Page 190 - Algae Anatomy, Biochemistry, and Biotechnology

Biogeochemical Role of Algae 173

oxic and anoxic environments. Most of the reduced sulfur is fixed by the intracellular assimilation
process and only a minor fraction of the reduced sulfur is released as volatile gaseous compounds,
as long as the organisms are alive. However, after the death of organisms, microbial degradation
liberates reduced sulfur compounds (mainly in the form of hydrogen sulfide [H 2 S] and dimethyl
sulfide [DMS], but also as organic sulfides) to the environment. During this stage, volatile sulfur
compounds may escape to the atmosphere. However, as with sulfides formed from dissimilatory
sulfate reduction, the sulfides released during decomposition are chemically unstable in an oxic
environment and are reoxidized to sulfate by a variety of microorganisms.
Sulfate is taken up into the cell by an active transport mechanism, and inserted into an energe-
0
tically activated molecule, APS (adenosine-5 -phosphosulfate), which can be further activated at
0
0
the expense of one more ATP molecule to PAPS (3 -phosphoadenosine-5 -phosphosulphate). It
is then transferred to a thiol carrier (a molecule with a -SH group) and reduced to the 22 oxidation
state. In contrast to nitrate assimilation, where the various intermediates are present free in the
cytoplasm, sulfur remains attached to a carrier during the reduction sequence. In a final step, the
carrier-bound sulfide reacts with O-acetyl-serine to form cysteine. Cysteine serves as the starting
compound for the biosynthesis of all other sulfur metabolites, especially the other sulfur-containing
amino acids homocysteine and methionine. Cysteine and methionine are the major sulfur amino
acids and represent a very large fraction of the sulfur content of biological materials.
One aspect of the sulfur metabolism of algae deserves special mention because of its atmos-
pheric consequences: many types of marine algae including planktonic algae, such as prymnesio-
phytes, dinophytes, diatoms, chrysophytes, and prasinophytes, and macroalgae, such as
chlorophytes and rhodophytes, produce large amount of dimethylsulfonium propionate (DMSP)
from sulfur-containing amino acids (methionine). DMSP is the precursor of DMS, its enzymatic
cleavage product, which is a gas with a strong smell, and in turn is a major source of atmospheric
sulfur. Marine organisms generate about half the biogenic sulfur emitted to the atmosphere
annually, and the majority of this sulfur is produced as DMS. Because reduced sulfur compounds
such as DMS are rapidly oxidized to sulfur dioxides that function as cloud condensation nuclei
(CCN), the production of DMS can potentially affect climate on a global scale. DMS has also a
peculiar role: birds use DMS as a foraging cue, as algae being consumed by fish release DMS,
as a consequence bringing the presence of the fish to the attention of the birds.
DMSP can function as osmoregulator, buoyancy controller, cryoprotectant, and antioxidant.
Freshwater algae do not produce DMSP, because, being an osmolyte, it has no significance in fresh-
water systems. It has also been postulated that DMSP production is indirectly related to the nitrogen
nutrition of algae, with DMSP being a store for excess sulfate taken up while assimilating the
molybdenum necessary to synthesize nitrate reductase or nitrogenase (this could also be true for
vascular plants with high nitrate-reductase activities or with symbiotic nitrogen-fixing bacteria).
The cleavage of DMSP by means of the enzyme DMSP lyase produces gaseous DMS and
acrylic acid. DMSP can also be released from phytoplankton cells during senescence, whereas zoo-
plankton grazing on healthy cells is thought to facilitate the release of DMSP from ruptured algal
cells during sloppy feeding. Once released from algal cells, DMSP undergoes microbially mediated
conversion by cleavage into gaseous DMS and acrylate.
It had been known for many years that the global budget of sulfur could not be balanced without
a substantial flux of this element from the oceans to the atmosphere and then to land. Once emitted
from the sea, DMS is transformed in the atmosphere by free radicals (particularly hydroxyl and
nitrate) to form a variety of products, most importantly sulfur dioxide and sulfate in the form of
small particles. As already stated, these products are acidic and are responsible for the natural
acidity of atmospheric particles; man’s activities in burning fossil fuels add further sulfur acidity
to this natural process. In addition, the sulfate particles (natural and man-made) can alter
the amount of radiation reaching Earth’s surface both directly by scattering of solar energy and
indirectly by acting as the nuclei on which cloud droplets form (CCN), thereby affecting the
energy reflected back to space by clouds (denser cloud albedo). The reduction of the amount of