Although we now know that dinitrogen (N2) fixation is a significant source of new nitrogen (N) fueling primary production in the marine environment (sensu Dugdale and Goering, 1967), little is known about the fate of this production, whether it is exported or stimulates remineralization (e.g., Eppley and Peterson, 1979), despite the importance of diazotrophs to global carbon and nutrient cycles (Karl et al., 2002; LaRoche and Breitbarth, 2005). A variety of marine cyanobacteria and bacteria are now known to fix N2 in marine environments, however, Trichodesmium spp. remain the most studied and most quantitatively significant pelagic nitrogen fixer based on available information. Trichodesmium spp. occur throughout the subtropical and tropical ocean where they can represent up to half of the primary production (Carpenter et al., 2004). Based on direct rate measurements, Trichodesmium accounts for a quarter to half of geochemically derived estimates of marine N2 fixation (Mahaffey et al., 2005). In addition to Trichodesmium, pelagic nitrogen fixers include other filamentous cyanobacteria, unicellular cyanobacteria, bacterioplankton, and cyanobacterial endosymbionts (Carpenter et al., 1999; Zehr et al., 2001; Montoya et al., 2004; Carpenter and Capone, 2007).
Global estimates of N2 fixation and possible controls on marine N2 fixation, at least by Trichodesmium, have been recently summarized and reviewed (LaRoche and Breitbarth, 2005; Mahaffey et al., 2005; Carpenter and Capone, 2007) and so will not be re-reviewed here. Growth rates of these organisms vary by orders of magnitude as do rates of N2 and carbon fixation (see Mulholland et al., 2006) and reasons for this variability are not well understood. Inputs of N and carbon (C) via diazotrophic growth have been measured directly or extrapolated in a variety of systems, however, the quantification of loss terms for N and C (e.g., export) are poorly constrained. Trichodesmium are rarely found in sediment traps and are positively buoyant (Walsby, 1992) and so sinking appears to be a minor loss term compared with cell lysis (Ohki, 1999; Hewson et al., 2004), extracellular release (Capone et al., 1994; Glibert and Bronk, 1994; Mulholland et al., 2004a), and grazing (O’Neil et al., 1996; O’Neil, 1999), each of which are discussed below. These observations suggest that material produced by diazotrophs may be primarily recycled in the surface ocean, unless there is another, as-yet unidentified pathway to export surface N fluxes due to diazotrophy to depth. In this paper, the fate of primary production from diazotrophy will be reviewed based on collected observations (published and not yet published) of N2 fixation and associated carbon fixation rates, N and C release, and trophodynamics associated with nutrient cycling.