Iron isotope geochemistry of biogenic magnetite-bearing sediments from the Bay of Vidy, Lake Geneva

Dissimilatory microbial iron oxide reduction (DIR) has been hypothesized to be an important respiratory pathway on early Earth, potentially generating significant quantities of Fe(II) that have been preserved in Proterozoic and Archean sedimentary rocks. In particular, DIR has been implicated in the formation of magnetite in Precambrian marine sediments. To date, however, only one modern sedimentary environment existswhere in situ magnetite formation has been linked to DIR: the Bay of Vidy in Lake Geneva, Switzerland. Previous work at this locality has characterized a magnetic susceptibility anomaly that reflects the presence of fine-grained magnetite produced via microbial reduction of amorphous Fe(III) oxides that enter the Bay of Vidy froma nearby sewage treatment plant. In this study, we report on the Fe isotope composition of aqueous and solid-phase Fe in the Bay of Vidy sediments. Extensive Fe(III) reduction has occurred, resulting in the conversion of nearly all reactive (non-silicate) Fe(III) to a variety of Fe(II)-bearing phases, with mixed Fe valence magnetite being a minor but easily detectable component (0.5–8wt.%). Very little Fe isotope variation was observed in any solid phase Fe components, including magnetite, although significant fractionation was observed between aqueous and solid-phase Fe(II). Because Fe mass-balance was dominated by the solid phase, little net change in δ56Fe values for Fe(II)-bearing components was produced despite clear evidence for DIR. This study provides a basis for interpreting instances in the rock recordwhere DIR was the driving force for Fe(II) production and magnetite formation, yet no significant deviations in δ56Fe valueswere preserved. A key implication of the results is that Fe isotope homogeneity is not sufficient to rule out a biological mechanismfor magnetite formation, and this should be taken into account when examining the Precambrian rock record

The microbial ferrous wheel in a neutral pH groundwater seep

Evidence for microbial Fe redox cycling was documented in a circumneutral pH groundwater seep near Bloomington, Indiana. Geochemical and microbiological analyses were conducted at two sites, a semi-consolidated microbial mat and a floating puffball structure. In situ voltammetric microelectrode measurements revealed steep opposing gradients of O2 and Fe(II) at both sites, similar to other groundwater seep and sedimentary environments known to support microbial Fe redox cycling. The puffball structure showed an abrupt increase in dissolved Fe(II) just at its surface (~ 5 cm depth), suggesting an internal Fe(II) source coupled to active Fe(III) reduction. MPN enumerations detected microaerophilic Fe(II)-oxidizing bacteria (FeOB) and dissimilatory Fe(III)-reducing bacteria (FeRB) at densities of 102-105 cells mL-1 in samples from both sites. In vitro Fe(III) reduction experiments revealed the potential for immediate reduction (no lag period) of native Fe(III) oxides. Conventional full-length 16S rRNA gene clone libraries were compared withhigh throughput barcode sequencing of the V1, V4 or V6 variable regions of 16S rRNA genes in order to evaluate the extent to which new sequencing approaches could provide enhanced insight into the composition of Fe redox cycling microbial community structure. The composition of the clone libraries suggested a lithotroph-dominated microbial community centered around taxa related to known FeOB (e.g. Gallionella, Sideroxydans, Aquabacterium). Sequences related to recognized FeRB (e.g. Rhodoferax, Aeromonas, Geobacter, Desulfovibrio) were also well represented. Overall, sequences related to known FeOB and FeRB accounted for 88 and 59% of total clone sequences in the mat and puffball libraries, respectively. Taxa identified in the barcode libraries showed partial overlap with the clone libraries, but were not always consistent across different variable regions and sequencing platforms. However, the barcode libraries provided confirmati