Ocean Environment -Zones

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.

Density, salinity and temperature of the marine environment

Under conditions  of pressure of 1013 hPa, or hectoPascal, temperature of 3.98 °C, zero salinity, the density of the water is exactly 10³ kg/m³. The density of seawater varies mainly with temperature and salinity, much less with pressure, which often leads to considering this fluid as incompressible.

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). On the contrary, at great depths (below -120 m), exchanges are almost limited to pure conduction 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. The sea bed and water column are subdivided further
  
  
  

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. The aphotic zone is subdivided based on depth.

               Photic zone is further divided into epipelagic, mesopelagic and bathypelagic zones, while, aphotic zone consists of abyssal pelagic and hadal zones.

 

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.

 

The amazing 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.

Only a few species have managed to pass through and differentiate themselves on land, which means that most of the biodiversity on our planet is found in the marine environment.

The most common estimates of the number of living species in the marine environment range from 5 to 10 million, but most of these species, especially those living in deep water, are still unknown. By way of comparison, the number of species living on land is about 1.3 million, including 850,000 varieties of insects

 

Bioremediation of pesticides

 Pesticides are the chemical substances use to kill or manage pests at tolerable levels. The extensive use of pesticides has resulted in serious environmental as well as health problems and has effected biodiversity. The use of pesticides not only degrade the soil quality but also aquatic environment. Among the pesticides 98% were classified as acutely toxic for fishes and crustaceans. 

    The pesticide contamination of surface and ground water pose a serious threat to surrounding ecosystems. The organochlorines (DDT, methoxychlor, dieldrin, chlordane, toxaphene, mirex, kepone, lindane, and benzene hexachloride) and organophosphates (malathion, parathion, diazinon, fenthion, dichlorvos, chlorpyrifos, ethion) cause tumors, irritability and convulsions. They also cause serious environmental issues due to biomagnifications

The fate of pesticides is often uncertain, thus decontamination of pesticide polluted areas is very complex process. Low degree of biodegradability has made them as persistent toxic substances. To reduce or eliminate them from the environment, earlier techniques or technologies which were used were landfills, recycling, pyrolysis etc., but these can lead to formation of toxic intermediates and are expensive and difficult to execute especially in case of pesticides.

Bioremediation is a promising technology which utilizes the ability of microorganisms to remove pollution from the environment. It is an eco-friendly, economical and versatile approach.

 

Pesticide	Persistence (Half-life)	Health Effects
Aldrin	20 days to 1 year	Nervous system effects. Probable carcinogen. Large doses: convulsions, death.
Dichlorodiphenyltrichloroethane (DDT)	2 to15 years	Nervous system effects (tremors, seizures); probable carcinogen
Chlordane	4 years	Nervous system, digestive system, liver effects. Higher doses: convulsions and death.
Dieldrin	Up to 7 years	Nervous system effects. Probable carcinogen. Large doses: Convulsions, death.
Heptachlor	0.4 to 2 years	Nervous system damage, liver and adrenal gland damage, tremors

 Factors affecting biodegradation process

Pesticide pollution is a serious environmental problem and their remediation is necessary. Ideally treatment should result in destruction of the compounds without generation of intermediates. In some cases, intrinsic bioremediation occurs because of microbes that are already present in polluted ecosystems, but intrinsic bioremediation is not adequate.

Any factor which can alter growth or metabolism, would also affect biodegradation. Hence, physicochemical characteristics of the environmental matrix, such as temperature, pH, water potential, oxygen and substrate availability, would influence the biodegradation efficiency

The requirements for the process of bioremediation of pesticides are summarised as

Factor	Conditions required
Micro organisms	Aerobic or Anaerobic
Natural biological processes of micro organisms	Catabolism and Anabolism
Environmental factors	Oxygen content
Nutrients	Carbon, Nitrogen, oxygen etc.,
Soil moisture	25-28 % of water holding capacity
Type of soil	Low clay or slit content

 The other factors include co-metabolism and action of microbial consortia. Some biodegraders need other substrates to degrade pollutants. This phenomenon is called co-metabolism and is especially required for organochlorine compounds. In contrast, it has been shown that the presence of other carbon sources decreases organophosphate biodegradation. When pesticide degradation occurs, it usually involves more than one microorganism (microbial consortia), i.e. each microorganism contributes to biodegradation reactions on pesticides, but no single strain can carry out ineralization. Thus, the presence of different microorganisms is essential for an adequate biodegradation.

Fungi belonging to Basidiomycetes or bacteria like Pseudomonas, Aerobacter, Acinetobacter, Moraxella, Plesiomonas, Burkholderia, Neisseria, Sphingomonas, Micrococcus and Flavobacterium perform biodegradation effectively by reactions like de-chlorination, cleavage, oxidation, reduction involving different enzymes such as oxidoreductases, hydrolases, transferases and translocases.

Bioremediation could be by bio stimulating the indigenous biodegraders (bio stimulation) or adding exogenous to the site (bio augmentation).

