Relationship between prokaryotic GC contents and environmental salinity

BackgroundThe correlation between GC content and halophilicity has received limited attention, despite the numerous environmental factors associated with GC content evolution. While higher GC content has been linked to halophiles in some archaeal cases, it is widely believed that selective pressure from high-intensity ultraviolet radiation in halophilic archaea drives GC content increase, as it prevents DNA photoproduct formation. However, this assumption has not been statistically analyzed in a phylogenetically independent manner prior to our study. ResultsUsing phylogenetic generalized least squares, we investigated the relationship between GC content and halophilicity in 1226 bacteria and 181 archaea. Our analysis found significant positive correlations in bacteria but not in archaea. Resampling analysis indicates that the absence of significant correlation in archaea may be due to the relatively small sample size. We also observed that the strength of the correlation is negatively influenced by the functional constraint of genomic components. Additionally, we found that halophilic bacteria and archaea do not have lower photoreactivity (a measure of DNA vulnerability to ultraviolet radiation) than the photoreactivity expected from their GC contents. ConclusionsIn contrast to previous assumptions, we did not find evidence to support the widespread photoprotection hypothesis or another hypothesis that high GC content in halophiles stabilizes nucleic acid structures. Instead, our findings align with a nonadaptive hypothesis. Halophilic prokaryotes likely evolved high GC content due to frequent GC-biased gene conversion in response to DNA double-strand breaks induced directly or indirectly by high salt concentrations.