Marine Microbiology · Web viewThe range of saline concentrations creates three types of niches: fresh water, brackish water, and saline water. Each niche is occupied by organisms

Marine Microbiology

The study of microorganisms living in the sea is known as marine microbiology. Nearly three fourths of the earth's surface is covered by the sea .The sea therefore is the largest natural environment inhabited by the microorganisms. The marine environment, as does land, contains bacteria, Protozoa, algae, yeasts and moulds, as well as viruses forms organisms that live in the sea.

Microorganisms which are found in the surface layers of the sea and other aquatic environment are collectively known as plankton. Plankton may be further sub divided into phytoplankton (photosynthetic microorganisms, algae) and zooplankton (Protozoa, and other microscopic animals). The former are more important plankton organisms, as they arc the primary producers of organic matter.

Bacteria and fungi are also present in large numbers in the plankton. Microorganisms present in the bottom regions (sediments) arc designated the benthos or benthic community. A variety of microorganisms arc found in this region. Bacteria, however, predominate.

The microbial flora characteristic of tile sea has one factor in its environmental pattern which differs from freshwater lakes, streams, and rivers, for the sea is salty. There is a high concentration of salt and mineral ions in the sea. The principal salts are the chlorides, sulphates, and carbonates of sodium, potassium, calcium and magnesium.

The concentration of dissolved salts (salinity) averages about 35 gin/kg or 3.5% by weight. Out of this 75% is NaCl. Marine microorganisms, therefore; may be divided into two general types: (I) those indigenous to the sea and not growing on media without sea water or high salt concentration and (2) transient organisms whose natural habitat is terrestrial and which are able to grow in media without sea water, but can tolerate high salt concentration.

There is also another group of microorganisms in the sea, in the depths of the sea microorganisms live at tremendous hydrostatic pressures, upto 1000 atmospheres. These organisms when brought to the surface require not only sea water but high pressure in order to grow. Such pressures are toxic to organisms normally living at one atmosphere at the surface.

Organization of the Ecosystem:

The marine environment is characterized by saline water (Atlas, 1998). The ocean is rich in water soluble minerals including larger amounts of sodium, chlorine, magnesium, sulfate, calcium, potassium; and, lesser amounts of nitrogen, phosphorous and iron. The coastal areas provide a wide diversity of ecological niches where fresh water and saline water converge in estuaries; however, the ocean is fairly uniform due to mechanical mixing mechanisms. Ocean saline concentrations average 3.5 parts per thousand whereas estuary saline concentrations range between 5 and 25 parts per thousand creating a wide range of ecosystem environments.

The marine environment is stratified vertically and horizontally; the key factors that change are light penetration, temperature, nutrients and oxygen content. The horizontal zones that extend from the shore are the littoral or intertidal zone, the sublittoral or neritic zone, and the pelagic or open water zone. The pleuston is the tension layer at the water and air interface. The water column is stratified by light

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penetration; photosynthesis occurs in the upper euphotic water layer, to a depth of 25 metres from the surface, and below is the disphotic or aphotic zone. The vertical regions that extend downward from the intertidal zone are the benthic region, the continental slope or bathyl region, the deep-sea floor or abyssal plain, and the deep ocean trenches or hadal region. Each zone or region has different characteristics; photon penetration, available nutrients, oxygen content and temperature differentials create specific microbial niches.

Advantages and Limitations to Microbial Growth:

The estuarine environment is unique for microbes because of the constantly changing environmental parameters that create a wide diversity of ecological niches (Atlas, 1998). Estuaries have high nutrients and high photon energy; consequently, they are the most productive marine ecosystem for photosynthetic aerobes. The range of saline concentrations creates three types of niches: fresh water, brackish water, and saline water. Each niche is occupied by organisms that are adapted for those conditions. This form of ecological partitioning reduces exploitative competition and enhances growth of different types of microbial communities (Campbell, 1993). Similarly, the continental shelf and the coral reefs are areas of high productivity due to high nutrients and photon energy, but without the extreme salinity gradient (Atlas, 1998).

Conversely, the ocean zones are not as highly productive except for the pleuston layer where there is adequate light for bacteria that are primary producers. The pelagic offshore zone does not have enough nutrients at the surface to support significant microbial growth. The primary producers lyse and sink to the deep benthic zone. The benthic zone, rich in nutrients, does not have enough light energy to support primary productivity.

Other forms of energy deep in the hadal trenches, combined with fresh outpourings of chemical nutrients from the molten core, provide the environmental conditions for islands of deep undersea communities of rare bacteria and invertebrate species (Geographic, 1979). The deep sea vents occur on the ocean floor where the ocean crustal plates spread apart, and cause plumes of hot lava to erupt into the ocean. The high concentrations of electron rich elemental compounds and the thermal heat from the lava vents create an extreme environment that is most akin to primordial earth conditions. This is the realm of the anaerobes such as methanogenic Archaebacteria, and aphotic eubacteria such as the chemoautotrophs (Campbell, 1993). Each type of bacteria has special adaptations that enable the organism to obtain metabolic energy, and to withstand the extreme environmental conditions of high pressure and temperatures over one hundred degrees Celsius.

