Molecular Diversity Studies in Lake Pavin Reveal the Ecological Importance of Parasitic True Fungi in the Plankton

Abstract

Parasitism is one of the most common symbiotic interactions, occurring in almost all environments. Microbial parasites are typically characterized by their small size, a short generation time, high rates of reproduction, and a simple life cycle usually completed within a single host. Microbial parasites are phylogenetically diverse and ubiquitous in aquatic ecosystems, comprising viruses, prokaryotes and eukaryotes. Pioneering environmental 18S-rDNA surveys of microbial eukaryotes in Lake Pavin, France, revealed the presence of a high diversity of undescribed eukaryotes, primarily affiliated to the fungal phylum Chytridiomycota (chytrids), and more than likely playing major roles as infecting agents in the system. These early diverging fungi produce free-swimming dispersal propagules characterized by a small size (2–8 μm) and a single, posterior flagellum. These characteristics make chytrids part of the so-called ‘zoosporic fungi’. Chytrids are particularly adapted to a planktonic lifestyle and have been shown to infect a wide variety of hosts, including fish, eggs, zooplankton, other aquatic fungi, and primarily phytoplankton in aquatic environments. Related ecological implications are important for pelagic food webs. Released organic particles resulting from the death of infected hosts can be used as substrates for other microbial processes, and zoospores themselves can provide nutrient-rich particles for planktonic grazers. The application of the plankton ecology group (PEG) model on Lake Pavin indicated that chytrid epidemics could represent an important driving factor for phytoplanktonic seasonal successions. Besides, the observation that phytoplankton chytridiomycosis preferentially impacts larger size species (e.g., filamentous cyanobacteria) suggests that bloom of such species may not represent a trophic bottleneck for the system as previously thought. In this chapter, based on studies we conducted in Lake Pavin and other lakes in the vicinity, we summarize knowledge on diversity, community structure, quantitative importance, and functional roles of planktonic chytrids. We primarily focus on parasitic chytrids of phytoplankton, the potential ecological implications for food web dynamics, as well as the methodological challenges related to their study. We conclude that phytoplankton chytridiomycosis is an important but still overlooked ecological driving force in aquatic food web dynamics.