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1 Aquatic Microorganisms The biosphere of our planet contains a large number of ecosystems in which microorganisms play an important part. Water is the dominant environment of these ecosystems, since ocean covers about 71 % of the globe's surface. Although water forms the basis for all aquatic environments, the difference in the characteristics of natural water distinguishes several different aquatic environments. Thus, microbial ecology of natural waters remains an important and diversified field. Ingeneral, aquatic microorganisms experience highly fluctuating, highly varying environmental factors.These factors vary not only among different aquatic environments, even within the same aquatic environment. This contributes to the well pronounced microbial biodiversity in aquatic environments. Inspite of the continuing researches in aquatic microbiology, much have to be deciphered about the aquatic microorganisms. The difficulties in the study of aquatic microorganisms and their environment are slowly being rectified with the introduction of modern techniques. There are two major types of water. 1. Ground water - It originates from deep wells and subterranean springs. This is virtually free of bacteria due to filtering action of soil, deep sand and rock. However, it may become contaminated when it flows along the channels. 2. Surface water - It is found in streams, lakes, and shallow wells. The air through which the rain passes may contaminate the water. Other sources are the various types of establishments and agricultural farms etc. by the sides the water flows. Possible sources of microbial contamination of a body of water are soil and agricultural run off, farm animals, rain water, industrial waste, discharges from sewage treatment plants and storm water run off from urban areas. In water microbiology the water is contaminated when it contains a chemical or biological poison or an infectious agent. These conditions also apply to water which is polluted except that the agent or poison is often obvious and 2 the water carries an unpleasant taste or appearance. Potability refers to the drinkability of water. When potable, it is fit for drinking. When unpotable it is unfit due to some contaminant or pollutant. Atmospheric Waters Rain, snow and hail which fan on land tend to carry down particles of dust, soot, and other materials suspended in the air. These often bear bacteria and other microorganisms -on their surface. The number of organisms depends upon local conditions. After heavy rain or snow the atmosphere is washed free of organisms. Surface Waters - As soon as ram or snow reaches the earth and flows over the soil, some of the soil organisms are gathered up by the water. Bodies, of water such as streams, rivers, and oceans represent surface water. Microbial populations depend upon their numbers in the soil and, also, upon the kinds and quantities of food material dissolved out of the soil by water. Climatic, geographical and biological conditions bring about great variations in microbial populations of surface waters. Rivers and streams show their highest count during the rainy period Dust blowing into rivers and streams also contributes many microorganisms. Animals also make considerable contribution to the microbial flora of the surface waters. They bathe and often drop their excreta in the water. Stored Waters - Inland waters held in ponds, lakes or reservoirs represent stored waters. Storage generally reduces the numbers of organisms in water. A certain degree (If purity and stability is established. Study of Aquatic Microorganisms -The study of aquatic microorganisms is quite difficult because of the nature of their habitat and their unique characteristic features. The study is particularly difficult when the samples are from sea. Pelczar et.al. (1986) have analysed some of the problems associated with the study of aquatic microflora which are as follows: Isolation of many aquatic microorganisms is very difficult since they will not readily grow on the usual laboratory media such as nutrient agar or nutrient broth. Thus, a number of microbes still have to be discovered from estuaries and oceans. A large proportion of aquatic bacteria have a natural tendency to grow attached on solid surfaces, like particulate matter or on large organisms. The time period between the sample collection and its transport to the laboratory usually leads to loss of viability of many microorganisms. This necessitates the use of laboratory-equipped ships for on-location culturing of specimens, which is very costly. The study requires special sampling devices especially for collecting samples from the deeper regions of the estuary or ocean. 