On the photosynthetic properties of marine bacterium COL2P belonging to Roseobacter clade

Aerobic anoxygenic phototrophs (AAPs) are prokaryotic microorganisms capable of harvesting light using bacteriochlorophyll-based reaction centres. Marine AAP communities are generally dominated by species belonging to the Roseobacter clade. For this reason, we used marine Roseobacter-related strain COL2P as a model organism to characterize its photosynthetic apparatus, level of pigmentation and expression of photosynthetic complexes. This strain contained functional photosynthetic reaction centres with bacteriochlorophyll a and spheroidenone as the main light-harvesting pigments, but the expression of the photosynthetic apparatus was significantly reduced when compared to truly photoautotrophic species. Moreover, the absence of peripheral light-harvesting complexes largely reduced its light-harvesting capacity. The size of the photosynthetic unit was limited to 35.4 ± 1.0 BChl a molecules supplemented by the same number of spheroidenone molecules. The contribution of oxidative phosphorylation and photophosphorylation was analysed by respiration and fluorometric measurements. Our results indicate that even with a such reduced photosynthetic apparatus, photophosphorylation provides up to three times higher electron fluxes than aerobic respiration. These results suggest that light-derived energy can provide a substantial fraction of COL2P metabolic needs.