small handheld devices to large systems. 

UV produces far fewer disinfection byproducts compared to other chemical disinfectants (e.g., chlorine, ozone, chlorine dioxide). UV does not leave any odour, flavor in the treated water. 

The disadvantage of using UV for disinfection is its inability to provide a residual activity.  

Small Scale /household disinfection 

Several types of filters are used to remove suspended matter from water and reduce turbidity and microorganisms, or to remove specific inorganic iron, aluminium or manganese compounds. Devices incorporating activated carbon filtration, reverse osmosis (RO) or UV disinfection can be used. 

Simplest and surest way of disinfecting water at households is boiling water at 100⁰C for 5-10 minutes.     

After disinfection, water is pumped into reservoirs/tanks for subsequent domestic distribution. Different methods of water treatment mentioned so far can be used singly or in combination depending on the raw water quality and treatment purpose. These ensure clean and safe water for healthier living.

Rhizosphere & rhizosphere microflora

 Rhizosphere is the region of intense microbial activity, extending several millimeters from the root system of vascular plants. It is where the soil and root of plant make contact. Rhizosphere soil is the thin layer of soil adhering to the root system after shaking and removing the loose soil.

In 1904 the German agronomist and plant physiologist Lorenz Hiltner first coined the term "rhizosphere” [Greek word "rhiza", meaning root]. 

Hiltner described the rhizosphere as; “The area around a plant root that is inhabited by a unique population of microorganisms, influenced by the chemicals released from plant roots”.

Rhizosphere microflora is quantitatively and qualitatively different from the non-rhizosphere microflora. Also, rhizosphere microflora of one plant differs from that of another. Thus, rhizosphere is a unique subterranean habitat for microorganisms.

Structure of Rhizosphere

It has three zones which are defined based on their relative proximity to, and thus influence from, the root.

1. Endorhizosphere; in close proximity with the plant cortex and endodermis in which microbes can occupy the "free space" between cells

2. Rhizoplane; is the root surface

3. Ectorhizosphere; the outermost zone which extends from the rhizoplane into the adjacent soil.

Rhizosphere effect

The direct influence of plant roots on microbes and microbes on plant root within the rhizosphere is known as Rhizosphere effect. The growth of a soil microorganism is enhanced by the excretions and organic debris of roots within a rhizosphere. These bring about physical and chemical alteration of the soil.

 Rhizosphere effect is expressed by R:S ratio, which is the ratio between number of microorganisms in the rhizosphere soil to the number of microorganisms in the non-rhizosphere soil. R: S ratio is different for different plants and changes with the stage of growth of a plant. The values are high for bacteria in rhizosphere region.

During seed germination and seedling growth, the developing plant interacts with microorganisms present in the surrounding soil. As seeds germinate and roots grow through the soil, the release of organic material lead to the development of active microbial populations in rhizosphere region (that includes plant root and surrounding soil in a few mm of thickness). 

Root exudation is the release of organic compounds from living plant roots into the surrounding soil. Rates of exudation vary widely among plant species and environmental conditions. It has been estimated that 12-40% of the total amount of carbohydrates produced by photosynthesis is released into the soil surrounding roots. 

Root exudates are mainly composed of water-soluble sugars, organic acids, and amino acids, but also contain hormones, vitamins, amino compounds, phenolics and sugar phosphate esters. The qualitative and quantitative compositions of root exudates are affected by various environmental factors including; pH, soil type, oxygen status, light intensity, soil temperature, nutrient availability and the presence of microorganisms. The rhizosphere, is a hot spot for numerous organisms and is considered as one of the most complex ecosystems on Earth. Microorganisms found in the soil include bacteria, fungi, nematodes, protozoa, algae, viruses etc.

Rhizosphere microflora

Rhizosphere organisms that have been well studied for their beneficial effects on plant growth and health are; the nitrogen-fixing bacteria, mycorrhizal fungi, plant growth-promoting rhizobacteria (PGPR), biocontrol microorganisms, myco parasitic fungi, and protozoa. Rhizosphere microorganisms that are deleterious to plant growth and health include the pathogenic fungi, bacteria, and nematodes. A third group of microorganisms that can be found in the rhizosphere are the human pathogens.

