# Carboxydotrophic Acetogenesis in Alkaline Conditions Results in Transient Formate Production by the Halo‐Alkaliphilic Acetogen Haloacetibacter carboxydivorans Gen. Nov. sp. Nov

## TL;DR

A new bacterium, Haloacetibacter carboxydivorans, produces formate under alkaline and high-salt conditions, which helps maintain redox balance during carbon monoxide-based growth.

## Contribution

The discovery of a novel halo-alkaliphilic acetogen that uses formate for redox homeostasis in alkaline environments.

## Key findings

- Haloacetibacter carboxydivorans produces formate during CO-based growth under high salt and alkaline conditions.
- Formate is consumed when CO is limited, supporting acetogenesis without producing ethanol or hydrogen.
- Genomic and transcriptomic data suggest specific enzymes may be involved in dynamic formate metabolism.

## Abstract

Carboxydotrophic acetogens are found widespread in the environment, yet the strains characterised to date are almost exclusively mild acidophiles or neutrophiles, often isolated from gut or freshwater systems. Here, we describe a novel carboxydotrophic halo‐alkaliphilic, acetogenic bacterium, strain MD4, isolated from a CO‐fed bioreactor operated under high salt and alkaline conditions. Phylogenetic analysis suggests that strain MD4 is the first representative of a novel genus, branching between the Alkalibacter and Alkalibaculum genera, for which we propose the name Haloacetibacter carboxydivorans. The bacterium tolerates a wide range of sodium (0.01–2.5 M) and pH (7–10), but was not exceptionally tolerant to metals such as copper, nickel and cobalt. During growth on CO, strain MD4 produced formate and acetate, the former being co‐consumed upon low CO availability to drive acetogenesis. Interestingly, common by‐products of carboxydotrophic acetogenesis—ethanol or hydrogen—were not produced, suggesting that formate production may serve as a form of redox homeostasis during alkaliphilic carboxydotrophy. Genome analyses revealed no clear bifurcating formate dehydrogenase or formate hydrogen lyase, but during carboxydotrophy the transcriptome showed high expression of two putative bifurcating hydrogenases, and a NADH‐dependent formate dehydrogenase, potentially playing a role in the dynamic formate metabolism.

This work presents the physiological exploration of the halo‐alkaliphilic, carboxydotrophic acetogen bacterium MD4. In contrast to well‐characterised neutrophilic and slightly acidophilic acetogenic carboxydotrophs, this work shows an important role for formate production, instead of ethanol, as a redox exhaust mechanism in alkaline conditions.