Retraction for Yadav et al., "The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition".

In our paper, we reported the bidirectional extracellular electron transfer capability of Geoalkalibacter halelectricus based on biochemical (i.e., with insoluble Fe-oxide and Fe(0)) and bioelectrochemical (i.e., with electrodes) experimental approaches. We noticed some issues and limitations of the methods and techniques that were used to analyze Fe species, particularly the reduced Fe ions in insoluble and precipitated Fe(II) minerals that led to incorrect interpretation of the results, specifically the reduction of Fe(III) to Fe(0) in this study. We were made aware of thermodynamic constraints that would make the biological reduction of Fe(III) to Fe(0) implausible. Hence, the conclusion about microbial Fe(III)-oxide reduction to Fe(0) is invalid. We also noticed errors in estimating the protein-based iron reduction/oxidation rates and faradic efficiencies, besides the limitations of the methods used for Fe analysis. For these reasons, we retract this article and sincerely apologize for the inconvenience it may have caused to the readers.

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

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 Fe3+ oxide (Fe2O3) to Fe0 at a 0.75 ± 0.08 mM/mgprotein/d rate linked to acetate oxidation via outward EET and oxidizes Fe0 to Fe3+ at a 0.24 ± 0.03 mM/mgprotein/d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm2 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/cm2) via inward EET. The cyclic voltammograms of G. halelectricus 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. halelecctricus in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe0 and Fe3+ 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. halelectricus 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 Fe3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.

Extremophilic electroactive microorganisms: Promising biocatalysts for bioprocessing applications.

Electroactive microorganisms (EAMs) use extracellular electron transfer (EET) processes to access insoluble electron donors or acceptors in cellular respiration. These are used in developing microbial electrochemical technologies (METs) for biosensing and bioelectronics applications and the valorization of liquid and gaseous wastes. EAMs from extreme environments can be useful to overcome the existing limitations of METs operated with non-extreme microorganisms. Studying extreme EAMs is also necessary to improve understanding of respiratory processes involving EET. This article first discusses the advantages of using extreme EAMs in METs and summarizes the diversity of EAMs from different extreme environments. It is followed by a detailed discussion on their use as biocatalysts in various bioprocessing applications via bioelectrochemical systems. Finally, the challenges associated with operating METs under extreme conditions and promising research opportunities on fundamental and applied aspects of extreme EAMs are presented.