Algae and the Sulfur Cycle

Sulfur is an essential element for autotrophs and heterotrophs. In its reduced oxidation state, the nutrient sulfur plays an important part in the structure and function of proteins. Three amino acids found in almost all proteins (cysteine, cystine, and methionine) contain carbon-bounded sulfur. Sulfur is also found in sulfolipids, some vitamins, sulfate esters, and a variety of other compounds.

In its fully oxidized state, sulfur exists as sulfate and is the major cause of acidity in both natural and polluted rainwater. This link to acidity makes sulfur important to geochemical, atmospheric, and biological processes such as the natural weathering of rocks, acid precipitation, and rates of denitrification. Sulfur cycle is also one of the main elemental cycles most heavily perturbed by 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 compounds 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 containing 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 (SO₂) or the particles of sulfate (SO₄^-2) 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 (H₂S). In the oceans, certain phytoplankton can produce a chemical that transforms organic sulfur to SO₂ 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 phosphorus. 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^2-) have the ability to take up, reduce, and assimilate SO₄^2- into amino acids and convert SO₄^2- 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 +4), sulfuric acid, particulate sulfate (oxidation state +6), and methane sulfonate (oxidation state +6). 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 +6) 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: dissimilatory and assimilatory sulfate reduction. Dissimilatory reduction of sulfate is a strictly anaerobic 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, including salt marshes, because sulfate is easily available due to its high concentration in seawater (28 mM; 900 mg L^-1 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 compounds. 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 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 [H₂S] 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 energetically activated molecule, APS (adenosine-5'-phosphosulfate), which can be further activated at 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 thecytoplasm, 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 atmospheric consequences: many types of marine algae including planktonic algae, such as prymnesiophytes, 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 freshwater 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 zooplankton 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 sunlight that reaches the Earth’s surface leads to the consequent reduction of global temperature. This drop in temperature is suggested to cause a decrease in the primary production of DMSP (in other words DMS). Thus, DMS is considered to be counteractive to the behavior of greenhouse gases like CO₂ and CH₄. Removal of the algae-DMS-derived sulfur from the air by rain and deposition of particles is a significant source of this biologically important element for some terrestrial ecosystems.