The model alga Chlamydomonas reinhardtii forms mutualistic interactions with Verticillium fungi

Algal-fungal interactions are widespread in natural environments and contribute to key ecological processes, such as primary production, nutrient cycling, and soil formation. Yet, molecular mechanisms underlying these associations remain poorly understood due to the complexity of natural systems and the lack of experimentally tractable models. Here, we describe a genetically tractable interaction between the model alga Chlamydomonas reinhardtii and the soil-borne fungus Verticillium dahliae. We show that these organisms form a contact-associated mutualistic interaction characterized by increased algal and fungal biomass. The algal physiological benefits persist under abiotic and microbial stress conditions. Across multiple Verticillium species, the interaction consistently promotes C. reinhardtii growth. In contrast, interaction outcomes vary across different Chlamydomonas species, where mutualistic interactions appear restricted to C. reinhardtii. Together, these findings demonstrate that algal-fungal interactions are shaped by asymmetric partner specificity and establish a tractable experimental system to dissect the mechanisms underlying partner compatibility in algal-fungal associations.