Nitrogen cycling in oxygen deficient zones

The major marine oxygen deficient zones (ODZs), namely, Eastern Tropical North Pacific (ETNP), Eastern Tropical South Pacific (ETSP) and the Arabian Sea, comprise only 0.1 - 0.2% of the total volume of the world ocean, yet have long been recognized to be important in global nitrogen (N) cycling. At least a quarter of marine fixed N is lost (i.e. converted to gaseous end products) in these regions and the impact of this is observed in both the fixed N mass and stable isotope budgets. The major ODZs also contribute disproportionately to the atmospheric budget for nitrous oxide, a potent greenhouse gas. Despite intense study, there is much uncertainty about the rates and pathways of N transformations in these regions.  The main fixed N loss processes are denitrification and anammox (anaerobic ammonium oxidation).  The substrates that support these processes are ultimately derived from the rain of organic matter produced in surface waters as well as the diffusion of inorganic N. Thus denitrification and anammox are linked to other processes in the N cycle to supply the N substrates that they convert to N₂. (Figure 1) Most of these processes are the subject of research currently underway in the Ward laboratory.

Figure 1. Marine nitrogen cycle: Production of organic matter by assimilation of inorganic nitrogen occurs in the surface water, including N₂ fixation. This material is mineralized at depth, leading to the release of NH₄. Ammonia oxidation, followed by nitrite oxidation produced the nitrate that is the main DIN component in deep water. In oxygen minimum zones, the nitrate is denitrified to N₂, via transformations including dissimilatory reduction to ammonium, anammox and canonical denitrification. The net input term for fixed N is N₂ fixation, and the main loss terms are denitrification and anammox.

Phytoplankton bloom dynamics

Photosynthesis by marine phytoplankton is thought to account for half of the oxygen in Earth’s atmosphere. Phytoplankton form the basis of the marine food web and thus are a crucial element in the biological pump whereby CO₂ from the atmosphere is sequestered in the deep ocean. Biologically available nitrogen (N) is essential for photosynthesis and phytoplankton growth, and yet much of the global ocean appears to be N limited. Thus, N availability has the potential to not only limit marine productivity, but may also determine the composition/diversity of the phytoplankton community.

For decades, biological oceanography focused on the eukaryotic phytoplankton. In the 1970-80’s, it was discovered that the single celled picocyanobacteria are numerically dominant in the oceans and are responsible for a large fraction of ocean photosynthesis, leading to a new paradigm in which the picocyanobacteria dominate upper ocean biology and biogeochemistry. In fact, new data support the classic view that the eukaryotic phytoplankton are disproportionately important in both N and C cycling, even in regions where very small cells dominate the biomass and where the eukaryotes themselves are in the “pico” size fraction. On the basis of molecular surveys, we now recognize that the diversity of both large and small eukaryotic phytoplankton is greater than previously thought and that the most abundant and widespread eukaryotes are probably not in culture and may not be closely related to known cultivated organisms. 

The structure of phytoplankton communities is a major determinant in the magnitude and fate of primary production. Diatoms contribute disproportionately to new production by virtue of their dominance in upwelling and other coastal systems, where they exhibit episodic blooms in response to nitrate supply. Although temporary escape from predation may be the ultimate reason that bloom biomass accumulates, the ability of diatoms to exploit episodic nitrate supply must also be a factor.

We are investigating the taxonomic, genetic and functional diversity of eukaryotic phytoplankton in many ocean environments and in the context of several different experiments.