Significant SAR11 removal by a hard-bottom community

Microorganisms are a key component of the marine food webs through the microbial loop. In previous work, we have shown that some bacteria, including Candidatus Pelagibacter spp (SAR11)—the most abundant bacterium in the ocean—can evade filtration by benthic and planktonic tunicates. Here we tested whether differential removal of microbial taxa by benthic filter-feeders can be observed in the distribution and abundance of microbial taxa from hard-bottom subtidal communities, a common coastal habitat in the Eastern Mediterranean Sea towards the open sea. The abundance of microbial groups along crosss-hore transects was measured based on combined flow cytometry and SSU rRNA gene metabarcoding. Our results show that most groups were depleted (up to 50%) over the hard-bottom compared to the open sea, but unexpectedly we did not observe a clear differential removal of different taxa, SAR 11 notably. This study indicates a strong top-down control of the abundance of pelagic microorganisms over shallow hard-bottom where suspension feeders are common.