Proteomic insight into functional changes of proteorhodopsin-containing bacterial species Psychroflexus torquis under different illumination and salinity levels

The extremely psychrophilic proteorhodopsin-containing bacterial species Psychroflexus torquis is considered to be a model sea-ice microorganism, which has adapted to an epiphytic lifestyle. So far, not much is known about proteorhodopsin-based phototrophy and associated life strategies of sea ice bacteria, although it has been previously shown that P. torquis can gain growth advantage from light using a proteorhodopsin proton pump, the activity of which is influenced by environmental salinity. The comprehensive quantitative proteomic study performed here indicated that P. torquis responds to changing salinity and illumination conditions. Proteins in the electron-transfer chain were down-regulated at a suboptimal salinity level, TonB-dependent transporters increased in abundance under supra-optimal salinity and decreased under suboptimal salinity. In addition, several anaplerotic CO₂ fixation proteins and three putative light sensing proteins that contain PAS and GAF domains became more abundant under illumination. Furthermore, central metabolic pathways (TCA and glycolysis) were also induced by both salinity stress and illumination. The data suggest that P. torquis responded to changes in both light energy and salinity to modulate membrane and central metabolic proteins that are involved in energy production as well as nutrient uptake and gliding motility processes that would be especially advantageous during the polar summer ice algal bloom.