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.