| Summary: | 13 years ago metagenomic analyses revealed widespread and diverse proteorhodopsin amongst numerous marine bacteria and archaea, which challenged the conventional view that chlorophyll a is the only important light capturing pigment in ocean surface waters. Thus understanding the role of proteorhodopsin may renew our opinion of matter and energy transfer through the microbial loop. Since its discovery, important questions remain regarding the effect of light on marine microbial physiology, as well as how light might influence the activity and function of 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 the sea-ice species Psychroflexus torquis occurs under conditions of salinity stress rather than nutrient limitation and that elevated salinity is related to increased growth yields. Our gel-free proteomic data 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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