Plant-Microbe interactions in the rhizosphere

Microorganisms present in the rhizosphere play important roles in ecological fitness of their plant host.

Important microbial processes that are expected to occur in the rhizosphere include;

✓ pathogenesis

✓ plant protection/growth promotion

✓ production of antibiotics

✓ geochemical cycling of minerals

✓ plant colonization

Plant-microbe interactions may thus be considered beneficial, neutral, or harmful to the plant, depending on the specific microorganisms and plants involved and on the prevailing environmental conditions.

Pathogenic interactions

Roots exudates can attract beneficial organisms, but they can also be equally attractive to pathogenic populations. Many pathogenic organisms, bacteria as well as fungi, have coevolved with plants and show a high degree of host specificity. But even though plants are in permanent contact with potential pathogens such as fungi, bacteria or viruses, successful infection is rarely established. Such a general resistance against most pathogens has been named “horizontal resistance”. These resistance mechanisms include structural barriers and toxic compounds that are present in the unaffected, healthy plant. Phytoanticipins; is a toxin which resist the entry and colonization of pathogenic fungi in plants. In some instances, pathogens can overcome the pre-formed barriers and develop virulent infection processes leading to plant disease.

Beneficial microorganisms and modes of action

Plant-beneficial microbial interactions can be roughly divided into three categories. 

First, those microorganisms that, in association with plants, are responsible for its nutrition (i.e., microorganisms that can increase the supply of mineral nutrients to the plant). In this case, most organisms may not directly interact with the plant, but their effects on soil biotic and abiotic parameters  have an impact on plant growth. e.g. Microbes involved in biogeochemical cycles. 

Second, there is a group of microorganisms that stimulate plant growth indirectly by preventing the growth or activity of pathogens. Such microorganisms are referred to as biocontrol agents, and they have been well documented.

 A third group involves those microorganisms responsible for direct growth promotion. For example, by production of phytohormones.

1. Plant growth promoting Rhizobacteria- PGPR  improve plant growth.  PGPR strains have been used successfully for crop inoculations such as Bacillus, Pseudomonas, Rhizobium etc. PGPR  promotes plant growth, development and protection by 

1. Biofertilization -directly, by helping to provide nutrient to the host plant, or helps in root growth and morphology or other beneficial symbiotic relationships- Rhizobium, Azospirillum etc, 
2. Phyto stimulation (plant growth promoting, usually by the production of phytohormones [auxins, cytokinins, and gibberellins) 
3. Biocontrol (controlling diseases, mainly by the production of antibiotics and antifungal metabolites, lytic enzymes etc). Eg., Bacillus thuringiensis (BT)

• Pathogen inhibition can also  be by
• Antagonism, including; Antibiosis i.e. the inhibition of microbial growth by diffusible antibiotics, toxins, and biosurfactants,  
• Competition  for resources such as nutrients and oxygen occur generally in soil between soil-inhabiting organisms. 
• Induced resistance -Plant-associated bacteria can reduce the activity of pathogenic microorganisms by helping the plant to better defend itself, a phenomenon termed “induced systemic resistance”, “ISR”.

Neutral interactions

Saprophytic microorganisms are responsible for decomposition of organic residues in soil and associated soil nutrient mineralization or turnover processes. Whereas these organisms do not appear to benefit or harm the plant directly (hence the term neutral), their presence is vital for soil dynamics, and their absence would influence plant health and productivity.

AQUATIC ENVIRONMENT- freshwater, marine, estuarine

 An aquatic ecosystem includes freshwater habitats like lakes, ponds, rivers, oceans and streams, wetlands, swamp, etc. and marine habitats include oceans, intertidal zone, reefs, seabed. The aquatic ecosystem is the habitat for various animals, plants, and microbes.

 FRESHWATER ENVIRONMENTS

Fresh water habitats are classified based on their chemical and physical properties. Those with standing water such as lakes and ponds are called lentic habitats and those with running water are lotic habitats.  Most fresh water is locked up in ice sheets, glaciers, or is in ground waters such as lakes and rivers. These provide microbial environments that are different from the larger oceanic systems in many important ways. 

    For example, in lakes, mixing and water exchange can be limited. This creates vertical gradients over much shorter distances. Changes in rivers occur over distance and/or time as water flows through river channels.

