Foraminiferal selectivity towards bacteria: an experimental approach using a cell-permeant stain

The behaviour of two species of foraminifera (Allogromia laticollaris and Ammonia beccarii) towards living and dead bacteria and inorganic particles was monitored using a cell-permeant fluorescent nucleic acid stain. The foraminifera were studied in seawater containing fluorescently labelled dead and living bacteria, and/or polystyrene particles of the same size as a control. Time-lapse observations under a fluorescence microscope clearly revealed pseudopodial transport of stained bacteria and uptake of bacteria inside the foraminifera. In contrast, no uptake of polystyrene particles was ever observed, although the foraminifera collected these particles and deposited them along the outside of the test. We conclude that foraminifera distinguish food and non-food particles while collecting. There appears to be a variation of uptake rate and final amount of ingested and digested bacteria. These variations occur between individuals of different size and species, and between sampling times (September 2001 and July 2002).