Growth Promotion of Chlamydomonas reinhardtii by Cupriavidus oxalaticus MEYA8

Microalgal growth-promoting bacteria (MGPBs) represent a promising strategy to enhance biomass productivity, yet the mechanistic basis of these mutualistic interactions remains poorly understood. Here, we isolated a new MGPB, Cupriavidus oxalaticus MEYA8, and characterized its mutualistic interaction with the model microalga Chlamydomonas reinhardtii. Co-cultivation at an optimal MEYA8: Chlamydomonas ratio greatly enhanced microalgal cell density and chlorophyll content, which reached 2.1-fold and 1.6-fold those of the control, respectively, with markedly improved photosynthetic efficiency across both photosystems. Transwell assays confirmed that this promotion is mediated by diffusible metabolites rather than direct cell contact. Metabolites and multi-omics analyses revealed that MEYA8 can produce compounds similar to indole-3-acetic acid (IAA) and is adapted to the microalgal phycosphere by preferentially utilizing organic acids and amino acid derivatives. In response, Chlamydomonas upregulated proteins involved in photosynthetic electron transport, energy metabolism, and nitrogen assimilation, consistent with the observed enhancement in photosynthetic performance. These findings suggest a metabolically reciprocal interaction model: MEYA8 supplies diffusible growth-promoting factors such as IAA-like compounds to enhance Chlamydomonas photosynthesis and growth, while Chlamydomonas provides organic substrates that sustain bacterial proliferation. Our work provides new insights into algal–bacterial mutualism and may provide a new microbial resource for engineering microalgal and beneficial bacterial consortia.