Beyond an Ocean of Chlorophyll: The Role of Proteorhodopsin Phototrophy in Plankton Ecology.

Until the early 2000s, light-driven energy capture in the ocean was viewed almost exclusively through the lens of chlorophyll-based photosynthesis. The discovery of proteorhodopsins (PRs) in marine bacterioplankton fundamentally expanded this paradigm, revealing that the surface ocean is also inhabited by photoheterotrophs that exploit sunlight using simple retinal-bound proton pumps. Over the past 25 years, multiomic and physiological studies have shown that PRs are widespread across marine microorganisms, especially in prokaryotes. Analyzing global ocean metatranscriptomes, we find that PR genes consistently rank among the most highly expressed functions in situ, often rivaling genes for core metabolic processes. The combined evidence suggests that PR-driven phototrophy enables cells to pay essential energetic tolls to maintain membrane potential, enhance substrate uptake and utilization, and sustain motility during carbon scarcity. Spectral tuning and carotenoid antennae further adapt PRs to distinct light niches across productivity gradients. By supporting heterotrophic metabolism across ocean regimes, PR phototrophy likely influences dissolved organic matter processing, underscoring the need to integrate light-driven heterotrophy into marine carbon cycle models.