1Rufus Scientific, Royston, United Kingdom.
2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts.
Astrobiology. 2019 Jul;19(7):885-902. doi: 10.1089/ast.2018.1958. Epub 2019 Mar 21.
Phosphorus is an essential element for all life on Earth, yet trivalent phosphorus (, in phosphines) appears to be almost completely absent from biology. Instead phosphorus is utilized by life almost exclusively as phosphate, apart from a small contingent of other pentavalent phosphorus compounds containing structurally similar chemical groups. In this work, we address four previously stated arguments as to why life does not explore trivalent phosphorus: (1) precedent (lack of confirmed instances of trivalent phosphorus in biochemicals suggests that life does not have the means to exploit this chemistry), (2) thermodynamic limitations (synthesizing trivalent phosphorus compounds is too energetically costly), (3) stability (phosphines are too reactive and readily oxidize in an oxygen (O)-rich atmosphere), and (4) toxicity (the trivalent phosphorus compounds are broadly toxic). We argue that the first two of these arguments are invalid, and the third and fourth arguments only apply to the O-rich environment of modern Earth. Specifically, both the reactivity and toxicity of phosphines are specific to aerobic life and strictly dependent on O-rich environment. We postulate that anaerobic life persisting in anoxic (O-free) environments may exploit trivalent phosphorus chemistry much more extensively. We review the production of trivalent phosphorus compounds by anaerobic organisms, including phosphine gas and an alkyl phosphine, phospholane. We suggest that the failure to find more such compounds in modern terrestrial life may be a result of the strong bias of the search for natural products toward aerobic organisms. We postulate that a more thorough identification of metabolites of the anaerobic biosphere could reveal many more trivalent phosphorus compounds. We conclude with a discussion of the implications of our work for the origin and early evolution of life, and suggest that trivalent phosphorus compounds could be valuable markers for both extraterrestrial life and the Shadow Biosphere on Earth.
磷是地球上所有生命的必需元素，但三价磷（在膦中）似乎几乎完全不存在于生物学中。除了少数其他含有结构相似化学基团的五价磷化合物外，生命几乎完全将磷用作磷酸盐。在这项工作中，我们解决了为什么生命不探索三价磷的四个先前提出的论点：（1）先例（缺乏生物化学中三价磷的确认实例表明，生命没有利用这种化学物质的手段），（2）热力学限制（合成三价磷化合物的能量成本过高），（3）稳定性（膦太容易反应，在富含氧气（O）的大气中容易氧化），以及（4）毒性（三价磷化合物具有广泛的毒性）。我们认为，前两个论点是无效的，第三个和第四个论点仅适用于现代地球富含 O 的环境。具体来说，膦的反应性和毒性都是有氧生命所特有的，并且严格依赖于富含 O 的环境。我们假设，在缺氧（无 O）环境中持续存在的厌氧生命可能会更广泛地利用三价磷化学。我们回顾了厌氧生物产生三价磷化合物的情况，包括磷化氢气体和烷基膦，磷杂环戊烷。我们认为，在现代陆地生命中没有发现更多此类化合物可能是由于对天然产物的搜索强烈偏向于有氧生物的结果。我们假设，更彻底地确定厌氧生物圈的代谢物可能会发现更多的三价磷化合物。我们最后讨论了我们的工作对生命的起源和早期进化的影响，并认为三价磷化合物可能是外星生命和地球暗影生物圈的有价值的标志物。
