Angela N. Knapp

The sensitivity of marine N2 fixation to dissolved inorganic nitrogen

The dominant process adding nitrogen (N) to the ocean, di-nitrogen (N2) fixation, is mediated by prokaryotes (diazotrophs) sensitive to a variety of environmental factors. In particular, it is often assumed that consequential rates of marine N2 fixation do not occur where concentrations of nitrate (NO−3) and/or ammonium (NH+4) exceed 1μM because of the additional energetic cost associated with assimilating N2 gas relative to NO−3 or NH+4. However, an examination of culturing studies and in situ N2 fixation rate measurements from marine euphotic, mesopelagic, and benthic environments indicates that while elevated concentrations of NO−3 and/or NH+4 can depress N2 fixation rates, the process can continue at substantial rates in the presence of as much as 30μM NO−3 and/or 200μM NH+4. These findings challenge expectations of the degree to which inorganic N inhibits this process. The high rates of N2 fixation measured in some benthic environments suggest that certain benthic diazotrophs may be less sensitive to prolonged exposure to NO−3 and/or NH+4 than cyanobacterial diazotrophs. Additionally, recent work indicates that cyanobacterial diazotrophs may have mechanisms for mitigating NO−3 inhibition of N2 fixation. In particular, it has been recently shown that increasing phosphorus (P) availability increases diazotroph abundance, thus compensating for lower per-cell rates of N2 fixation that result from NO−3 inhibition. Consequently, low ambient surface ocean N:P ratios such as those generated by the increasing rates of N loss thought to occur during the last glacial to interglacial transition may create conditions favorable for N2 fixation and thus help to stabilize the marine N inventory on relevant time scales. These findings suggest that restricting measurements of marine N2 fixation to oligotrophic surface waters may underestimate global rates of this process and contribute to uncertainties in the marine N budget.

Oceanography: a marine nitrogen cycle fix?

Some of our suppositions about the marine nitrogen cycle may be wrong. An innovative analysis of nutrients at the oceans surface reveals a feedback mechanism that might hold the whole cycle in balance.

Dissolved organic nitrogen in the global surface ocean: Distribution and fate

[1] The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean’s oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphotic zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphotic zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphotic zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface-accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphotic zone.

Placing an upper limit on cryptic marine sulphur cycling.

A quantitative understanding of sources and sinks of fixed nitrogen in low-oxygen waters is required to explain the role of oxygen-minimum zones (OMZs) in controlling the fixed nitrogen inventory of the global ocean. Apparent imbalances in geochemical nitrogen budgets have spurred numerous studies to measure the contributions of heterotrophic and autotrophic N2-producing metabolisms (denitrification and anaerobic ammonia oxidation, respectively). Recently, ‘cryptic’ sulphur cycling was proposed as a partial solution to the fundamental biogeochemical problem of closing marine fixed-nitrogen budgets in intensely oxygen-deficient regions. The degree to which the cryptic sulphur cycle can fuel a loss of fixed nitrogen in the modern ocean requires the quantification of sulphur recycling in OMZ settings. Here we provide a new constraint for OMZ sulphate reduction based on isotopic profiles of oxygen (18O/16O) and sulphur (33S/32S, 34S/32S) in seawater sulphate through oxygenated open-ocean and OMZ-bearing water columns. When coupled with observations and models of sulphate isotope dynamics and data-constrained model estimates of OMZ water-mass residence time, we find that previous estimates for sulphur-driven remineralization and loss of fixed nitrogen from the oceans are near the upper limit for what is possible given in situ sulphate isotope data.

Low rates of nitrogen fixation in eastern tropical South Pacific surface waters.

Significance We present direct, field-based measurements of low nitrogen fixation rates in the eastern tropical South Pacific (ETSP) Ocean demonstrating that N2 fixation plays a minor role supporting export production regionally. These results are in contrast to indirect estimates that the highest global rates of N2 fixation occur in the ETSP. The low N2-fixation rates occur in a region with relatively high surface ocean phosphate concentrations (and low nitrate concentrations) but where atmospheric iron deposition rates are diminishingly low. Consequently, these results indicate that the ETSP hosts a minor fraction of global N2-fixation fluxes and that low nitrate to phosphate concentration ratios alone are insufficient to support high N2-fixation fluxes. An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N2 fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N2 fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N2 fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 µmol N m−2 d−1 based on sediment trap fluxes. Whereas the fractional importance of N2 fixation did not change, the N2-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N2-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N2 fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N2-fixation rates were higher, up to 100 µmol N m−2 d−1, and detected N2 fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N2 fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N2 fixation in the ETSP.

Estimates of vertical turbulent mixing used to determine a vertical gradient in net and gross oxygen production in the oligotrophic South Pacific Gyre

Mixed layer (ML) gross (GOP) and net (NOP) oxygen production rates based on in situ mass balances of triple oxygen isotopes (TOI) and O2/Ar are influenced by vertical transport from below, a term traditionally difficult to constrain. Here, we present a new approach to estimate vertical eddy diffusivity (Kz) based on density gradients in the upper thermocline and wind-speed based rates of turbulent shear at the ML depth. As an example, we use this Kz, verified by an independent 7Be-based estimate, in an O2/TOI budget at a site in the oligotrophic South Pacific Gyre (SPG). NOP equaled 0.31 ± 0.16 mmol m-2 d-1 in the ML (~55-65 m depth) and 1.2 ± 0.4 mmol m-2 d-1 (80%) beneath the ML, while GOP equaled 74 ± 27 mmol m-2 d-1 (86%) in the ML and 12 ± 4 mmol m-2 d-1 (14%) below, revealing a vertical gradient in production rates unquantifiable without the Kz estimate.

Eastern Tropical South Pacific Nitrogen fixation

15N2 incubation-based N2 fixation rates, nitrate plus nitrite concentration and d15N, sediment trap PN mass flux and isotopic composition from the Eastern Tropical South Pacific, 2010 and 2011