Role of Sulfur for Algae: Acquisition, Metabolism, Ecology and Evolution

Abstract

Algae, like most photolithotrophs, acquire sulfur as sulfate. In the oceans sulfate concentration is never limiting and is consistently very high (29 mM). Freshwaters, however, are characterized by daily and seasonal variations and by concentrations that cover a very broad range and can, in some cases, be very low. The decrease in anthropogenic sulfur emission may reduce further these concentrations and in the future sulfur may becomes limiting in some lakes. The strategies that algae adopt in acquiring and assimilating sulfur in different environments often reflect these differences. The availability of sulfate also has repercussion on the overall metabolism of algal cells, because of its many pivotal roles in cell physiology. The maintenance of homeostasis and the responses to sulfate deprivation have a strong impact on photosynthesis, and carbon and nitrogen acquisition and metabolism. In a number of algae, most sulfur is allocated into dimethylsulfonioproprionate, whose cleavage into acrylate and dimethylsulfide is of great relevance for the ecology of extant phytoplankton and may have been important in the radiation of some algal groups. Dimethylsulfide, furthermore, is the main source of biogenic atmospheric sulfur and it is believed to have a major role in the control of global climate. These and other related matters are discussed in this review.