Cyanobacteria in hypersaline environments: biodiversity and physiological properties

Within the cyanobacterial world there are many species adapted to life in hypersaline environments. Some can even grow at salt concentrations approaching NaCl saturation. Halophilic cyanobacteria often form dense mats in salt lakes, and on the bottom of solar saltern ponds, hypersaline lagoons, and saline sulfur springs, and they may be found in evaporite crusts of gypsum and halite. A wide range of species were reported to live at high salinities. These include unicellular types (Aphanothece halophytica and similar morphotypes described as Euhalothece and Halothece), as well as non-heterocystous filamentous species (Coleofasciculus chthonoplastes, species of Phormidium, Halospirulina tapeticola, Halomicronema excentricum, and others). Cyanobacterial diversity in high-salt environments has been explored using both classic, morphology-based taxonomy and molecular, small subunit rRNA sequence-based techniques. This paper reviews the diversity of the cyanobacterial communities in hypersaline environments worldwide, as well as the physiological adaptations that enable these cyanobacteria to grow at high salt concentrations. To withstand the high osmotic pressure of their surrounding medium, halophilic cyanobacteria accumulate organic solutes: glycine betaine is the preferred solute in the most salt-tolerant types; Coleofasciculus produces the heteroside glucosylglycerol, and the less salt-tolerant cyanobacteria generally accumulate the disaccharides sucrose and trehalose under salt stress. Some cyanobacteria growing in benthic mats in hypersaline environments are adapted to life under anoxic conditions and they can use sulfide as an alternative electron donor in an anoxygenic type of photosynthesis through a process which involves photosystem I only.