Dissimilatory reduction of nitrate to ammonium, not denitrification or anammox, dominates benthic nitrate reduction in tropical estuaries

We measured benthic denitrification (DN) and dissimilatory reduction of nitrate to ammonium (DNRA) using the isotope-pairing technique in three tropical estuaries in Thailand (Mae Klong), Indonesia (Cisadane), and Fiji (Vunidawa-Rewa) during rainy, dry, and intermediate seasons along the salinity gradient of each estuary. DNRA dominated. Anammox (AN) was measured initially but neither AN activity nor AN bacteria-related 16S ribosomal RNA genes were detected in any of the estuaries. DN was either zero or extremely low, driven by water column nitrate and not from benthic nitrification-DN. N2O was not formed during DN. N2O saturations in estuary water were low, except in the nutrified Indonesian estuary, and tropical estuaries are therefore likely to be only small sources of N2O. Benthic nitrate reduction was nitrate limited; when nitrate was enhanced experimentally, DN increased slightly, but DNRA increased proportionately much more. Predominance of DNRA over DN in tropical estuaries may be due both to an energetic advantage (greater standard free energy change, ΔG°) of nitrate ammonifiers over denitrifiers when competing for limited nitrate, and also to higher affinity for nitrate by the nitrate ammonifiers. At tropical temperatures the three processes occur in the order DNRA > DN > AN. In contrast, temperate estuaries, at lower temperature and higher nitrate concentrations, exhibit proportionately greater levels of AN and DN. The Cisadane estuary became anoxic during the dry season, with high ammonium and sulfide, but no nitrate reduction because of lack of nitrate. Addition of nitrate stimulated high rates of autotrophic DN driven by sulfide, but not DNRA.