Structure elucidation of the first sex‐inducing pheromone of a diatom

Diatoms are abundant unicellular microalgae, responsible for ~ 20% of global photosynthetic CO2 fixation. Nevertheless, we know little about fundamental aspects of their biology, such as their sexual reproduction. Pheromone‐mediated chemical communication is crucial for successful mating. An attraction pheromone was identified in the diatom Seminavis robusta, but metabolites priming cells for sex and synchronizing search and mating behavior remained elusive. These sex‐inducing pheromones (SIP) induce cell cycle arrest and trigger the production of the attraction pheromone. Here we describe the challenging structure elucidation of an S. robusta SIP. Guided by metabolomics, a candidate metabolite was identified and elucidated by labeling experiments, NMR, ESI MSn analyses, and chemical transformations. Essential was the use of negative ion mode MS to decipher the unprecedented hydroxyproline and β‐sulfated aspartate‐containing cyclic heptapeptide that acts in femtomolar concentrations.