The Physiological Cost Of Diazotrophy For Trichodesmium Erythraeum Ims101

Trichodesmium plays a significant role in the oligotrophic oceans, fixing nitrogen in an area corresponding to half of the Earth's surface, representing up to 50% of new production in some oligotrophic tropical and subtropical oceans. Whilst Trichodesmium blooms at the surface exhibit a strong dependence on diazotrophy, colonies at depth or at the surface after a mixing event could be utilising additional N-sources. We conducted experiments to establish how acclimation to varying N-sources affects the growth, elemental composition, light absorption coefficient, N-2 fixation, PSII electron transport rate and the relationship between net and gross photosynthetic O-2 exchange in T. erythraeum IMS101. To do this, cultures were acclimated to growth medium containing NH4 (+) and NO3 -(replete concentrations) or N-2 only (diazotrophic control). The light dependencies of O-2 evolution and O-2 uptake were measured using membrane inlet mass spectrometry (MIMS), while PSII electron transport rates were measured from fluorescence light curves (FLCs). We found that at a saturating light intensity, Trichodesmium growth was similar to 10% and 13% lower when grown on N-2 than with NH4 (+) and NO3 (-), respectively. Oxygen uptake increased linearly with net photosynthesis across all light intensities ranging from darkness to 1100 mu mol photons m(-2) s(-1). The maximum rates and initial slopes of light response curves for C-specific gross and net photosynthesis and the slope of the relationship between gross and net photosynthesis increased significantly under non-diazotrophic conditions. We attribute these observations to a reduced expenditure of reductant and ATP for nitrogenase activity under non-diazotrophic conditions which allows NADPH and ATP to be re-directed to CO2 fixation and/ or biosynthesis. The energy and reductant conserved through utilising additional N-sources could enhance Trichodesmium's productivity and growth and have major implications for its role in ocean C and N cycles.