Distribution of acI ‐ A ctinorhodopsin genes in B altic S ea salinity gradients indicates adaptation of facultative freshwater photoheterotrophs to brackish waters

Abstract

Summary Knowledge on A ctinobacteria rhodopsin gene ( actR ) diversity and spatial distribution is scarce. The B altic S ea is characterized by strong salinity gradients leading to the coexistence of marine and freshwater bacteria and hence is an ideal study area to elucidate the dispersion and phylogenetic affiliation of actR in dependence on salinity . ActR DGGE fingerprints in summer 2008 revealed between 3 and 19 distinct bands within a salinity range of 2.4–27 PSU . Environmental actR clone sequences were obtained from stations distributed along the whole salinity gradient. Overall, 20 different actR sequence groups (operational taxonomic units) were found, with up to 11 different ones per station. Phylogenetically, the actR sequences were predominantly (80%) affiliated with freshwater acI ‐ A ctinobacteria whose 16 S rRNA gene accounted for 2–33% of total 16 S rRNA genes in both the B othnian S ea and central B altic S ea. However, at salinities above 14 PSU , acI ‐16 S rRNA gene accounted for less than 1%. In contrast, the diversity of actR remained high. Changes in actR gene diversity were significantly correlated with salinity, oxygen, silica or abundance of S ynechococcus sp. Our results demonstrate a wide distribution of freshwater actR along the B altic S ea salinity gradient indicating that some freshwater A ctinobacteria might have adapted to higher salinities.

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