Some asterionella are more equal than others: diary of a chytrid-diatom pairing

Parasitic chytrids are ecologically significant in various aquatic ecosystems through their roles in controlling bloom-forming phytoplankton populations; until recently, their diversity had been overlooked, but they are now a proven link in the trophic food web. As a model, we brought into stable laboratory culture a pairing between the bloom-forming diatom Asterionella formosa and a pathogenic chytrid identified as Rhizophydium planktonicum, isolated from Pavin Lake, France. We present here some data on the life cycle, host range and infectivity of the chytrid, as well as its impact on the growth of diatom cultures. We show that clonal diatom strains present a gradient of susceptibility of infection that is positively correlated with the host cell survival, implying that the chytrid requires a live host cell in order to grow and complete its cycle. We also report on the successful long term preservation of the culture by a colligative cryopreservation protocol employing a two-step cooling approach. Our next step is to explore the gene complement and metabolic potential of the pairing using transcriptomics. In this way, we are aiming to gain useful knowledge of the ecology (life cycle, host range, infectivity) and the biology (metabolic pathways express during the parasitic and dissemination phase) of the chytrids and their host.