Page 189 - Algae Anatomy, Biochemistry, and Biotechnology

172 Algae: Anatomy, Biochemistry, and Biotechnology

human activity. Estimates suggest that emissions of sulfur to the atmosphere from human activity
are at least equal or probably larger in magnitude than those from natural processes. Like nitrogen,
sulfur can exist in many forms: as gases or sulfuric acid particles. The lifetime of most sulfur com-
pounds in the air is relatively short (e.g., days). Superimposed on these fast cycles of sulfur are the
extremely slow sedimentary-cycle processes or erosion, sedimentation, and uplift of rocks contain-
ing sulfur. Sulfur compounds from volcanoes are intermittently injected into the atmosphere, and a
continual stream of these compounds is produced from industrial activities. These compounds mix
with water vapor and form sulfuric acid smog. In addition to contributing to acid rain, the sulfuric
acid droplets of smog form a haze layer that reflects solar radiation and can cause a cooling of the
Earth’s surface. While many questions remain concerning specifics, the sulfur cycle in general, and
acid rain and smog issues in particular are becoming major physical, biological, and social
problems.
The sulfur cycle can be thought of as beginning with the gas sulfur dioxide (SO 2 ) or the par-
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ticles of sulfate (SO 4 ) compounds in the air. These compounds either fall out or are rained out
of the atmosphere. Algae and plants take up some forms of these compounds and incorporate
them into their tissues. Then, as with nitrogen, these organic sulfur compounds are returned to
the land or water after the algae and plants die or are consumed by heterotrophs. Bacteria are
important here as well because they can transform the organic sulfur to hydrogen sulfide gas
(H 2 S). In the oceans, certain phytoplankton can produce a chemical that transforms organic
sulfur to SO 2 that resides in the atmosphere. These gases can re-enter the atmosphere, water,
and soil, and continue the cycle.
All living organisms require S as a minor nutrient, in roughly the same atom proportion as phos-
phorus. Sulfur is present in freshwater algae at a ratio of about 1 S atom to 100 C atoms (0.15–
1.96% by dry weight), and the S content varies with species, environmental conditions, and
season. Vascular plants, algae, and bacteria (except some anaerobes that require S 22 ) have the
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ability to take up, reduce, and assimilate SO 4 into amino acids and convert SO 4 into ester
sulfate compounds.
Reduced volatile sulfur compounds, which are released to the oxygen-rich atmosphere, are
chemically oxidized during their atmospheric lifetime and end up finally as sulfur dioxide
(oxidation state þ4), sulfuric acid, particulate sulfate (oxidation state þ6), and methane sulfonate
(oxidation state þ6). It is mainly these compounds that are removed from the atmosphere and
brought back to the Earth by dry and wet deposition.
As the oxidation state of sulfur in sulfuric acid (oxidation state þ6) is the most stable under oxic
conditions, sulfate is the predominant form of sulfur in oxic waters and soils. Thus, the reduction of
sulfate to a more reduced sulfur species is a necessary prerequisite for the formation of volatile
sulfur compounds and their emission to the atmosphere. Biochemical processes which lead to
this reduction can be considered as the driving force of the atmospheric sulfur cycle.
Two types of biochemical pathways of sulfate reduction are important in the global cycles: dis-
similatory and assimilatory sulfate reduction. Dissimilatory reduction of sulfate is a strictly anaero-
bic process that takes place only in anoxic environments. Sulfate-reducing bacteria reduce sulfate
and other sulfur oxides to support respiratory metabolism, using sulfate as a terminal electron
acceptor instead of molecular oxygen. As the process is strictly anaerobic, dissimilatory sulfate
reduction occurs largely in stratified, anoxic water basins and in sediments of wetlands, lakes,
and coastal marine ecosystems. The process is particularly important in marine ecosystems, includ-
ing salt marshes, because sulfate is easily available due to its high concentration in seawater
(28 mM; 900 mg l 21 S).
In contrast to animals, which are dependent on organosulfur compounds in their food to supply
their sulfur requirement, other biota (bacteria, cyanobacteria, fungi, eukaryotic algae, and vascular
plants) can obtain sulfur from assimilatory sulfate reduction for synthesis of organosulfur com-
pounds. Sulfate is assimilated from the environment, reduced inside the cell, and fixed into
sulfur-containing amino acids and other organic compounds. The process is ubiquitous in both