Interactions between the bloom-forming dinoflagellate Peridinium gatunense and the chytrid fungus Phlyctochytrium sp.

Epidemic chytrid fungal infection of Peridinium gatunense, the major bloom-forming dinoflagellate in Lake Kinneret, Israel, is reported for the first time. The fungus, identified as Phlyctochytrium sp. based on morphological and life cycle characteristics, was first observed in December 2000. It rapidly developed into an epidemic, coinciding with a 1000-fold decline in Peridinium density within 9 days of the initial observation of the chytrid. In August 2001 a second fungal epidemic occurred, which was again associated with a Peridinium population crash. In 2002-2003 less dramatic fungal epidemics were recorded, infection prevalence was low and the Peridinium population did not crash. Laboratory experiments demonstrated that healthy Peridinium cells are not prone to fungal infection, mostly stressed or dead cells were infected. In other words, Phlyctochytrium is a saprophyte, possibly a facultative parasite of Peridinium. Probably another pathogen was responsible for the observed bloom crashes and the saprophytic Phlyctochytrium was a secondary parasite, infecting the already moribund cells. The emergence of fungal epidemics in Lake Kinneret could be interpreted as an early signal of ecosystem response to increasing anthropogenic stress.