3 Another important problem is the lack of routine techniques for the isolation of aquatic viruses. Microbial Communities in Aquatic Environments- Various habitats can be recognized within an aquatic environment and their actual size may vary from one another. Each habitat is characterized by one or more microbial communities and thus the microorganisms within an aquatic environment can be grouped as distinct communities. Microbial Communities - In neutral and slightly alkaline waters of ponds and lakes, planktons are represented by the group diatoms which includes Fragilaria and Melosira varians. Acidic, oligotrophic lakes have flagellated planktons like Euglena and Chlamydomonas. Eutrophic lakes include planktons of the groups Chlorococcales such as Scenedesmus, Chlorella etc., Coccoid cyanophyta such as Microcystis and Coelosphaerium and filamentons cyanophyta such as Anabaena and Oscillatoria etc. Ground Water Microbiology - About 29% of the earth's surface is represented by continental surface. Beneath all these continental surface is the, groundwater habitat. Thus, groundwater ecosystems include vast area on a global scale. The typical composition of the continental areas of the earth includes, in vertical sequence, the following: the A and B soil horizons; the C soil horizon from which the above two horizons might have been derived; an unsaturated or vadose zone which is bound between the C soil horizon and the water table; a capillary fringe zone just above the water table and a saturated zone which may extend through several geological strata. Major Groundwater Microbial Groups - Only limited informations are available on the major groups of microorganisms and their spatial distribution within the groundwater ecosystem. Comparatively, groundwater ecosystem contains less number of microorganisms than any other aquatic habitats or the terrestrial surface as well. This can be related to the infiltration of water through various geological strata. Bacteria- The major group of microorganisms found in groundwater ecosystem is bacteria. The abundance and distribution of bacteria in a given subsurface zone is actually determined by the nature of geological stratum, mineral type, particle size distribution, texture, hydraulic conductivity etc. The nutrient levels decline, when moving down from A and B soil horizons to C horizon accompanied by a proportional decline in bacterial abundance. However, the number of bacteria does not continue to diminish with depth. Indeed there is again an increase in the bacterial numbers at the capillary fringe zone and at the water table. This is due to the horizontal saturated flow as well as the vertical seasonal fluctuations of the water table which lead to a dynamic mixing of oxygen and nutrients in these regions. The high relative abundance of bacteria in the saturated zone can be attributed to the hydrological, Filamentous Spore Forming Microorganisms - Both actinomycetes and fungi are spore-forming filamentous microorganisms. Inspite of their spore-forming ability they are found in very low numbers in groundwater ecosystems. However their contributions to the aerobic heterotrophic metabolism is quite high within the larger subsurface microbial community. Metabolic diversity of Microorganisms- Groundwater ecosystem is inhabitated by metabolically diverse groups of microorganisms each mediating its own characteristic metabolic activity. 4 Some important microbial activity in the groundwater ecosystem includes carbon dioxide production, denitrification, methanogenesis, methane oxidation and biodegradation of polycyclic aromatic hydrocarbons. Freshwater Ecosystem The study of freshwater habitats is called limnology. Fresh water environments include the flowing waters (lotic environment) and standing bodies of water (lentic environment). The dominant feature of lotic environments, is the continuous movement of water and currents, which cuts the channel, moulds the character of the stream and influences the chemical and organic composition of the water. Water running off the land follows courses of least resistance and develops these as distinct channels by erosion. Young or rejuvenated streams, with a high velocity, erode more than they deposit. Water in slow-moving rivers reflects the characteristic of the terrain; nutrient level and sediment load vary according to region. The slow-moving stream often develops floodplains, meanders, and associated features and terminates in a lake or estuary. The lentic and lotic ecosystems are fundamentally different from one another because of differences in energy input and flow, and mineral input and circulation. Microorganisms of The Aquatic (Freshwater) Habitat 1. Bacteria Most aquatic bacteria are gram negative because of the following reasons. Gram negative envelope is suited for the nutrient diluted environment (oligotrophic). Important hydrolytic enzymes are retained in the periplasmic space rather than being excreted and lost to the aquatic environment as in gram positive bacteria. Gram negative bacterial cell wall contains lipopolysaccharide (LPS) that protects the cell against toxic molecules like fatty acids and antibiotics. Specially found are Pseudomonas sp., Flavobacterium, Achromobacter and Alcaligenes, Vibrio sp., Acinetobacter, Staphylococcus sp. Bacteria in surface region are pigmented. 