A novel strategy for halophilicity in the photoautotrophic proteobacterium <i>Halorhodospira halophila</i>
Halorhodospira halophila is an extremophilic photoautotrophic proteobacterium found in highly saline desert lakes. It is one of the most halophilic organisms known and provides a system to investigate adaptive mechanisms for survival of abiotic stress. This report describes genome-based experimental studies of halophilic adaptations in H. halophila. Two distinct strategies are known to be used by halophilic organisms to cope with high salt conditions, namely: 'High-salt-in-' where organisms accumulate KCl (up to 5 M) in their cytoplasm, which requires them to have an acidic proteome, and 'Low-salt, organic-solute-in': where compatible solutes are accumulated in the cytoplasm. The salt in cytoplasm strategy is mainly used by extreme halophiles, which gives them ability to grow in high salt environments (up to saturation levels) while the organic osmolyte strategy is often used by moderate halophiles, which gives them adaptability to grow over wide range of salt concentrations. In the work described here, it was found that H. halophila has an acidic proteome as examined by bioinformatics analysis and isoelectric focusing gel electrophoresis. In line with this, based on Inductively Coupled Plasma (ICP) and X-ray micro probe analysis revealed that H. halophila accumulates up to 3 M KCl in its cytoplasm. However it can grow over a broad range of NaCl concentrations (3.5-35% NaCl). When grown in 5% NaCl, it had KCl concentration similar to E. coli despite its acidic proteome. Determination of cellular glycine betaine content showed that H. halophila switches to accumulation of compatible solutes when grown in media containing high NaCl but a reduced KCl concentration. These data indicate that H. halophila uses both halophilic strategies and can switch between them depending on the environmental conditions. This capability is likely to be important in enabling H. halophila to grow in high salt environments but also over wide range of salt concentrations. The potassium concentration at which H. halphila switches its halophilic strategy (1 mM KCl) is similar to that of its natural habitat (Wadi Natrun Lakes, Egypt), and therefore this osmoprotectant switch is likely to be ecologically relevant. Unexpectedly, the closely related organism Halorhodospira halochloris does not accumulate KCl but only glycine betaine. In line with this, isoelectric focusing gel electrophoresis revealed it does not have acidic proteome. This suggests recent rapid evolution in halophilic strategy in the genus Halorhodospira.