Gas fermentation: process for conversion of CO and CO₂ into ethanol using Clostridium ljungdahlii DSM 13,528

Abstract

In the present study, a novel biological process has been demonstrated in which Clostridium ljungdahlii DSM 13528, an acetogenic gas-fermenting organism, produces ethanol as the primary metabolic product. The process offers environmental sustainability, as the strain utilizes carbon dioxide and carbon monoxide, as the carbon source and electron donor. Modulation of the headspace gas composition (CO₂:CO ratio) and reactor headspace volume yielded a maximum biomass titer of 159.1 ± 10.8 mg/L with a productivity of 1.5 ± 0.1 mg/L/h. Sequential adaptation over 11–14 cultivation cycles resulted in a 26% increase in the specific growth rate in C. ljungdahlii medium with 1 g/L yeast extract (CLY) and a 160% increase in C. ljungdahlii medium without yeast extract (CLNY). Subsequently, a medium engineering strategy involving modulation of metal ions and nitrogen sources (nitrate and nitrite) was implemented to redirect carbon flux from acetate towards ethanol, as the main product. These combined strategies resulted in a maximum biomass titer of 274.5 ± 26.7 mg/L and a productivity of 2.14 mg/L/h, along with a maximum ethanol titer of 18.5 ± 3.4 mmol/L and a productivity of 13.2 mg/L/h in CLY medium supplemented with tungstate (2.5 mg/L) and nitrite (18.7 mmol/L). These values are among the highest reported for C. ljungdahlii DSM 13528 in batch mode of cultivation. Notably, when nitrite was used as the nitrogen source, the carbon flux was predominantly redirected towards ethanol, with only trace quantities of other products detected.

Graphical abstract

Keywords

Clostridium ljungdahlii DSM 13,528 / Gas fermentation / Process optimization / Medium engineering / Ethanol biosynthesis.

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