The Majority of Active Rhodobacteraceae in Marine Sediments Belong to Uncultured Genera: A Molecular Approach to Link Their Distribution to Environmental Conditions

General studies on benthic microbial communities focus on fundamental biogeochemical processes or the most abundant constituents. Thereby, minor fractions such as the <i>Rhodobacteraceae</i> are frequently neglected. Even though this family belongs to the most widely distributed bacteria in the marine environment, their proportion on benthic microbial communities is usually within or below the single digit range. Thus, knowledge on these community members is limited, even though their absolute numbers might exceed those from the pelagic zone by orders of magnitudes. To unravel the distribution and diversity of benthic, metabolically active <i>Rhodobacteraceae</i>, we have now analyzed an already existing library of bacterial 16S rRNA transcripts. The dataset originated from 154 individual sediment samples comprising seven oceanic regions and a broad variety of environmental conditions. Across all samples, a total of 0.7% of all 16S rRNA transcripts was annotated as <i>Rhodobacteraceae.</i> Among those, <i>Sulfitobacter</i>, <i>Paracoccus</i>, and <i>Phaeomarinomonas</i> were the most abundant cultured representatives, but the majority (78%) was affiliated to uncultured family members. To define them, the 45 most abundant <i>Rhodobacteraceae</i>-OTUs assigned as "uncultured" were phylogenetically assembled in new clusters. Their next relatives particularly belonged to different subgroups other than the <i>Roseobacter</i> group, reflecting a large part of the hidden diversity within the benthic <i>Rhodobacteraceae</i> with unknown functions. The general composition of active <i>Rhodobacteraceae</i> communities was found to be specific for the geographical location, exhibiting a decreasing richness with sediment depth. One-third of the <i>Rhodobacteraceae</i>-OTUs significantly responded to the prevailing redox regime, suggesting an adaption to anoxic conditions. A possible approach to predict their physiological properties is to identify the metabolic capabilities of their nearest relatives. Those need to be proven by physiological experiments, as soon an isolate is available. Because many uncultured members of these subgroups likely thrive under anoxic conditions, in future research, a molecular-guided cultivation strategy can be pursued to isolate novel <i>Rhodobacteraceae</i> from sediments.