Lichen-like association ofChlamydomonas reinhardtiiandAspergillus nidulansprotects algal cells from bacteria

Organismal interactions within microbial consortia and their responses to harmful intruders remain largely understudied. An important step toward the goal of understanding functional ecological interactions and their evolutionary selection is the study of increasingly complex microbial interaction systems. Here, we discovered a tripartite biosystem consisting of the fungusAspergillus nidulans, the unicellular green algaChlamydomonas reinhardtii, and the algicidal bacteriumStreptomyces iranensis. Genetic analyses and MALDI-IMS demonstrate that the bacterium secretes the algicidal compound azalomycin F upon contact withC. reinhardtii. In co-culture,A. nidulansattracts the motile algaC. reinhardtii, which becomes embedded and surrounded by fungal mycelium and is shielded from the algicide. The filamentous fungusSordaria macrosporawas susceptible to azalomycin F and failed to protectC. reinhardtiidespite chemotactically attracting the alga. BecauseS. macrosporawas susceptible to azalomycin F, this data imply that for protection the fungus needs to be resistant. Formation of the lichen-like association betweenC. reinhardtiiandA. nidulansincreased algal growth. The protection depends on the increased amounts of membrane lipids provided by resistant fungi, thereby generating a protective shelter against the bacterial toxin. Our findings reveal a strategy whereby algae survive lethal environmental algicides through cooperation with fungi.