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 assemblagesand the grazing of nanoheterotrophs upon prokaryotes in oxicand suboxic waters were examined in two coastal upwelling areasoff northern Chile where a shallow Oxygen Minimum Zone (OMZ)is characteristic. Prokaryotic prey included bacterioplanktonand cyanobacteria (Synechococcus); both displayed a bimodaldistribution, with abundance maxima above and within the upperOMZ. Flagellates numerically dominated the nanoplankton andwere mostly concentrated in the oxic layer. Mean ingestion ratesof cyanobacteria by nanoflagellates (vacuole content method)ranged from 0.2 to 1.1 cells flagellate^–1 h^–1 andmean consumption rates (34–160 cells mL^–1 h^–1)were four times higher in the oxic layer. With the selectiveinhibitors technique, specific grazing rates on bacteria werelow (<0.1 h^–1) and consumption did not control bacterialproduction in the surface layer but did so in the suboxic layer(accounting for >100% of bacterial production). With thesame method, the specific grazing rate on cyanobacteria rangedbetween zero and 0.23 h^–1 with no clear differences betweenoxygen conditions; prey growth and production were always higherthan the grazing pressure (accounting for <17% of cyanobacterialproduction). The impact of grazing by nanoheterotrophs in regulatingthe production of prokaryotes in oxic and suboxic waters inthis region is discussed.