2. Molds Deuteromycetes members, Phycomycetes and Myxomycetes members are found in the marine environments. 3. Protozoa Species of Foraminifera and Rodidaxia and many ciliate species are common. Aquatic Microbiology-Lakes Eutrophic Lakes - Eutrophic lakes are those which have a high nutrient content. They are shallower than the oligotrophic lakes. Because of the high nutrient content, extensive aerobic decomposition of organic matter occurs in eutrophic lakes. And this makes them to have a low dissolved oxygen concentration. However, they are characterized by very high primary productivity. Thermal Stratification - Biological processes within the lake water are influenced by the physical state of water which in turn is governed by the temperature. For example, at 4°C water has its maximum density. 5 In lakes, the increase in depth with a corresponding change in the temperature leads to a more or less stable seasonal stratification called thermal stratification. Hypolimnion- In this region the temperature will be more or less the same throughout its depth. This region is subjected to wide variation as the temperature fluctuates with that of the atmosphere. Below the epilimnion is a colder deep region known as hypolimnion which has a more or less constant temperature. Metalimnion Thermolimnion - In between these two regions is a transition zone characterized by an abrupt change in the temperature. This region is called as metalimnion or thermocline. The lithosphere underlying the lake is referred to as sediment which is enriched with organic nutrients. Redox discontinuity Layer Chemocline- Oxygen consuming catabolic process from the sediment render the hypolimnion anaerobic. But the epilimnion is always in contact with atmospheric air and regularly mixed up by the air current. This facilitates the aeration of the region and makes epilimnion aerobic. Thus, thermal stratification also leads to a gradient of redox potential and chemical parameters. Since the redox potential and chemical parameters are quite different in the regions both above and below the thermocline, it is also called as redox discontinuity layer or chemocline. Seasonal Changes of Lakes - In spring, the lake water in the epilimnion is heated by sun, creating a less dense layer. The water in the hypolimnion is cooler and remains at the bottom. The same stratification continues throughout the summer. In autumn, the epilimnion cools off by radiating the heat. When the temperature in both the epilimnion and the hypolinmion is equal, the two layers mix with each other. This mixing is assisted by autumn winds. Holomictic Lakes - Lakes which have a complete circulation cycle like this are referred to as holomictic lakes. Meromictic Lakes- If the mixing of water is incomplete that type of lakes are referred to as meromictic lakes. Amictic Lakes- If the mixing of water is completely absent that type of lakes are referred to as amictic lakes . Biological Process in Holomictic Lakes -Various biological activities taking place in a holomictic lake allows a complete cycling of nutrients and oxygen. In the epilimnion light penetration allows photosynthesis to take place. Carbon dioxide, produced in the bottom region, is made available in the form of dissolved carbon dioxide. Oxygenic Photosynthesis - Phytoplanktons are the most common microflora in the epilimnion which include diatoms, flagellates, green algae and cyanobacteria. Thus epilimnion is characterized by the primary production through oxygenic photosynthesis. Anoxygenic Photosynthesis - When hydrogen sulphide is present and sufficient illumination is available purple as well as green sulphur bacteria like Chlorobium and Chromatium start primary production through anoxygenic Photosynthesis. These bacterial species are provided with either gas vacuoles or flagella so that they can be maintained just below the metalimnion to trap any available light. Gas vacuole containing bacteria occurring in this region include Amoebobacter, Lamprocystis, Thiopedia, Thiodictyon and Ancalochloris. Zonation - Penetration of light through water varies with the depth in lakes and ponds. Based on this various zones can be recognized within the lake. The depth upto which effective penetra60n of light 6 occurs is referred to as light compensation level. The open area found above the light compensation level in a lake is called limnetic zone. Photosynthetic activity is maximum