 Lakes

Lakes vary in nutrient status. Some are oligotrophic or nutrient poor; others are eutrophic or nutrient-rich. Nutrient-poor lakes are oxygen saturated, has low organic matter and a low microbial population.  Nutrient-rich lakes have bottom sediments that contain organic matter and oxygen poor bottoms.

  When large amounts of nutrients are added to water, eutrophication (nutrient enrichment) takes place and stimulates the growth of plants, algae, and bacteria. Because nitrogen and phosphorus frequently limit microbial growth in freshwater habitats, the addition of nitrogen and phosphorus compounds has a particularly large impact on freshwater systems. Both cyanobacteria and algae can contribute to massive blooms in strongly eutrophied lakes.

Based on penetration of the light, lakes are divided into three zones

 1) Littoral zone- where light penetrates into the bottom- occupied by submerged or partially submerged higher plants and attached filamentous and epiphytic algae.

2) Limnetic zone – area of open water away from the shore.

Littoral and limnetic zone together form the euphotic zone, where photosynthesis occurs. Limnetic zone extends to a level known as compensation depth.

The compensation depth is the lowest level having effective light penetration. Photosynthetic activity balances respiratory activity, here.

 3)      Profundal zone- the area of deeper water beyond the depth of effective light penetration. Absent in shallow lakes. In deeper lakes, it extends from the light compensation level to the bottom.

Littoral and limnetic zone form the euphotic zone. Below euphotic zone is the aphotic zone with no light penetration.

Euphotic zone mainly has primary producers such as phototrophs. Profundal zone has secondary producers who depend on the transport of organic compounds from the upper zone. The lake bottom called, benthos is the interface between hydrosphere and lithosphere. The lithosphere seen at the bottom of lake is called sediment and is occupied by many microorganisms.

 Thermal stratification of Lake

Lakes have different layers/strata depending on the temperature of water. This thermal stratification of a lake according to temperature and depth is seasonal. There is an aerobic  epilimnion (warm, upper layer) and usually anaerobic hypolimnion (deeper, colder, bottom layer) if the lake is nutrient-rich). The epilimnion and hypolimnion are separated by a one of rapid temperature decrease called the thermocline, and there is little mixing of water between the two layers.

 

Epilimnion- warm, oxygen rich with vigorous photosynthesis

Hypolimnion – low temperature, low oxygen, poor light penetration, restricted photosynthesis

 

         As seasons change, the aerobic surface water and the anaerobic subsurface water will turn over as the result of differences in temperature and specific gravity. After such mixing occurs, motile bacteria and algae migrate within the water column to again find their most suitable environment.

In addition to temperature and light, concentrations of organic and inorganic nutrients, oxygen, salt concentrations and acidity affect microbial growth and distributions in lake.

 

Freshwater Microflora- Freshwater Microbial Diversity

  Freshwater environments provide excellent habitats for microorganisms. Large numbers of microorganisms in a body of water generally indicate high nutrient levels in the water. Water contaminated by inflows from sewage systems or from biodegradable industrial organic wastes is relatively high in bacterial numbers. Freshwater environments are highly variable in the resources and conditions available for microbial growth. Both oxygen producing and oxygen consuming organisms are present in aquatic environments, and the balance between photosynthesis and respiration controls the natural cycles of oxygen, carbon, and other nutrients (nitrogen, phosphorus, metals).

Neuston

Neuston is the uppermost layer of hydrosphere-it is the interface between hydrosphere and atmosphere and is occupied by phototrophic microorganisms. Organisms float on the top of the water -(Epineuston) or live right under the surface - (Hyponeuston).  Primary producers are abundant here because of the availability of unrestricted light, carbon dioxide & mineral nutrients. Secondary producers also proliferate here. Microbial numbers in the surface layer are 10 to 100 fold higher than the underlying water column. Bubbles arising from the neuston layer burst out liberating bacteria and other microorganisms to air.  

Autochthonous (native) neuston microbiota include algae, bacteria, fungi and protozoa. Common bacteria are Pseudomonas, Caulobacter, Achromobacter, Flavobacterium, Alcaligenes etc. Gram positive and negative, pigmented and non-pigmented, motile and non-motile, rod and cocci, stalked and un-stalked forms seen. Common blue green algae or Cyanobacteria include Anabaena & Microcystis. Filamentous fungi like Cladosporium and various yeasts, algae like Nautococcus, Chromulina, and protozoa like Vorticella, Arcella etc. are present in neuston.

