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Abstract In Clear Creek, which runs through the Iowa State University campus in Ames, Iowa, USA, several types of iron mineralisation occur within stagnant pools and slow-moving water. This includes rusty flocs, commonly observed in mineral springs, rust-stained sediments and iridescent films (‘schwimmeisen’) on the pool surfaces. Observations of iron mineralisation over the course of more than a year in a single reach indicated that mineralisation occurred after precipitation events once water levels in the stream had dropped. Iron extracted and quantified from Clear Creek sediments and pool waters indicated the stream and its sediments were unlikely to be supplying the iron for mineralisation. We hypothesise that the observed mineralisation could result from the discharge of shallow, reducing groundwater-bearing Fe(II) into stagnant pools that form in debris-dammed areas of the stream. Piezometers installed next to the creek documented that shallow groundwater contained dissolved Fe, with the source of Fe being the floodplain sediments and the hydraulic gradient promoted groundwater discharge into the stream. Microorganisms identified in mineralised pools using 16S rRNA amplicon sequencing revealed an elevated presence of putative iron-oxidizing and iron-reducing microorganisms in mineralised vs. non-mineralised pools. Further investigation of the iridescent films revealed them to be composed of amorphous Fe(III) minerals. We further hypothesise that microbial exudates reduce surface tension and potential micro-zones for subsequent microbial iron redox cycling with dissolved organic matter in the pools. Determining the processes controlling mineralisation can lead to a better understanding of the ecological role of iron mineralisation in agricultural watersheds and the importance of contaminant degradation and nutrient cycling. 

Anoxygenic phototrophic bacteria can be important primary producers in some meromictic lakes. Green sulfur bacteria (GSB) have been detected in ferruginous lakes, with some evidence that they are photosynthesizing using Fe(II) as an electron donor (i.e., photoferrotrophy). However, some photoferrotrophic GSB can also utilize reduced sulfur compounds, complicating the interpretation of Fe-dependent photosynthetic primary productivity. An enrichment (BLA1) from meromictic ferruginous Brownie Lake, Minnesota, United States, contains an Fe(II)-oxidizing GSB and a metabolically flexible putative Fe(III)-reducing anaerobe. “ Candidatus Chlorobium masyuteum” grows photoautotrophically with Fe(II) and possesses the putative Fe(II) oxidase-encoding cyc2 gene also known from oxygen-dependent Fe(II)-oxidizing bacteria. It lacks genes for oxidation of reduced sulfur compounds. Its genome encodes for hydrogenases and a reverse TCA cycle that may allow it to utilize H 2 and acetate as electron donors, an inference supported by the abundance of this organism when the enrichment was supplied by these substrates and light. The anaerobe “ Candidatus Pseudopelobacter ferreus” is in low abundance (∼1%) in BLA1 and is a putative Fe(III)-reducing bacterium from the Geobacterales ord. nov. While “ Ca. C. masyuteum” is closely related to the photoferrotrophs C. ferroooxidans strain KoFox and C. phaeoferrooxidans strain KB01, it is unique at the genomic level. The main light-harvesting molecule was identified as bacteriochlorophyll c with accessory carotenoids of the chlorobactene series. BLA1 optimally oxidizes Fe(II) at a pH of 6.8, and the rate of Fe(II) oxidation was 0.63 ± 0.069 mmol day –1 , comparable to other photoferrotrophic GSB cultures or enrichments. Investigation of BLA1 expands the genetic basis for phototrophic Fe(II) oxidation by GSB and highlights the role these organisms may play in Fe(II) oxidation and carbon cycling in ferruginous lakes.