Time series data of water column (0−700 m) inventories (a) of nitrate and nitrite and (b) Chl a and mixing depth. Mixing and stratification periods are separated by the dashed vertical lines.

Anton Post joined the MBL faculty in 2008 and is a world’s authority on nitrogen (N) stress in photosynthetic microbes. He explores his interests in the evolution of genes and genomes in cyanobacteria and their role in N acquisition. Oligotrophic oceans are often characterized by a short supply of available N. The unicellular marine cyanobacteria Prochlorococcus and Synechococcus are among the few organisms that thrive and sustain primary production in such waters and they contribute roughly 25% of global carbon dioxide drawdown from the atmosphere. They are able to acquire a variety of different (in)organic N species: ammonium, nitrate and nitrite. The distribution of the latter was the subject of the study reported here. Two alternative mechanisms are suggested for nitrite accumulation in the oxygenated oligotrophic water column: (1) excretion by phytoplankton or (2) microbial oxidation of ammonium (nitrification). We assessed the role of these 2 mechanisms, based on high-resolution seasonal and diurnal depth profiles of the dissolved inorganic nitrogen (DIN) species (nitrite, ammonium, nitrate) and chlorophyll a in the Gulf of Aqaba, Red Sea. Both mechanisms operated in the water column, but in different seasons; nitrification was the prime process responsible for nitrite accumulation during the stratified summer season and phytoplankton nitrite excretion operated during winter mixing. At the onset of summer stratification two N peaks developed below the photic zone, an ammonium maximum (AM) and below it the primary nitrite maximum (PNM). Both peaks were located at a depth range where phytoplankton are thought to be inactive and not excreting nitrite. During summer stratification, the water column deep chlorophyll maximum (DCM), AM, PNM and the nitracline were ordered by a downward increase in N oxidation state similar to the temporal order of the N-species during nitrification. This similarity, together with the diurnal stability of the PNM and its co-existence with oscillating chlorophyll profiles above the DCM, is consistent with nitrification as the key process forming the PNM. Thus, we suggest that transport and reaction control the vertical order and separation of N-species in the water column. The ratios between the rate constants for ammonification, ammonium oxidation, nitrite oxidation and nitrate assimilation were estimated by a simple box model to be 1:3:1.5:0.15, respectively. These field estimates are similar to the ratios between the rate constants measured in laboratory experiments. This study currently informs metagenome and metatranscriptome analyses of microbial N acquisition genes in oligotrophic Pacific waters as well as in productive waters over the New England continental shelf.