What are the sources of water?
Natural waters are of 3 type:
- Atmospheric water – Present in cloud and falls back as rain, snow and hail
- Surface water – Found in water bodies
- Groundwater – Water beneath the earth’s surface
Hydrologic Cycle / Water cycle
It begins when the earth’s surface water from lakes, rivers, oceans etc. evaporates and enters the vapour phase in the atmosphere. Living beings also contribute to the same – plants emit water vapour by transpiration while animals do so by respiration. This water vapour accumulates and condenses as visible clouds due to lowered temperature at high altitudes. Water is returned to earth by precipitation as snow or rain. A large proportion of it falls back to the surface water sources. 2% of it either seeps into the ground or is bound in ice. The percolated water gets collected in extensive subterranean pockets formed by underlying rocks, gravel and sand to form aquifers. Aquifers are deep groundwater sources that resurface as geysers, springs or hot vents.
How does water serve as suitable microbial habitat?
The nature of water as a microbial habitat depends on a number of physical factors like temperature, pH, light penetration, dissolved oxygen content (DOC) etc.
- Gases – Flux rate of oxygen in water is 1/10,000 times less than that in air. However, in some marine habitats, oxygen diffusion is altered due to the increased solubility of oxygen at colder temperatures and increasing pressure. Hence, for very deep seas and oceans the oxygen content actually increases with depth, even though air-water interface is miles away. In tropical lakes and summertime-temperate lakes, oxygen becomes limited only few metres below the surface. This happens because the aerobic microorganisms consume the oxygen near the surface faster than it can be replenished, frequently leading to the formation of hypoxic or anoxic zones in these aquatic habitats. This permits the growth of anaerobic microbes, both phototrophic and chemotrophic to grow in the lower anoxic regions of the water body where light can penetrate.
CO2 also plays an important role. pH of non-buffered distilled water is determined by the dissolved CO2 in equilibrium with air and is approximately 5.0-5.5. The pH of fresh water-bodies like lakes, ponds, rivers and streams, which are weakly buffered, is controlled greatly by the kind of terrestrial input and the rate at which CO2 is removed by photosynthesis. When autotrophic microorganisms such as diatoms utilize CO2, the pH of water becomes alkaline. Seawater is strongly buffered by the balance of CO2, carbonate and bicarbonate ions. Atmospheric CO2 enters the sea where it is converted to organic biomass by photosynthesis. Alternatively, it may react with sea water to form carbonic acid, which quickly dissociates to form bicarbonate and carbonate ions.
The pH ocean water is 7.6-8.2 and buffered by the above mentioned carbonate equilibrium system. The pH is determined by the relative concentrations of the weak acid and the ions. Some environmentalists predict that the pH of the ocean will drop by 0.35 units by 2100 unless effective means of limiting greenhouse gas emissions are executed.
Other important gases in aquatic habitats influencing the microflora include nitrogen, hydrogen and methane. N2 is utilized by nitrogen fixers. H2 is both a waste product and a vital substrate. CH4 is produced under the anaerobic conditions by methanogens. It leaves the microbe’s environment by diffusing up the water column where it is either oxidized by methanotrophs or released into the atmosphere. This prevents toxic waste accumulation in the aquatic environment. These gases vary in their solubility in water with methane being the least soluble of the three.