Mixotrophy broadens the ecological niche range of the iron oxidizer Sideroxydans sp. CL21 isolated from an iron-rich peatland

The genome of the iron-oxidizing bacterium Sideroxydans sp. CL21 contains additional metabolic traits to adapt to its unique habitat, where it grows by simultaneously using substrate combinations. Sideroxydans sp. CL21 is a microaerobic, acid-tolerant Fe(II)-oxidizer, isolated from the Schloppnerbrunnen fen. Since the genome size of Sideroxydans sp. CL21 is 21% larger than that of the neutrophilic Sideroxydans lithotrophicus ES-1, we hypothesized that strain CL21 contains additional metabolic traits to thrive in the fen. The common genomic content of both strains contains homologs of the putative Fe(II) oxidation genes, mtoAB and cyc2. A large part of the accessory genome in strain CL21 contains genes linked to utilization of alternative electron donors, including NiFe uptake hydrogenases, and genes encoding lactate uptake and utilization proteins, motility and biofilm formation, transposable elements, and pH homeostasis mechanisms. Next, we incubated the strain in different combinations of electron donors and characterized the fen microbial communities. Sideroxydans spp. comprised 3.33% and 3.94% of the total relative abundance in the peatland soil and peatland water, respectively. Incubation results indicate Sideroxydans sp. CL21 uses H-2 and thiosulfate, while lactate only enhances growth when combined with Fe, H-2, or thiosulfate. Rates of H-2 utilization were highest in combination with other substrates. Thus, Sideroxydans sp. CL21 is a mixotroph, growing best by simultaneously using substrate combinations, which helps to thrive in dynamic and complex habitats.

Automated summary

The genome of iron-oxidizing bacterium Sideroxydans sp. CL21 contains additional metabolic traits to adapt to its unique habitat, and it grows by simultaneously using substrate combinations. Sideroxydans sp. CL21 is a microaerobic, acid-tolerant Fe(II)-oxidizer, isolated from the Schloppnerbrunnen fen. The larger genome size of strain CL21 compared to strain ES-1 suggests the presence of additional metabolic traits for survival in the fen.