Microbial rhodopsins are major contributors to the solar energy captured in the sea

Abstract

All known phototrophic metabolisms on Earth rely on one of three categories of energy-converting pigments: chlorophyll-<i>a</i> (rarely -<i>d</i>), bacteriochlorophyll-<i>a</i> (rarely -<i>b</i>), and retinal, which is the chromophore in rhodopsins. While the significance of chlorophylls in solar energy capture has been studied for decades, the contribution of retinal-based phototrophy to this process remains largely unexplored. We report the first vertical distributions of the three energy-converting pigments measured along a contrasting nutrient gradient through the Mediterranean Sea and the Atlantic Ocean. The highest rhodopsin concentrations were observed above the deep chlorophyll-<i>a</i> maxima, and their geographical distribution tended to be inversely related to that of chlorophyll-<i>a</i>. We further show that proton-pumping proteorhodopsins potentially absorb as much light energy as chlorophyll-<i>a</i>-based phototrophy and that this energy is sufficient to sustain bacterial basal metabolism. This suggests that proteorhodopsins are a major energy-transducing mechanism to harvest solar energy in the surface ocean.