in this zone. Organisms include photosynthetic bacteria, flagellates and diatoms. Microbial Communities - In neutral and slightly alkaline waters of ponds and lakes, planktons are represented by the group diatoms which includes Fragilaria and Melosira varians. Acidic, oligotrophic lakes have flagellated planktons like Euglena and Chlamydomonas. Eutrophic lakes include planktons of the groups Chlorococcales such as Scenedesmus, Chlorella etc., Coccoid cyanophyta such as Microcystis and Coelosphaerium and filamentons cyanophyta such as Anabaena and Oscillatoria etc. Biofilm - Any wetted surface submerged in a river tend to be coated with microorganisms within an hour. These include algae, bacteria and protozoa and the presence of algae is limited by the availability of light. These attached forms are generally regarded as biofilm. Mucilage - The biotic elements of the epilithic biofilm are fixed to the rock surfaces by a continuous matrix of extracellular polysaccharides, also called mucilage, produced by the microbes involved. Functions of Mucilage-1. A site for entrapping both soluble and particulate matter. 2. A retaining as well as protective mechanism for the extracellular enzymes produced by the microorganisms involved. 3. A retaining mechanism for the products of extracellular enzymatic hydrolyses. 4. A framework for different groups of autotrophic and heterotrophic microorganisms forming a complex under suitable conditions. Nutrient Spiralling - The path of nutrients in a river is spiral rather than a cycle and this phenomenon in known as nutrient spiralling. The suspended nutrients in a river is usually absorbed by attached microbial communities which act upon them. After the death of these attached forms the absorbed nutrients are left free in the flowing water. After a flow for a small distance downstream these nutrients are again absorbed by some other attached forms. As a result of this interruption, the nutrients carried along with the running water do not move with the speed of current. Thus, the nutrients involve a downstream movement before a cycle. Spiral Length - Spiral length has been defined as the distance traveled by a substance in a stream before assimilation by attached organisms of the streambed. Thus, if a nutrient is at limiting concentrations in a stream it may be possible that the nutrient will be rapidly recycled and hence it will have short spiral length. On the other hand, if the nutrient is found abundant, it will be recycled less rapidly and hence will have a long spiral length. Ground Water Microbiology - About 29% of the earth's surface is represented by continental surface. Beneath all these continental surface is the, groundwater habitat. Thus, groundwater ecosystems include vast area on a global scale. The typical composition of the continental areas of the earth includes, in vertical sequence, the following: 7 the A and B soil horizons; the C soil horizon from which the above two horizons might have been derived; an unsaturated or vadose zone which is bound between the C soil horizon and the water table; a capillary fringe zone just above the water table and a saturated zone which may extend through several geological strata. It should be noted that the transition between soil and groundwater habitats is not delineated by soil horizons, thus making the terms 'groundwater' habitat and 'terrestrial subsurface' synonyms. Aquifers- In its simplest hydrogeological sense ground water has been referred to water that is easily extractable from saturated, highly permeable geological strata known as aquifers. In addition to these aquifers, groundwater also includes capillary water and water vapors from the unsaturated zones. For microorganisms which live within these microhabitats all available forms of water may certainly be important. Moreover, unsaturated zones may be transiently saturated during recharge events and they may influence the microbiology of the saturated zone. Thus, in a broader sense, groundwater refers to all subsurface water found beneath the soil A and B horizons that is available to sustain and influence microbial life in the terrestrial subsurface (Madsen & Ghiorse, 1993). Major Groundwater Microbial Groups - Only limited informations are available on the major groups of microorganisms and their spatial distribution within the groundwater ecosystem. Comparatively, groundwater ecosystem contains less number of microorganisms than any other aquatic habitats or the terrestrial surface as well. This can be related to the infiltration of water through various geological strata. Bacteria- The major group of microorganisms found in groundwater ecosystem is bacteria. The abundance and distribution of bacteria in a given subsurface zone is actually determined by the nature of geological stratum, mineral type, particle size