Most isolated nitrate-reducing Fe(II)-oxidizing microorganisms are mixotrophic, meaning that Fe(II) is chemically oxidized by nitrite that forms during heterotrophic denitrification, and it is debated to which extent Fe(II) is enzymatically oxidized. One exception is the chemolithoautotrophic enrichment culture KS, a consortium consisting of a dominant Fe(II) oxidizer, Gallionellaceae sp., and less abundant heterotrophic strains (e.g., Bradyrhizobium sp., Nocardioides sp.). Currently, this is the only nitrate-reducing Fe(II)-oxidizing culture for which autotrophic growth has been demonstrated convincingly for many transfers over more than 2 decades. We used 16S rRNA gene amplicon sequencing and physiological growth experiments to analyze the community composition and dynamics of culture KS with various electron donors and acceptors. Under autotrophic conditions, an operational taxonomic unit (OTU) related to known microaerophilic Fe(II) oxidizers within the family Gallionellaceae dominated culture KS. With acetate as an electron donor, most 16S rRNA gene sequences were affiliated with Bradyrhizobium sp. Gallionellaceae sp. not only was able to oxidize Fe(II) under autotrophic and mixotrophic conditions but also survived over several transfers of the culture on only acetate, although it then lost the ability to oxidize Fe(II). Bradyrhizobium spp. became and remained dominant when culture KS was cultivated for only one transfer under heterotrophic conditions, even when conditions were reverted back to autotrophic in the next transfer. This study showed a dynamic microbial community in culture KS that responded to changing substrate conditions, opening up questions regarding carbon cross-feeding, metabolic flexibility of the individual strains in KS, and the mechanism of Fe(II) oxidation by a microaerophile in the absence of O2.
Bibliographical noteFunding Information:
This work was supported by the German Research Foundation (DFG)-funded research training group RTG 1708 "Molecular principles of bacterial survival strategies." We thank Ellen Struve and Kay Simmack for acetate, nitrate, and Fe(II) analysis as well as Larissa Lohmueller for preparation of media. We declare that we have no conflict of interest
© 2018 American Society for Microbiology.
- Autotrophic nitrate-dependent Fe(II) oxidation