Chytrid Epidemics Research Articles

Taxonomic and functional dynamics during chytrid epidemics in an aquatic ecosystem.

Fungal parasitism is common in plankton communities and plays a crucial role in the ecosystem by balancing nutrient cycling in the food web. Previous studies of aquatic ecosystems revealed that zoosporic chytrid epidemics represent an important driving factor in phytoplankton seasonal successions. In this study, host-parasite dynamics in Lake Pavin (France) were investigated during the spring diatom bloom while following chytrid epidemics using next generation sequencing (NGS). Metabarcoding analyses were applied to study changes in the eukaryotic microbial community throughout diatom bloom-chytrid epidemics. Relative read abundances of metabarcoding data revealed potential "beneficiaries" and "victims" during the studied period. Subsequently, metatranscriptomic analyses on samples before and during the chytrid epidemic unveiled the active part of the community and functional/metabolic dynamics in association with the progress of chytrid infection. Diatom functions involving lipases, transporters, histones, vacuolar systems, the proteasome, proteases and DNA/RNA polymerases were more abundant during the diatom bloom. Chytrid functions related to a parasitic lifestyle including invasion, colonization and stress tolerance were up-regulated during the chytrid epidemic. In addition, functions related to the degradation/metabolism of proteins, lipids and chitin were in higher proportion in the community during the epidemic event. Results of NGS and bioinformatics analyses offered a panorama of dynamic biodiversity and biological functioning of the community.

Fate of hepatotoxin microcystin during infection of cyanobacteria by fungal chytrid parasites

Chytrid parasites are increasingly recognized as ubiquitous and potent control agents of phytoplankton, including bloom-forming toxigenic cyanobacteria. In order to explore the fate of the cyanobacterial toxin microcystins (MCs) and assess potential upregulation of their production under parasite attack, a laboratory experiment was conducted to evaluate short- and long-term variation in extracellular and intracellular MC in the cyanobacteria Planktothrix agardhii and P. rubescens, both under chytrid infection and in the presence of lysates of previously infected cyanobacteria. MCs release under parasite infection was limited and not different to uninfected cyanobacteria, with extracellular toxin shares never exceeding 10%, substantially below those caused by mechanical lysis induced by a cold-shock. Intracellular MC contents in P. rubescens under infection were not significantly different from uninfected controls, whereas infected P. agardhii showed a 1.5-fold increase in intracellular MC concentrations, but this was detected within the first 48 hours after parasite inoculation and not later, indicating no substantial MC upregulation in cells being infected. The presence of lysates of previously infected cyanobacteria did not elicit higher intracellular MC contents in exposed cyanobacteria, speaking against a putative upregulation of toxin production induced via quorum sensing in response to parasite attack. These results indicate that chytrid epidemics can constitute a bloom decay mechanism that is not accompanied by massive release of toxins into the medium.

Intercomparison of Two Fluorescent Dyes to Visualize Parasitic Fungi (Chytridiomycota) on Phytoplankton

Fungal microparasites (here chytrids) are widely distributed and yet, they are often overlooked in aquatic environments. To facilitate the detection of microparasites, we revisited the applicability of two fungal cell wall markers, Calcofluor White (CFW) and wheat germ agglutinin (WGA), for the direct visualization of chytrid infections on phytoplankton in laboratory-maintained isolates and field-sampled communities. Using a comprehensive set of chytrid–phytoplankton model pathosystems, we verified the staining pattern on diverse morphological structures of chytrids via fluorescence microscopy. Empty sporangia were stained most effectively, followed by encysted zoospores and im-/mature sporangia, while the staining success was more variable for rhizoids, stalks, and resting spores. In a few instances, the staining was unsuccessful (mostly with WGA), presumably due to insufficient cell fixation, gelatinous cell coatings, and multilayered cell walls. CFW and WGA staining could be done in Utermöhl chambers or on polycarbonate filters, but CFW staining on filters seemed less advisable due to high background fluorescence. To visualize chytrids, 1 µg dye mL−1 was sufficient (but 5 µg mL−1 are recommended). Using a dual CFW–WGA staining protocol, we detected multiple, mostly undescribed chytrids in two natural systems (freshwater and coastal), while falsely positive or negative stained cells were well detectable. As a proof-of-concept, we moreover conducted imaging flow cytometry, as a potential high-throughput technology for quantifying chytrid infections. Our guidelines and recommendations are expected to facilitate the detection of chytrid epidemics and to unveil their ecological and economical imprint in natural and engineered aquatic systems.