 

 Other Freshwater Microflora           

A variety of microorganisms live in fresh water. The region of a water body near the shoreline (the littoral zone) is well lighted, shallow, and warmer than other regions of the water. Photosynthetic algae and bacteria that use light as energy flourish in this zone. Further away from the shore is the limnetic zone. Areas of the limnetic zone with sufficient oxygen contain bacteria like Pseudomonads and species of Cytophaga, Caulobacter, and Hyphomicrobium. Photosynthetic algae are also located in the limnetic zone. 

Deeper waters of the profundal and benthic zones have low oxygen concentrations and less light. Algal growth near the surface often filters the light, and photosynthetic microbes in deeper zones  use different wavelengths of light from those used by surface-layer photosynthesizers. Purple and green sulfur bacteria are found in the profundal zone. These bacteria are anaerobic photosynthetic organisms that metabolize H₂S to sulfur and sulfate in the bottom sediments of the benthic zone. Finally, at the bottom of fresh waters is the benthic zone containing the sediments, where few microbes survive. Bacteria that can survive in the absence of oxygen and sunlight, such as methane producing bacteria, thrive here. Clostridium species are common in bottom sediments and may include botulism organisms, particularly those causing outbreaks of botulism in waterfowl. 

Microbial photosynthesizers mainly include algae and cyanobacteria . Others feed on these organisms, forming the next link in the food chain . Plant material from the land also enters lakes and streams at their edges, providing an important nutrient source for many water bodies. Decomposers form an especially important part of fresh-water ecosystems because they consume dead bodies of plants, animals, and other microbes. These microbial agents of decay are an important part of the ecosystem because they convert detritus (dead and decaying matter) and organic materials into needed nutrients, such as nitrate, phosphate, and sulfate. Decomposers  are essential to the major biogeochemical cycles by which nutrients are exchanged between the various parts of the ecosystem, both living and nonliving.

 Aerobic decomposers in water need oxygen to survive and do their work which is ensured by the flowing water and waves. If there is not enough oxygen in the water, many parts of the system suffer-the aerobic decomposers cannot digest plant matter, insects cannot develop and mature, and the fish cannot grow properly. Eventually, the stream or pond will be changed, starting at the microbial level. Human interaction can jeopardize parts of this system in a variety of ways.

Fresh water is host to numerous microorganisms that affect human health directly.  Polluted drinking water is a major source of illness and death throughout the world, particularly in developing countries. Some common microbes in lakes and streams that are responsible for disease include:

· The protozoa Giardia lamblia, found in fresh-water bodies throughout the world. Giardiasis is a common waterborne illness.

· The bacterium Vibrio cholerae, remains a significant source of disease and death .

· The bacterium Escherichia coli, is a very common waterborne pollutant. Humans have a large and harmless population of E. coli in their large intestines, and bacteria make up a large fraction of the volume of human feces. When released into drinking water or recreational water sources, E. coli can be ingested causing diarrhea. 

Thus, many microorganisms are found naturally in fresh water including bacteria, cyanobacteria, protozoa, algae and tiny animals such as rotifers. These can be important in the food chain that forms the basis of life in the water.

 

OCEAN ENVIRONMENT

 

During the slow evolution of our planet, plate tectonics has continuously changed the positions of continents and oceans. At the time of the Pangea, there was only one large ocean surrounding this one continent. Today, according to the classification of the International Hydrological Organization (IHO), there are three oceans.

The Pacific Ocean is the largest, its surface area is about half that of the oceans as a whole, and it alone covers one-third of the Earth’s surface. because of its predominance on the surface that the median meridian of this ocean was chosen as the date change line. The Atlantic Ocean is the second largest by area, accounting for about 30% of the total. It is much better supplied with fresh water than other oceans, since it receives flows from large rivers such as the Amazon, Congo and St. Lawrence. The Indian Ocean, the third largest by area, accounts for about 20% of the total. It is almost entirely located in the southern hemisphere, between Asia, Africa and Australia.

