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.