Measurement of bacterivory by protists in open ocean waters

Protozoa are the main consumers of heterotrophic bacteria in aquatic habitats. The numbers of these bacteria and protozoa in oligotrophic areas of the open ocean are low, and current methods lack the sensitivity to assess rates of bacterivory in such waters. A new method is proposed for estimating bacterivory on dual radioactively labelled natural bacteria using living ambient prey bacteria and separation of predators from prey by fractionation. This approach is sufficiently sensitive to measure the consumption of less than 1% of the labelled bacteria during a 13-h incubation period. When tested on samples collected from 27 stations in mesotrophic and oligotrophic regions of the North and South Atlantic Oceans, about 17% of metabolically active bacteria were grazed per day and about 60% of consumed prey biomass labelled with 14C-leucine was retained by the predators.