Physical and Chemical Characteristics of the Marine EnvironmentIn general, water is most essential for the maintenance and activities in all life forms. Water is an important component of cell constituting about 80% of the weight of protoplasm. For the photosynthetic process in autotrophic plants, water itself is an important raw material. It is a universal solvent, which carries the necessary gases like oxygen and carbon dioxide and also the growth regulating minerals in dissolved condition. In aquatic environments, there is no problem of dessication and consequently no specialisation in the organisms.

In view of high transparency of water, photosynthesis is possible even at a relatively deeper level. The sea water can change from acid to alkaline condition and vice versa. This buffering nature is useful to organisms because an abundant supply of carbon dioxide is necessary for plants for photosynthesis. In case of alkaline condition, the construction of shells by marine organisms by using calcium carbonate is

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enhanced. The lower specific gravity of sea water is most beneficial to marine organisms. As the sea water contains large number of salts it is a most suitable, environment for living cells. Further it has been found that the ratio of total saIt content of seawater is almost same as that of body fluids of many invertebrates.

In general, the marine environment offers a wide range of living conditions. The salinity ranges from very dilute estuarine condition to as high as 37 %. Varying light conditions exist with brilliant sunlight at the surface waters to no light in the deep waters. Likewise, the pressure varies from 1 atmosphere at the surface to 1000 atmospheres at greater depths. These gradients of environmental  parameters are favourable to a number of sensitive animals. The more fluctuations of environmental features are especially encountered in coastal areas due to their peculiar physiographic characters. The water movement/  circulation is useful in the oxygenation of subsurface water, for the dispersal of metabolic wastes and plant and animal (growth) nutrients, and also for the disposal of spores, eggs, larvae and even adults

Structure of the SeaThe littoral (eulittoral) zone occurs at the seashore. This zone is subjected to alternate periods of flooding and drying at high and low tides respectively. The sublittoral zone extends from the low tide mark to the edge of the continental shelf. This region is also called as the neritic, or near shore zone. The term pelagic is used to designate open water or the high sea and includes portions of the neritic and the entirety of the oceanic province. The benthos or benthic region is the bottom, regardless of the overlying zone

The benthic region begins at the intertidal zone (littoral zone) and extends downwards. The continental shelf is a gently sloping benthic region that extends away from the land mass. At the continental shelf edge, the slope greatly increases. The continental slope, also known as the bathy region, drops down to the sea floor. The deep-sea floor is known as the abyssal plain and usually lies at about 4000m. The ocean floor is not flat but has deep ocean trenches and submarine ridges. The deep ocean trenches are called a the hadal region.

Archaebacteria Autotrophic eubacteria Chemoheterotrophic eubacteria Eukaryotes

Chemoautotrophs Photoautotrophs Gram-positive Photoautotrophs

Methanogens Anoxygenic photosynthesis

Endospore-forming rods and cocci

Microalgae Chemoheterotrophs

Thermoacidphiles Purple and green photosynthetic bacteria (Order Rhodospirillales)

Non-spore forming rods

Non-spore forming cocci

(Family Micrococaeae)

Protozoan flagellates

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Chemoheterotrophs

Oxygenic photosynthesis

Cyanobacteria

(Order Cyanobacteriales)

Prochlorophytes

(Order Prochlorales)

Chemoautotrophs

Nitrifying bacteria

(Family Nitrobacteriaceae)

Colourless sulphur

oxidising bacteria

Methane oxidisingbacteria

(Family Methylococcaceae)

Actinomycetes (Order

Actinomycetales) and

related organisms.

Aerobic Gram-negative

rods and cocci (Family

Pseudomonadaceae)

Facultative aerobe

(Family Vibrionaceae)

Anaerobic sulphur reducing

bacteria Gliding bacteria

(Order Cytophagales and

Beggiatoales) Spirochaetes

(Order Spirochaetales)

Spiral and curved bacteria

(Family Spirillaceae)

Budding and/ or appendaged

bacteria Mycoplasmas

(class Mollicutes)

Amoebae

Ciliates

Fungi

Higher Fungi

Ascomycetes

Deuteromycetes

Basidiomycetes

Lower fungi (Class

Phycomycetes)

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Bacterial Flora of the SeaThey are more abundant near the shore particularly in polluted areas. They are sparsest at great depths in open oceans. The generation time of bacteria ranges from less than an hour to months or even longer. The shortest generation time reported is 9.8 minutes for Pseudomonas natrieganus. The most characteristic feature of marine bacteria is their capacity to survive and grow in the sea. In coastal regions (where river water enters the sea) there is no sharp distinction between freshwater forms and marine forms. Excepting spores, most freshwater forms perish within a few hours under oceanic conditions. Certain bacterial endospores or fungal spores seem to survive in a dormant state for prolonged periods.