distribution, texture, hydraulic conductivity etc. The nutrient levels decline, when moving down from A and B soil horizons to C horizon accompanied by a proportional decline in bacterial abundance. However, the number of bacteria does not continue to diminish with depth. Indeed there is again an increase in the bacterial numbers at the capillary fringe zone and at the water table. This is due to the horizontal saturated flow as well as the vertical seasonal fluctuations of the water table which lead to a dynamic mixing of oxygen and nutrients in these regions. The high relative abundance of bacteria in the saturated zone can be attributed to the hydrological, physical and geochemical properties of each stratum rather than the depth. Also, there occurs a wide variation in the morphology and physiology of bacteria found in each geological stratum. The occurrence of endospore-forming bacteria in significant numbers is a notable point in the groundwater ecosystem. Because endospores are appreciated for their long term survival. Filamentous Spore Forming Microorganisms - Both actinomycetes and fungi are spore-forming filamentous microorganisms. Inspite of their spore-forming ability they are found in very low numbers in groundwater ecosystems. However their contributions to the aerobic heterotrophic metabolism is quite high within the larger subsurface microbial community. 8 Estuary - Estuary, which is the coastal adjunct of the marine ecosystem, can itself be studied as an ecosystem since its physical, chemical and biological characteristics are much different from the marine environment. An estuary is a semiepclosed coastal body of water which has a free connection with the open sea (pelczar et al, 1986) Unlike those of ocean waters, the characteristics of an estuary such as salinity, temperature, turbidity, and nutrient load are not generally constant. These characteristics fluctuate over a wide range in relation to space and time. Bacterial Production - Estuary, which is the coastal adjunct of the marine ecosystem, can itself be studied as an ecosystem since its physical, chemical and biological characteristics are much different from the marine environment. An estuary is a semiepclosed coastal body of water which has a free connection with the open sea (pelczar et al, 1986). Unlike those of ocean waters, the characteristics of an estuary such as salinity, temperature, turbidity, and nutrient load are not generally constant. These characteristics fluctuate over a wide range in relation to space and time. For example, an ideal estuary has a salinity gradient from less than 5% at the upper end to more than 25% at the mouth. Estuaries are complex and are characterized by intensive exchange of water, energy and materials across the margin between marine and freshwater environments. Structure of Lentic Habitat Lentic ecosystems include all standing water (freshwater) habitats such as lakes, ponds, marshes, swamps, bogs, etc. Lakes are inland depressions containing standing water. They may vary in size from small ponds of less than a hectare to large lakes covering thousands of square kilometres. They may range in depth from a few centimetres to over 1666 metres. Ponds however are considered as small bodies of standing water so shallow that rooted plants can grow over most of the bottom. Most ponds and lakes have outlet streams and both are more or less temporary features on the landscape. 9 The aquatic habitats of lake and pond remain vertically stratified in relation to light intensity, wavelength absorption, hydrostatic pressure, temperature etc. In a lake there are well recognised horizontal strata which include: i. Shallow water near the shore forms the littoral zone. It contains upper warm and oxygen-rich circulating water layer which is called epilimnion. The littoral zone includes rooted vegetation. ii. Sublittoral zone extends from rooted vegetation to the non-circulating cold water with poor oxygen zone, i.e. hypolimnion. iii. Limnetic zone is the open water zone away from the shore. It is the zone upto the depth of effective light penetration where rate of photosynthesis is equal to the rate of respiration. iv. Profundal zone is the deep water area beneath limnetic zone and beyond the depth of effective light penetration. v. Abyssal zone is found only in deep lakes, since it begins at about 2,000 meters from the surface. Ponds have little vertical stratification. In ponds, the littoral zone is larger than the limnetic zone and Profundal zone. In a small pond the limnetic and Profundal zone are not found. 