Microorganisms in freshwater system- streams & rivers, wetlands, ice

• Streams and Rivers

Streams and rivers are a different from lakes due to the presence of flowing waters. They have zones of rapid water movements and pools of reduced currents. Rocky substrates underlie the rapid zones. In pool zones deposition of silt is seen due to decreased current velocity. Rivers are closely connected with lithosphere along its banks and there is constant transfer of substances through rain water runoff and erosion of river banks. 

Upper course of a river has swift flow, high degree of oxygenation and low temperature. Primary production is low due to shading by trees and the organic material input occurs mainly from lithosphere. Middle course of river has low flow velocity, huig temperature, less shading and significant primary production. Lower course has high levels of silt depositions, is subject to tidal influences, and is inhabited by less of freshwater microbes and more of salt-tolerant estuarine microorganisms.

Microbial organisms in rivers are seen attached to submerged rocks and other structures. Dissolved nutrients are rapidly absorbed by them and liberated upon their death and decay. These are then reabsorbed at a little distance downstream. Thus the nutrients do not move swiftly along with the water flow. Rather nutrient spiraling is seen where it is absorbed, released and then moves little downstream again to be absorbed, thus completing a cycle.

Nutrient spiraling

Rivers receive high amounts of effluents from industries and municipalities. High amounts of organic compounds are released which can lead to depletion of available oxygen since it is used in microbial decomposition of these organic compounds. Industrial effluents may contain toxic heavy metals, which affect microbial life and activities. Agricultural runoff containing chemical toxins or fertilizers can either harm or cause proliferation of microbial communities.

Thus, in rivers and streams, there is more of horizontal water movement and most of the functional microbial biomass is attached to surfaces. Depending on the size of the stream or river, the source of nutrients may vary. The source may be in-stream production based on photosynthetic microorganisms. Nutrients also may come from outside the stream, including runoff sediment from riparian areas (the edge of a river), or leaves and other organic matter falling directly into the water. Chemoorganotrophic microorganisms metabolize the available organic material and provide an energy base for the ecosystem.

Under most conditions the amounts of organic matter added to streams and rivers will not exceed the system’s oxidative capacity and productive, aesthetically pleasing streams and rivers will be maintained. The capacity of streams and rivers to process such added organic matter is, however, limited. If too much organic matter is added, the water may become anaerobic. This is especially the case with urban and agricultural areas located adjacent to streams and rivers. The release of inadequately treated municipal wastes and other materials from a specific location along a river or stream represents a point source of pollution. Such point source additions of organic matter can produce distinct and predictable changes in the microbial community and available oxygen creating an oxygen sag curve.  Runoff from fields and feedlots, which causes algal blooms in eutrophic water bodies, is an example of a nonpoint source of pollution. 

The Dissolved Oxygen Sag Curve.

Dissolved Oxygen Sag Curve- Microorganisms and their activities can create gradients over distance and time when nutrients are added to rivers. When organic wastes are added to a clean river system, there are changes in dissolved oxygen levels and amount of aquatic life. During the later stages of self-purification, the phototrophic community will again become dominant, resulting in diurnal changes in river oxygen levels.

           When the amount of organic matter added is not excessive, the algae will grow using the minerals released from the organic matter. This leads to the production of O₂ during the daylight hours, and respiration will occur at night farther down the river, resulting in diurnal oxygen shifts. Eventually the O₂ level approaches saturation, completing the self-purification process.

Along with the stresses of added nutrients, removal of silicon from rivers by the construction of dams and trapping of sediments causes major ecological disturbances. The decreased silicon availability inhibits the growth of diatoms because the ratio of silicon to nitrate has been altered (silicon is required for diatom shell formation). With this shift in resources, diatoms are not able to grow and immobilize nutrients. There may be an increase in nitrate levels and a massive development of toxic algae. Thus the delicate balance of rivers can be altered in unexpected ways by dams, leading to effects on aquatic microbiological processes and whole ecosystems. With the damaging effects of dams now being recognized, there are attempts to stop constructing more and to restore normal water and sediment movements. This also allows fish migration into upper regions of rivers where they often have been excluded for decades.

• Wetlands

Shallow aquatic environments with plants dominating. Depending on the kind of plants

·         Freshwater marshes- grassy plants dominate reeds, cattails etc

·         Fens- mineral rich wetlands, neutral to alkaline pH; contain sedge plant

·         Shrub-carrs- dense growth of willows, birches

·         Swamps with coniferous/hardwood trees

                    Shrub -carr

Marsh

                                                                Fen with sedge plants

                                                                    Swamp

    High productivity is seen but waterlogging and oxygen limited conditions inhibit biodegradation of plant polymers especially lignocellulose resulting in accumulation of plant residues. Accumulated plant residues can be transformed geochemically to form coal or peat.

Bogs are a kind of wetlands. Dominant plant is sphagnum moss which form a thick mass. Less productive than other wetlands. Sphagnum moss holds water effectively resembling a water soaked sponge. As its height increases by accumulation of peat,  it is isolated from the soil and bedrock and cannot receive nutrients from running water. It becomes ombrotrophic-cloud nourished (nutrients form atmosphere and rain).

• Freshwater Ice

Fresh waters include glaciers, as well as vast ice sheets in the Arctic and Antarctic polar regions. These regions, although far away from most people, are important as habitats for microorganisms. Of particular interest are deep frozen lakes in the Antarctic region. An excellent example is the McMurdo Dry Valley Lakes, which have 3 to 6 meter thick ice layers with windblown sediments occupying various levels in the ice profile. In the summer, melting of the ice in these sediment-containing zones provides an opportunity for microbial growth. There is interest in studying ice-inhabiting microorganisms from the north and south poles.

One of the most interesting deep-frozen habitats lies above Lake Vostok in the Antarctic, where cores are at a depth of 3,600 meters (11,800 feet) in ice that has been frozen for over 420,000 years. The actual Lake Vostok lies below another 120 meters of ice. Scientists are actively searching for microorganisms in these unique older freshwater frozen environments.