Chemical warfare in the rice plant rhizosphere: Diverse Pseudomonas antagonize the green alga Chlamydomonas

Algae and bacteria form dynamic partnerships that shape diverse ecosystems. The model green microalga Chlamydomonas reinhardtii is often found in paddy rice fields, where the diversity and functional characteristics of its bacterial partners remain largely unexplored. Here, we isolated 12 bacterial strains from the rice rhizosphere and its surrounding soil. Phylogenetic analysis based on 16S rRNA gene sequences revealed that eight isolates belong to the genus Pseudomonas. Three Pseudomonas rhizosphere strains exhibited antagonistic effects against C. reinhardtii, negatively impacting its growth, pigmentation and cell morphology. Close physical associations between the three antagonistic Pseudomonas strains and C. reinhardtii were observed in liquid cultures. Whole genome sequencing identified the antagonistic strains as Pseudomonas alloputida, Pseudomonas tohonis, and Pseudomonas asiatica. The three bacterial genomes encode several factors well-known for mediating chemical warfare. Notably, P. tohonis bears a gene cluster encoding the phenolic toxin, 2,4-diacetylphloroglucinol (DAPG). DAPG inhibits the growth and alters the cell morphology of C. reinhardtii. These findings expand our understanding of the diversity of bacterial partners associated with C. reinhardtii and highlight the role of Pseudomonas rice rhizosphere-associated bacteria as potent green algal antagonists. They harbor a reservoir of common functional traits that negatively influence green algal population dynamics.