Nitrite is a central molecule in the marine nitrogen cycle because it is a substrate or product in several microbially mediated reactions.  As an intermediate in both aerobic and anaerobic pathways, it is the pivot point between loss and retention of fixed nitrogen in oxygen minimum zones (OMZs). Although nitrite oxidation has been found only in obligately aerobic nitrite oxidizing bacteria (NOB), tracer incubation experiments detect high rates in the oxygen depleted zone of the major OMZs in the ocean.  These rates can now be attributed to a novel clade of aerobic NOB that are adapted to very low or intermittently present oxygen conditions.  The geographic and depth distributions of draft genomes (MAGs) representing these new NOB show that they are the dominant NOB in OMZs and are absent from oxygenated waters. Conversely, all other NOB, cultivated or identified from metagenomic data, have opposite distributions.  High rates of nitrite oxidation are supported by high rates of nitrate reduction, the first step in the modular denitrification pathway. The rate of nitrate reduction greatly exceeds the rates of denitrification and anammox in the same water column, because most of the nitrite is reoxidized.  This results in accumulation of nitrite and the retention of fixed nitrogen.  This nitrate – nitrite cycle can also provide an explanation for the uncoupling between denitrification and anammox, by producing ammonium (via anaerobic organic matter decomposition by nitrate reducers).

nitrite, nitrification, oxygen