Grazing by the Antarctic sea-ice ciliate Pseudocohnilembus

Potential uptake and clearance rates of fluorescent microspheres (FM) from 0.25 to 4.05 mum diameter were determined for the non-loricate ciliate Pseu docohnilembus sp. from Antarctic sea ice. The percentage of ciliate cells t hat ingested FM after 20 min incubation decreased with increasing particle diameter. Pseudocohnilembus sp. ingested FM between 0.25 and 4.05 mum in di ameter. We offered FM at concentrations less than natural concentrations fo r plankton plus detrital material and obtained clearance rates less than th ose previously reported for bactivorous ciliates. Clearance rates were 3.6- 5.4 nl cell(-1) h(-1) for FM 0.5 and 1 mum diameter, respectively, but decr eased to 1.1 nl cell(-1) h(-1) for 1.97 mum diameter and 1.4 nl cell(-1) h( -1) for 4.05-mum-diameter FM. Clearance and uptake rates of FM 0.5 and 1 mu m diameter indicate that Pseudocohnilembus sp. principally grazes on bacter ia-sized particles. However, it can also ingest organisms as large as nanop lankton and may graze particles as small as femtoplankton and colloids. Thi s suggests a feeding strategy that may suit the temporal and spatial change s in food availability in the sea-ice habitat.