Appendicularians are ubiquitous pico-planktivores found in most of the world`s oceans. Their discarded houses and fecal pellets play a key role in the export of the oceanic primary production to the deep sea. We used blue-water SCUBA techniques, in situ incubations, flow cytometry, qPCR and next generation sequencing to measure the diet composition and grazing rates of three appendicularian species (Oikopleura fusiformis, O. longicauda and O. albicans). All three species efficiently removed pico- and nano- eukaryotic algae as well as the cyanobacteria Prochlorococcus and Synechococcus (although the latter was removed at a reduced efficiency). Surprisingly, non- photosynthetic bacteria were removed at a much lower rate. We used next generation sequencing and qPCR to quantify the relative abundance of different microbial populations before and after the in situ incubations, focusing on the hypothesis that Pelagibacter ubique, of the SAR11 clade, could evade the appendicularian mucus based filter apparatus. Selective, size-independent, and taxon-specific filtration of marine microbes by pelagic tunicates may have profound ramifications for microbial community composition, particle size spectra, the microbial loop, and biogeochemical cycles in the ocean.
