Chytrids Exhibit Trophic Versatility in Their Interactions with a Microalga

Chytrids and other zoosporic fungi are key regulators of primary production and carbon cycling in aquatic ecosystems. Yet besides severe crashes of microalgal populations in both natural water bodies and industrial cultivation systems, an understanding of the underlying ecology of alga-chytrid interactions is lacking. To investigate biotic and abiotic drivers of chytrid infection, from Haematococcus ponds we isolated two fungal strains: Rhizophydiales sp. (Chytridiomycota) and Paraphysoderma sedebokerense (Blastocladiomycota). Culture-based assays indicated fungal infectivity’s dependence on algal growth stage, with highest infectivity of both strains on cyst-stage algae. Furthermore, we discovered that these fungal strains possess a previously unrecognized non-infective lifestyle in which they can feed on algal exudate. Quantitative PCR of chytrid and blastoclad groups in production ponds revealed high fungal abundances during low-infection periods, validating that the noninfective lifestyle is a critical component of fungal dynamics in algal ponds. We explored features of the P. sedebokerense genome that may underlie its trophic versatility and infectivity. Ongoing work will address the mechanistic underpinnings of alga-chytrid interactions, including the molecular changes underlying the switch in trophic strategies and subsequent physiologies, using metatranscriptomics and exometabolomics. While the full significance of multiple trophic strategies in outdoor algal ponds has yet to resolved, identifying the molecular mechanisms and ecological drivers controlling chytrid metabolism is critical for deciphering their complex environmental roles, and will ultimately benefit our understanding of aquatic carbon cycling and algal biofuel production.