Isolation of ‘Candidatus Ferrigenium straubiae’ – a microaerophilic Fe(II)-oxidizing bacterium and nitrate-reducing Fe(II)-oxidizer within the community of culture KS

'<i>Candidatus</i> Ferrigenium straubiae' strain KS (KCTC 25982, DSM 118991) is a neutrophilic, Fe(II)-oxidizing bacterium representing up to 98% of the community in culture KS, the most extensively studied mixed culture known for autotrophic nitrate-reducing Fe(II) oxidation. The phylogeny and genome of this bacterium were previously analysed and identified as '<i>Candidatus</i> Ferrigenium straubiae'. In this study, we report the first-time successful isolation of '<i>Candidatus</i> Ferrigenium straubiae' strain KS and its experimental physiological characterization. The bacterium was identified as a non-stalk-forming, rod-shaped and non-halophilic strain with a Gram-negative classification. We characterized its physiology when grown in agarose-stabilized Fe(II)-O₂ gradient tubes where Fe(II) stemming from FeS functions as the electron donor and O₂ as the electron acceptor. It showed growth at temperatures of 20-30 °C (optimal at 25°C) and at pH levels of 6.0-7.5 (optimal at pH 6.5-7.0). The doubling time at 20 °C and pH 6.5 was 16 h. We further optimized the gradient tubes for sustainable culture maintenance using modified Wolfe's mineral medium (MWMM; 1 g l^-1 NH₄Cl) supplemented with 7-vitamin solution, SL-10 trace elements, selenite-tungstate solution and selenite-molybdate-nickel-copper-arsenic-vanadium solution (final concentrations of 10 µM Se, 10 µM Mo, 0.1 µM Ni, 0.1 µM Cu, 0.1 µM As and 5 nM V). We also evaluated several Fe(II) sources (with O₂ as electron acceptor), as well as both inorganic and organic substrates for their influence on growth. Although a known member of the denitrifying community in culture KS, the isolated strain '<i>Candidatus</i> Ferrigenium straubiae' KS exhibited exclusively microaerobic and autotrophic growth in agarose-stabilized Fe(II)-O₂ gradients, utilizing Fe(II) from FeS as the electron donor.