Microbial synergy between <i>Rhodospirillum rubrum</i> and <i>Acetobacterium woodii</i> enables anaerobic CO conversion to polyhydroxyalkanoates

The high cost of traditional substrates has hindered the large-scale adoption of polyhydroxyalkanoates (PHAs) as sustainable alternatives to petrochemical plastics. One-carbon (C1) substrates like carbon monoxide (CO) offer a low-cost, sustainable feedstock, but efficient biocatalytic systems for their conversion to PHAs have been lacking. Here, we report the first successful anaerobic production of PHAs from CO using a synthetic co-culture of <i>Rhodospirillum rubrum</i> and <i>Acetobacterium woodii</i>. In this system, <i>R. rubrum</i> catalyzes the water-gas shift reaction, converting CO into H₂ and CO₂. <i>A. woodii</i> subsequently transforms these products into acetate, serving as an organic carbon source for PHA accumulation by <i>R. rubrum</i>. Neither organism, in monoculture, was able to grow on CO alone, underscoring the importance of the microbial synergy. While continuous cultivation in chemostats proved unstable, fed-batch cultivation achieved a PHA production rate of 58 ± 11 mg L_medium ^-1 day^-1 with a final PHA content of 38 ± 5% (dry weight). This study introduces a pioneering anaerobic route for PHA synthesis from CO, representing a significant advance toward sustainable PHA production from C1 substrates.