Despite this official classification, we mention 5 oceans on our planet with the Antarctic Ocean to the south, which surrounds the Antarctic continent up to about 60 degrees and whose area represents about 6% of the total, and the Arctic Ocean to the north, which is bordered by the lands of Siberia, Scandinavia, Greenland and North America, whose area represents about 4% of the total.

The seas are the marine sub-domains, of relatively small sizes- Mediterranean Sea, North Sea Baltic Sea Caribbean Sea and the English Channel.

The salinity, or mass fraction of salt is expressed in g/kg (gram of salt per kilogram of sea water). In lakes and rivers, salinity is almost zero, rarely exceeding a few units. It can reach and sometimes exceed 50 g/kg in the seas, its average value is around 35 g/kg. It is 12 g/kg in the Black Sea. In the Dead Sea, its very high value, close to 275 g/kg, practically prohibits any animal or plant life.

In summer, the surface water of the warmest seas can reach temperatures of 26 to 30°C, which often leads to cyclones. In the upper layers of the marine environment, surface water, heated by solar radiation, is subjected to constant thermal exchanges by conduction and convection with the atmosphere. Agitation by waves and turbulence then manages to homogenize the temperature in the first tens of meters (between 0 and -50 m). At great depths (below -120 m), exchanges are limited and become much weaker yielding a resting marine environment.

Between these two areas, a relatively thin layer (between -50 and -120 m) called the thermocline, where the temperature can vary by about ten degrees between the water above and the water below. The temperature of the water above the thermocline experiences significant seasonal variations, due to variations in sunlight, without any change in the temperature of the deep layers.

There is a vertical movement of water which plays a key role in the deep life of the marine environment by regenerating oxygen by supplying surface water and also bring nutrients from the seabed to the surface. In the Mediterranean, this phenomenon of deep convection is mainly located in the Gulf of Lions, making it a major center of biological activity. However, this convection only occurs when the conditions are adequate.

The Black Sea lacks these which this prevents the penetration of oxygen beyond a depth of 200 m. Only very specific species can live in this marine environment under these anoxic conditions.

The amount of sunlight that reaches the water in ocean depend mainly on two factors: distance from shore and depth of water. Oceans are divided into zones based on these two factors. The ocean floor makes up another zone (benthos) .

Horizontal Divisions- Zones Based on distance from Shore

The ocean is divided horizontally by distance from the shore. There are three main Horizontal Divisions -the intertidal zone, neritic zone, and oceanic zone.

· Nearest to the shore lies the intertidal zone (also called the littoral zone), the region between the high and low tidal marks. The important feature of the intertidal is change: water is in constant motion in the form of waves, tides, and currents. The land is sometimes under water and sometimes exposed.

· The neritic zone is from low tide mark and slopes gradually downward to the edge of the seaward side of the continental shelf. Some sunlight penetrates to the seabed here.

· The oceanic zone is the entire rest of the ocean from the bottom edge of the neritic zone, where sunlight does not reach the bottom.

Vertical Divisions-  Zones based on depth of water

    The vertical extent of ocean water is the water column Two main zones based on depth of water and based on light penetration, vertically are the photic zone and aphotic zone. 

    Sunlight only penetrates the sea surface to a depth of about 200 m, creating the photic zone ("photic" means light). Organisms that photosynthesize depend on sunlight for food and so are restricted to the photic zone. Tiny photosynthetic organisms, known as phytoplankton, supply nearly all of the energy and nutrients to the rest of the marine food web and they occupy photic zone. In the aphotic zone there is not enough light for photosynthesis. The aphotic zone makes up the majority of the ocean, but has a relatively small amount of its life, both in diversity of type and in numbers. Photic zone is further divided into epipelagic, mesopelagic and bathypelagic zones, while, aphotic zone consists of abyssal pelagic and hadal zones, based on the depth.

 Composition of seawater

The chemical composition of seawater is quite complex. Most of the chemical elements are found in solution in the form of a complex mixture of anions, cations and molecules.

