Denitrification, nitrate turnover, and aerobic respiration by benthic foraminiferans in the oxygen minimum zone off Chile

Population density, nitrate turnover, and oxygen respiration of benthic foraminiferans were investigated in the oxygen minimum zone (OMZ) off the Chilean coast. Live foraminiferans were found predominantly in the upper 3 mm of the sediment, and the nitrate accumulating species Nonionella cf. stella and Stainforthia sp. dominated with a combined standing stock of 2.0 × 106 Rose Bengal stained specimens m− 2. The rate of denitrification in cells of N. cf. stella analyzed with nitrous oxide microsensors during acetylene inhibition was 84 ± 33 pmol C individual− 1 d− 1. Multiplied with the standing stock of N. cf. stella and Stainforthia sp. this yielded a minimum benthic denitrification rate of 173 µmol N m− 2 d− 1 by foraminiferans. Foraminiferal denitrification, which seemed to account for almost all benthic denitrification at the investigated site will be overlooked by most conventional methods measuring benthic denitrification. Compared to the denitrification rates, the potential rates of nitrate accumulation and oxygen respiration by N. cf. stella were an order of magnitude higher (864 pmol N individual− 1 d− 1 and 760 ± 87 pmol C individual− 1 d− 1, respectively), which seems an adaptation to the infrequent availability of nitrate and oxygen in the sediment surface.