Sulphur and Algae: Metabolism, Ecology and Evolution

Abstract

Sulphur is one of the main components of algal cells, with a cell quota typically very similar to that of phosphorus. S is present in numerous pivotal structural and functional compounds such as the amino acids cysteine and methionine, non-protein thiols (glutathione), sulpholipids, vitamins and cofactors, cell wall constituents. Sulphur is also a constituent of dimethylsulphoniopropionate (DMSP), which in some algae can represent a very large portion of cell S and is involved in algal responses to a variety of abiotic and biotic stresses, in addition to being indicted (controversially) of an important role in climate control. Algae acquire S as sulphate (SO42−), the most abundant form of inorganic S in nature. Sulphur is however assimilated in the organic matter as sulphide (S2−). A non-trivial amount of reducing power is thus required for S assimilation. In both algae and plants, S assimilation mostly takes place in the chloroplast. In eukaryotic algae (except dinoflagellates) and oceanic cyanobacteria the first step in sulphate assimilation, catalysed by ATP sulfurylase (ATPS) is subject to redox regulation, whereas in vascular plants APS reductase is the main control point in the pathway. This chapter describes in details the sulphate reduction and sulphation pathways. Attention is also given to the synthesis of glutathione and phytochelatins from cysteine and to the production of DMSP from methionine. The interactions among S assimilation and C, N and P metabolism are also addressed. Current hypotheses on the role of spatial and temporal changes of S availability on algae evolutionary trajectories are discussed.