Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138;
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138.
Proc Natl Acad Sci U S A. 2020 Dec 29;117(52):32996-33004. doi: 10.1073/pnas.2012357117. Epub 2020 Dec 14.
Bacterial hopanoid lipids are ubiquitous in the geologic record and serve as biomarkers for reconstructing Earth's climatic and biogeochemical evolution. Specifically, the abundance of 2-methylhopanoids deposited during Mesozoic ocean anoxic events (OAEs) and other intervals has been interpreted to reflect proliferation of nitrogen-fixing marine cyanobacteria. However, there currently is no conclusive evidence for 2-methylhopanoid production by extant marine cyanobacteria. As an alternative explanation, here we report 2-methylhopanoid production by bacteria of the genus , cosmopolitan nitrite oxidizers that inhabit nutrient-rich freshwater, brackish, and marine environments. The model organism produced only trace amounts of 2-methylhopanoids when grown in minimal medium or with added methionine, the presumed biosynthetic methyl donor. Supplementation of cultures with cobalamin (vitamin B) increased nitrite oxidation rates and stimulated a 33-fold increase of 2-methylhopanoid abundance, indicating that the biosynthetic reaction mechanism is cobalamin dependent. Because spp. cannot synthesize cobalamin, we postulate that they acquire it from organisms inhabiting a shared ecological niche-for example, ammonia-oxidizing archaea. We propose that during nutrient-rich conditions, cobalamin-based mutualism intensifies upper water column nitrification, thus promoting 2-methylhopanoid deposition. In contrast, anoxia underlying oligotrophic surface ocean conditions in restricted basins would prompt shoaling of anaerobic ammonium oxidation, leading to low observed 2-methylhopanoid abundances. The first scenario is consistent with hypotheses of enhanced nutrient loading during OAEs, while the second is consistent with the sedimentary record of Pliocene-Pleistocene Mediterranean sapropel events. We thus hypothesize that nitrogen cycling in the Pliocene-Pleistocene Mediterranean resembled modern, highly stratified basins, whereas no modern analog exists for OAEs.
细菌藿烷类脂质在地质记录中普遍存在，可作为重建地球气候和生物地球化学演化的生物标志物。具体来说，在中生代海洋缺氧事件（OAE）和其他时期沉积的 2-甲基藿烷的丰度被解释为反映了固氮海洋蓝细菌的增殖。然而，目前尚无确凿证据表明现存海洋蓝细菌能够产生 2-甲基藿烷。作为一种替代解释，我们在这里报告了广泛存在于富营养淡水、半咸水和海洋环境中的硝化细菌属的细菌能够产生 2-甲基藿烷。当在最小培养基或添加蛋氨酸（假定的生物合成甲基供体）中生长时，模式生物 产生的 2-甲基藿烷只有痕量。向培养物中补充钴胺素（维生素 B）可提高亚硝酸盐氧化速率，并刺激 2-甲基藿烷丰度增加 33 倍，表明生物合成反应机制依赖于钴胺素。由于 种不能合成钴胺素，我们推测它们从栖息在共享生态位的生物中获得钴胺素，例如氨氧化古菌。我们假设，在富营养条件下，基于钴胺素的共生作用会加剧上层水柱的硝化作用，从而促进 2-甲基藿烷的沉积。相比之下，贫营养表层海洋缺氧条件下限制盆地的缺氧会促使厌氧氨氧化的浅层化，导致观察到的 2-甲基藿烷丰度较低。第一种情况与 OAE 期间营养负荷增加的假说一致，而第二种情况与上新世-更新世地中海塞浦路斯事件的沉积记录一致。因此，我们假设上新世-更新世地中海的氮循环与现代高度分层的盆地相似，而 OAE 则没有现代类似物。
