Grazing of intertidal benthic foraminifera on bacteria: Assessment using pulse-chase radiotracing

Although foraminifera are a dominant component of many marine benthic communities, quantification of their predation on prokaryotes remains an experimental challenge. We have developed an approach that allows us to study grazing by adult specimens of the calcareous species Haynesina germanica and Ammonia beccarii, and the single-chambered agglutinated species Psammophaga sp., on bacteria (Halomonas sp.), pulse-chase-labelled with ³H- and ¹⁴C-Leucine. The bacterivorous ciliate Uronema sp. and flagellate Pteridomonas sp. were used as positive controls. The rate of release of ³H when protozoa were incubated with the labelled bacteria indicated the predator's grazing rate; the proportion of ¹⁴C found in the foraminiferal biomass and shell indicated the prey assimilation rate. All three foraminiferal species grazed bacteria at a rate of 3.2-5.7 ng C ind⁻¹ h⁻¹ depending on bacterial concentrations. About 23% of the biomass of the ¹⁴C-labelled prey was most likely assimilated into foraminiferal pseudopodia, 12% was expelled in dissolved waste material, about 62% was respired and only 0.1% was incorporated into the carbonate shell. Extracellular digestion associated with pseudopodia could explain the very low proportion of the labelled food assimilated in the cell body and the significant proportion located in pseudopodial networks. These experiments also suggest that very little of the carbon ingested by adult calcareous foraminifera is incorporated into the shell. However, we cannot conclude that diet has no influence on the stable isotope composition of the shell since none of our calcareous specimens grew new chambers during the experiments.