Oxygen minimum zones - microbial reactors of global significance

Oxygen Minimum Zones (OMZ), as mentioned in my last blog, are currently spreading due to climate change which will have major impacts on marine ecosystems and organisms. There is always hope that we don`t have to assume the worst case scenario, although reading a paper by Ulloa et al. (2012) makes me wonder. Advanced, more sensitive technology has recently revolved anoxic conditions in OMZ‘s nitrite rich cores. These so called anoxic marine zones (AMZ) form suitable habitats for bacteria and archaea using anaerobic or microaerophilic metabolisms which contribute towards the marine nitrogen cycle. Many nitrogen-processes only occur under strictly anoxic conditions and Ulloa et al., as many others, spend a lot of time wondering if it is denitrification or annamox, which contribute mostly to the loss of fixed nitrogen. Most recently, annamox is thought to be the more dominant pathway in many OMZ, however it was shown that denitrification is very significant in AMZs of arabian sea and ETSP. 

Data from autonomous continuously profiling floats recorded episodic mixing of oxygen into the AMZ core. This makes the debate of the importance of annamox and denitrification even more important; considering that aerobic microbial communities can be established and maintained in essentially anoxic water, regulating biogeochemical cycling in AMZs. As far as I can tell, annammox seems to be more important due to the fact that heterotrophic denitrification is limited by the availability of organic matter and hence more variable in space and time.

Now to the scary part: Recent studies recognised a cryptic pelagic sulfur cycle connected with AMZs. Sulfur cycles were thought to only occur when nitrate, nitrite and oxygen is fully depleted and was therefore not thought to be important in nitrate and nitrite rich AMZs. Studies which show abundant and diverse sulfur oxidising microbial communities uncovered in AMZ water columns change this point of view. Sulfur cycles fuel nitrate reduction, thereby supplying additional substrates (nitrate and ammonia) for annammox bacteria. Next to this, autotrophic sulfur oxidation has also been found coupled  with denitrification. 

In short (Fig.1): If denitrification becomes too intense and nitrogen further limiting, primary production may be reduced; which in turn reduces the organic carbon flux into OMZs. Therefore, the rate of denitrification drops which allows nitrate and nitrite to persist. However this scenario might change if nitrogen fixation balances the N₂ loss in the AMZs. When upwelling rates increase to a point where sufficient primary production is generated and all nitrate and nitrite by denitrification and anammox are removed by downward carbon flux, sulfidic conditions can develop.

Figure 1: Development of different chemistries in OMZs depending on the relative mix of different driving parameters

Modern AMZs support the same biogeochemical cycles as sulfidic water bodies today and therefore represent a chemical intermediate state between fully oxic and fully sulfidic systems. Daily increasing anthropogenic input also contributes to AMZ expansion and intensification. We don`t even want to imagine what happens if they eventually develop sulfidic conditions. The potential impact of AMZs on marine ecosystem structures and in global geochemical cycling emphasises the need to increase the pace of exploration and discovery. We have just begun to understand the role of sulfur cycling in AMZ waters… and the clock is ticking.

Ulloa, O., Canfield, D. E. , DeLong, E. F., Letelier, R. M. and Stewart, F. J. (2012) Microbial oceanography of anoxic oxygen minimum zones. PNAS. 109(40): 15996-16003.