Biorefinery application of Rhodopseudomonas faecalis PA2 for co-production of carotenoids and hydrogen from coconut residue

The production of numerous high-value products from complex agro-industrial wastes remains a substantial challenge in biorefinery processes. In this study, we investigated the valorization of coconut residue into carotenoid-rich biomass and hydrogen (H 2 ) energy as a co-product using the carotenoid-producing strain Rhodopseudomonas faecalis PA2. Repeated batch cultivation with an optimal light strategy enabled efficient production of carotenoids and H 2 . Continuous 24-h artificial light optimized carotenoid production, whereas natural light enhanced H 2 production. The cumulative carotenoid production achieved over five consecutive repeated batch cycles under 24-h light was 1,573 mg/L. This yield translates to an estimated unit production cost of approximately 12.62 USD/g of carotenoids (based on a five-batch cost of 7.94 USD). The overall H 2 yield across the five cycles under natural light was 71.82 mL H 2 /L of coconut residue filtrate, and the unit cost calculated was 14.62 USD/L of the produced H 2 , attributed to a five-batch cost of 0.42 USD. This study is the first to report gene expression associated with carotenoids ( crt ) and H 2 ( nif ) during the co-production of this strain, providing a genetic blueprint for future strain engineering to enhance productivity. This study demonstrates a cost-effective biorefinery platform that integrates high-value products with energy generation. • Coconut residue was valorized into carotenoids and H 2 via R. faecalis PA2 • Repeated batch with optimized light enabled co-production of carotenoids and H 2 • Continuous 24-h light maximized carotenoid yield across five consecutive batches • Natural light enhanced hydrogen production, achieving 71.82 mL H 2 per liter • First report linking crt & nif expression during carotenoid–hydrogen co-production