Rates of benthic bacterivory of marine ciliates as a function of prey concentration

Feeding rates of protozoa on bacteria attached to particles have been determined much less frequently than grazing rates on suspended bacteria, yet in sediments most bacteria are associated with particles. This study measured the ingestion rates of benthic bacterivorous ciliates over a range of attached bacteria concentrations. Using bacteria that were transformed to express green fluorescent protein as a natural tracer, bacteria were cultured in sediment microcosms to concentrations ranging from 108 to 1010 bacteria per milliliter of pore water (ml− 1 PW). Ingestion rates of the marine ciliates Euplotes vannus, Euplotes plicatum, and Uronema marinum increased with attached prey density, ranging from 102 to 105 bacteria ciliate− 1 h− 1. Ingestion rates of attached bacteria were 10-1000 times greater than rates previously reported for benthic ciliates. Ingestion rates at a typical sediment prey concentration (109 ml− 1 PW) were significantly higher for all grazer species when feeding on attached prey versus ingestion rates when grazing on suspended prey at similar concentrations. This, combined with species-specific responses to prey density when grazing attached v. suspended prey, leads to the conclusion that ciliates are capable of grazing a much larger portion of benthic bacterial biomass than has previously been measured.