Bacterivory in the common foraminifer Ammonia tepida: isotope tracer experiment and the controlling factors

7 The majority of sediment dweller foraminifera are deposit feeders. They use their 8 pseudopodia to gather sediment with associated algae, organic detritus and bacteria. Uptake of 9 bacteria by foraminifera have been observed but rarely quantified. We measured uptake of 10 bacteria by the common foraminifera Ammonia tepida using 15 N pre-enriched bacteria as 11 tracers. In intertidal flats, seasonal, tidal and circadian cycles induce strong variations in 12 environmental parameters. Grazing experiments were performed in order to measure effects 13 of abiotic (temperature, salinity and irradiance) and biotic (bacterial and algal abundances) 14 factors on uptake rates of bacteria. In mean conditions, A. tepida grazed 78 pgC ind -1 h -1 15 during the first eight hours of incubation, after which this uptake rate decreased. Uptake of 16 bacteria was optimal at 30°C, decreased with salinity and was unaffected by light. Above 7 ! 17 10 8 bacteria ml wt sed -1 , uptake of bacteria remained unchanged when bacterial abundance 18 increased. Algal abundance strongly affected algal uptake but did not affect uptake of 19 bacteria. As uptake of bacteria represented 8 to 19% of microbes (algae plus bacteria) uptake, 20 Ammonia seemed to be mainly dependant on algal resource. 21