The potential of zooplankton in constraining chytrid epidemics in phytoplankton hosts.

Fungal diseases threaten natural and man‐made ecosystems. Chytridiomycota (chytrids) infect a wide host range, including phytoplankton species that form the basis of aquatic food webs and produce roughly half of Earth's oxygen. However, blooms of large or toxic phytoplankton form trophic bottlenecks, as they are inedible to zooplankton. Chytrids infecting inedible phytoplankton provide a trophic link to zooplankton by producing edible zoospores of high nutritional quality. By grazing chytrid zoospores, zooplankton may induce a trophic cascade, as a decreased zoospore density will reduce new infections. Conversely, fewer infections will not produce enough zoospores to sustain long‐term zooplankton growth and reproduction. This intricate balance between zoospore density necessary for zooplankton energetic demands (growth/survival), and the loss in new infections (and thus new zoospores) because of grazing was tested empirically. To this end, we exposed a cyanobacterial host (Planktothrix rubescens) infected by a chytrid (Rizophydium megarrhizum) to a grazer density gradient (the rotifer Keratella cf. cochlearis). Rotifers survived and reproduced on a zoospore diet, but the Keratella population growth was limited by the amount of zoospores provided by chytrid infections, resulting in a situation where zooplankton survived but were restricted in their ability to control disease in the cyanobacterial host. We subsequently developed and parameterized a dynamical food‐chain model using an allometric relationship for clearance rate to assess theoretically the potential of different‐sized zooplankton groups to restrict disease in phytoplankton hosts. Our model suggests that smaller‐sized zooplankton may have a high potential to reduce chytrid infections on inedible phytoplankton. Together, our results point out the complexity of three‐way interactions between hosts, parasites, and grazers and highlight that trophic cascades are not always sustainable and may depend on the grazer's energetic demand.

Fungicides at environmentally relevant concentrations can promote the proliferation of toxic bloom-forming cyanobacteria by inhibiting natural fungal parasite epidemics

Fungal parasites of the phylum Chytridiomycota (chytrids) are increasingly recognized as potent control agents of phytoplankton, including toxic bloom-forming cyanobacteria. We experimentally tested whether agricultural fungicides can interfere with natural epidemics caused by parasitic chytrid fungi and thereby favor cyanobacterial bloom formation. Specifically, we exposed the toxic bloom-forming cyanobacterium Planktothrix and its chytrid parasite Rhizophydium megarrhizum to different concentrations of the widely used agricultural fungicides tebuconazole and azoxystrobin, as well as the medical fungicide itraconazole (the latter was included to test its potential to suppress infection in vitro). Environmentally relevant concentrations of tebuconazole (20–200 μg/L) and azoxystrobin (1–30 μg/L) significantly decreased infection prevalence over a timespan of seven days, while not affecting the growth of uninfected cyanobacteria. Itraconazole suppressed infection completely. Our findings demonstrate that agricultural fungicide run-off has the potential to inhibit natural chytrid epidemics and, thereby, to promote the proliferation of toxic cyanobacteria.

