Microbiology of oxygen minimum zones
The amount of N2 fixation and the distribution of diazotrophs in the ocean is determined by the availability of oxygen (O2), light, phosphorus (P), iron (Fe), and organic matter, as well as habitat temperature.
[6] In OMZs, different steps in the denitrification processes are performed by separate groups of bacteria, and these denitrifiers are often found directly on sinking organic matter particles, which are hotspots of microbial activity.
Genomic studies conducted in these ecosystems reveal a growing abundance of the genes encoding for the proteins responsible for the dissimilatory nitrate reduction to ammonium (DNRA) and anammox at the core of these OMZs.
[12] Such studies provide information to map out the nitrogen cycle and demystify missing links and unexplored pathways in the water column.
[7][6] DNRA has been found to be the dominant process supplying NH4+ near the shelf and upper slope of sediments because of the presence of large bacterial mats made up of the giant sulfur-oxidizing bacteria Thioploca spp.
[17] Anaerobic ammonium oxidation with nitrite (anammox) is a major pathway of fixed nitrogen removal in the anoxic zones of the open ocean.
[20] Anammox requires a source of ammonium, which under anoxic conditions could be supplied by the breakdown of sinking organic matter via heterotrophic denitrification.
[21] Alternative sources of NH4+ than denitrification, such as the DNRA, the diffusion and advection from sulfate-reducing sediments, or from microaerobic remineralization at the boundaries of anoxic waters, can supply NH4+ to anammox bacterial communities,[22] even though it is not yet clear how much they can influence the process.
[22][23] Another source of NH4+, which plays an important role in the N cycle of OMZs by contributing to the decoupling of anammox and denitrification, is the excretion of NH4+ by diel vertically migrating animals.
To escape predation, diel vertical migration (DVM) of zooplankton and micronekton can reach the anoxic layers of the major OMZs of the open ocean, and because animals excrete reduced N mostly as NH4+, they can fuel anammox directly and decouple it from denitrification.
The downward export of organic matter by migrating zooplankton and micronekton is generally smaller than that of particles at the base of the euphotic zone.
[25] Methanogens can also oxidize methane as they have the genes to do so, however this requires oxygen which they obtain from photosynthetic organisms in the upper anoxic zone.
[26] As ciliates have hydrogenosomes, which release hydrogen molecules under low oxygen conditions, they have the ability to host endosymbiotic methanogens.
[29] Aerobic respiration in OMZs helps remineralize organic matter and is a major source of ammonium for most of the upper oxygen minimal zones.
[31] While oxygen minimum zones (OMZs) occur naturally, they can be exacerbated by human impacts like climate change and land-based pollution from agriculture and sewage.
[41][40][42] Existing Earth system models project considerable reductions in oxygen and other physical-chemical variables in the ocean due to climate change, with potential ramifications for ecosystems and humans.