Some ions come from the dissolution of continental rocks by rivers that carry them to the oceans, where they stay for very long periods of time and where evaporation of water increases their concentration. A significant part of the cations comes from the original ocean floor. And the origin of the chloride ion is often attributed to the degassing of hydrogen chloride from volcanoes, which is soluble in water

In addition to water and salts, there are also various low-concentration molecules, such as boric acid (0.0198 g/kg) and carbon dioxide (0.0004 g/kg), as well as nitrogen and oxygen. The amount of carbon dioxide in seawater is much greater than in the air- the possibility of sequestering carbon dioxide in the oceans, with a view to reducing the content of this greenhouse gas in the atmosphere is a much discussed topic now.

 Biodiversity of the marine environment

On Earth, formed 4.6 billion years ago, life appeared in the oceans about 3.8 billion years ago. And it was only very recently, about 400 million years before our era, that it conquered the land. As a result, extremely diverse lifestyles have developed in the oceans, where light only penetrates the upper layers, gradually adapting to the specific conditions of this marine environment. Most of the biodiversity on our planet is found in the marine environment but most of these species, especially those living in deep water, are still unknown.

 Marine Microflora

Neuston and pleuston are organisms that live near the surface of a water body. The small aquatic organisms inhabiting the surface layer or moving on the surface film, are neuston and the organisms that live at the air-water interface is the pleuston. These organisms are exposed to harsh environmental conditions such as high-temperature variations, high-light variations, including UV irradiation, marine, and aerial predators, etc. . 

Marine bacteria

Pelagibacter ubique and its relatives may be the most abundant organisms in the ocean, and they are possibly the most abundant bacteria in the world. They make up about 25% of all microbial plankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water.

The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).

Marine bacteria perform all kinds of chemical processes in the open ocean, including most of the steps in nitrogen cycling. Cyanobacteria are a large group of photosynthetic bacteria, they “fix” nitrogen, converting nitrogen gas into more biologically useful compounds.

Trichodesimium is one of the most important and well-studied nitrogen-fixing cyanobacteria found in open-ocean areas such as Station ALOHA. It is one of the few organisms involved in the oceanic nitrogen cycle that is visible to the naked eye. Other cyanobacteria in open-ocean areas is Richelia, which is found living inside diatoms, a type of marine algae. Uncultivated cyanobacteria group A (UCYN-A) is a group of nitrogen-fixing cyanobacteria that cannot perform photosynthesis and possibly form “partnerships” (symbioses) with other photosynthetic organisms.

Proteobacteria are an extremely diverse group of bacteria. Some alphaproteobacteria and gammaproteobacteria fix nitrogen. Other important ones are the ammonium oxidizing bacteria (AOB) such as betaproteobacteria and gammaproteobacteria involved in nitrification and nitrite oxidizing bacteria including Nitrobacter, Nitrospira, and Nitrospina etc. Photosynthetic cyanobacteria which do not fix nitrogen include Prochlorococcus, Synechococcus.

Marine archaea are also common in oceans, including ammonium oxidizing archaea (AOA) in the group Crenarchaeota. These archaea are common in the deeper parts of the open ocean, where there are little light and oxygen concentrations are relatively low.

 Archaea are extremophiles living in harsh environments, such as the yellow archaea in a hot spring, but they are also found in a much broader range of habitats. Archaea are particularly numerous in the oceans and may play roles in both the carbon cycle and the nitrogen cycle. Thermoproteota (also known as eocytes or Crenarchaeota) are a phylum of archaea thought to be very abundant in marine environments and one of the main contributors to the fixation of carbon. Eocytes may be the most abundant of marine archaea. Halobacteria, found in water near saturated with salt, are now recognised as archaea. Methanosarcina barkeri, is a marine archaea that produces methane. Marine thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.

 Redtide

Red tide is a colloquial term used to refer the natural phenomena harmful algal blooms (HABs), (or excessive algae growth) that causes negative impacts to other organisms by production of toxins, mechanical damage to other organisms, or by other means. After the bloom dies, the microbes that decompose the dead algae use up more of the oxygen, generating a "dead zone" which can cause fish death. When these zones cover a large area for an extended period of time, neither fish nor plants are able to survive. HABs are induced by eutrophication, which is an overabundance of nutrients (nitrogen and phosphate) in the water. The excess nutrients are emitted by agriculture, industrial pollution, excessive fertilizer use in urban/suburban areas, and associated urban runoff.

