2 N2 fixation
Pelagic N2 fixation is an important source of new N to otherwise oligotrophic marine systems. The most widely studied pelagic marine diazotrophs, Trichodesmium spp., play a pivotal role in marine elemental cycles in otherwise oligotrophic tropical and subtropical seas (Capone et al., 1997; Karl et al., 2002; LaRoche and Breitbarth, 2005). Globally, based on direct measurements, oceanic N2 fixation by Trichodesmium has been estimated to be 60 (Mahaffey et al., 2005) to 80 Tg N year−1 (Capone and Carpenter, 1999), and represents upwards of 50% of the new production in some oligotrophic tropical and subtropical oceans (Karl et al., 1997; Capone et al., 2005; Mahaffey et al., 2005). Based on observed and derived N2 fixation rates by Trichodesmium, N2 fixation by these species alone is comparable in magnitude to the estimated diffusive nitrate flux across the base of the euphotic zone in tropical and subtropical systems (Karl et al., 1997; Capone, 2001; Capone et al., 2005).
However, Trichodesmium still represent only 40 to 59% of the geochemically inferred N2 fixation for the North Atlantic and Pacific (Mahaffey et al., 2005). The recent discoveries of diazotrophic unicellular cyanobacteria and bacterioplankton in marine systems (e.g., Zehr et al., 2001; Falc´on et al., 2004; Montoya et al., 2004) suggest that there are additional sources of N2 fixation that may yet reconcile measurements with geochemically predicted rates of N2 fixation in the ocean. Although, the full range of diazotrophic marine organisms is as yet, unknown, it is thought that unicellular diazotrophs may contribute up to 10% of global new production (Montoya et al., 2004). Richelia intracellularis, an endosymbiotic cyanobacterium that can inhabit a diverse group of diatoms, fixes significant amounts of nitrogen where diatom/Richelia associations occur (Carpenter et al., 1999). Based on the available rate measurements and regional and global abundance estimates in the euphotic zone, endosymbiotic and free-living unicellular cyanobacteria and bacteria are now believed to fix at least as much nitrogen as Trichodesmium in the ocean (Table 1; Carpenter et al., 1999; Zehr et al., 2001; Montoya et al., 2004). As a result, recent estimates for total pelagic marine N2 fixation are now between 100 and 200 Tg N year−1 (Karl et al., 2002; Galloway et al., 2004).
Extrapolation of N2 fixation rates made in laboratory or field populations of Trichodesmium to the world’s ocean can yield a wide range of global marine N2 fixation rates. For example, rates of N2 fixation by Trichodesmium from field populations vary by six orders of magnitude (LaRoche and Breitbarth, 2005; Mulholland et al., 2006). Laboratory estimates vary only by about 4 orders of magnitude, but still, which rates do we choose for our global estimate? Based on laboratory studies, rates of N2 fixation vary with environmental factors and according to physiological state (e.g., Mulholland et al. 1999, 2001; Mulholland and Capone, 2001; Mulholland and Bernhardt, 2005; Breitbarth et al., 2006; Hutchins et al., 2007) and yet the physiological state of natural populations is impossible to assess at the time of sampling. It is thought that rates of N2 fixation and growth by Trichodesmium are limited by phosphorus (P), iron (Fe), or light (Sa˜nudo-Wilhelmy et al., 2001; Mills et al., 2004; Fu et al., 2005; Mulholland and Bernhardt, 2005). However, the range of responses to these variables and their interactions is unknown (Mulholland and Bernhardt, 2005).
Besides real physiological variability, rates vary depending on the method used to estimate N2 fixation. The two most commonly used methods are the acetylene reduction method and 15N2 uptake; the former measures gross N2 fixation and the latter measures net N2 uptake into biomass (Gallon et al., 2002; Mulholland et al., 2004, 2006). The acetylene reduction method relies on a conversion factor to convert moles of acetylene to moles N2 reduced and the value of this conversion factor has been a matter of debate (see Capone, 1993; Mulholland et al., 2004, 2006; LaRoche and Breitbarth, 2005). Paired comparisons between the two methods used to calibrate one against the other demonstrate that the ratio between acetylene reduction and N2 uptake varies widely both within and among systems and studies (Table 2). Consequently, we are left with an unsatisfying set of data with which to make direct estimates of global N2 fixation. In addition to extrapolations from direct measurements of N2 fixation rates, global estimates of marine N2 fixation have been inferred based on geochemical arguments that rely on elemental stoichiometry of particles and dissolved nutrients in the ocean (see Mahaffey et al., 2005, for a more complete discussion). There are limitations to both of these approaches, however, because of methodological constraints and the physiological peculiarities of the dominant marine N2 fixer, Trichodesmium, discussed below. The physiology of more recently identified N2 fixers is still being elucidated and so it is premature to speculate on how these groups may influence estimates of global new production and carbon export.