Microbial diversity involved in iron and cryptic sulfur cycling in the ferruginous, low-sulfate waters of Lake Pavin

Both iron- and sulfur- reducing bacteria strongly impact the mineralogy of iron, but their activity has long been thought to be spatially and temporally segregated based on the higher thermodynamic yields of iron over sulfate reduction. However, recent evidence suggests that sulfur cycling can predominate even under ferruginous conditions. In this study, we investigated the potential for bacterial iron and sulfur metabolisms in the iron-rich (1.2 mM dissolved Fe2+), sulfate-poor (< 20 μM) Lake Pavin which is expected to host large populations of iron-reducing and iron-oxidizing microorganisms influencing the mineralogy of iron precipitates in its permanently anoxic bottom waters and sediments. 16S rRNA gene amplicon libraries from at and below the oxycline revealed that highly diverse populations of sulfur/sulfate-reducing (SRB) and sulfur/sulfide-oxidizing bacteria represented up to 10% and 5% of the total recovered sequences in situ, respectively, which together was roughly equivalent to the fraction of putative iron cycling bacteria. In enrichment cultures amended with key iron phases identified in situ (ferric iron phosphate, ferrihydrite) or with soluble iron (Fe2+), SRB were the most competitive microorganisms, both in the presence and absence of added sulfate. The large fraction of Sulfurospirillum, which are known to reduce thiosulfate and sulfur but not sulfate, present in all cultures was likely supported by Fe(III)-driven sulfide oxidation. These results support the hypothesis that an active cryptic sulfur cycle interacts with iron cycling in the lake. Analyses of mineral phases showed that ferric phosphate in cultures dominated by SRB was transformed to vivianite with concomitant precipitation of iron sulfides. As colloidal FeS and vivianite have been reported in the monimolimnion, we suggest that SRB along with iron-reducing bacteria strongly influence iron mineralogy in the water column and sediments of Lake Pavin.

铁还原细菌与硫还原细菌均会显著影响铁的矿物学特征，但长期以来学界基于铁还原相较于硫酸盐还原更高的热力学产率，认为二者的活性在时空上彼此分离。然而近期研究证据表明，即便在富铁环境中，硫循环仍可占据主导地位。本研究以帕万湖（Lake Pavin）为研究对象，该湖富铁（溶解态亚铁离子浓度达1.2 mM）且贫硫酸盐（浓度低于20 μM），其永久性缺氧底层水体与沉积物中存在大量铁还原与铁氧化微生物，可调控铁沉淀物的矿物学特征，本研究旨在探究该环境中细菌铁代谢与硫代谢的潜在活性。对氧化跃层及跃层下方水体的16S rRNA基因扩增子文库分析结果显示，原位环境中高度多样的硫/硫酸盐还原菌（sulfur/sulfate-reducing bacteria, SRB）与硫/硫化物氧化细菌的序列数分别占总回收序列的10%与5%，二者总占比大致与推测的铁循环细菌相当。在添加了原位鉴定出的关键铁物相（磷酸铁、水铁矿）或可溶性亚铁离子的富集培养体系中，无论是否添加硫酸盐，SRB均为最具竞争优势的微生物类群。所有培养体系中均存在大量硫螺旋菌属（Sulfurospirillum），该类群已知可还原硫代硫酸盐与单质硫，但无法还原硫酸盐，其丰度大概率依赖于三价铁驱动的硫化物氧化过程。上述结果支持了本研究的假说：帕万湖中活跃的隐秘硫循环与铁循环存在相互作用。矿物物相分析结果显示，在以SRB为主导的培养体系中，磷酸铁可被转化为蓝铁矿，同时伴随硫化铁的沉淀。由于学界已在帕万湖的底层滞水层中检测到胶体硫化亚铁与蓝铁矿，本研究认为，SRB与铁还原细菌共同显著调控了帕万湖水体层与沉积物中的铁矿物学特征。