         Bacteria are preferred for bioremediation due to their fast growth, easy handling and low cost. The complete biodegradation of the pesticide involves the oxidation of the parent compound resulting in to carbon dioxide and water, this provides energy to microbes. The soil where innate microbial population cannot be able to manage pesticides, the external addition of pesticide degrading micro flora is recommended. Degradation of pesticides by microbes not only depends on the enzyme system but also the conditions like temperature, pH and nutrients. Some of the pesticides are easily degraded however some are recalcitrant because of presence of anionic species in the compound. 

Although different enzymes participate in each condition, it seems that both, aerobic and anaerobic degradation are needed for  mineralization. Anaerobic metabolism is more adequate for dechlorination and aerobic metabolism produces a cleavage in aromatic or aliphatic cyclic metabolites. Organochlorine pesticides show higher persistance in aerobic conditions  compared to anaerobic conditions. The removal of heteroatoms (like halogens) or heteroatom-containing groups are among the first steps in biodegradation. These steps are catalyzed by dehalogenases under anaerobic conditions. Thus anaerbic conditions are more adequate for biodegradation of organochlorine pesticides, while aerobic are better for biodegrading hydrocarbon metabolites from pesticides.

Anaerobic biodegradation of dichlorodiphenyltrichloroethane (DDT) occurs by reductive dehalogenation- removal of only on chlorine atom from DDT. Pseudomonas, Aerobacter, Trichoderma, Neisseria, Moraxella and Acinetobacter can degrade  DDT . DDT is converted to TDE (2,2-bis(4-chlor-phenyl)-1,1 -dichloro ethane) and TDE is further degraded to DDE (1,1-dichlor-2,2, bis-(4,chloro phenyl) ethylene). Mineralization of DDT does not usually occur in the environement. 

Anabaena,  Pseudomonas aeruginosa and Burkholderia were shown to be good biodegraders of endosulfan. No mineralization of endosulfan has been observed. Microorganisms from the Pseudomonas, Bacillus, Trichoderma, Aerobacter, Mucor, Micrococcus and Burkholderia genera have been shown to biodegrade dieldrin and endrin. 

    Fungal enzymes especially, oxidoreductases, laccase and peroxidases play a key role in the biodegradation of  xenobiotics compounds. 

Species of fungi	Potential for degrading pesticide
Dichomitus spp,  Hypholoma spp,  Auricularia spp,  Pleurotus spp,	triazine, phenylurea, chlorinated organophosphorus compounds
White-rot fungi	Heptachlor,   atrazine, lindane, metalaxyl, chlordane, mirex,  dieldrin, diuron, aldrin, DDT

        Bioremediations has a tremendous potential for remediation of the soils that are affected by pesticides. Microorganisms that are present in the soils can remove pesticides from the environment. Biopesticide enzymatic degradation of polluted environment represents most important strategy for pollutant removal and degradation of persistent chemical substances that can surely solve the problem of pesticide pollution of soils.

  

Bioremediation of polluted environment – Oil spills

Bioremediation is the use of living microorganisms to degrade environmental pollutants -it is a technology for removing pollutants from the environment thus restoring the original natural surroundings and preventing further pollution. 

 "Remediate" means to solve a problem, and "bio-remediate" means to use biological organisms to solve an environmental problem such as contaminated soil or groundwater.

 Bioremediation- Two types

1. In situ bioremediation

2. Ex situ bioremediation 

“In Situ” bioremediation means it is done on the site of pollution. Most often, in situ bioremediation is applied to the degradation of contaminants in soils and groundwater. It is a superior method to cleaning contaminated environments since it is cheaper and uses harmless microbial organisms to degrade the chemicals.

In situ bioremediation methods have many potential advantages- it can be done on the site of contamination and hence proves to be cost effective,  but the method is time consuming compared to the other remedial methods. There may be seasonal variation of the microbial activity due to changes in environmental factors that cannot be controlled.

Genetically engineered microorganisms can be used, although stimulating indigenous microorganisms is preferred.

The use of microorganisms is not limited to one field of study of bioremediation, it has an extensive use; Petroleum, its products and oils constitute hydrocarbons and if present in the environment causes pollution. Oil spills caused by oil tankers and petrol leakage into the marine environment is common. A number of microorganisms can utilize oil as a source of food, and many of them produce potent surface-active compounds that can emulsify oil in water and facilitate its removal. Apart from degrading hydrocarbons, microbes also have the ability to remove industrial wastes, reduce the toxic cations of heavy metals (such as Selenium) to a much less toxic soluble form. Many algae and bacteria accumulate metals that are toxic in high levels. The metals are in effect removed from the food chain by being bound in this way. Degradation of pesticides and dyes are also brought about by some anaerobic bacteria and fungi.

Oil spills and oil pollution 

Petroleum (crude oil) is a liquid fossil fuel. It is a product of decaying organic matter, such as algae and zooplankton. It is one of the major energy sources in the world. However, oil drilling or transportation can cause spills which contaminate the environment. Oil spills in marine environments are especially damaging because they cannot be contained and can spread over huge areas. The aromatic compounds in oil are toxic to living organisms and such spills can cause disaster in an ecosystem. 

