The physiological cost of diazotrophy for <i>Trichodesmium erythraeum</i> IMS101

<div><p><i>Trichodesmium</i> 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 <i>Trichodesmium</i> 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, N₂ fixation, PSII electron transport rate and the relationship between net and gross photosynthetic O₂ exchange in <i>T</i>. <i>erythraeum</i> IMS101. To do this, cultures were acclimated to growth medium containing NH₄⁺ and NO₃^- (replete concentrations) or N₂ only (diazotrophic control). The light dependencies of O₂ evolution and O₂ 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, <i>Trichodesmium</i> growth was ~ 10% and 13% lower when grown on N₂ than with NH₄⁺ and NO₃^-, respectively. Oxygen uptake increased linearly with net photosynthesis across all light intensities ranging from darkness to 1100 μmol 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 <i>Trichodesmium’s</i> productivity and growth and have major implications for its role in ocean C and N cycles.</p>