Response of bacterial grazing rates to experimental manipulation of an Antarctic coastal nanoflagellate community

We examined changes of bacterial losses related to heterotrophic nanoflagellate (HNF) size distribution in late spring/early summer (1998/1999) using 9 grazing experiments in a coastal Antarctic area (Johnson's Dock, Livingston Island, Bransfield Sector). Water samples were subjected to size fractionation through 50 and 5 μm pore sizes to obtain a truncation of the microbial food web. In each fraction, we estimated bacterial loss rates and abundance and biomass of HNF grouped into 4 size classes (≤2, 2 to 5, 5 to 10, 10 to 20 μm). We also investigated whether grazing on bacteria was mainly due to HNF, and which HNF size class had a major impact on bacteria. We expected that in the 50 μm fraction, large protists (ciliates, dinoflagellates) would prey preferentially on nanoprotists and relieve bacterial pressure from HNF. Bacterial grazing rates were estimated by disappearance of fluorescently labeled bacteria over 24 h. These showed similar values in both experimental treatments, although they were slightly higher for the 50 μm fraction. Average grazing rates were 4.8 × 10⁵ ± 3.6 × 10⁵ cells ml^-1 d^-1 in the 5 μm treatment and 6.9 × 10⁵ ± 3.2 × 10⁵ cells ml^-1 d^-1 for the 50 μm fraction. In the 5 μm fraction, HNF abundance (integrated over 24 h, HNFi) and bacterial grazing rates were significantly related. The best relationship was obtained with the smallest HNFi size classes (from ≤2 and 2 to 5 μm). In the 50 μm fraction, no relationships were found between bacterial loss rate and both total HNFi and any HNFi size class in terms of abundance and biomass. However, microozooplankton was negatively related to total bacteria and both HNFi abundance and biomass. The major contributor to this negative relationship was the HNFi size classes from ≤2 and 2 to 5 μm. Consequently, and against our expectations, large protists contributed to microbial food-web complexity by masking carbon fluxes from bacteria to HNF, and by feeding on both bacteria and nanoprotists.