School of Biological Sciences, University of Essex, Colchester, United Kingdom.
PLoS One. 2018 Apr 11;13(4):e0195638. doi: 10.1371/journal.pone.0195638. eCollection 2018.
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.
束毛藻在贫营养海洋中起着重要作用，在地球表面一半的区域固定氮，在一些贫营养热带和亚热带海洋中，其固定的氮占新生产力的 50%。虽然表面的束毛藻大量繁殖强烈依赖固氮作用，但在混合事件后深处或表面的藻丛可能会利用其他氮源。我们进行了实验，以确定适应不同氮源如何影响束毛藻 IMS101 的生长、元素组成、光吸收系数、N2 固定、PSII 电子传递速率以及净光合 O2 交换与总光合 O2 交换之间的关系。为此，将培养物适应于含有 NH4+ 和 NO3-（充足浓度）或仅 N2（固氮对照）的生长培养基中。使用膜入口质谱法（MIMS）测量 O2 释放和 O2 吸收的光依赖性，而 PSII 电子传递速率则从荧光光曲线（FLC）中测量。我们发现，在饱和光强下，与 NH4+ 和 NO3-相比，在 N2 上生长时，束毛藻的生长速度分别降低了约 10%和 13%。在从黑暗到 1100 μmol 光子 m-2 s-1 的所有光强度范围内，O2 吸收与净光合作用呈线性增加。非固氮条件下，C 特异性总光合作用和净光合作用的光响应曲线的最大速率和初始斜率以及总光合作用和净光合作用之间关系的斜率显著增加。我们将这些观察结果归因于非固氮条件下固氮酶活性所需的还原剂和 ATP 减少，这使得 NADPH 和 ATP 可以重新定向用于 CO2 固定和/或生物合成。通过利用其他氮源节省的能量和还原剂可以增强束毛藻的生产力和生长，并对其在海洋 C 和 N 循环中的作用产生重大影响。