The, typical characteristic of marine bacteria are their small size (0.1 mm3) and low DNA content. They are obligately oligotrophic. Most oceanic bacteria grow more slowly and form smaller colonies on submerged slide (a technique used to study the occurrence of marine flora). Oceanic bacteria are more

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proteolytic than soil bacteria that are saccharolytic and more often they are facultatively aerobic. Most of the marine bacteria liquefy agar.

Most bioluminescent bacteria come from the sea including the saprophytic, symbiotic and commensal species. Oceanic flora can survive extremely low concentrations of nutrients (20 mg/l) hence they are most often associated with algal surfaces or detrital particles which offer nutritional advantages

Most of the oceanic bacteria grow at 0°C (optima is12-25°C). Heating sea samples for 10 min at 30-40°C kills 80% of the bacteria. Most of the sea flora are barotolerant and grow at pH of 7.2-7.6 at the surface and pH 6.4-9.4 in marine sediments. Marine bacteria have a highly specific need for sodium and chlorine. Some marine bacteria have multiple membranes surrounding their cells. Exposure to fresh water, disrupts these membrane layers causing a loss of viability.

A high percentage of marine bacteria have a tendency to grow attached to solid surfaces (hence submerged slide technique is used extensively to study the occurrence of marine flora). Numerous species are attached to plankton or to larger organisms.

The highest biomass of microflora in marine waters is normally seen near the surface and they decrease with depth. Some microflora growing in the surface go more downwards often attached to sediment particles. They provide food sources to organisms growing in pelagic habitats. High numbers of heterotrophic bacteria and cyanobacteria are transported into the deep sea attached to rapidly sedimenting particles.

Hence, there is every possibility of genetic exchange between populations previously assumed to be genetically isolated.

Most marine bacteria are gram negative and motile. For example, Pseudomonas, Vibrio are dominant genera followed by Flavobacterium, Alcaligenes and  Microcystis. Actinomycetes are also found in marine waters. Some gram positive bacteria like Bacillus are found in marine sediments. Below the sediments, anaerobic bacteria are composed of the autochthonous microorganisms including Desulfovibrio

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and methanogenic bacteria. Chemolithotrophs include Nitrosomonas, Nitrosococcus, Nitrospira and Nitrobacter.

Fungal flora of the sea Yeast are more common, e.g. Torulopsis, Candida, Cryptococcus, Saccharomyces, Rhodosporidium.

Algal flora of the sea The algal flora usually belong to Chlorophyta, Phaeophyta, Rhodophyta, Cyanophyta.e.g. Sargassum, Fucus. Chlorophyta and Chrysophyta members are prominent among phytoplankton found in the upper regions of surface (0-50 m). Planktonic cyanobacteria are common forms, e.g. Trichodesmium. Occasional blooms of Pyrrophycophyta members are common in the oceans causing the 'red tides' (ocean becomes red brown in colour). The toxins produced by some of the dinoflagellates kill fishes and other marine animals.

Some of the dinoflagellates produce neurotoxins which accumulate in shell fishes that feed on them. When ingested by humans such tainted sea food can cause paralytic shell fish poisoning (PSP).

Protozoan flora of the sea They are important component of marine zooplankton. They can tolerate up to 10% NaCl concentration. They include flagellates, rhizopods and ciliates, e.g. Radiolaria, Acantharia. They occur in the deeper layers of the sea. Marine protozoa graze on bacteria, phytoplankton and smaller forms of zooplanktons. Grazing provides a critical link in the marine food web between Very small primary producers and the higher members of the marine food web.

Distribution of Microorganisms in Ocean and Sea - Microorganisms have been found at all depths and at all latitudes within the ocean/sea. Their numbers vary from a few per litre to about 108 per ml. Bacteria are abundant in near shore regions, where organic pollution as well as microbial pollution from the surrounding environment occurs.The topmost layer of marine environment has a very high bacterial content, about 10 to 05 bacteria per m] upto a depth of l000m. These regions usually contain aerobic photosynthetic bacteria and algae. With the increasing depth bacterial abundance tend to decrease, usually less than 104 per ml at depths exceeding l000m.

The decrease in bacterial numbers with increasing depth appears to be a function of hydrostatic pressure. The upper layers of sediments contain about 103 to 109 bacteria per gram.Chemolithotrophic Bacteria in Marine Environment - Various chemolithotrophic bacteria have been reported to be abundant in marine environment. These organisms have the ability to oxidise inorganic compounds as an energy source.

