Osmoadaptation mechanisms of cyanobacteria and archaea from the stromatolites of hamelin pool, Western Australia.

摘要

The stromatolites of Shark Bay Western Australia, located in a hypersaline environment, is an ideal biological system for studying survival strategies of cyanobacteria and halophilic archaea to high salt and their metabolic cooperation with other bacteria. To-date, little is known of the mechanisms by which these stromatolite microorganisms adapt to hypersalinity. To understand the formation of these sedimentary structures, detailed analysis of the microbial communities and their physiology for adaptation in this environment are crucial. In this study, microbial communities were investigated using culturing and molecular methods. Phylogenetic analysis of the 16S rRNA gene was carried out to investigate the diversity of microorganisms present. Unique phylotypes from the bacteria, cyanobacteria and archaea clone libraries were identified. Representative cyanobacteria isolates and Halococcus hamelinensis, a halophilic archaea isolated from in this study, were the focus for identifying osmoadaptation mechanisms. The presence of osmolytes in these microorganisms was detected by Nuclear magnetic resonance spectroscopy (NMR). It was found that the cyanobacterial isolates studied utilised different osmolytes. Glucosylglycerol, unique to marine cyanobacteria was not identified; instead various saccharides, glycine betaine and TMAO were the predominant solutes present. Thus cyanobacteria are likely to possess more complex mechanisms of adaptation to osmotic stress than previously thought. Findings here also indicated that H. hamelinensis accumulates glycine betaine and glutamate instead of potassium ions. DNA molecular methods were employed to identify candidate genes for the uptake of osmoprotectants. Three putative glycine betaine transporters from Halococcus hamelinensis were identified. Functionality of one of these glycine betaine transporters was determined by complementation studies. For the first time, an archaeal glycine betaine transporter was shown to be successfully complemented in a glycine betaine transport deficient mutant (E. coli MKH13). This study has increased our understanding of how microorganisms co-exist in fluctuating environments in response to solubilisation/precipitation or dilution/evaporation processes, resulting in a hypersaline environment. It also provides an excellent platform for the identification of any novel osmolytes/compatible solutes that might have been produced by these microorganisms that have been isolated for the first time from stromatolites.