Life on Earth depends on N₂-fixing microbes to make ammonia from atmospheric N₂ gas by the nitrogenase enzyme. Most nitrogenases use Mo as a cofactor; however, V and Fe are also possible. N₂ fixation was once believed to have evolved during the Archean-Proterozoic times using Fe as a cofactor. However, δ¹⁵N values of paleo-ocean sediments suggest Mo and V cofactors despite their low concentrations in the paleo-oceans. This apparent paradox is based on an untested assumption that only soluble metals are bioavailable. In this study, laboratory experiments were performed to test the bioavailability of mineral-associated trace metals to a model N₂-fixing bacterium Azotobacter vinelandii. N₂ fixation was observed when Mo in molybdenite, V in cavansite, and Fe in ferrihydrite were used as the sole sources of cofactors, but the rate of N₂ fixation was greatly reduced. A physical separation between minerals and cells further reduced the rate of N₂ fixation. Biochemical assays detected five siderophores, including aminochelin, azotochelin, azotobactin, protochelin, and vibrioferrin, as possible chelators to extract metals from minerals. The results of this study demonstrate that mineral-associated trace metals are bioavailable as cofactors of nitrogenases to support N₂ fixation in those environments that lack soluble trace metals and may offer a partial answer to the paradox.

中文翻译：

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矿物相关微量金属作为固氮固氮辅助因子的生物利用度

地球上的生命依靠固氮微生物通过固氮酶从大气中的N _2气体中制造氨。大多数固氮酶使用 Mo 作为辅助因子；然而，V和Fe也是可能的。N ₂固定曾被认为是在太古宙-元古代时期使用 Fe 作为辅助因子进化而来的。然而，古海洋沉积物的δ ^{15 N 值表明 Mo 和 V 是辅助因子，尽管它们在古海洋中的浓度较低。}这种明显的悖论是基于未经检验的假设，即只有可溶性金属才具有生物可利用性。在本研究中，进行了实验室实验，以测试与矿物质相关的微量金属对模型 N ₂固定细菌Azotobacter vinelandii的生物利用度。当辉钼矿中的Mo、钙锰矿中的V和水铁矿中的Fe作为唯一辅助因子来源时，观察到N _{2固定，但N 2}固定率大大降低。_{矿物质和细胞之间的物理分离进一步降低了N 2}固定率。生化测定检测到五种铁载体，包括氨基螯合素、偶氮螯合素、偶氮菌素、原螯合素和弧菌铁蛋白，它们可能是从矿物中提取金属的螯合剂。这项研究的结果表明，与矿物相关的微量金属作为固氮酶的辅助因子具有生物可利用性，可在缺乏可溶性微量金属的环境中支持 N _{2固定，并可能为这一悖论提供部分答案。}