Salts and Brines

Abstract

Cyanobacteria form a major component of the biota of hypersaline environments including salt lakes, solar salterns, hypersaline lagoons, salt flats, and hypersaline sulfur springs. Light-exposed sediments in such environments are often covered with dense communities of filamentous and unicellular cyanobacteria that may be responsible for much of the primary production of these environments. Cyanobacteria are often found in evaporite crusts of gypsum and even halite. A wide range of species, belonging to different taxonomic groups, were reported to live at high salinities, but two major types characteristically dominate hypersaline environments: the filamentous Microcoleus chthonoplustes, a cosmopolitan mat-building cyanobacterium found from seawater salinity to salinities exceeding 200%o; and the unicellular Aphanothece halophytica. In many hypersaline environments, including the hypolimnion of Solar Lake, Sinai, hypersaline sulfur springs, and benthic mats in salt lakes and salt flats, cyanobacteria are exposed to high sulfide concentrations, either permanently or periodically. Certain species are able to use sulfide as an electron donor in an anoxygenic type of photosynthesis through a process which involves photosystem I only and that produces elemental sulfur or thiosulfate. Some forms are able to grow in the absence of molecular oxygen. Oscillatoria limnetica, isolated from Solar Lake, was used as a laboratory model for the study of anoxygenic photosynthesis, and analysis of the phenomenon at the molecular level commenced. To be able to withstand the high osmotic pressure caused by the salt concentrations in their surrounding medium, cyanobacteria living at high salinities possess mechanisms to maintain osmotic equilibrium and cell turgor. Ions (Na+, K+, C1-) can temporarily enter the cells to counteract rapid increases in medium salinity, however, for the long term, organic solutes are accumulated to provide osmotic balance, as expected in organisms whose enzymatic machinery is inhibited by salt. Different organic osmolytes were found in cyanobacteria: the disaccharides sucrose and trehalose (especially in the less salt tolerant types); glucosylglycerol (in moderately halotolerant species); and glycine betaine (in A. halophytica and a few other forms that tolerate very high salt concentrations). Our understanding of the osmoregulatory mechanisms in cyanobacteria increased greatly in recent years, especially with regard to the molecular analysis of glucosylglycerol metabolism in Synechocystis PCC 6803. Accumulation of organic osmotic solutes has important implications for the carbon cycle in hypersaline environments where halophilic cyanobacteria provide the greatest contribution to the primary productivity.