# Careful CO Addition Enhances Autotrophic d‐Lactate Formation With Engineered Acetobacterium woodii

## TL;DR

Researchers found that adding small amounts of CO to gas fermentation improves d-lactate production by engineered A. woodii, but too much CO inhibits growth.

## Contribution

The study shows that precise CO control enhances autotrophic d-lactate production in A. woodii through metabolic engineering.

## Key findings

- 3% CO increased biomass by 89% compared to no CO.
- 0.8% CO led to 189% higher d-lactate accumulation than without CO.
- Higher CO levels (6%) inhibited growth and product formation.

## Abstract

The acetogen A. woodii efficiently converts CO2 and H2 to acetate. Metabolic engineering enabled the autotrophic production of non‐native products, for example, d‐lactate from CO2 by overexpression of d‐lactate dehydrogenase from Leuconostoc mesenteroides and knockout of the native lactate dehydrogenase. During gas fermentation with acetogens, the addition of CO leads to increased provision of reducing equivalents, and thus increased biomass formation. However, literature data reveal that already small CO partial pressures in the gas phase inhibit the autotrophic growth of A. woodii. This study aims to investigate adding 0.6%–6.0% CO to batch‐operated stirred tank bioreactors with continuous gassing to study autotrophic growth and product formation with the d‐lactate producing A. woodii mutant. No growth and product formation were observed with 6% CO. Surprisingly, cell growth and metabolic product concentrations are non‐linearly dependent on lower CO concentrations in the inlet gas phase. Highest biomass concentrations were observed with 3% CO (3.24 g L−1, 89% improvement compared to the reference process without CO addition), and the highest d‐lactate accumulation was achieved with 0.8% CO (6.2 g L−1
d‐lactate, 189% improvement compared to the reference without CO) after a prolonged lag phase. In conclusion, CO‐sensitive A. woodii cells need tight control of CO in syngas to affect autotrophic product selectivities.