Rantamäki Susanne, Meriluoto Jussi, Spoof Lisa, Puputti Eeva-Maija, Tyystjärvi Taina, Tyystjärvi Esa
Department of Biochemistry/Molecular Plant Biology, University of Turku, 20014, Turku, Finland.
Biochemistry, Faculty of Biosciences, Åbo Akademi University, Tykistökatu 6 A, 20520, Turku, Finland.
Photosynth Res. 2016 Dec;130(1-3):103-111. doi: 10.1007/s11120-016-0231-4. Epub 2016 Feb 19.
The Earth has had a permanently oxic atmosphere only since the great oxygenation event (GOE) 2.3-2.4 billion years ago but recent geochemical research has revealed short periods of oxygen in the atmosphere up to a billion years earlier before the permanent oxygenation. If these "whiffs" of oxygen truly occurred, then oxygen-evolving (proto)cyanobacteria must have existed throughout the Archaean aeon. Trapping of oxygen by ferrous iron and other reduced substances present in Archaean oceans has often been suggested to explain why the oxygen content of the atmosphere remained negligible before the GOE although cyanobacteria produced oxygen. We tested this hypothesis by growing cyanobacteria in anaerobic high-CO atmosphere in a medium with a high concentration of ferrous iron. Microcystins are known to chelate iron, which prompted us also to test the effects of microcystins and nodularins on iron tolerance. The results show that all tested cyanobacteria, especially nitrogen-fixing species grown in the absence of nitrate, and irrespective of the ability to produce cyanotoxins, were iron sensitive in aerobic conditions but tolerated high concentrations of iron in anaerobicity. This result suggests that current cyanobacteria would have tolerated the high-iron content of Archaean oceans. However, only 1 % of the oxygen produced by the cyanobacterial culture was trapped by iron, suggesting that large-scale cyanobacterial photosynthesis would have oxygenated the atmosphere even if cyanobacteria grew in a reducing ocean. Recent genomic analysis suggesting that ability to colonize seawater is a secondary trait in cyanobacteria may offer a partial explanation for the sustained inefficiency of cyanobacterial photosynthesis during the Archaean aeon, as fresh water has always covered a very small fraction of the Earth's surface. If oxygenic photosynthesis originated in fresh water, then the GOE marks the adaptation of cyanobacteria to seawater, and the late-Proterozoic increase in oxygen concentration of the atmosphere is caused by full oxidation of the oceans.
地球自23亿至24亿年前的大氧化事件（GOE）以来才拥有永久性的含氧大气，但最近的地球化学研究表明，在永久性氧化之前的十亿年里，大气中曾有过短暂的氧气存在。如果这些短暂的氧气“喷发”确实发生过，那么产氧（原始）蓝细菌必定在整个太古宙时期都存在。人们常常认为，太古宙海洋中存在的亚铁和其他还原物质会捕获氧气，这就解释了为什么在大氧化事件之前，尽管蓝细菌产生了氧气，但大气中的氧气含量仍然可以忽略不计。我们通过在含有高浓度亚铁的培养基中，在厌氧高二氧化碳大气环境下培养蓝细菌来验证这一假设。已知微囊藻毒素能螯合铁，这促使我们也测试微囊藻毒素和节球藻毒素对铁耐受性的影响。结果表明，所有测试的蓝细菌，尤其是在没有硝酸盐的情况下生长的固氮物种，无论其产生蓝藻毒素的能力如何，在有氧条件下对铁敏感，但在厌氧条件下能耐受高浓度的铁。这一结果表明，现代蓝细菌能够耐受太古宙海洋中的高铁含量。然而，蓝细菌培养产生的氧气中只有1%被铁捕获，这表明即使蓝细菌在还原性海洋中生长，大规模的蓝细菌光合作用也会使大气氧化。最近的基因组分析表明，在海水中定殖的能力是蓝细菌的次要特征，这可能部分解释了太古宙时期蓝细菌光合作用持续低效的原因，因为淡水在地球表面所占的比例一直很小。如果有氧光合作用起源于淡水，那么大氧化事件标志着蓝细菌对海水的适应，而大气中氧气浓度在元古代晚期的增加是由海洋的完全氧化引起的。