In the environment, such spills are naturally cleaned by microorganisms that can break down the oil. The success of bioremediation technologies applied to hydrocarbon-polluted environments highly depends on the biodegrading capabilities of native microbial populations. The communities which were exposed to hydrocarbons become adapted. The adapted microbial communities can respond to the presence of hydrocarbon pollutants within hours and exhibit higher biodegradation rates than communities with no history of hydrocarbon contamination.

 Since crude oil is made of a mixture of compounds, and since individual microorganisms metabolize only a limited range of hydrocarbon substrates, biodegradation of crude oil requires mixture of different bacterial groups or mixed consortia to degrade a wider range of hydrocarbons.

Microbial degradation occurs by attack on aliphatic or light aromatic fractions of oil. High molecular weight aromatics, resins and asphaltenes are recalcitrant or have low rates of biodegradation. 

The dominant group of biodegrading bacteria are the hydrocarbonoclastic bacteria (HCB). The concentration of these bacteria increases significantly in areas of oil spill. One of the best studied representative of this group is Alcanivorax borkumensis. This species contains individual genes responsible for breaking down certain alkanes into harmless products. It also possesses genes to  produce a biosurfactant which help to enhance the oil emulsification. The addition of nitrogen and phosphorus to the Alcanivorax environment increases its growth rate. 

Aside from hydrocarbons, crude oil contains additional toxic compounds, such as pyridine. These are degraded by representatives of other genera such as Micrococcus and Rhodococcus. Oil tarballs are biodegraded slowly by species from the genera Chromobacterium, Micrococcus, Bacillus, Pseudomonas, Candida, Saccharomyces and others. 

Microorganisms inhabiting locations contaminated with PHs (petroleum hydrocarbons) often contain enzymes that can degrade oil and thus might be of interest for scientific and industrial applications. Microbes capable of degrading PHs can operate on a wide range of metabolic substrates, and can degrade PHs anaerobically or aerobically by use of a variety of enzymes encoded by key functional genes, such as alkane hydroxylase genes, ring-hydroxylating dioxygenases etc.,

Bioremediation is the most efficient, economical way to deal with contamination of soils by petroleum hydrocarbons (PHs). It also does not generate toxic metabolites. Thus, it has been widely accepted and used for remediation of areas that have been contaminated long-term with petroleum. . Highly hazardous oily materials can be mineralized to harmless products using suitable microorganisms. A limiting factor for successful bioremediation of oil polluted sites is the low aqueous solubility and strong adsorptive capacity of the hydrophobic contaminants to soil. So, hydrocarbons presented in crude oil require solubilization before being degraded by microorganisms. 

 In aquatic ecosystems, dispersion and emulsification of oil is important for biodegradation. Large masses of tar balls persist because of limited surface area available for microbial activity. Biodegradation is limited by resistant and toxic components of oil, low temperatures in water, scarcity of mineral nutruients (N & P), exhaustion of dissolved oxygen, scarcity of hydrocarbon degrading organisms (in case of new areas of oil spills)

Accidental spills are easier to contain in land and clean than on water. Oil spills are destructive to vegetation because of contact toxicity and generation of anoxic conditions in soil.

Microbial bioremediation method is generally categorized into two classes—bioaugmentation and biostimulation. 

Bioaugmentation involves the addition of microbial culture or microbial consortium into the contamination site to speed up the biodegradation process of specific contaminants. Because indigenous microbes may not be efficient in the degradation of the complex mixtures such as petroleum or may be stressed as a result of a current exposure to the oil spill or when the rate of degradation is slow, the introduction of oil-degrading microorganisms to supplement the indigenous populations (bioaugmentation) can accelerate bioremediation of oil-contaminated sites.

Biostimulation is one where the environment is modified to incite existing bacteria that can efficiently carry out bioremediation. Additives are usually added to the subsurface through injection wells. Thus, when nutrients are added, the indigenous microorganism population grows rapidly, potentially increasing the rate of biodegradation. The primary advantage of biostimulation is that biodegradation will be undertaken by already present native microorganisms that are well suited to the subsurface environment and are well distributed in the environment. 

Bioventing is a process of stimulating the natural in situ biodegradation of contaminants in soil by providing air or oxygen to existing soil microorganisms.

Superbug/“an oil eating bacteria” 

        In 1971, the very first superbug was created to degrade oil  when Anand Mohan Chakrabarty, a distinguished Indianborn American microbiologist  and co-workers reported the development of a new strain of bacterium by transfer of plasmids. It was named  superbug since it could utilize a number of toxic organic chemicals like octane, hexane, xylene, toluene, camphor and naphthalene. He transferred the genes required for degradation of oil using plasmid transfer  and as a result produced a new stable bacterial species (Pseudomonas putida). He called it as a “multi-plasmid hydrocarbon-degrading Pseudomonas” which was capable of digesting two thirds of hydrocarbons found in typical oil spill and at a faster rate than previously existing strains of oil-eating microbes. This discovery became a biological remedy for removing oil pollution especially during disastrous oil spills and leakages in marine ecosystems.

    In 1980, United States granted the patent to this superbug making it the first genetically engineered microorganism to be patented.  This superbug was then used for cleaning an oil spill in Texas in 1990.