Oligo-heterotrophic Activity of Marinobacter subterrani Creates an Indirect Fe(II) Oxidation Phenotype in Gradient Tubes

Abstract

Autotrophic bacteria utilizing Fe(II) as their energy and electron sources for growth affect multiple biogeochemical cycles. Some chemoheterotrophic bacteria have also been considered to exhibit an Fe(II) oxidation phenotype. For example, several Marinobacter strains have been reported to oxidize Fe(II) based on formation of oxidized iron bands in semi-solid gradient tubes that produce opposing concentration gradients of Fe(II) and oxygen. While gradient tubes are a simple and visually compelling method to test for Fe(II) oxidation, this method alone cannot confirm if, and to what extent, Fe(II) oxidation is linked to metabolism in chemoheterotrophic bacteria. Here we probe the possibility of protein-mediated and metabolic byproduct-mediated Fe(II) oxidation in Marinobacter subterrani JG233, a chemoheterotroph previously proposed to oxidize Fe(II). Results from conditional and mutant studies, along with measurements of Fe(II) oxidation rates suggest M. subterrani is unlikely to facilitate Fe(II) oxidation under microaerobic conditions. We conclude that the Fe(II) oxidation phenotype observed in gradient tubes inoculated with M. subterrani JG233 is a result of oligo-heterotrophic activity, shifting the location where oxygen dependent chemical Fe(II) oxidation occurs, rather than a biologically-mediated process. Importance Gradient tubes are the most commonly used method to isolate and identify neutrophilic Fe(II)-oxidizing bacteria. The formation of oxidized iron bands in gradient tubes provides a compelling assay to ascribe the ability to oxidize Fe(II) to autotrophic bacteria whose growth is dependent on Fe(II) oxidation. However, the physiological significance of Fe(II) oxidation in chemoheterotrophic bacteria is less well understood. Our work suggests that oligo-heterotrophic activity of certain bacteria may create a false-positive phenotype in gradient tubes by altering the location of the abiotic, oxygen-mediated oxidized iron band. Based on the results and analysis presented here, we caution against utilizing gradient tubes as the sole evidence for the capability of a strain to oxidize Fe(II) and that additional experiments are necessary to ascribe this phenotype to new isolates.

Metagenomic Features Characterized with Microbial Iron Oxidoreduction and Mineral Interaction in Southwest Indian Ridge

“…Several autotrophic iron-oxidizing species have been reported functionally versatile and metabolically active, such as Zetaproteobacteria and Gammaproteobacteria ( Thiomicrospira ) ( 26 – 28 ). Conversely, up to now, heterotrophic iron-oxidizing bacteria have been rarely detected and most of them are presumed to have limited abilities of conducting iron oxidation by subsidiary abiotic processes ( 29 , 30 ). Heterotrophic bacteria that can direct enzymatic iron oxidation are scarcely reported across all nature ecosystems, let alone hydrothermal vent environments ( 29 , 31 ).…”

Sulfur cycling likely obscures dynamic biologically‐driven iron redox cycling in contemporary methane seep environments

“…The generally discrete nature of some of the iron (oxyhydr)oxide bands in the sulfate‐free gradient tubes is a feature consistent with the presence of FeOB (Lueder et al, 2018 ), or at the very least, that the localisation of precipitation was caused by biological activity (Jain et al, 2021 ). A biosignature of some FeOB is the presence of structured iron (oxyhydr)oxides that are undeniably biogenic in origin, taking on forms like twisted stalks and tubular sheaths that, according to current understanding, cannot be formed abiotically.…”

Marinobacter: A case study in bioelectrochemical chassis evaluation

“…These attributes suggest a wide range of operational conditions for this genus, making them attractive chassis for applications in polluted and non‐freshwater environments. It is also worth noting that they can survive and even grow under extremely low‐carbon/energy conditions (Jain et al, 2021 ). This can serve as a potential advantage in field applications, when periods of starvation may occur.…”