The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition

Abstract

Bidirectional extracellular electron transfer (EET) is crucial to upholding microbial metabolism with insoluble electron acceptors or donors in anoxic environments. Investigating bidirectional EET-capable microorganisms is desired to understand the cell-cell and microbe-mineral interactions and their role in mineral cycling besides leveraging their energy generation and conversion, biosensing, and bio-battery applications. Here, we report on iron cycling by haloalkaliphilic Geoalkalibacter halelectricus via bidirectional EET under haloalkaline conditions. It efficiently reduces Fe 3+ oxide (Fe 2 O 3 ) to Fe 0 at a 0.75 ± 0.08 mM/mg protein /d rate linked to acetate oxidation via outward EET and oxidizes Fe 0 to Fe 3+ at a 0.24 ± 0.03 mM/mg protein /d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm 2 current by linking acetate oxidation to anode reduction via outward EET and reduced fumarate by drawing electrons from the cathode (‒2.5 ± 0.3 µA/cm 2 ) via inward EET. The cyclic voltammograms of G. halelectricu s biofilms revealed redox moieties with different formal potentials, suggesting the involvement of different membrane components in bidirectional EET. The cyclic voltammetry and GC-MS analysis of the cell-free spent medium revealed the lack of soluble redox mediators, suggesting direct electron transfer by G. halelecctricu s in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe 0 and Fe 3+ oxide, respectively, this study advances the limited understanding of the metabolic capabilities of extremophiles to respire on insoluble electron acceptors or donors via bidirectional EET and invokes the possible role of G. halelectricu s in iron cycling in barely studied haloalkaline environments. IMPORTANCE Bidirectional extracellular electron transfer (EET) appears to be a key microbial metabolic process in anoxic environments that are depleted in soluble electron donor and acceptor molecules. Though it is an ecologically important and applied microbial phenomenon, it has been reported with a few microorganisms, mostly from nonextreme environments. Moreover, direct electron transfer-based bidirectional EET is studied for very few microorganisms with electrodes in engineered systems and barely with the natural insoluble electron acceptor and donor molecules in anoxic conditions. This study advances the understanding of extremophilic microbial taxa capable of bidirectional EET and its role in barely investigated Fe cycling in highly saline-alkaline environments. It also offers research opportunities for understanding the membrane components involved in the bidirectional EET of G. halelectricus . The high rate of Fe 3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.