Most of them are Gram-negative bacteria and are grouped on the basis of their energy source.Nitrite Oxidising Bacteria - Four genera have been recognized in nitrite oxidising bacteria. They are Nitrobacter, Nitrococcus, Nitrospina and Nitrospira which occupy strictly aerobic habitats. Among these genera Nitrobacter is the important one, with a single species Nitrobacter winogradskyi.

They are Gram-negative rods which reproduce by budding. Nitrosococcus contains a single species Nitrococcus mobilis and Nitrospina also includes a single species Nitrospina gracilis. Nitrospira marina is the only member of the newly recognized genus Nitrospira.Ammonia Oxidising Bacteria - Ammonia oxidising bacteria include three genera, each having a single species. They are Nitrosococcus oceanus, Nitrosomonas europaea and Nitrospora briensis.

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Sulphur Bacteria - Sulphur bacteria are also important in chemolithotrophic activity. They are characterized by the presence of intracellular droplets or inclusions. Macromonas bipunctata, M. mobilis and Thiobacterium bovista are important species in this group. Other examples include Achromatium, Thiobacillus, Thiospira, Thiomicrospora and Thiovulum.Methane Producing Bacteria in Marine environment - Methane production is an important anaerobic process in marine environment taking place in anaerobic marine sediments. Five families have been recognized in methane bacteria, Methano-bacteriaceae, Methanococcaceae, Methanomicrobiaceae, Methanoplanaceae and Methanosarcinaceae. Most of which are anaerobes.Characterestics of Marine Microorganisms - Since the marine environment is characterized by environmental conditions which are entirely different from other environments, its microflora is also very much different from that of others.

Marine microflora has its own special characteristics which enable it to survive in that environment. The main characteristic of marine flora is its capacity to survive and grow in sea/ocean.Most of the bacterial flora in ocean are Gram-negative rods. The dominance of Gram-negative bacteria in aquatic environment is due to their cell structure. Aquatic environments are nutritionally dilute when compared with terrestrial environment. Under such conditions the outer membrane, especially the lipopolysaccharide (LPS), of Gram-negative bacteria helps to absorb nutrients.

Various important hydrolytic enzymes are retained within the periplasmic space. Thus, the possible dilution of enzymes, which may occur when they are excreted out into the environment, is prevented. In addition LPS gives protection against toxic molecules like fatty acids and antibiotics. A larger proportion of the marine bacteria is motile and pigments producers.Facultative aerobes dominate sea and there are relatively very few obligate aerobes and obligate anaerobes. When cultured most oceanic bacteria grow more slowly and form smaller colonies than those from other environments. Many of them are capable of proteolysis.As with any other aquatic bacteria, marine bacteria can grow in extremely low concentrations of nutrients and hence are called oligotrophic bacteria. Generally, the concentration of organic matter in seawater will be less than mg per liter.

Other interesting features of marine microorganisms are their ability to survive at very low temperature and at high salinity. The groups exhibiting the above characteristics are referred to as psychrophiles and halophiles respectively. Marine bacteria are also characterized by their pressure tolerance, especially those at depths. These forms belong to the group of barophiles.The generation time of marine bacteria is quite long, ranging from less than one hour to many months. The shortest generation time, 9.8 minutes, has been reported for Pseudomonas natriegens at 37°C. In marine environment, when freeliving bacteria are compared with attached bacteria the latter are bigger in size.

This may be due to the fact that attached forms assimilate more dissolved organic matter than the free living formsBacterioplankton - 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.

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Factors Affecting The Microbial Population in Natural Water - The various factors that affect the microbial population in natural waters include the physical and chemical conditions prevailing in that particular habitat. These conditions vary over a wide range in different aquatic habitats.

Temperature- Temperature is one of the main factor that has a profound effect on 'the growth of microorganisms. Microorganisms which require very low temperature for their growth are known as psychrophiles and which require a high temperature are known as thermophiles. The group which falls between these two are known as mesophiles. The temperature of natural waters varies from about 0 C in polar regions to even upto 75 to 80oe in hot springs.

Since the specific thermal capacity of water is very high, large water bodies are able to either absorb or lose great amount of heat energy without any change in their temperature. As a result of this many aquatic organisms including microbes experience very stable climatic conditions. For instance, about 90% of the marine habitats maintain more or less a constant temperature of about 4°C which favours Psychrophilic microorganisms.

Vibrio marinus is the best known example for Psychrophilic bacterium from marine ecosystem. Since most aquatic habitats maintain low temperatures majority of the aquatic microorganisms are psychrophiles. Thermus aquatic us, a common thermophilic bacterium, survives in hot springs with an optimum growth temperature of 70 to 72°C. Recent, unconfirmed report suggest that certain extreme thermophilic microorganisms are capable of growing at 250°C and 265 atmospheric pressure.

