Sustainable Co-Production of Carotenoids and Lipids by <i>Rhodotorula toruloides</i> Metabolizing Acetate Derived from Carbon Dioxide Fermentation

The ability of <i>Rhodotorula toruloides</i> DSM 4444 to metabolize low-cost carbon sources such as fatty acids was comprehensively studied. This organism is shown, for the first time, to simultaneously accumulate microbial oils (biofuel precursors) and carotenoids from acetic acid obtained from CO₂ fermentation. This fatty acid is typically the single end product of acetogenic bioconversion of one-carbon gas pollutants (e.g., CO₂ and CO). In the first set of experiments, different aerobic fermentations were carried out in automated bioreactors, with acetic acid in one case and with glucose, a more conventional carbon source, as a control, in another bioreactor. <i>R. toruloides</i> consumed around 80 g/L substrate under both conditions. Maximum lipid content (27.2% g/g dry weight) was reached from 38 g/L glucose, while carotenoid content was higher with acetic acid (1.4 mg/g cell after 54.1 g/L acetic acid consumed), representing a 40% increase compared to glucose (1.0 mg/g cell after 64.2 g/L glucose consumed). Additionally, in the second set of assays, a fermented broth produced by <i>Acetobacterium woodii</i> from CO₂ fermentation, containing residual nutrients and metabolites, was tested. Despite its complex composition, <i>R. toruloides</i> grew and produced carotenoids (up to 0.141 mg/g), showing potential adaptability. To the best of our knowledge, this is the first report on a greenhouse gas-based biotechnological process as a promising sustainable alternative for the valorization of pollutants, e.g., gas emissions, their bioconversion to VFAs, such as acetic acid, and subsequent fermentation of the carboxylic acid into microbial oils, as a source of renewable energy, as well as carotenoids as a high-value nutraceutical product.