The harmful effects from such blooms is due to the toxins they produce or from using up oxygen in the water which can lead to fish die-offs. Some only discolor water, producing a smelly odor, or adding a bad taste to the water. There are three main types of phytoplankton which can form into harmful algal blooms: cyanobacteria, dinoflagellates and diatoms.  Some cyanobacteria, such as Microsystis, can produce hazardous cyanotoxins such as microcystins, which are hepatotoxins that harm the liver of mammals. Other types of cyanobacteria can also produce hepatoxins, as well as neurotoxins, cytotoxins, and endotoxins. Diatoms and dinoflagellates (in marine coastal areas) also cause HAB. Diatoms produce domoic acid, another neurotoxin, which can cause seizures in higher vertebrates and birds as it concentrates up the food chain. Domoic acid  accumulates in the bodies of shellfish, sardines, and affect the nervous system of the consumers such as sea lions, otters, cetaceans, birds or people causing serious injury or death. Blooms of harmful algae can have large and varied impacts on marine ecosystems, depending on the species involved, the environment where they are found, and the mechanism by which they exert negative effects.

Redfield ratio

 In 1934, Alfred Redfield discovered that the ratio of carbon to nitrogen to phosphorus is a nearly constant 106:16:1 throughout the world's oceans, in both phytoplankton biomass and in dissolved nutrient pools. Redfield noticed that the ratio between the quantities of Carbon, Nitrogen and Phosphorus constituting the healthy oceanic phytoplankton, as well as the Nitrogen and Phosphorus in the waters of healthy seas remained close to this value. Thus, "Redfield Ratio" is maintained as the optimal ratio between Carbon, Nitrogen and Phosphorus in natural aquatic ecosystems.

 Marine viruses

Marine phages parasite marine bacteria and archaea, such as cyanobacteria. They are the most abundant biological entity in marine environments. Tailed phages of the order Caudovirales, non-tailed viruses, Phages belonging to the families Corticoviridae, Inoviridae, Microviridae are also known to infect diverse marine bacteria. There are also archaean viruses which replicate within archaea and giant viruses such as the giant mimivirus and the largest known virus, Tupanvirus, as marine microflora.

Microorganisms make up about 70% of the marine biomass. There are 15 times as many viruses in the oceans as there are bacteria and archaea. The viruses kill 20% of microbial biomass and harmful algal blooms. Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.

 Marine protists

Protists are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Common examples are red and brown algae, diatoms, some dinoflagellates, foraminiferans, radiolarian, some marine amoebae, ciliates and flagellates, slime moulds and slime nets. Diatoms have glass like cell walls made of silica. Diatoms generate about 20% of world oxygen production. Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by a shell covered with  circular plates or scales called coccoliths, made from calcium carbonate.

 Single-celled alga, Gephyrocapsa oceanica,and groups of small-sized algae- Zoochlorellae or zooxanthellae -that live inside freshwater protozoans and invertebrate hosts like coral are common in marine environments. They have a symbiotic association with the host and use carbon dioxide, nitrogenous, phosphorous waste, and provide oxygen and essential nutrients to the host. Most protists are single-celled and microscopic. Some single-celled marine protists are macroscopic. Spiculosiphon oceana, a unicellular foraminiferan has an appearance and lifestyle that mimics a sponge. Xenophyophore, another single-celled foraminiferan, lives in abyssal zones. Giant kelp, a brown algae, is not a true plant, but is multicellular and can grow to 50m. 

Marine fungi

Over 1500 species of fungi are known from marine environments. These are parasitic on marine algae or animals, or are saprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, and other substrata. Marine fungi can also be found in sea foam and around hydrothermal areas of the ocean. Many unusual secondary metabolites is produced by marine fungi. The aquatic fungi play a significant role in heterotrophic mineralization and nutrient cycling. Lower fungi  adapted to marine habitats include mastigomycetes: oomycetes and chytridiomycetes. Higher fungi are filamentous hyphomycetes, ascomycetes, basidiomycetes.

 Lichens are mutualistic associations between a fungus, usually an ascomycete, and an alga or a cyanobacterium. Several lichens are found on rocks in marine environments or covering sea snails. Fossil marine lichens 600 million years old have been discovered in China

 Estuaries

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and opening to the sea.

