Phytoplankton Chytridiomycosis: Fungal Parasites of Phytoplankton and Their Imprints on the Food Web Dynamics

Télesphore Sime-Ngando

doi:10.3389/fmicb.2012.00361

Abstract

Parasitism is one of the earlier and common ecological interactions in the nature, occurring in almost all environments. Microbial parasites typically are characterized by their small size, short generation time, and high rates of reproduction, with simple life cycle occurring generally within a single host. They are diverse and ubiquitous in aquatic ecosystems, comprising viruses, prokaryotes, and eukaryotes. Recently, environmental 18S rDNA surveys of microbial eukaryotes have unveiled major infecting agents in pelagic systems, consisting primarily of the fungal order of Chytridiales (chytrids). Chytrids are considered the earlier branch of the Eumycetes and produce motile, flagellated zoospores, characterized by a small size (2–6 μm), and a single, posterior flagellum. The existence of these dispersal propagules includes chytrids within the so-called group of zoosporic fungi, which are particularly adapted to the plankton lifestyle where they infect a wide variety of hosts, including fishes, eggs, zooplankton, algae, and other aquatic fungi but primarily freshwater phytoplankton. Related ecological implications are huge because chytrids can killed their hosts, release substrates for microbial processes, and provide nutrient-rich particles as zoospores and short fragments of filamentous inedible hosts for the grazer food chain. Furthermore, based on the observation that phytoplankton chytridiomycosis preferentially impacts the larger size species, blooms of such species (e.g., filamentous cyanobacteria) may not totally represent trophic bottlenecks. Besides, chytrid epidemics represent an important driving factor in phytoplankton seasonal successions. In this review, I summarize the knowledge on the diversity, community structure, quantitative importance, and functional roles of fungal chytrids, primarily those who are parasites of phytoplankton, and infer the ecological implications and potentials for the food web dynamics and properties. I reach the conclusion that phytoplankton chytridiomycosis represents an important but as yet overlooked ecological driving force in aquatic food web dynamics and network organization.

Highlights

Parasitism is one of the earlier known and most common ecological interactions in nature (Cavalier-Smith, 1993), occurring in almost all environments (Lafferty et al, 2006)
The existence of these dispersal propagules includes chytrids within the so-called group of zoosporic fungi, which are adapted to the plankton lifestyle where they infect a wide variety of hosts, including fishes, eggs, zooplankton, algae, and other aquatic fungi but primarily freshwater phytoplankton
I summarize the knowledge on the diversity, community structure, quantitative importance, and functional roles of fungal chytrids, primarily those who are parasites of phytoplankton, and infer the ecological implications and potentials for the food web dynamics and properties

Summary

INTRODUCTION

Parasitism is one of the earlier known and most common ecological interactions in nature (Cavalier-Smith, 1993), occurring in almost all environments (Lafferty et al, 2006). A recent extensive seasonal study in temperate freshwater lakes (Rasconi et al, 2012), based on CFW staining of infective sporangia and phenotypic identification, has identified up to 15 different chytrid species on diverse host populations, with specific biovolume ranging from 7 to 72 μm sporangium−1 (Table 1). Except the study by Rasconi et al (2012) that has proposed a general empirical model on chytrid seasonality and trophodynamics (i.e., with their hosts) based on the theoretical PEG model of seasonal succession of planktonic events in freshwaters (Sommer et al, 1986), there is still no study assessing the fungal species successions in natural environments This contrasts with the general hypotheses and patterns of plankton (primarily phytoplankton and zooplankton) successions and community structure, which are well described in temperate lakes (Sommer et al, 1986). Nutrient conditions and persistent stratification favor the www.frontiersin.org

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CONCLUSION