The temperature in other water bodies like lakes, streams and estuaries shows seasonal variations with corresponding changes in the microbial population.

Light- Almost all forms of life in aquatic habitats are either directly or indirectly influenced by light, because primary production is mainly through photosynthesis. Since algae and photosynthetic bacteria are involved in photosynthesis, they are the most important forms with respect to light. Light is also important in the spatial distribution of microorganisms, especially the photosynthetic forms.

Photosynthetic micro-organisms are mainly restricted to the upper layers of aquatic systems, the photic zone, where effective penetration of light occurs. Although photosynthesis is confined to the upper 50 to 125m of the water bodies, the depth of the photic zone may vary depending upon the latitude, season and turbidity. Apart from the decrease in the quantity of light with depth there is also a change in the color of the light. Blue light is transmitted most while the red is the least.

Light may also be bactericidal. It has been suggested that the reason for the death of sewage bacteria in sea water is due to light. Sometimes there may be a reduction in the bacterial activity without killing. For example, reduction in the rate of oxidation occur in Nitrosomonas and Nitrobacter due to high light intensities.

Cell division may also be related to the day and night variation. The diatom Nitzschia divides mostly in the light, while the dinoflagellate Ceratium divides during darkness.

Salinity- A wide range of salinity occurs in natural waters. For example, salinity is near zero in fresh water and almost at saturation level in salt lakes.

A clear distinction can be made between the flora and fauna of fresh and salt water systems due to the

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variation in the salinity levels. The dissolved salt concentration of sea water varies from 33 to 37g per kg of water. Nutrients - Supply of nutrients is one of the major determinants of microbial biomass and variety. Both organic and inorganic nutrients playa vital role. Nutrients may occur either as dissolved material or as a particulate material.

The amount may vary from near zero in spring to 1-2 mg per litre in deep ocean and certain lakes. Aquatic environments with limited nutrient content are said to be oligotrophic and those with a high nutrient content are said to be eutrophic. Turbidity- Since turbidity of water in an aquatic environment inhibits the effective penetration of light, it is also considered as an important factor affecting microbial life.

Turbidity is mainly caused by suspended materials, which include particles of mineral material originating from land, detritus, particulate organic material such as cellulose, hemicellulose and chitin fragments and suspended microorganisms.Water Currents-Water currents are obvious in lentic habitats like rivers. Although washing off by water current helps in dispersal, removal from suitable habitat may also occur. The water current in rivers leads to a phenomenon known as nutrient spiralling. In ocean, water currents or geothermal vents are responsible for the mixing up of nutrients.

Usually in deep waters the water currents may move in opposite direction to those on the surface and usually they are slower. The velocity of water within a water body especially in rivers, have been found to influence the nutrient uptake and metabolism.

Studies on biofilms have also shown that, under phosphorus limiting conditions, a small increase in water velocity maximized the uptake of phosphorus.Lakes and Ponds- Lakes and ponds constitute freshwater habitats along with rivers and streams. The study of flora and fauna of lakes and ponds and their abiotic characteristics is referred to as limnology. They are regarded as well defined ecosystems. Lakes can be classified on the basis of their productivity and nutrient contents into two types oligotrophic lakes and eutrophic lakes.Oligotrophic Lakes- Oligotrophic lakes are those which have a very low nutrient content. They are comparatively deeper than the other type. They have a larger hypolimnion and a smaller epilimnion. Because of the limited nutrient content oligotrophic lakes are characterized by low primary productivity.

Functions of Marine Flora1. They cause spoilage of fish and other marine food products. Clostridium botulinum is quite commonly present in fishes, clams and oysters.

2. Harmful bacteria bring about the deterioration of various man-made structures such a fish nets, ropes, soil cloth, etc.

3. The most important activity of bacteria in the sea is the mineralization, modification and synthesis of organic matter (carbohydrates, proteins, hydrocarbons, chitin)

4. Both chemosynthetic and photosynthetic autotrophs occur in the sea, thus increasing the primary productivity of the sea.5. More than 100 species of microbes have been shown to fix nitrogen found in sea water /bottom deposits.

6. Certain chemical transformations caused by bacteria tend to affect

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the pH of the environment.

pH may be increased by (a) formation of ammonia, (b) reduction of nitrates, nitrites and sulphates, (c) oxidation or decarboxylation of organic acids, or (d) reduction of carbon dioxide.

pH can be decreased by (a) bacterial oxidation of ammonia to nitrite andnitrate, (b) oxidation of sulphur and hydrogen sulphide to sulphate,(c) formation of organic acids/carbon dioxide from carbohydrates, or(d) liberation of phosphates from phospholipids.

Generally these activities go hand in hand thus maintaining neutrality.

