The grazing potential of some unusual marine benthic amoebae feeding on bacteria

The grazing rates of some atypical marine benthic amoebae were estimated at 18 degrees C. These rates were based on the uptake of fluorescently labelled bacteria (Planococcus citreus) fed to a range of morphotypes which included a filose amoeba, a testate amoeba, plasmodial amoebae, branched amoebae and a large lobose amoeba. The smallest amoeba, with an estimated volume of only 57.3 mu m(3) consumed 10 bacterial cells h(-1) while the largest cell examined, Trichosehaerium sieboldi, with an estimated volume of 61,021 mu m(3), consumed around 1,465 bacteria h(-1). Despite the wide range of morphotypes examined, amoeba consumption was well described by the relationship, log consumption (h(-1)) = 0.124 + 0.636.log cell volume (mu m(3)). The number of reticulate and branched amoebae in sediments from the Clyde Sea area were found to range from below detection to 66 amoebae cm(-3) of sediment. However, complementary observations based on the frequency of attachment of amoebae to buried coverglasses suggest that these estimates are low. Although the reticulate amoeba population are unlikely to play a major role in removing bacteria from marine sediments, the consumption rates found suggest that amoeboid protozoa, in general, may be important grazers of bacteria in this habitat.