Hydro-ecosphere is considered as a more suitable habitat for microorganisms than the atmosphere and lithosphere, mainly because hydrosphere contains water, which is important in microbial metabolism (Atlas and Bartha, 1993). The unique properties of water are also attributes of hydrosphere as a suitable habitat. Due to polarity of water (slightly negative and slightly positive), water is considered as a great solvent, capable of forming hydrogen bonding. Large heat capacity of water is due to its high specific heat (1 calorie/gram); hence a large amount of energy is needed before a 1oC-rise in temperature. Water also has high heat of fusion (80 cal/g) so it does not freeze easily. Surface tension is high because water molecules stick together, and have ability to attract molecules of surface in contact. This is important in accumulation of non-polar organic compound on the surface layer, which could be used as source of nutrients for microorganisms. With these characteristics of water, aquatic systems are more resistant to extreme environmental fluctuations. However, such water properties can be modified by the presence of dissolved substances. Aquatic environments are classified as ocean waters and inland waters (groundwater and surface water). Ocean waters contain approximately 35 ppt salt; freshwaters which include lakes, ponds, groundwater, river and spring, has 0.05 ppt salinity. Special habitats in hydrosphere include bottom sediments, biological and non-biological surface, and interface (between atmosphere and hydrosphere or lithosphere and hydrosphere). There are at least five factors that affect the hydro-ecosphere: light intensity, temperature, pressure, dissolved gases and dissolved solids. Due to presence of autochthonous microbiota, certain limited general characteristics can be ascribed.
Light. The light from the sun provides energy for primary producers. During daytime, more light is absorbed in water due to the directness of the light; at sunset, lesser light is absorbed because light strikes water more acutely. Dissolved substances, which can confer turbidity, can cause light extinction (since some molecules could absorb and/or reflect light). At different depths, there is a difference in light penetration, which gives rise to zonation (specifically, vertical zonation) in marine and lake habitats. In lake habitat, the littoral zone is the region where light penetrates the bottom, the limnetic zone is an area of open water away from the shore that descends to the light compensation level, the profundal zone is area of deeper water beyond the depth of effective light penetration, and the benthos (bottom of the lake) represents the interface between the hydrosphere and lithosphere. The profundal zone is not present in shallow waters. In marine habitat, the euphotic zone is the area of effective light penetration to the compensation level, about 0-200 m. The light compensation level is the depth at which photosynthesis just balances respiration. The aphotic (or disphotic) zone, about 200-6000 m, lies below the euphotic zone. Temperature. Amount of solar energy received, geographical latitude and altitude, and weather conditions determine the temperature in an aquatic environment. Usually, temperature ranges from -1-7oC in the polar regions and 25-30oC in tropical and subtropical waters. In streams and ponds, there are large temperature fluctuations, but in large bodies of water, temperature is quite stable. The heat distribution in aquatic systems is influenced by depth, amount of water, and differential heating (which part is lighted and not). During summer, differences in temperature in some regions of lake result to layer turnover: epilimnion (warmer, aerobic, uppermost layer with mixing of water), thermocline (the transition between mixed water and bottom) and hypolimnion (colder, anaerobic, bottom layer). The thermocline is...
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