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Microaerophillic Zone (Oxygen Scarce)
In this zone oxygen diffuses down from the surface but is limited in concentraction. Sulfur from the lower part of the column has begun
to move up in the form of H2S. This diffusion of H2S from the sediment into the water column enables anaerobic photosynthetic bacteria to grow. They are seen usually as two narrow, brightly colored bands
immediately above the sediment - a zone of green sulfur bacteria, such as Chlorobium, characterized by a green/olive color indicative of growing anaerobic conditions, then a zone of
purple sulfur bacteria, such as Rhodospirilum and Rhodopseudomonas, which takes on a red/ orange or rust color.
The green and purple sulfur bacteria gain energy from light reactions and produce their cellular materials from CO2 in much the same way as plants do. However, there is one essential difference: they do not generate
oxygen during photosynthesis because they do not use water as the reducer; instead they use H2S. The following simplified equations show the parallel processes:
6 CO2 + 6 H20 = C6H12O6 + 6 O2 (plant photosynthesis)
6 CO2 + 6 H2S = C6H12O6 + 6 S (bacterial anoxygenic photosynthesis)
Anaerobic Zone (Oxygen Depleted)
The only organisms that can grow in anaerobic conditions are those that ferment organic matter and those that perform anaerobic respiration.
Fermentation is a process in which organic compounds are degraded incompletely; for example, yeasts ferment sugars to alcohol. Anaerobic respiration
is a process in which organic substrates are degraded completely to CO2, but using a substance other than oxygen as the terminal electron acceptor
There are three basic levels that form in the lower level of the column. At one level purple sulfur bacteria such as Chromatium, in a Red to Purple layer, are processing Sulfates into Sulfur. At another
point Gallionella, a stalked bacteria, processes Iron to help create the black layer that forms just below. This level is marked by a strong rust/orange color.
Some cellulose-degrading Clostridium
species start to grow when the oxygen is depleted in the sediment. All Clostridium species are strictly anaerobic because their vegetative cells are killed by exposure to oxygen, but they can survive as spores in aerobic conditions. They degrade the cellulose to glucose and then ferment the glucose to gain energy, producing a range of simple organic compounds (ethanol, acetic acid, etc.) as the fermentation end products.
Deeper in the column, the sulfur-reducing bacteria, marked by a deep black layer and typified by Desulfovibrio,canutilize these fermentation products by anaerobic respiration, using either sulfate or
other partly oxidized forms of sulfur (e.g. thiosulfate) generating large amounts of H2S by this process. The H2S will react with any iron in the sediment, producing black ferrous sulfide. This is why lake sediments
(and our household drains) are frequently black. However, some of the H2S diffuses upwards into the water column, where other organisms utilize it.
Finally, at the bottom, depending on the source of the mud, a pink layer will develop due to purple sulfur bacteria with gas vesicles. A characteristic species is Amoebobacter. This environment is very high in
H2S and is more tolerant of air and light
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