Unassisted stimulation of autotrophic ethanol bioproduction by visible light

Illuminated semiconductors stimulate CO2 bioconversion by acetogens and other autotrophic microorganisms. Visible light by itself is known to alter the lifestyle of non-photosynthetic aerobic microbes via photoexcitation stress. The direct effect of illumination on the metabolism of acetogens in the absence of photocatalytic semiconductors remains unknown. Here, the industrial acetogen Clostridium autoethanogenum was exposed to visible light at different intensities when metabolizing CO2 or organic carbon. Upon irradiation, an important metabolic shift was observed when ethanol production was stimulated. The illumination of C. autoethanogenum augmented ethanol synthesis 1.8 and 2.2 times from C1 gases and fructose, respectively. Blue light was responsible for the metabolic alteration through O-2-independent photostress involving organic chromophores. The transcriptomic analysis revealed that visible light increased the pathways expression for ethanol production from acetyl-CoA and acetate. These transcriptional changes were aimed at limiting medium acidification, which when combined with light, became highly toxic. Our results uncover the impacts of illumination on industrial acetogens and show this ubiquitous stimulus can be harnessed to improve biofuel bioproduction by C. autoethanogenum from the greenhouse gas CO2 without assistance from abiotic photosystems.