Widespread distribution of proteorhodopsins in freshwater and brackish ecosystems.

Proteorhodopsins (PRs) are light-driven proton pumps that have been found in a variety of marine environments. The goal of this study was to search for PR presence in different freshwater and brackish environments and to explore the diversity of non-marine PR protein. Here, we show that PRs exist in distinctly different aquatic environments, ranging from clear water lakes to peat lakes and in the Baltic Sea. Some of the PRs observed in this study formed unique clades that were not previously observed in marine environments, whereas others were similar to PRs found in non-marine samples of the Global Ocean Sampling (GOS) expedition. Furthermore, the similarity of several PRs isolated from lakes in different parts of the world suggests that these genes are dispersed globally and that they may encode unique functional capabilities enabling successful competition in a wide range of freshwater environments. Phylogenomic analysis of genes found on these GOS scaffolds suggests that some of the freshwater PRs are found in freshwater Flavobacteria and freshwater SAR11-like bacteria.