Speaker: Douglas G. Capone
Institution: University of Southern California
The importance of biological nitrogen fixation in ocean biogeochemistry has only recently come to be fully appreciated. While results from early field studies, largely from observations at mid-latitudes and in marginal tropical and subtropical seas, indicated a relatively limited role for nitrogen fixation in the oceanic N cycle, several lines of geochemical evidence emerged in the late 1990s which suggested otherwise. This prompted a resurgence in field efforts examining this process which in turn has provided direct evidence to support the biogeochemical significance of nitrogen fixation in the oligotrophic ocean.
However, there are still major puzzles to be solved. Fully assessing the phylogenetic and physiological diversity of marine diazotrophs is an ongoing process as is the recognition of the diverse marine habitat in which they occur. Controls on marine nitrogen fixation are also being explored more thoroughly. Field observations and experimental and modeling results suggest diazotrophs, which are not limited by nitrogen availability, may be constrained by other macro- and micro-nutrient factors in different ocean basins. Indeed, a mosaic of factors which may limit nitrogen fixation in situ is emerging. Accumulating molecular evidence also suggests that the relative dominance of different diazotrophic groups varies among ocean basins. While early research focused largely on photosynthetic cyanobacteria, the presence of heterotrophic forms are widely reported and their quantitative role is an area of active research globally.
Biogeochemical conundrums include an apparent mismatch in some analysis of the balance between oceanic nitrogen fixation relative to nitrogen removal through denitrification and anammox. Methodological issues in tracer assays have been noted which may affect interpretation of some field data. Several geochemical models suggest a close coupling between inputs and removal (nitrogen homeostat), and mechanisms for that coupling have been proposed. We have recently tested the hypothesis that nitrogen fixation is enhanced in surface waters with excess phosphorus advecting offshore from major oxygen minimum zones.
Upper ocean warming, ocean acidification and increases in dissolved inorganic carbon are all likely to affect the extent and distribution of oceanic nitrogen fixation in the future. Finally, atmospheric N deposition to the ocean is rapidly accelerating and will soon exceed current estimates of oceanic nitrogen fixation.
Ref: Zehr, J. P., & Capone, D. G. (2020). Changing perspectives in marine nitrogen fixation. Science, 368(6492).