The Sulfur Cycle

Sulfur is a component of a couple of vitamins and essential metabolites and it occurs in two amino acids, cysteine and methionine. In spite of its paucity in cells, it is an absolutely essential element for living systems. Like nitrogen and carbon, the microbes can transform sulfur from its most oxidized form (sulfate or SO₄) to its most reduced state (sulfide or H₂S). The sulfur cycle, in particular, involves some unique groups of procaryotes and procaryotic processes. Two unrelated groups of procaryotes oxidize H₂S to S and S to SO₄. The first is the anoxygenic photosynthetic purple and green sulfur bacteria that oxidize H₂S as a source of electrons for cyclic photophosphorylation. The second is the "colorless sulfur bacteria" (now a misnomer because the group contains many Archaea) which oxidize H₂S and S as sources of energy. In either case, the organisms can usually mediate the complete oxidation of H₂S to SO₄.

H₂S----------------> S ----------------> SO₄ litho or phototrophic sulfur oxidation

Sulfur-oxidizing procaryotes are frequently thermophiles found in hot (volcanic) springs and near deep sea thermal vents that are rich in H₂S. They may be acidophiles, as well, since they acidify their own environment by the production of sulfuric acid.

Since SO₄ and S may be used as electron acceptors for respiration, sulfate reducing bacteria produce H₂S during a process of anaerobic respiration analogous to denitrification. The use of SO₄ as an electron acceptor is an obligatory process that takes place only in anaerobic environments. The process results in the distinctive odor of H₂S in anaerobic bogs, soils and sediments where it occurs.

Sulfur is assimilated by bacteria and plants as SO₄ for use and reduction to sulfide. Animals and bacteria can remove the sulfide group from proteins as a source of S during decomposition. These processes complete the sulfur cycle.

Figure 3. The Sulfur Cycle

The Phosphorus cycle

The phosphorus cycle is comparatively simple. Inorganic phosphate exists in only one form. It is interconverted from an inorganic to an organic form and back again, and there is no gaseous intermediate.

Phosphorus is an essential element in biological systems because it is a constituent of nucleic acids, (DNA and RNA) and it occurs in the phospholipids of cell membranes. Phosphate is also a constituent of ADP and ATP which are universally involved in energy exchange in biological systems.

Dissolved phosphate (PO₄) inevitably ends up in the oceans. It is returned to land  by shore animals and birds that feed on phosphorus containing sea creatures and then deposit their feces on land. Dissolved PO₄ is also returned to land by a geological process, the uplift of ocean floors to form land masses, but the process is very slow. However, the figure below considers how PO₄  is recycled among land-based groups of organisms.

Figure 4. The Phosphorus Cycle. Plants, algae and photosynthetic bacteria can absorb phosphate (PO₄) dissolved in water, or if it washes out of rocks and soils. They incorporate the PO₄ into various organic forms, including such molecules as DNA, RNA, ATP, and phospholipid. The plants are consumed by animals wherein the organic phosphate in the plant becomes organic phosphate in the animal and in the bacteria that live with the animal. Animal waste returns inorganic PO₄ to the environment and also organic phosphate in the form of microbial cells. Dead plants and animals, as well as animal waste, are decomposed by microbes in the soil. The phosphate eventually is mineralized to the soluble PO₄ form in water and soil, to be taken up again by photosynthetic organisms.
 

Ecology of a Stratified Lake

The role of microbes in the global cycle of elements (described above) can be visited on a smaller scale, in a lake, for example, like Lake Mendota, which may become stratified as illustrated in Figure 5. The surface of the lake is well-lighted by the sun and is aerobic. The bottom of the lake and its sediments are dark and anaerobic. Generally there is less O₂ and less light as the water column is penetrated from the surface. Assuming that the nutrient supply is stable and there is no mixing between layers of lake water, we should, for the time being, have a stable ecosystem with recycling of essential elements among the living systems. Here is how it would work.

At the surface, light and O₂ are plentiful, CO₂ is fixed and O₂ is produced. Photosynthetic plants, algae and cyanobacteria produce O₂, cyanobacteria can even fix N₂; aerobic bacteria, insects, animals and plants live here.

At the bottom of the lake and in the sediments, conditions are dark and anaerobic. Fermentative bacteria produce fatty acids, H₂ and CO₂, which are used by methanogens to produce CH₄. Anaerobic respiring bacteria use NO₃ and SO₄ as electron acceptors, producing NH₃ and H₂S. Several soluble gases are in the water: H₂, CO₂, CH₄, NH₃ and H₂S.

The biological activity at the surface of the lake and at the bottom of the lake may have a lot to do with what will be going on in the middle of the water column, especially near the interface of the aerobic and anaerobic zones. This area, called the thermocline, is biologically very active. Bacterial photosynthesis, which is anaerobic, occurs here, using longer wave lengths of light that will penetrate the water column and are not absorbed by all the plant chlorophyll above. The methanotrophs will stay just within the aerobic area taking up the CH₄ from the sediments as a carbon source, and returning it as CO₂. Lithotrophic nitrogen and sulfur utilizing bacteria do something analogous: they are aerobes that use NH₃ and H₂S from the sediments, returning them to NO₃ and SO₄.

Figure 5. Ecology of a Stratified lake