Enabling strong acetogenic growth on CO2 and H2: H2 solubility limits Clostridium ljungdahlii growth on CO2 and H2

Abstract

Due to their ability to convert CO2, a greenhouse gas, into useful products, certain acetogenic bacterial species, such as Clostridium ljungdahlii, have been proposed as promising platform strains for renewable, carbon-negative chemical production. C. ljungdahlii, and similar acetogens, grows slowly and produce primarily acetate when grown on CO2 with H2 as the electron donor, but it grows quickly and can produce ethanol when grown on higher energy substrates, notably CO or fructose. Here, by utilizing different mixing strategies (and notably the first time use of roller bottles) to modulate the volumetric gas interfacial mass transfer coefficient (kLa), we show that, under both mixotrophic (sugar and gas utilization) and autotrophic conditions, C. ljungdahlii growth and CO2 fixation are primarily electron-limited due to the low solubility of H2 relative to CO and CO2. We demonstrate that, with sufficiently high H2 mass transfer, C. ljungdahlii can grow at similar high rates using CO2 as its sole carbon source compared to CO or fructose, a finding with significant implications for the use of acetogens in CO2-negative biomanufacturing, especially because at least 50% of CO used is oxidized and released as CO2. We also show that accumulation of fructose inhibits CO2 utilization by C. ljungdahlii under mixotrophic growth conditions, suggesting that a non-classical "catabolite repression" by fructose inhibits CO2 utilization.