Grazing Ecophysiology of Florenciella and other mixotrophs that consume Prochlorococcus at Station ALOHA

Mixotrophic nanoflagellates are thought to contribute to more than half of the bacterivory in the open ocean. Florenciella, a heterokont nanoflagellate in the class Dictyocophyceae, may be particularly important as one of the major consumers of Prochlorococcus and Synechococcus in the North Pacific Subtropical Gyre. However, little is known about the ecophysiology of Florenciella, or open-ocean mixotrophs in general. Here we present the results of grazing experiments conducted on multiple strains of Florenciella and other nanoflagellates recently isolated from Station ALOHA. We find that Florenciella readily consumes Prochlorococcus, Synechococcus and heterotrophic bacteria. The maximal ingestion rate is estimated to be 10 prey cells Florenciella^-1 hr^-1, with maximal clearance rates of 1.5 nL Florenciella^-1 hr^-1 when fed Prochlorococcus and 5 nL Florenciella^-1 hr^-1 when fed Synechococcus. Grazing rates are low to moderate when compared to heterotrophic nanoflagellates, but are sufficient to allow substantial growth rates in the absence of added dissolved N, exceeding 3 per day at high prey concentrations. Their grazing behavior is also plastic in a way that appears adaptive, with substantially higher grazing rates when nutrients are limiting, and an apparent tradeoff between the maximal clearance rate and the maximal ingestion rate in response to prey density. Together, our study provides essential parameters for better understanding the grazing ecology of mixotrophs and suggests that dictyochophytes may play an important role as bacterivores in the ocean.