Regulation of proteorhodopsin phototrophy in the flavobacterium Dokdonia sp. MED134

Abstract

Recent advances reveal the benefit of proteorhodopsin (PR) phototrophy for promoting growth and survival in marine bacteria. Still, the metabolic pathways actually mediating these light responses remain unknown. We therefore investigated metabolic activities and gene expression patterns in Dokdonia sp. MED134 growing in seawater with low concentrations of complex (yeast extract and peptone) or simple (alanine) dissolved organic carbon (DOC). Remarkably, anaplerotic CO2 fixation supplied 30% of the carbon demand in these carbon-limited cultures, with much stronger light responses in alanine. Expression analysis of 11 key genes in bicarbonate uptake, anaplerotic CO2 fixation and TCA cycle pathways showed the PR gene was up-regulated 40-fold in the light, independently of DOC source. Strikingly, the two genes in the glyoxylate shunt (icl and ms) were up-regulated 40- to 100-fold in the light – but only in seawater with alanine. This implies the glyoxylate shunt, which is widespread in marine bacteria, efficiently allows house holding with carbon when PR phototrophy provides ATP. Thus, regulatory interactions between DOC quality and central metabolic pathways critically determine the fitness of surface ocean bacteria engaging in PR phototrophy