| Summary: | The discovery of Proteorhodopsin challenged the conventional view that chlorophyll a is the only important light capturing pigment in ocean surface waters. Thus understand the role of proteorhodopsin may renew our opinion of matter and energy transfer through the microbial loop. The current hypothesis is that proteorhodopsin could provide an adaptive advantage to the bacteria under oligotrophic conditions. To date all studies have focused on explaining the physiological role of proteorhodopsin that might affect bacterial growth during periods of low-nutrient or carbon limited conditions. However, since proteorhodopsins are widespread in natural environments and these environments may have many other stressful conditions in which proteorhodopsin may be advantageous. We hypothesize that proteorhodopsin may be an important provider of light derived energy under stress conditions that are associated with a specific ! econiche in a nutrient independent manner. Here we report for the first time that light stimulated growth of a sea-ice isolates Psychroflexus torquis occurs under conditions of salinity stress rather than nutrient limitation and that elevated salinity is related to increased growth yields. Genome analysis revealed that a cluster of putative ice-binding/adhesion genes are next to proteorhodopsin gene and its cognate carotenoid monoogygenase, thus proteorhodopsin is suspected to have a role aiding sea-ice inhabitation and persistence in this particular strain. It as well showed the genome harbors quite a few light sensing bacteriophytochrome genes compare to another Psychroflexus strain which also produce proteorhodopsin gene but does not response to light. We also used gel-free 1D LC/MS to obtain whole proteome data which suggested that proteorhodopsin levels and associated proton pumping activity, and proteorhodopsin abundance in P. torquis also is post-transcriptionally regulated by both light and salinity and thus could represent an adaptation to its sea-ice habitat. Our findings extend the existing paradigm that light provides an energy source for marine prokaryotes under stress conditions other than nutrient limitation.
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