Field strains of the unicellular alga <i>Chlamydomonas reinhardtii</i> exhibit multicellular characteristics that shape their interactions

Abstract Chlamydomonas reinhardtii is a unicellular green alga long studied as a biological model system but rarely considered from the perspective of its own ecology, thus epitomizing the disconnection between reductionist biology in the laboratory and life in nature. Here we present insights into its ecology understood from field strains. We examined bacterial communities that coenriched with C. reinhardtii from the field, revealing specific associations. We then compared the biology of C. reinhardtii field strains to laboratory strains, illuminating strain level heterogeneity and adaptations to life in the field vs. laboratory. Field strains exhibited more robust photosynthesis, higher abundances of pherophorin proteins, a propensity for pallmeloid formation, and high cell wall permeability. Finally, we phenotyped cocultures of C. reinhardtii with a coenriched bacterial partner, demonstrating how differences between field and laboratory strains manifest in biotic interactions. Although the organisms in question are classically understood as unicellular, our observations of field strains highlighted their participation in multicellular units, challenging the utility of unicellular frameworks in extending our knowledge of model organism biology in the laboratory towards understanding microbial ecology.