N2 fixation by subsurface populations of Trichodesmium : an important source of new nitrogen to the North Atlantic Ocean

Abstract

Trichodesmium, a genus of diazotrophic cyanobacteria, is an important contributor to the marine nitrogen (N) and carbon (C) cycles. The extent to which Trichodesmium dinitrogen (N2) fixation contributes to the marine N cycle has been modeled based on abundance data and rate estimates from surface populations. However, recent data show that Trichodesmium populations have a broad vertical distribution. The presence of previously unaccounted for subsurface populations suggests that past estimates of the contribution of new N by Trichodesmium to the North Atlantic may be artificially low. Herein, culture and field studies were combined to examine trends in N2 fixation in discrete surface and subsurface Trichodesmium populations in the western North Atlantic. Surface populations were dominated by the raft colony morphology of Trichodesmium and surface N2 fixation rates ranged from (33 to 156 μmol h-1 mol C-1). Subsurface populations were dominated by the puff colony morphology. Subsurface N2 fixation was typically detectable, but consistently lower than surface population rates (9 to 88 μmol h-1 mol C-1). In an analysis of the entire field dataset, N2 fixation rates varied non-linearly as a function of in situ irradiance. This trend in N2 fixation versus in situ irradiance is consistent with field and culture observations in the literature (Bell et al., 2005; Capone et al., 2005), however other models that predict N2 fixation based on light predict higher subsurface N2 fixation than what was detected in this study. In culture, N2 fixation in Trichodesmium was proportional to light level over the range of irradiances tested (10 to 70 μmol quanta m-2 s-1) and over long and short time scales, suggesting subtle changes in the light field could depress subsurface N2 fixation. Since the subsurface samples were dominated by the puff colony morphology, it is unclear if the subsurface N2 fixation rates are the result of the intrinsic responses of different species of Trichodesmium, or light driven population segregation within a single species, among other possibilities including the effects of temperature and nutrient availability. Regardless, the subsurface rates presented herein indicate that N2 fixation by subsurface populations represents an undersampled source of new N to the western north Atlantic. This result is consistent with the findings of Davis and McGillicuddy (2006), who suggest that subsurface populations of Trichodesmium increase the average N2 fixation rate in the North Atlantic by 2.9 to 3.3 times over estimates based solely on surface estimates (Davis and McGillicuddy, 2006).