SEMINARIO DEL DEPARTAMENTO DE PLANCTON
Y ECOLOGÍA MARINA
MIÉRCOLES 1 DE JUNIO
10:00 AM-12:00 PM
Emerging Patterns of Nitrogen Fixation in the Oceans
Douglas G. Capone
Department of Biological Sciences & the Wrigley Institute for Environmental StudiesUniversity of Southern California
The importance of biological nitrogen fixation in ocean biogeochemistry has only recently come to be fully appreciated (Mahaffey et al. 2005, Carpenter & Capone 2008). Results from field studies in the 1980s largely at mid-latitudes and in marginal tropical and subtropical seas, indicated that nitrogen fixation could be of regional significance in some of these areas. However, aggregation of these early studies suggested that there was a relatively limited role for marine nitrogen fixation in the global N cycle and in supporting primary production. However, several lines of geochemical evidence which emerged in the late 1990s suggested otherwise. This has prompted a resurgence in field efforts examining this process which in turn has provided direct evidence to support the biogeochemical significance of nitrogen fixation and its role in supporting net (or new) primary production the oligotrophic ocean. The recognition that oceanic nitrogen fixation may directly promote atmospheric carbon sequestration has fueled further interest in this process (Michaels et al. 2001).
Research on marine nitrogen fixation continues to move rapidly apace. Macro-diazotrophs such as the non-heterocystous cyanobacterium Trichodesmium spp. have been recognized to contribute to marine nitrogen fixation since the late 1960’s and have been well studied (Capone et al. 1997). Symbiotic associations between oligotrophic diatoms and heterocystous cyanobacteria have also been observed in the past but their quantitative significance in nitrogen fixation has only recently come to be appreciated.
The infusion of molecular biological methods into biological oceanography has allowed scientists to make a more realistic assessment of the diversity of marine diazotrophs and especially those that cannot be seen with the naked eye or using microscopy. Indeed, diazotrophic coccoid cyanobacteria and proteobacteria have been found that can be very abundant in some regions of the ocean (Zehr et al. 2001) and provide substantial inputs (Montoya et al. 2005).
While nitrogen fixation was long ignored in ocean biogeochemical modeling efforts (with a few notable exceptions e.g. Schaffer 1989), many current biogeochemical models are now incorporating nitrogen fixation as an explicit function providing input of new reactive nitrogen into marine ecosystems and evaluating its importance in supporting carbon uptake and sequestration in the sea.
However, there are still major puzzles to be solved. Two current and related conundrums are whether rates of denitrification and nitrogen fixation are near balance in the current ocean, and discerning how closely they are coupled in time and space. Depending on the source, recent scale-ups of oceanic inputs by nitrogen fixation and removal by “denitrifying” processes are either near steady-state, or substantially out of balance (see Carpenter & Capone 2008 and references therein) with removal far exceeding inputs. Open ocean nitrogen fixation is generally associated with the upper, photic layers of oligotrophic waters which represent about 50-60% of the global ocean. Denitrification (including the anammox reaction) in the oceanic water column is thought to be restricted to the major oxygen minimum zones (OMZ) of the Arabian Sea, eastern tropical north Pacific and eastern tropical South Pacific. The denitrifying processes occurring in OMZs result in deep waters that are enriched in phosphate relative to nitrate. Recent modeling efforts suggest that as these waters reach the surface (e.g. through upwelling) and advect offshore, nitrogen fixers may proliferate by exploiting the excess phosphate thereby providing a potential for close spatial and temporal (~102 years) coupling between these opposing processes (Deutsch et al. 2007). In contrast, the excess nitrogen generated in the North Atlantic in the absence of a substantial loss pathway appears to be exported into South Atlantic and beyond to provide an input which offsets deficits over ocean mixing (~103 years) time-scales (Moore et al. 2009).
Controls on nitrogen fixation are also being actively explored. Field observations and experimental and modeling results suggest diazotrophs, which are not likely limited by dinitrogen availability, may be limited by other macro and micronutrient factors in different ocean basins. Indeed, a mosaic of factors which may constrain nitrogen fixation in situ is emerging. Large dust inputs into the tropical N. Atlantic from the Sahel appear to foster higher availability of iron in the tropical & subtropical N. Atlantic, where phosphate availability may be a more important constraint on nitrogen fixation. In contrast, excess phosphate in surface waters of the eastern tropical and subtropical Pacific (both north & south) coupled with low deposition rates of iron from the atmosphere drives iron limitation with respect to diazotrophic growth and activity. Diazotrophy in the tropical and sub-tropical S. Atlantic & S. Pacific may also be strongly constrained by iron availability although the data density for these areas is substantially lower than from the northern hemisphere. Interestingly, elevated CO2 concentrations have been shown to stimulate nitrogen fixation by some cyanobacterial marine diazotrophs.
Recent molecular evidence suggests that the relative dominance of different diazotrophic groups may also vary among ocean basins. Microdiazotrophs such as Trichodesmium spp. appear to play a more predominant role in the N. Atlantic compared to nanodiazotrophs such as Crocosphaera and other small coccoid cyanobacteria (Langlois et al. 2008, Foster et al. 2007). In contrast, nanodiazotrophs are seemingly more dominant in the central N. Pacific (Church et al. 2008). Diazotrophs may also be constrained by the relative availability of other elemental (e.g. Mo, Co) and organic factors. Some recent evidence suggests that cyanobacterial diazotrophs may specifically contribute to certain trace vitamin (e.g. B12) fluxes in the upper ocean.
With the prospect of upper ocean warming as a result of global climate change, the magnitude, distribution and importance of marine nitrogen fixation is likely to change, although research into how is just beginning. Upper ocean warming, ocean acidification and increases in dissolved inorganic carbon are all likely to affect the extent and distribution of oceanic nitrogen fixation. Finally, atmospheric N deposition to the ocean is rapidly accelerating and will soon exceed current estimates of oceanic nitrogen fixation. With the current state of the nitrogen cycle still poorly constrained, and climatically and human induced changes on the horizon, the continued study of oceanic nitrogen fixation is important to our broader understanding of ocean biogeochemistry now and in the future.