Ritu's Biology Hub

Continued from Part 7

Limnology – The study of freshwater environments such as lakes, ponds, streams, rivers, frozen lakes and glaciers and estuaries

Zones of a Lake or Pond:

1. Limnetic zone – It is the zone along the shore where light penetrates to the bottom. There is a lot of rooted vegetation
2. Littoral zone – It is the upper region of the open sea away from the shore. Its depth is determined by effective light penetration
3. Profundal zone – It refers to the deeper regions of the open area of the sea. The photosynthetic activity gradually decrease in this zone
4. Benthic zone – It is the soft mud or the sediment at the bottom

Microorganisms found in Freshwater environments

1. Glaciers and Permanently frozen lakes

• Studies have determined that microbes in glaciers are not dormant as would be expected in the extreme cold conditions
• One permanently frozen lake is Antarctica’s McMurdo Dry Valley Lakes where the ice is 3-6m deep. Ecosystem depends on the photosynthetic activity of psychrophilic microorganisms
• Lakes underlying glaciers are blocked from sunlight. Hence, the ecosystem is run by chemosynthesis
• Lake Vostok in Antarctica is covered with huge masses of icebergs. Geothermal heating, pressure and insulation of the overlying ice keep the lake in a liquid state. This stable environment supports growth of gram-negative proteobacteria and gram positive actinomycetes. The overlying ice also harbours similar flora but at lower densities
• To find out if the microbes were metabolically active, radiolabeled substrates such as ¹⁴C-acetate and ¹⁴C-glucose were added to a microbial culture. Respiration measured as ¹⁴C-CO2 demonstrated that the microbes were indeed alive and active.

2. Streams and Rivers

• Continuous flow of water in streams and rivers prevents the establishment of a significant microbial community
• Depending upon the size of the water body, the source of nutrients may vary. Autochthonous production is in-stream production by photosynthetic microbes. Allochthonous nutrients are those that enter the water body through run-offs or leaves falling off trees
• Chemoorganotrophs metabolize the available organic matter and recycle nutrients within the ecosystem. Autotrophs grow utilizing the inorganic minerals released from the organic materials. This leads to production of oxygen during daytime and respiration occurs at night in deeper parts of the water body resulting in diurnal oxygen shifts
• If too much organic matter is added then the river is said to be eutrophic and respiration exceeds photosynthesis making the water anoxic. This is seen in rivers and streams adjacent to urban areas
• The release of inadequately treated municipal wastes represent point source pollution which predictably affects the microbial community of that habitat and O₂ availability resulting in an oxygen sag curve
• Runoffs from agricultural fields are examples of non-point source pollution. This can cause disequilibrium in the microbial community leading to eutrophication
• In addition to adding nutrients, removal of silica from rivers due to construction of dams and trapping of sediments cause major ecological disturbances.

3. Lakes

• Lakes vary in nutrient status. Some are oligotrophic (nutrient deficient) while others are eutrophic (nutrient rich). Oligotrophic lakes remain oxic throughout the year and seasonal shifts usually do not result in oxygen stratification. On the contrary, eutrophic lakes have a nutrient rich sediment.
• In thermally stratified lakes the epilimnion (warmer, upper layer) is oxic whereas the hypolimnion (cold, bottom layer) is anoxic. These two layers are separated by thermocline
• In tropical eutrophic lakes, since there is little or no mixing between the epilimnion and hypolimnion, the bottom waters may become oxygen deprived. This situation also occurs in the summer time in temperate lakes
• If the nutrient load is high then the benthos becomes dominant by anaerobic microbial activity which release toxic gases like H₂ Human activities add to this by releasing nitrogen and phosphorus rich fertilizers into the water body. This can result in bloom of microbes in the epilimnion. During autumn cooling, temperate lakes lose their thermocline resulting in mixing of water. If the benthos had become full of anaerobes then the sudden upwelling of H₂S causes fish kills
• If nutrients are added to oligotrophic lakes, cyanobacteria – Anabaena, Nostoc, Cylindrospermum bloom which fix nitrogen. Oscillatoria can fix nitrogen under anoxic conditions using H₂S as the electron donor
• Cyanobacteria compete with algae for the limiting nutrients. By using CO₂ at rapid rates, they also increase the pH of the water making the environment less suitable for protists
• Cyanobacteria have a competitive advantage over protists because –
  • They produce hydroxamates, which bind iron, making this essential nutrient less available to protists
  • Some resist predation by synthesizing toxins
• Cyanobacteria also produce odorous compounds that affect the quality of water