Antagonistic bacteria disrupt calcium homeostasis and immobilize algal cells. Issue 1 (December 2017) - Aiyar, Prasad
Abstract Photosynthetic unicellular organisms, known as microalgae, are key contributors to carbon fixation on Earth. Their biotic interactions with other microbes shape aquatic microbial communities and influence the global photosynthetic capacity. So far, limited information is available on molecular factors that govern these interactions. We show that the bacteriumPseudomonas protegens strongly inhibits the growth and alters the morphology of the biflagellated green algaChlamydomonas reinhardtii . This antagonistic effect is decreased in a bacterial mutant lacking orfamides, demonstrating that these secreted cyclic lipopeptides play an important role in the algal–bacterial interaction. Using an aequorin Ca2+ -reporter assay, we show that orfamide A triggers an increase in cytosolic Ca2+ inC .reinhardtii and causes deflagellation of algal cells. These effects of orfamide A, which are specific to the algal class of Chlorophyceae and appear to target a Ca2+ channel in the plasma membrane, represent a novel biological activity for cyclic lipopeptides. Predatory or competitive interactions between microbes are poorly understood but likely influence global nutrient cycles. Here, the authors show that Pseudomonas bacteria could immobilize algal cells, potential prey, by releasing secondary metabolites that induce a Ca2+ signal and algal deflagellation.