10 Biota of Lentic Habitat Different organisms of the lentic environment can be ecologically classified based on whether they are dependent on the substratum or free from it. Organisms depending on the substratum are called pedonic forms and those that are free from it are the limnetic forms. Further, the aquatic organisms may also be classified into the following groups depending upon their sizes and habits. Neuston These are unattached organisms that live at the air-water interface. They may consist of microbial forms, plants and animals. Animals and microbial forms that spend their lives on top of the airwater interface, such as water striders are termed epineuston while others including insects such as beetles which spend most of their time on the underside of the air-water interface and obtain much of their food from within water, are termed hyponeuston Plankton These are forms which are found in all aquatic ecosystems except fast moving rivers. They are small plants and microbial forms whose powers of self-locomotion are so limited that they cannot overcome currents. Thus their distribution is controlled largely by the currents in their ecosystems. The phytoplanktons and zooplanktons can move a bit, either to control their vertical distribution or to seize prey. Certain zooplanktons are exceedingly active and move relatively great distances considering their small size, but they are so small that their range is still controlled largely by currents and such plank tons are also called nektoplankton. Nekton Nektonic animals are swimmers and are found in all aquatic systems except for fast moving rivers. In order to overcome currents, these animals are relatively large and powerful. Benthos The benthos include the organisms living at the bottom of the water mass. They occur virtually in all aquatic ecosystems. The benthos organisms living above the sediment-water interface are termed benthic epifauna and those living in the sediment itself are termed infauna. The water surface of a lake or pond contains certain free floating hydrophytes such as Azolla, Lemma, Wolffa, etc. The microbial community lives associated with the surface film. The habitat is unusual in many ways since it is subjected to rapid temperature fluctuations, increasing to high light intensity and is 11 very well aerated. Organic matter especially particulate and inorganic nutrients such as phosphates, accumulate on the surface of the film. The habitat is favourable for photoautotrophs. There are increased numbers of bacteria especially Pseudomonas and Caulobacter Neuston is a stressed habitat and microbial growth rate is low. Bacteria in neuston have hydrophobic surfaces and produce extracellular polysaccharides both of which are concerned with adsorption on the surface film. Bubbles arising through the neuston layer and bursting play a major role in water-to-air transfer of bacteria and viruses. Characteristic autochthonous neuston microbiota include algae, fungi and protozoa. Cyanobacterial species include Aphanizomenon, Anabaena, Microcytis. Filamentous fungus include Cladosporium. Yeasts are also found. Among protozoan species include Diffugia, Arcella, Acinata, etc. Aquatic life is most prolific in the littoral zone. The littoral zone of a lake remains rich in pedonic flora especially up to the depth to which effective light penetration is possible facilitating the growth of rooted vegetation. Microorganisms exhibit different absorption spectra determining which wavelength of light can be utilised for photosynthesis. Green and purple sulphur bacteria grow at the sediment-water interface below the layers of short wavelength absorbing algal and cyanobacterial growth by utilising wavelength of light not absorbed by the overlying phytoplankton because the purple and green sulphur bacteria obtain electrons from hydrogen sulphide at lower energy loss than water splitting photoautotrophs and thus require lower light intensities for carrying out photosynthesis. Conditions in the euphotic zone (area of light penetration) are favourable for the growth of photoautotrophs. The bottom of the lake (benthos) represents the interface between hydrosphere and lithosphere. Sedimented organisms in the profundal zone are largely secondary producers and are dependent on the transport of organic compounds from the overlying zone. Particulate nutrients sedimented by gravitational forces concentrate on the surface of the sediment. Growth occurs on the sand grains (sediment) if they are relatively undisturbed by currents and wave action. Flora consists of bacteria and small diatoms (Fragilaria, Opephora). There are also some motile organisms like diatoms (Nitzhia), cyanobacteria and bacteria. In a fresh water lake, mud found at the sediment, the acidity, water type and the nutrient status controls the microorganisms found there. All muds have diatoms as the major group of microflora. Bacteria are more on the surface and anaerobes are found for some depth in the mud. Oxygen can diffuse only very slowly in the waterfilled pore spaces of sediments. Concentration of inorganic nutrients (nitrogen and