Anoxygenic phototrophic Fe(II) oxidizers (photoferrotrophs) are thought to have thrived in Earth’s ancient ferruginous oceans and played a primary role in the precipitation of Archaean and Palaeoproterozoic (3.8–1.85-billion-year-old) banded iron formations (BIFs). The end of BIF deposition by photoferrotrophs has been interpreted as the result of a deepening of water-column oxygenation below the photic zone, concomitant with the proliferation of cyanobacteria. However, photoferrotrophs may have experienced competition from other anaerobic Fe(II)-oxidizing microorganisms, altering the formation mechanism of BIFs. Here we utilize microbial incubations to show that nitrate-reducing Fe(II) oxidizers metabolically outcompete photoferrotrophs for dissolved Fe(II). Moreover, both experiments and numerical modelling show that the nitrate-reducing Fe(II) oxidizers inhibit photoferrotrophy via the production of toxic intermediates. Four different photoferrotrophs, representing both green sulfur and purple non-sulfur bacteria, are susceptible to this toxic effect despite having genomic capabilities for nitric oxide detoxification. Indeed, despite nitric oxide detoxification mechanisms being ubiquitous in some groups of phototrophs at the genomic level (for example, Chlorobi and Cyanobacteria) it is likely that they would still be affected. We suggest that the production of reactive nitrogen species during nitrate-reducing Fe(II) oxidation in ferruginous environments may have inhibited the activity of photoferrotrophs in the ancient oceans and thus impeded their role in the precipitation of BIFs.

无氧光养 Fe（II） 氧化剂（光铁营养生物）被认为在地球古老的铁质海洋中茁壮成长，并在太古生代和古生代（3.8-18.5 亿年）带状铁层 （BIF） 的沉淀中发挥了主要作用。光铁营养生物 BIF 沉积的结束被解释为光区以下水柱氧合加深的结果，伴随着蓝细菌的增殖。然而，光铁营养生物可能经历了来自其他厌氧 Fe（II） 氧化微生物的竞争，改变了 BIF 的形成机制。在这里，我们利用微生物培养来表明硝酸盐还原 Fe（II） 氧化剂在代谢上优于光铁营养生物对溶解的 Fe（II）。此外，实验和数值建模都表明，硝酸盐还原型 Fe（II） 氧化剂通过产生有毒中间体来抑制光铁营养。四种不同的光铁营养生物，分别代表绿色硫细菌和紫色非硫细菌，尽管具有一氧化氮解毒的基因组能力，但容易受到这种毒性作用的影响。事实上，尽管一氧化氮解毒机制在基因组水平的某些光养生物群（例如，Chlorobi 和 Cyanobacteria）中无处不在，但它们很可能仍然会受到影响。我们认为，在铁质环境中硝酸盐还原 Fe（II） 氧化过程中活性氮的产生可能抑制了古代海洋中光铁营养生物的活性，从而阻碍了它们在 BIF 沉淀中的作用。