The co-existence of motile macroorganisms and mat-building cyanobacteria in the Paleoproterozoic FB₂ Member of the Franceville sub-basin, Gabon, points to the possible emergence of multi-trophic-level biological interaction by 2.1 billion years (Ga) ago. However, it is uncertain how these shallow-marine communities acquired and cycled nitrogen, a key, biolimiting nutrient required to sustain life at all trophic levels. Here, we use carbon and nitrogen isotope data from ancient microbial mats and host sediments, in combination with bottom-water redox proxies, to constrain biogeochemical processes operating in these settings. In this shallow-marine upwelling zone, iron speciation data and redox-sensitive metal concentrations point to oxygen-deficient bottom waters, which were episodically renewed with upwelling deep anoxic waters rich in nutrients and manganese. Organic carbon and nitrogen isotopes show little difference between the mat-related structures (MRS) and host sediments, suggesting either that similar metabolisms operated in benthic and planktonic microbial communities or that benthic carbon fixation contributed organic matter to the host sediments. The isotopic fractionation between organic and inorganic carbon is as large as 44‰, implying the involvement of multiple levels of heterotrophic carbon processing, linked to phototrophy, secondary productivity, and methanotrophy. Whole-rock nitrogen isotope values in the range of −3.5 to + 1.9‰ are consistent with microbial community nitrogen fixation in a nitrate-limited ecosystem. These data suggest that nitrogen fixation, common in photosynthetic microbial mats in modern environments, operated in benthic settings in the coastal area of the mid-Paleoproterozoic Franceville sub-basin. The upwelling of deep, anoxic waters invoked for deposition of the upper part of the underlying FB₁ Member suggests that basin-scale redox structure modulated nitrate availability in this otherwise oxic, shallow-marine basin shelf environment.

运动的大型有机体与垫层构建的蓝细菌在古元古生物FB _2中并存Franceville次流域的成员加蓬指出，到21亿年前，可能会出现多营养水平的生物相互作用。但是，尚不确定这些浅海生物群落如何获取并循环利用氮，氮是维持所有营养水平的生命所必需的关键性生物限制营养素。在这里，我们使用来自古代微生物垫和宿主沉积物的碳和氮同位素数据，结合底水氧化还原代理，来约束在这些环境中运行的生物地球化学过程。在这个浅海上升流区，铁的形态数据和氧化还原敏感的金属浓度都指向缺氧的底水，这些水被富含营养和锰的上升流深层缺氧水明显地更新了。有机碳和氮同位素在垫层相关结构（MRS）和宿主沉积物之间显示出很小的差异，表明底栖生物和浮游微生物群落中相似的新陈代谢或底栖碳固着作用为宿主沉积物提供了有机物质。有机碳和无机碳之间的同位素分馏高达44‰，这暗示了多级异养碳加工的参与，这与光养，次生生产力和甲烷营养有关。全岩层氮同位素值在-3.5至+ 1.9‰范围内，与硝酸盐受限生态系统中的微生物群落固氮作用一致。这些数据表明，固氮在现代环境中在光合微生物垫中很常见，在古古生代弗朗西维尔次流域中部沿海地区的底栖环境中进行作业。深层缺氧水的上升流被调用以沉积下层FB的上部_1位成员认为，在这种有氧，浅海盆地盆地环境中，盆地规模的氧化还原结构调节了硝酸盐的有效性。