Grazing rates on bacteria by marine heterotrophic microflagellates compared to uptake rates of bacterial-sized monodisperse fluorescent latex beads

Abstract

Uptake rates of monodlsperse fluorescent latex beads by heterotrophic microflagellates were investigated under controlled laboratory conditions and in Masfjorden, S Norway. In 2-stage continuous cultures it was shown that for a Bodo sp., ingestion of latex beads was considerably less efficient than ingestion of bacteria. The selection coefficient (clearance rate of beaddclearance rate of bacteria) decreased with increasing dilution rate, implying that slow-growing or starving micro-flagellates discriminated less efficiently against latex beads. Only between 17 and 50% of the flagellates grown at dfferent dllution rates contained latex beads after incubation for 20 min, whereas 80 % were found to ingest FITC-stained bacteria. In a comparison among 8 species of bacterivorous microflagellates, one species did not take up beads at all, while the other species ingested beads to a variable extent. In seawater samples from Masfjorden, the number of organisms found to take up beads was, as a mean, only 60 % (n = 6) of the number found to take up FITC-stained bacteria. Results imply that if latex beads are used to measure grazing rates on bacteria, corrections for selectivity are required to avoid serious underestimates. With selectivity being growth-rate-dependent such corrections may be complicated. The use of stained bacteria as markers seems more promising.