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. The use of negative ion mode MS was essential to decipher the unprecedented hydroxyproline and ß-sulfated aspartate-containing cyclic heptapeptide that acts in femtomolar concentrations.

Pheromone Mediated Sexual Reproduction of Pennate Diatom Cylindrotheca closterium.

Benthic diatoms dominate primary production in marine subtidal and intertidal environments. Their extraordinary species diversity and ecological success is thought to be linked with their predominantly heterothallic sexual reproduction. Little is known about pheromone involvement during mating of pennate diatoms. Here we describe pheromone guided mating in the coastal raphid diatom Cylindrotheca closterium. We show that the two mating types (mt+ and mt-) have distinct functions. Similar to other benthic diatoms, mt+ cells are searching for the mt- cells to pair. To enhance mating efficiency mt- exudes an attraction pheromone which we proved by establishing a novel capillary assay. Further, two more pheromones produced by mt- promote the sexual events. One arrests the cell cycle progression of mt+ while the other induces gametogenesis of mt+. We suggest that C. closterium shares a functionally similar pheromone system with other pennate diatoms like Seminavis robusta and Pseudostaurosira trainorii which synchronize sexual events and mate attraction. Remarkably, we found no evidence of mt+ producing pheromones, which differentiates C. closterium from other pennates and suggests a less complex pheromone system in C. closterium.