Nanoheterotroph grazing on bacteria and cyanobacteria in oxic and suboxic waters in coastal upwelling areas off northern Chile

The vertical distribution and abundance of microbial assemblages and the grazing of nanoheterotrophs upon prokaryotes in oxic and suboxic waters were examined in two coastal upwelling areas off northern Chile where a shallow Oxygen Minimum Zone (OMZ) is characteristic. Prokaryotic prey included bacterioplankton and cyanobacteria (Synechococcus); both displayed a bimodal distribution, with abundance maxima above and within the upper OMZ. Flagellates numerically dominated the nanoplankton and were mostly concentrated in the oxic layer. Mean ingestion rates of cyanobacteria by nanoflagellates (vacuole content method) ranged from 0.2 to 1.1 cells flagellate(-1) h(-1) and mean consumption rates (34-160 cells mL(-1) h(-1)) were four times higher in the oxic layer. With the selective inhibitors technique, specific grazing rates on bacteria were low (< 0.1 h(-1)) and consumption did not control bacterial production in the surface layer but did so in the suboxic layer (accounting for > 100% of bacterial production). With the same method, the specific grazing rate on cyanobacteria ranged between zero and 0.23 h(-1) with no clear differences between oxygen conditions; prey growth and production were always higher than the grazing pressure (accounting for < 17% of cyanobacterial production). The impact of grazing by nanoheterotrophs in regulating the production of prokaryotes in oxic and suboxic waters in this region is discussed.