We measured the δ⁹⁸Mo of cells and media from molybdenum (Mo) assimilation experiments with the freshwater cyanobacterium Anabaena variabilis, grown with nitrate as a nitrogen (N) source or fixing atmospheric N₂. This organism uses a Mo‐based nitrate reductase during nitrate utilization and a Mo‐based dinitrogenase during N₂ fixation under culture conditions here. We also demonstrate that it has a high‐affinity Mo uptake system (ModABC) similar to other cyanobacteria, including marine N₂‐fixing strains. Anabaena variabilis preferentially assimilated light isotopes of Mo in all experiments, resulting in fractionations of −0.2‰ to −1.0‰ ± 0.2‰ between cells and media (ε_{cells–media}), extending the range of biological Mo fractionations previously reported. The fractionations were internally consistent within experiments, but varied with the N source utilized and for different growth phases sampled. During growth on nitrate, A. variabilis consistently produced fractionations of −0.3 ± 0.1‰ (mean ± standard deviation between experiments). When fixing N₂, A. variabilis produced fractionations of −0.9 ± 0.1‰ during exponential growth, and −0.5 ± 0.1‰ during stationary phase. This pattern is inconsistent with a simple kinetic isotope effect associated with Mo transport, because Mo is likely transported through the ModABC uptake system under all conditions studied. We present a reaction network model for Mo isotope fractionation that demonstrates how Mo transport and storage, coordination changes during enzymatic incorporation, and the distribution of Mo inside the cell could all contribute to the total biological fractionations. Additionally, we discuss the potential importance of biologically incorporated Mo to organic matter‐bound Mo in marine sediments.

中文翻译：

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在硝酸盐利用和 N 2 固定过程中通过蓝藻同化进行钼同位素分馏。

我们测量了来自钼 (Mo) 同化实验的细胞和培养基的 δ ⁹⁸ Mo，其中淡水蓝藻Anabaena variabilis以硝酸盐作为氮 (N) 源生长或固定大气 N ₂。这种生物在硝酸盐利用过程中使用基于 Mo 的硝酸盐还原酶，并在此处的培养条件下在 N ₂固定过程中使用基于 Mo 的二氮酶。我们还证明它具有类似于其他蓝藻的高亲和力 Mo 吸收系统 (ModABC)，包括海洋 N ₂固定菌株。在所有实验中，鱼腥藻优先同化钼的轻同位素，导致细胞和培养基之间的分馏为 -0.2‰ 至 -1.0‰ ± 0.2‰ (ε_{细胞-培养基}），扩大了先前报道的生物钼分馏的范围。分馏在实验中是内部一致的，但随着使用的 N 源和采样的不同生长阶段而变化。在硝酸盐生长期间，变异曲霉菌始终产生 -0.3 ± 0.1‰（实验之间的平均值 ± 标准偏差）的分馏。当固定 N _{2 时}，A. variabilis在指数生长期间产生 -0.9 ± 0.1‰ 的分馏，在稳定期产生 -0.5 ± 0.1‰ 的分馏。这种模式与与 Mo 传输相关的简单动力学同位素效应不一致，因为在研究的所有条件下，Mo 很可能通过 ModABC 吸收系统传输。我们提出了一个 Mo 同位素分馏的反应网络模型，该模型展示了 Mo 的运输和储存、酶促掺入过程中的配位变化以及 Mo 在细胞内的分布都可能对总的生物分馏有贡献。此外，我们讨论了生物结合的 Mo 对海洋沉积物中有机物结合的 Mo 的潜在重要性。