Fig. 5.

Schematic overview of the interaction of Streptomyces spp. with C. reinhardtii. C. reinhardtii induces the release of the marginolactone azalomycin F as well as the overproduction and release of desertomycin A and monazomycin in cocultures with the bacteria Streptomyces iranensis, Streptomyces macronensis, and Streptomyces mashuensis. Lethal concentrations of these marginolactones lead to bleaching and death of C. reinhardtii. Sublethal concentrations trigger the formation of multicellular gloeocapsoids in which the individual cells are surrounded by multiple cell walls, membranes, and a spacious polysaccharide matrix. These aggregates protect the individual cells against otherwise lethal azalomycin F concentrations as well as alkaline pH, indicating the presence of competing streptomycetes. We hypothesize that the additional membranes and the acidic polysaccharide matrix sequester the positively charged azalomycin F, and hence, the algicide cannot reach the individual algal cells. After stress is relieved, the gloeocapsoids disassemble and motile single cells emerge. We hypothesize that sustained sublethal stress leads to the evolution of a multicellular organism exhibiting differentiated cell types and division of labor.