phosphorus) are important in determining the ability of the habitat to support microbial growth and metabolism. Salt concentration of water and pH also influence the characteristic autochthonous microbes of some lakes with high salt concentration (28%) to develop halophilic (organisms which can grow at high salt concentrations) population (archaea and some algae, e.g. Dunaliella). Structure of the Lotic Habitat Running or moving water or lotic ecosystems include rivers, streams and related environment. They are remarkably variable, ranging in size from large rivers to the trickling small springs. The lotic habitat is primarily determined by the velocity of the current which can create either slow- 12 moving or fast-moving streams; each has very distinct characteristics. The base of the food chain is dependent on detritus from upstream or from the edges. In slow-moving streams, plant and animal communities largely resemble those found in lentic (lake and pond) habitats. The significant phytoplankton populations that usually exist contribute to a higher rate of primary productivity than that found in fast-moving streams. The level of productivity is dependent upon water temperature and the amount of nutrient input received from the surrounding environment, and therefore subject to seasonal variation. The diversity of consumer organisms varies according to the physical conditions and vegetation. Planktonic populations are relatively high, although not as dense as those found in lakes. In fast-moving streams, there is very little primary production in the open-water habitat, due to the velocity and turbulence of the current. Populations of consumer organisms (mainly particulate feeders) are low. Riffle areas provide valuable habitat for juvenile trout and salmon. Pools are important resting areas for several fish species, including Atlantic salmon. The quality of these areas can be adversely affected when shade trees are removed from the banks. Successional Sequence The normally understood process of ecological succession does not apply to open water. In slow-moving streams, the development of habitat depends upon the depositional and erosional characteristics of the river. The fast flowing, young streams will always be present as the river erodes the landscape. Over time, the young stream will mature into a slow-moving stream, but it can be rejuvenated when a geological obstacle (e.g. a waterfall) is encountered. In mature streams, there is a progressive downstream movement of meanders, leaving shallow or deep pools, backwaters, braided channels and oxbow ponds. There is an associated change in the character of the open water Biota of the Lotic Habitat Vegetation in the lotic open-water habitat consists mainly of phytoplankton found in slow-moving streams. There are no plankton species unique to rivers; those found there originate mostly from backwaters or lakes. Several species of desmids and diatoms present in slow-moving rivers, although not as abundant as in lakes. Some zooplankton species and rotifers can be found in slow-moving streams. Their abundance depends on the amount of the predation from invertebrates and small fish. Fish species such as redbelly dace and white sucker, and introduced species such as brown trout are commonly found in slow-moving streams. Fast-moving streams provide excellent habitat for many kinds of fish, including brook trout, Atlantic salmon parr, common shiner, white suckers and yellow perch. Several factors affect the microbial flora of stored waters. These are as follows Sedimentation: Microorganisms have a specific gravity slightly greater than that of' water, and therefore slowly settle down. However, the most important factor is their attachment to suspended particles. Microorganisms are removed from the upper layers of the water as the suspended particles settle down. Activities of other organisms: Predatory Protozoa engulf living or dead bacteria for food, provided the water contains sufficient dissolved oxygen. 13 Light ray: Direct sunlight is toxic to both vegetative cells and spores of microorganisms. The toxicity of ultraviolet rays is inversely proportional to the turbidity of water. In tropical countries direct sun light is a very effective sterilizing agent. Temperature. Temperature has variable effects. It may kill some organisms and may stimulate the growth of others. During colder months the multiplication rate of microorganisms is considerably reduced. Food supply. If there is considerable vegatation or suspended food particles in the body of water, it is likely to increase the number of organisms. On the other hand, certain toxic substances may bring about marked reduction in the number of, organisms. Microflora of Different Water Unpolluted Actinomycetes Yeasts Bacillus spores Clostridium spores Cellulose digesters