| Abstract: |
Planktothrix agardhii dominates the cyanobacterial harmful algal bloom community in Sandusky Bay, Lake Erie (USA), from May through September. This filamentous cyanobacterium is host to a known obligate parasite, the chytrid Rhizophydium sp. During the 2018 bloom season, by utilizing dilution and single-filament isolation techniques, 7 chytrid and 12 P. agardhii strains were isolated from Sandusky Bay. These 7 chytrids and a selection of P. agardhii hosts were then characterized with respect to infection rates. Infections by the isolated chytrids were specific to Planktothrix planktonic species and were not found on other filamentous cyanobacterial taxa present in the bay (Aphanizomenon sp. and Cuspidothrix sp.). Even among the potential P. agardhii host strains, individual chytrid isolates had different degrees of infectivity and showed preferences for different host isolates, suggesting possible ecological partitioning even within the same sample population. When mechanisms of chytrid pathogenesis were examined, the zoospores displayed a swarming pattern to attack and fracture the host filament and create new infection sites at the trichome termini. Infections by these parasitic chytrids also led to a release of intracellular microcystin toxins from the hosts. Additionally, infections were dependent on medium type, highlighting the importance of medium choice for experimental outcomes. Media in which chytrid swarming was observed closely matched the ionic strength of the natural environment. Understanding pathogenesis by fungal parasites will assist future efforts to determine environmental factors favoring loss mechanisms for Planktothrix agardhii-dominated blooms.
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