Sometimes either one may predominate as in the sediments

Marine bacteria are believed to promote the transformation of petroleum from marine sediments. In highly reducing environments (anaerobic), bacteria tend to convert organic matter partly into carbon compounds relatively richer in hydrogen and poorer in oxygen, nitrogen, sulphur and phosphorus. This

results in the formation of carbon compounds that are more like petroleum.

Some marine bacteria can oxidise oil by degrading the oil; they can thus reduce danger of oil pollution in the marine environment.

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Marine Zones and Stratifications

The neritic zone, also called the sublittoral zone, is the part of the ocean extending from the low tide mark to the edge of the continental shelf, with a relatively shallow depth extending to about 200 meters (100 fathoms). The neritic zone has generally well-oxygenated water, low water pressure, and relatively stable temperature and salinity levels. These, combined with presence of light and the resulting photosynthetic life, such as phytoplankton and floating sargassum, make the neritic zone the location of the majority of sea life.

The continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during glacial periods, but is undersea during interglacial periods such as the current epoch by relatively shallow seas (known as shelf seas) and gulfs.

The photic zone or euphotic zone (Greek 'well lit') is the depth of the water in a lake or ocean, that is exposed to sufficient sunlight for photosynthesis to occur. The depth of the photic zone can be greatly affected by seasonal turbidity.

The aphotic zone (aphotic from Greek "without light") is the portion of a lake or ocean where there is little or no sunlight.

It is formally defined as the depths beyond which less than 1% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone. Most food comes from dead organisms sinking to the bottom of the lake or ocean from overlying waters.

The mesopelagic (Greek μέσον, middle) (also known as the middle pelagic or twilight zone) is a pelagic zone extending from 200 m (650 ft.) down to around 1000 m (3280 ft.) below sea level. It is located between the photic epipelagic and the aphotic bathypelagic, where there is no light at all.

The bathyal zone or bathypelagic – from Greek (bathýs), deep – is the pelagic zone that extends from a depth of 1000 to 4000 meters below the ocean surface. It lies between the mesopelagic

above, and the abyssopelagic below. The average temperature hovers at about 4 °C (39 °F). Although larger by volume than the euphotic zone, the bathyal zone is less densely populated. Sunlight does not

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reach this zone, meaning there can be no primary production. It is known as the midnight zone because of this feature.

The abyssal zone is the abyssopelagic layer of pelagic zone that contains the very deep benthic communities near the bottom of oceans. "Abyss" is from the Greek word meaning "bottomless sea". At depths of 4,000 to 6,000 meters (13,123 to 19,685 feet), this zone remains in perpetual darkness and never receives daylight. Its permanent inhabitants (for example, the Black swallower, tripod fish, deep-sea anglerfish and the giant squid) are able to withstand the immense pressures of the ocean depths, up to 76 megapascals (11,000 psi).

Hadal zone or (trench zone) (Greek for "like Hades," in other words "unseen") or Hadopelagic zone is the delineation for the deepest trenches in the ocean. This zone is found from a depth of around 6,000 meters (20,000 ft) to the bottom of the ocean.The demersal zone is the part of the sea or ocean (or deep lake) comprising the water column that is near to (and is significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer of the larger profundal zone.

The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone are called benthos. They generally live in close relationship with the substrate bottom; many such organisms are permanently attached to the bottom. The superficial layer of the soil lining the given body of water is an integral part of the benthic zone, as it influences greatly the biological activity which takes place there. Examples of contact soil layers include sand bottoms, rock outcrops, coral, and bay mud.Stratifications of OceanA pycnocline is a rapid change in water density with depth. In freshwater environments such as lakes this density change is primarily caused by water temperature, while in seawater environments such as oceans the density change may be caused by changes in water temperature and/or salinity.

An isopycnal is a surface of constant potential density of water. In the ocean, as the depth increases, so too does the density. Varying degrees of salinity and temperature act to modify the density of water, and the denser water always lies below the less dense water. Because of the action of winds and currents, isopycnals are not always level.

The thermocline (sometimes metalimnion) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere), in which temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline may be thought of as an invisible blanket which separates the upper mixed layer from the calm deep water below.

In oceanography, a halocline is a strong, vertical salinity gradient. Because salinity (in concert with temperature) affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m3 results in an increase of seawater density of around 0.7 kg/m3.

The term thermohaline circulation (THC) refers to the part of the large-scale ocean circulation that is thought to be driven by global density gradients created by surface heat and freshwater fluxes.A chemocline is the border region or interface between two contrasting and predominating chemistries within a body of water. A chemocline is analogous to a thermocline, the border at which warmer and

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cooler waters meet in an ocean, sea, lake, or other body of water. (In some cases, the thermocline and chemocline coincide.)

