How does water serve as suitable microbial habitat?
- Temperature – The temperature of surface water widely ranges from 0-40°C. Beneath the surface, more than 90% of the living organisms live at a temperature below 5°C. This low temperature favours the growth of psychrophilic microbes. Some thermophiles have been isolated from anaerobic cracks and hydrothermal vents in the ocean floor. Pyrodictium occulltum, a thermophilic archaebacterium was isolated from submarine field of Vulcano, Italy where the water is as hot as 103°C. Lab studies have shown that the optimum growth temperature for this organism is 105°C. This anaerobic archaea is a chemolithoautotroph that grows by generating H2S from H2 gas and elemental S.
Solar radiation warms the surface water that may cause thermal stratification. Warm water, being less dense than cool water, floats on top in the water body. Thus, a thermocline develops. These two layers of water remain separate until there occurs substantial mixing event such as storm or the onset of autumn in temperate zones. As the weather cools, the upper layer of warm water is cooled and the water masses mix. This results in the nutrients being brought back to the upper regions from the sediments, called nutrient upwelling. This can stimulate a sudden and rapid increase in the population of some microorganisms.
- Hydrostatic Pressure – It is defined as the pressure along the vertical column of water. The hydrostatic pressure increases as the depth increases. This implies that the pressure near the ocean floor is enormous. Barophilic organisms are those that obligately require a high pressure and cannot grow at normal atmospheric pressure. Organisms isolated from Pacific ocean trench at depths of 1000-10,000m are some examples of barophilic microorganisms. Most of them grow best at a pressure slightly less than the pressure at the sites of their habitat.
- Light – Most aquatic organisms depend directly or indirectly on autotrophic organisms for food. Photosynthetic organisms found in aquatic habitats include algae, cyanobacteria and higher plants. In fresh water habitats, much of the carbon is fixed by macroscopic algae and plants. Organic sources of C enter through terrestrial run-offs. In the open ocean, all of the inorganic C is fixed by microorganisms. Photosynthetic microbes are found in the photic zone, the limit of the water upto which sunlight can penetrate. The size of this zone varies depending on a number of factors like position of the sun, season, turbidity of water etc. Generally, the limit is confined to 150-200m from the surface of ocean where there is minimal nutrients and turbidity. In lakes and estuaries where the water is turbid, the photic zone may be only 1-2m deep.
- Salinity – The salt concentration in natural water ranges from 0 in freshwater to saturation in salt lakes. The concentration of NaCl in sea water is 2.7% and the total salt concentration is 3.3-3.7%. Besides NaCl, the other commonly found salts in brackish water are sulfates and carbonates of Na and chlorides, sulfates and carbonates of K, Ca and Mg. The salt concentration is usually less in offshore regions and near the river mouths. In estuaries, salinity varies greatly from top to the bottom, upper end to lower end and season to season, creating ever-changing conditions for its inhabitants. Most marine organisms are halophilic ie., they grow optimally at 2-4% NaCl On the other hand, freshwater organisms are inhibited by concentrations above 1%.
- Turbidity –The clarity of surface water shows marked variations. The suspended materials responsible for turbidity of water bodies include:
- Particles of minerals due to coastal erosions
- Detritus – particulate organic matter that comes from dead and decaying plant and animal matter
- Suspended microorganisms
Greater the turbidity of water, lesser will be the effective light penetration and therefore, lesser will be the depth of photic zone. Many bacteria, called epibacteria, adhere to particulate matter in order to colonize. The suspended particulate matter also serve as substrate for metabolism by microbes.