Carbon Source Flexibility in Chlamydomonas reinhardtii: Impact of Glycolate on Mixotrophic Growth

Mixotrophy is a widespread trophic strategy adopted by microalgae to optimize the utilization of environmental resources. It combines phototrophic and heterotrophic growth, involving the simultaneous use of both light and exogenous organic carbon as energy inputs. In addition to addressing biotechnological challenges, studying mixotrophy offers a means to investigate the interplay between photosynthesis and carbon metabolism. Here, we explored the mixotrophic growth of the green alga Chlamydomonas reinhardtii, a facultative phototroph routinely cultured under mixotrophic conditions in acetate-supplemented medium. We used Phenotype MicroArray plates to simultaneously screen a wide range of substrates and identified glycolate as the most effective one, besides acetate, in promoting mixotrophic growth. While the role of glycolate as a photorespiratory metabolite has been widely studied, its utilization in mixotrophy is less documented. We therefore performed a broader characterization of the photosynthetic, photoprotective and metabolic responses associated with glycolate supply, and compared them to those observed under phototrophic and acetate-supplemented growth conditions. We showed that, in contrast to acetate, glycolate supply associates with sustained photosynthetic activity and photoprotection capacity while shifting carbon metabolism towards starch accumulation. This work provides new insights into the impact of glycolate on Chlamydomonas mixotrophic growth, highlighting its potential as a valuable substrate rather than merely a photorespiratory by-product.