Abstract
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, N2 fixation, PSII electron transport rate and the relationship between net and gross photosynthetic O2 exchange in T. erythraeum 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, Trichodesmium 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 Trichodesmium’s productivity and growth and have major implications for its role in ocean C and N cycles.
Highlights
In marine ecosystems, phytoplankton primary production is often limited by the bioavailability of fixed N [1,2,3], where N-sources (e.g. NO3, NO2, NH4+, urea etc) are quickly depleted by fast growing phytoplankton [4]
Balanced growth of T. erythraeum IMS101 was 0.34 d-1 when grown on N2, increasing by 10% and 13% when grown in the presence of NH4+ and NO3, respectively (Table 1)
Carbon and chlorophyll a (Chla)-specific N2 fixation rates were highest for the N2 treatment, decreasing significantly by 84% and 80% (Chla-specific) and 73% and 68% (C-specific) for the NH4+ and NO3- treatments, respectively (Table 1)
Summary
Phytoplankton primary production is often limited by the bioavailability of fixed N [1,2,3], where N-sources (e.g. NO3-, NO2-, NH4+, urea etc) are quickly depleted by fast growing phytoplankton [4]. Among the most important marine diazotrophs are Trichodesmium sp., which can form extensive surface blooms in the tropical and subtropical oceans [8,9,10,11,12]. It is commonly assumed that Trichodesmium obtains most of its nitrogen quota from N2 fixation, field-based measurements of N2 fixation show wide temporal and spatial variability [18]. The causes of this variability remain unclear, but environmental factors such as the availability of combined nitrogen may be a contributing factor
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