Estuaries are transition zones between rivers and the sea and differ from both in abiotic and biotic factors. They are also among the most highly productive ecosystems on the earth. The estuarine environment is characterized by a constant mixing of freshwater, saline seawater, and sediment, which is carried into the estuary from the sea and land. 

The mixture and fluctuation of salt and freshwater impose challenges to the animals and microbes. The salinity ranges from full strength seawater to freshwater. Associated change is sedimentary conditions from fine sediment to coarse sediments. Other changes include nutrient input, pollutant and chemical concentration along with estuarine flows.

The productivity and variety of estuarine habitats support a wonderful abundance and diversity of species. Thousands of species of fish, migratory birds, shore birds, marine mammals, clams, shellfish and other wildlife survive in and around estuarine habitats. In addition to serving as important habitats for wildlife, estuaries also provide valuable environmental services. The water flowing to the ocean carries sediments, organic and inorganic nutrients, and pollutants. The harmful pollutants deposited creates an environment for microbial biodegradation of these sediments. Estuaries also provide a great deal of aesthetic enjoyment for the people who live, work, or recreate in and around them. 

The activities of microorganisms dominate the functions and nutrient cycling of estuarine ecosystems. Large numbers of bacteria, fungi and protozoa have been found in estuaries and benthic sediments. Their distribution, species abundances and activities interact with their physical and chemical environment.

 Microbial communities

Bacteria

Bacteria are the most numerous organisms in the estuary. Sediments and salt marsh soil generally harbor more bacteria per unit volume than does the water column. Aerobic and facultative anaerobic bacteria are most common, and Pseudomonads and Vibrio are the most often isolated species. Higher bacteria densities have been found in most estuaries than in nearby coastal seawater and river water 

Fungi

The number of fungi living in estuaries is extremely large. Some of fungi are unique in estuaries, while others have a broader range of habitats. Aquatic fungi and yeast dominate species in aquatic environment, few of fungi associate with particles or solid matters in the water. In sediments, the active species of fungi primarily are found in surface aerobic zones. The densities of fungi decrease rapidly with soil depth, but the spores of fungi are found throughout sediments 

Most of the bacterioplankton are closely related to surrounding freshwater or marine bacterial groups and belong to the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Cyanobacteria play an important role as primary producers,  Oscillatoriales, chroococcoid colonies and  Synechococcus-like Cyanobacteria are prevalent. Methanogenic Archaea are important for the mineralization of organic matter in anoxic estuarine environments. Sulfate-reducing bacteria often outcompete methanogens for hydrogen and acetate in estuarine sediments.

 

Carbon & Nitrogen cycling

Bacteria show a variety of metabolic pathways related to carbon flow and cycling. Photosynthesis is mainly carried out by algae and phytoplankton in estuarine. As many of the sediment and water-logged soils of estuaries are anoxic, anaerobic decomposition is important. Complex organic matter is used by the fermenters and dissimilatory nitrogenous oxide reducers.

Nitrogen is a major limiting nutrient for primary production in estuaries. The N-cycling processes that are dominated by microbial activity include nitrification, dissimilatory nitrous oxide reduction, and nitrogen fixation. Nitrogen cycling in estuaries is related to the water mixing and microbial community dynamics.

Water movement is the dominant controlling factor in estuarine ecosystem. Circulation stimulates fluxes of dissolved constituents and particulate materials such as sediments, detritus, bacteria, and plankton. In Estuaries, salt water mixes with water derived from land drainage. The estuarine circulation movements are the primary mechanism of mixing. 

Much of the organic matter carried to an estuary by rivers, produced by phytoplankton, or derived from marshes, is deposited on the sediment surface. Oxygen is the most important electron acceptor in organic matter respiration, but at the water column of anerobic estuarine or saturated sediment sulfate become more significant electron acceptors. The major product of sulfate reduction is hydrogen sulfide, which gives the habitat a pungent smell. 

Autotrophic nutrients are important for the functional estuarine ecosystems, because they are the raw materials for the primary producers. The concentrations of these nutrients change in estuaries due to the mixing of river and ocean water. Microbial heterotrophic activity and primary production play very important roles in the formation and turnover of organic matter in eutrophic estuaries