Autotrophic bacteria Euglena Paramecium Polluted Coliform bacteria Escherichia coli Desulfovibrio sp. Clostridium sp. Fecal streptococcci Pritozoan cysts Blue-green algae Enteric viruses Marine Halophilic organisms Psychrophilic organisms Diatoms Dinoflagellates Mold spores Pseudomonas sp. Forminiferans Luminous microbes *** Microbial Communities in Aquatic Environments- Various habitats can be recognized within an aquatic environment and their actual size may vary from one another. Each habitat is characterized by one or more microbial communities and thus the microorganisms within an aquatic environment can be grouped as distinct communities. Neuston - The microorganisms which are found at the surface of an aquatic environment, exactly at the air-water interface, are referred to as neuston. Pleuston - specific term pleuston may be used to denote the organisms occupying the air-water interface in a marine biota. These neuston and pleuston can be regarded as specialized 14 communities as their air-water interface habitat is subjected to widely fluctuating environmental conditions. Plankton - Plankton refers to the free floating microorganisms, which are maintained in the water column. An important criterion for the plankton is that they should exist, at least for a part of their life, free in the open water. It should not include stray organisms washed in from the surrounding land or from other aquatic environments. Phytoplankton - Plankton can also be classified on the basis of the constituent organisms and their feeding habits. Phytoplankton includes photosynthetic forms like algae. Usually phytoplankton occupies the top layers of water bodies where light is available to carry out photosynthesis. Phytoplanktons are the major primary producers in an aquatic environment and they convert the radiant energy into chemical energy. Zooplankton - Zooplanktons are found in regions just below that of phytoplanktons. Zooplanktons are motile forms with cilia or flagella and show vertical migration. Vertical Migration - Zooplanktons are motile forms with cilia or flagella and show vertical migration. During day time and feed on their prey. Diurnal Migration - Zooplanktons are motile forms with cilia or flagella and show vertical migration. During day time and feed on their prey. Thus they exhibit a diurnal migration. Sometimes, bacteria in the water column may be considered as a separate planktonic group which are called bacterioplankton. Bacterioplankton - The second group namely zooplankton consists mainly of protozoans which feed on phytoplanktons, bacteria and detritus. Zooplanktons are found in regions just below that of phytoplanktons they try to escape from light and sink to the bottom regions. During night they come to the surface Zooplanktons are motile forms with cilia or flagella and show vertical migration. During day time and feed on their prey. Thus they exhibit a diurnal migration. Sometimes, bacteria in the water column may be considered as a separate planktonic group which are called bacterioplankton. Seston Epipelon- Seston produced by nekton, i.e. large swimming animals, may also act as a habitat along with other organic debris. Microorganisms inhabiting these detritus matters and other fine sediment surface are referred to as epipelon. Epibiotic Communities Epibacteria Periphytes - Inanimate as well as animate surfaces also provide habitats for microorganisms. These attached microorganisms are referred to as epibiotic 15 communities. Sometimes the attached bacteria are also referred to as epibacteria or periphytes. In contrast to the epibiotic community, endobiotic community inhabits the tissues of larger aquatic forms. Microorganisms belonging to this community may exhibit a mutualistic or parasitic relationship with the host organisms. Epizooic Community Epiphyton- The term epizooic community is also used to denote the microorganisms associated with aquatic animals. Similarly, epiphyton refers to microorganisms found on various aquatic plants and algae. Epilithon Epixylon Episammon- Epilithon is another community which includes the attached microorganisms found on the rocks and stones of the aquatic environment. Epixylon refers to the microorganisms found on the fallen woods in water bodies. Sand grains also support microbial growth and the associated microorganisms are referred to as episammon. Benthic Community Benthos-Organisms inhabiting the bottom sediment of aquatic environments constitute the benthic community or benthos. Peleagic Community-Apart from these, a separate waterborne community has also been recognized which occurs at precise depths. It is referred to as pelagic community. Starting from the surface of water and descending to the bottom four different pelagic habitats occur. They are epipelagic, mesopelagic, bathypelagic and abyssopelagic. **********