A red algal polysaccharide influences the multicellular development of the choanoflagellate .

We uncovered an interaction between a choanoflagellate and alga, in which porphyran, a polysaccharide produced by the red alga , induces multicellular development in the choanoflagellate . We first noticed this possible interaction when we tested the growth of in media that was steeped with as a nutritional source. Under those conditions, formed multicellular rosette colonies even in the absence of any bacterial species that can induce rosette development. In biochemical purifications, we identified porphyran, a extracellular polysaccharide produced by red algae, as the rosette inducing factor The response of to porphyran provides a biochemical insight for associations between choanoflagellates and algae that have been observed since the earliest descriptions of choanoflagellates. Moreover, this work provides complementary evidence to ecological and geochemical studies that show the profound impact algae have exerted on eukaryotes and their evolution, including a rise in algal productivity that coincided with the origin of animals, the closest living relatives of choanoflagellates.