The fate of microbial biomass and associated trace elements during the genesis of Banded Iron Formations

• Applicant: Dr. Manuel Schad
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525223979

Banded iron formations (BIFs) are iron (Fe)- and silica (Si)-rich, chemical sediments that were formed between 3.8-1.8 Ga. Their widespread presence in the rock record has made them useful archives to reconstruct Precambrian seawater composition and the redox state of the ancient atmosphere. A potential major pathway for their initial precipitation in form of Fe(III) (oxyhydr)oxides was via oxidation of dissolved Fe(II) by anoxygenic photoautotrophic bacteria in the upper water column. This microbial process would also have resulted in the co-deposition of microbially produced organic carbon (Corg) such as dead cells. This would have had two major implications. First, this Corg could have fueled down-stream microbial processes during sediment diagenesis (e.g. dissimilatory microbial Fe(III) reduction, methanogenesis and Fe(III)-based anaerobic oxidation of methane) as well as low-grade metamorphic thermochemical Fe(III) reduction. Second, Corg would also have provided a viable vector for trace elements to the sediment pile. However, the microbes that drive sediment diagenesis cannot utilize complex Corg (i.e., intact cells), thus requiring other metabolic processes which took place first. In this regard Corg degradation via hydrolysis and fermentation may be an overlooked but key metabolic process in BIF genesis. To move forwards, three issues need to be addressed: (1) if and to what extent Corg degraders can access phototroph biomass and to which extent this Corg is protected by co-precipitated Fe(III) minerals; and (2) which metabolic byproducts (e.g., volatile fatty acids, CH4, H2) are formed by such processes. (3) It is further unknown, if and how Corg degradation will influence the release of trace elements from microbial Corg and how this influences the interpretation of the BIF trace element budget. In this proposal, I suggest co-cultivating anoxygenic phototrophs and Corg-degrading bacteria under conditions mimicking the early ocean chemistry to determine to which extent phototroph biomass is degraded depending on the presence/ absence of Fe(III) minerals and which metabolic products are formed. I further propose to experimentally quantify the mobility of trace elements following the degradation of Corg. By providing a C mass balance, and then comparing my experimental results to the BIF rock record, I will ultimately be able to ascertain the role of microbial Corg degradation for BIF genesis and determine how this microbial process would have influenced the BIF trace element budget.

• DFG Programme: WBP Fellowship
• International Connection: Canada
• Host: Professor Dr. Kurt O. Konhauser