Bacteria and Archaea and the Cycles of Elements in the Environment (page 4)
(This chapter has 4 pages)
© Kenneth Todar, PhD
The Sulfur Cycle
Sulfur is a component of a couple of vitamins and essential
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
its most oxidized form (sulfate or SO4) to its most reduced
state (sulfide or H2S). The sulfur cycle, in particular,
some unique groups of procaryotes and procaryotic processes. Two
groups of procaryotes oxidize H2S to S and S to SO4.
The first is the anoxygenic photosynthetic purple and green sulfur
that oxidize H2S as a source of electrons for cyclic
The second is the "colorless sulfur bacteria" (now a misnomer because
group contains many Archaea) which oxidize H2S and S as
of energy. In either case, the organisms can usually mediate the
oxidation of H2S to SO4.
H2S----------------> S ----------------> SO4
or phototrophic sulfur oxidation
Sulfur-oxidizing procaryotes are frequently thermophiles found in
(volcanic) springs and near deep sea thermal vents that are rich in H2S.
They may be acidophiles, as well, since they acidify their own
by the production of sulfuric acid.
Since SO4 and S may be used as electron acceptors for
sulfate reducing bacteria produce H2S during a process of
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
and reduction to sulfide. Animals and bacteria can remove the sulfide
from proteins as a source of S during decomposition. These processes
the sulfur cycle.
Figure 3. The Sulfur Cycle
The Phosphorus cycle
The phosphorus cycle is comparatively simple. Inorganic phosphate
in only one form. It is interconverted from an inorganic to an organic
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
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
It is returned to land by shore animals and birds that feed on
containing sea creatures and then deposit their feces on land.
PO4 is also returned to land by a geological process, the
of ocean floors to form land masses, but the process is very slow.
the figure below considers how PO4 is recycled among
groups of organisms.
Figure 4. The Phosphorus
Plants, algae and photosynthetic bacteria can absorb phosphate (PO4)
dissolved in water, or if it washes out of rocks and soils. They
the PO4 into various organic forms, including such molecules
as DNA, RNA, ATP, and phospholipid. The plants are consumed by animals
the organic phosphate in the plant becomes organic phosphate in the
and in the bacteria that live with the animal. Animal waste returns
PO4 to the environment and also organic phosphate in the
of microbial cells. Dead plants and animals, as well as animal waste,
decomposed by microbes in the soil. The phosphate eventually is
to the soluble PO4 form in water and soil, to be taken up
by photosynthetic organisms.
Ecology of a Stratified Lake
The role of microbes in the global cycle of elements (described
can be visited on a smaller scale, in a lake, for example, like Lake
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
and its sediments are dark and anaerobic. Generally there is less O2
and less light as the water column is penetrated from the surface.
that the nutrient supply is stable and there is no mixing between
of lake water, we should, for the time being, have a stable ecosystem
recycling of essential elements among the living systems. Here is how
At the surface, light and O2 are plentiful, CO2
is fixed and O2 is produced. Photosynthetic plants, algae
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
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
of the water column, especially near the interface of the aerobic and
zones. This area, called the thermocline, is biologically very
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
just within the aerobic area taking up the CH4 from the
as a carbon source, and returning it as CO2. Lithotrophic
nitrogen and sulfur utilizing bacteria do something analogous: they are
that use NH3 and H2S from the sediments,
them to NO3 and SO4.
Figure 5. Ecology of a
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