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Estuarine Ecosystem

An estuary is a semi-enclosed body of water with variable salinity intermediate between salt and fresh water. Examples of estuaries are river mouths, coastal bays, tidal marshes and water bodies behind barrier beaches. Estuaries are among the most naturally fertile waters in the world (Odum, 1989). Their high productivity results from their unique juxtaposition at the edge of the continent. Nutrients from four sources contribute to the productivity of estuaries

1. fresh water flowing off the land;2. tidal exchange with the ocean;3. the atmosphere; and4. the recycling of material from the estuarine bottom sediments.

The most important nutrient is nitrogen-a component of all proteins. Phosphorus, silica, and other compounds in lesser amounts also serve as nutrients to living thing sin the estuary.

The estuary functions as an efficient nutrient trap that is partly physical and partly biological. Three major forms of photosynthesising organisms play key roles in maintaining high productivity by exploiting nutrient sources.

i. phytoplankton suspended within the sunlit Zone of the water column;

ii. benthic microflora which are microscopic plants living on the sediment surface wherever sufficient light reaches the bottom; and

iii. macroflora or rooted plants and rootless algae growing in shallow water and along the shoreline

These plants are the foundation of complex food webs and provide structural habitats which create natural habitat for most coastal shellfish and finfish. Physical processes contribute to the acquisition and transformation of nutrients by living things. For example, the importation of nutrients and exportation of waste products to and from the estuary are subsidised by gravitational energy in the form of streamflow and tidal exchange. As a result, the estuary becomes a productive seafood factory

One component of the estuary is shallow salt marshes that play an important role for organisms. Salt marshes are regularly rinsed by the ebb and flow of the tides. Waste products are diluted and removed and nutrients are brought in from the land and the sea. The materials of streams flocculate as the mixture of clay-sized particles and algae meet the sea waters. They settle down, leaving the water clear. In the salt marsh, plants need not expend energy in collecting minerals.They are assisted in obtaining minerals from many organisms. Clams, oysters and mussels trap nutrients as they feed, and deposit the wastes as pseudo-faeces. In these nutrient-rich waters, plants grow, die, and cycle rapidly as they are consumed by the inhabitants of the estuary. The major energy flow is by way of detritus food chain rather than the grazing food chain

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Biota of EstuariesIn estuaries, the plants of different groups are present, including phytoplankton, benthic diatoms, bacteria and fungi and larger macrophytes. Though estuaries are rich in their nutrients phytoplanktonic organisms are not abundant. This is due to the reduction in light penetration as a result of turbidity. Most of them are marine in origin and fresh water phytoplankton is poorly represented

The important estuarine phytoplanktonic organisms are diatoms and dinoflagellates. The diatoms are Skeletonema costatum, Parallia sulcata, Chaetoceras debills, etc. Ceratium furga and Ceratium buceros are the dinoflagellates of the estuaries. The benthic diatoms are bottom dwelling organisms which are the primary producers and serve as food for a variety of estuarine animals. Euglena obtuse is quite common in the bottom of the estuaries.

Both aerobic and anaerobic bacteria inhabit the estuarine water and also the bottom. Sulphate reducing bacteria are found in the mud. Generally all types of bacteria decompose the organic substances of the estuaries. The fungi such as Mucor and Pencillium also decompose the organic materials of the estuaries.

The macrophytes are larger plants which are also found in the estuaries. Brown algae like Fucus, Cladophora and Vaucheria, Chorophyceae and Xanthophyceae are the common algae of the estuaries. Besides, eel-grass are the typical vegetation of the estuaries

Mangrove EcosystemThe word 'mangrove' originates from 'mangue', a combination of the Malay, Spanish, French or Portuguese for 'wood' and' grove' English for' a small wood'. Mangroves are the special types of habitats found in the coasts of subtropical and tropical countries. They are bordered by shallow sea water and thus protected from direct wave action. Mangroves are defined as a type of coastal woody vegetation that fringes muddy saline shores and estuaries in tropical and subtropical regions.

Characteristic Features of Mangroves1. A unique characteristic feature of all mangroves is the muddy bottom. The bottom is muddy due to the absence of wave actions and strong water currents. The muddy bottom is exposed during low tides.

2. The mangroves are always shallow in nature, and rarely show high depth at certain regions.

3. Generally mangroves are developed on coasts where the atmospheric  temperature is never below 20°C even during the winter season.

4. A high rainfall is necessary for the development of mangroves.

5. The water of mangrove shows high salinity due to the concentration of salts such as sodium chloride, and magnesium sulphate. Besides, the water shows dissolved ions, manganese and molybdenum.

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6. They are water logged or stagnant with high nutrients brought in by the incoming fresh water. The muddy bottom also contains rich organic matter.

ZonationOn the basis of salinity, five zones of mangrove distribution are considered. These are the euhaline, polyhaline, mesohaline, oligohaline and limnetic zones. In India, the west coast is characterised by the rocky substratum and hence absence of mangroves in the mouth region. On the other hand, the same euhaline zone along the estuaries of the east coast, which is a delta region, shows the presence of luxuriant mangrove forests as observed in the Gangetic, Mahanadi and Godavari deltas.

