Ecological role of a cosmopolitan testate rhizopod, Paulinella ovalis , in the microbial food web of a eutrophic estuary

Paulinella ovalis is a heterotrophic, nanoplanktonic rhizopod that is poorly studied yet cosmopolitan and abundant in coastal waters. Heterotrophic nanoplankton ecology has focused primarily on heterotrophic nanoflagellates that are important picoplankton consumers and prey for larger heterotroph–recycling nutrients and transferring energy to higher trophic levels. We hypothesized a similar role for P. ovalis and assessed it by examining P. ovalis’s spatial and seasonal distribution relative to picocyanobacterial prey and potential microzooplankton predators in the Neuse River Estuary (North Carolina, USA), measuring its growth rate and estimating its potential grazing pressure on picocyanobacteria. P. ovalis abundance paralleled picocyanobacteria with higher abundance in warmer, saltier waters. Negative temporal correlations between P. ovalis and ciliate abundance and significant P. ovalis mortality at high ciliate biomass suggested that ciliates are important predators of P. ovalis. Based on an ~11 000 cells mL−1 average summertime abundance, estimated 0.37 d−1 growth rate and assumed growth efficiency of 0.3, P. ovalis potentially consumes ~60% of the picocyanobacteria and ~30% of total primary production in the summer per day. Results demonstrate P. ovalis’s potential importance within the microbial food web and should motivate future studies of grazing rates by and on P. ovalis to assess its role more quantitatively.