Impact of Pyrosoma antlanticum on the Microbial Loop: Results from Flow Cytometry and 16S rRNA Gene Sequencing

Pyrosomes are colonial pelagic tunicates that can occur in large blooms. Recent work has shown that some pelagic tunicates selectively graze on microbes, but the feeding impact of pyrosomes is not well understood. Recent blooms of Pyrosoma atlanticum off the Oregon coast raise questions about the impact they have on the microbial community of this economically and ecologically important area. P. atlanticum colonies were sampled from a MOCNESS. Background seawater was collected from Niskin bottles on a CTD rosette coinciding with the pyrosome sampling. Individual pyrosome guts were sequenced using 16S rRNA gene universal primers, which suggested feeding on diverse microbial prey. Using flow cytometry we also identified cyanobacteria and pigmented phytoplankton that had been consumed. Collaborators confirmed these results in a microscopy study of the same pyrosome colonies (Ward, et.al.). Comparisons of phytoplankton community structure between the background seawater and pyrosome guts may indicate preferential feeding on larger eukaryotic phytoplankton, suggesting selective grazing on the microbial community. This study indicates that pyrosome grazing habits may bypass the microbial loop, transferring energy and carbon from microbial communities directly to high trophic levels.