In the euhaline zone, the salinity ranges from 30 ppt to 40 ppt. Wave action is maximum and the gradient is not steep; thus sediment deposition will take place in the region of confluence which is known as delta. Otherwise, the entire sediment load will be washed into the sea.

The polyhaline region, characterised by salinity range of 18 ppt to 30 ppt, has a low wave action and substratum is sandy clay.

The mesohaline region, with 5 ppt and 18 ppt salinity, has silty clay bottom and feeble wave action.

In the oligohaline zone, the salinity is reduced further to 0.5 ppt to 5ppt as a result of more freshwater influx. Its substratum is silty.

The limnetic zone is almost freshwater with occasional intrusion of brackish water at the highest high tides. The salinity in this area is less than 0.5 ppt and the substratum is full of gravel and coarse grains of sand.

Different mangrove species, according to their salt tolerant ability and substratum preference occupy different zones along the estuaries. The deposition of sediment at the mouth of the major estuaries like Ganga, Mahanadi, Godavari, etc. is quite common.

During this process, the fine alluvial silt is deposited in the form of shallow islands in the mouth region. Slowly the new mangrove formation develops following a typical succession pattern. After the growth of pioneer species, the dominant or climax vegetation of mangrove species takes over and flourishes.

These new mangrove formations are very sensitive to the flood waters during rainy season. Depending upon the flood, the rate of erosion in the mangrove soil varies. During heavy to very heavy floods, these

new mangrove formation may be completely washed out. During the rainy season, the rate of erosion is higher while during the non-monsoon season the accretion is higher. The effect of accretion and erosion seem to be more prominent along the east coast than on the west coast of India.

Biota of MangroveThe succession of mangrove is dependent on the available seeds or

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propagules, their size or length and the tidal fluctuation. Seeds of grass, sedge or Excoecaria agallocha, which are minute in size, will always establish themselves at the uppermost limit of the intertidal region. At the same time, seedlings of taxa like Rhizophora, Kandelia, Ceriops or Bruguiera will be established according to their floating height.

The major ecological role of mangroves is the stabilisation of the shoreline and prevention of shore erosion. The dense network of prop roots, pneumatophores and stilt roots not only give mechanical support to the plant, but also trap the sediments.

The rate of sedimentation or accretion is generally much higher in these estuaries lined with mangroves.

The second important ecological role of the mangroves is the detritus, which helps in feeding and provides breeding and nursery grounds for thejuveniles of many commercially important shrimps and fishes. Major primary fraction of this production is consumed by herbivores. The remainder enters the mangrove water as litter fall. The decomposition of this litter fall produces detritus, which in turn is colonised by heterotrophic microorganisms, thus enhancing its nutritive value. The detritus, besides forming a food source for suspension and deposit feeders, is also consumed by the juveniles of a variety of bivalves, shrimps and fishes, which migrate into the mangrove environments in their life cycle for better feeding and protection.

There is a direct correlation between the extent of mangrove forests along a coastline and the fishery as well as shrimp catches from the coastal waters adjoining the mangroves thus demonstrating the importance of mangroves for sustaining coastal fisheries.

In mangrove forests, the floral elements responsible for the photosynthesis under brackish water condition are of different types, i.e. angiospermic flora, phytoplankton and marine algae.

Other Interrelated EcosystemsThere are certain other ecosystems interrelated with mangrove, which include:

Benthic community Major groups represented by the benthic organisms are molluscs, crustaceans, echinoderms, hydroids, actinarians, planarians, nematodes, polychaetes and larval forms of several other organisms.

Pelagic community The mangrove water usually rich in detritus, is highly suitable for fishing. The major fishery resources found in these waters are detritivorous species of fishes, crabs, crustaceans and molluscs

Roughly about 60% of India's coastal marine fish species is dependent on the mangrove-estuarine complex. The annual landing of the fish from the Hoogly-Matlah estauries of Gangetic Sundarbans have exceeded 10,000 tons.

Some of the most common fishes in Indian mangrove waters are species of Liza, Mugil, Lates, Polynemus, Sciaena, Setipinna, Pangasium, Hilsa, Ilisha and Atroplus. Prawns are represented by the species of Penaeus and Metapenaeus, while the crabs are represented mainly by Scylla serrata. The molluscs of mangrove waters are mainly represented by Crassostrea sp., Mytilus and clams.

In the upstream regions, giant prawns like Microbrachium rosenbergii are also found in large quantities.

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The important phytoplankton of the mangroves are Pleurosigma, Bacillaria, Navicula, Thalassiotirix, and Biddulphia (diatoms). Dinoflagellates such as Ceratium, Peridiniem and Procentrum are also quite common in the water of mangroves.

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