Acid-shifted isoelectric point profiles of the proteins in a hypersaline microbial mat: an adaptation to life at high salt concentrations?

In a metagenomic analysis of a stratified hypersaline (9 % salt) microbial mat in Guerrero Negro, Mexico, Kunin et al. (Mol Systems Biol 4:198, 2008) detected a significantly acid-shifted proteome, and concluded that adaptation by enriching proteins with acidic amino acids is more widespread than previously assumed. We here reevaluate these data and conclusions by comparing the isoelectric point profiles of the Guerrero Negro microbial mats (average isoelectric point 6.8) with those of the proteins encoded by the genomes of prokaryotes adapted to different salt concentrations ranges and belonging to different phylogenetic and physiological groups. Average isoelectric points below 6.8 were found not only in the proteomes of the moderately halophilic aerobic bacteria Halomonas elongata and Chromohalobacter salexigens, but even in common types of marine bacteria of the genera Alteromonas and Aliivibrio. We did not find clear evidence that the isoelectric point profile of the Guerrero Negro microbial mat can be considered to be the result of species-independent molecular convergence of the members of the microbial community determined by the salinity of the overlaying brine.