Chytrid infecting the bloom-forming marine diatom Skeletonema sp.: Morphology, phylogeny and distribution of a novel species within the Rhizophydiales

Chytrids have long been recognised as important parasites of microalgae in freshwater systems, able to shape the dynamics of blooms, the gene pool of their host and phytoplankton succession. In the sea however, where the presence of these organisms is erratic and ephemeral, studies concerning chytrids are sparse and confined to metabarcoding surveys or microscopy observations. Despite the scarcity of data, chytrid epidemics are supposed to play an important role in marine biogeochemical cycles, being one of the drivers of phytoplankton dynamics. Here we combine microscopy observations and in silico mining of a single-cell whole genome to molecularly and morphologically characterise a novel chytrid parasite of the dominant diatom genus Skeletonema. Morphological observations highlight features of the thallus and ascertain the parasitic nature of the interaction whilst the genetic markers obtained allows for a phylogenetic reconstruction, placing the new species in the order Rhizophydiales. Thanks to the molecular data obtained we are also able to provide a first investigation of the global distribution of this organism by screening the Ocean Sampling Day (OSD) dataset, highlighting a northern transatlantic dissemination.

Observations of a Diatom Chytrid Parasite in the Lower Columbia River

Often overlooked, zoosporic fungal parasites of phytoplankton (‘chytrids’) are present in aquatic systems worldwide. Although extensive studies in lakes give insight into potential impacts of chytrid epidemics on phytoplankton blooms and organic matter cycling, the ecological significance of chytrid infections of phytoplankton is still poorly understood in lotic systems. Here, we report the first observations of chytrid parasites attached to multiple diatom species in the lower Columbia River. We isolated a chytrid parasite of the dominant spring bloom diatom, Asterionella formosa, and sequenced DNA from several regions within the ribosomal RNA gene. We also investigated the specificity of the A. formosa chytrid to host and non-host diatoms with isolated cultures and found no cross infection at the species level. In the Columbia River, alterations in the hydrograph following the installation of hydroelectric dams may have opened a niche for chytrid parasites. Greater retention times may allow diatoms to bloom and provide a prolonged interaction period whereby chytrid parasites are able to infect hosts. Future research is needed to assess the seasonality and severity of chytrid infections on diatoms in the lower Columbia River and to evaluate the potential role of zoosporic fungi in influencing the food web structure and biogeochemical cycling in river systems.

Daphnia in tadpole mesocosms: trophic links and interactions with Batrachochytrium dendrobatidis

Summary1. Amphibians are in decline, and the disease chytridiomycosis, caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd), has been repeatedly implicated throughout the world. This chytrid reproduces via an infectious, motile zoospore stage that remains viable for weeks in the water column.2. Daphnia is a keystone zooplankton grazer in intact freshwater ecosystems, whose importance to amphibians may be overlooked. As an efficient grazer, Daphnia can suppress chytrid epidemics by consuming zoospores and may therefore play a role in Bd infection dynamics. Daphnia may also have important effects on tadpoles by mediating the properties of pond food webs. We tested the role of Daphnia in outdoor mesocosms containing the tadpoles of red‐legged frogs (Rana aurora) infected with Bd. We also tested the ability of Daphnia to filter Bd from the water column in laboratory microcosms.3. In the water of microcosms, Daphnia dramatically decreased the number of Bd genomic equivalents detectable using quantitative PCR. Bd genomic equivalents fell below the limit of detection at very high (>1 Daphnia mL−1) Daphnia densities.4. In mesocosms, Daphnia was critical to the development of tadpoles: in the presence of Daphnia, tadpoles were twofold heavier at metamorphosis than in their absence. Daphnia and Bd interacted to affect the tadpole survival: survival was highest in the presence of Daphnia and in the absence of Bd. We were unable to detect an effect of Daphnia on the transmission of Bd in mesocosms. However, Bd transmission among the tadpoles in mesocosms was unexpectedly low, limiting our power to detect an effect of Daphnia on transmission.5. Tadpole dissection showed that tadpoles also consumed large numbers of Daphnia. Current models of mesocosm food webs that assume no predation by tadpoles on zooplankton therefore probably overlook important features of both natural and experimental systems.

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

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.

Use of Calcofluor White for Detection, Identification, and Quantification of Phytoplanktonic Fungal Parasites

We propose a routine protocol based on size fractionation of pelagic samples and the use of the fluorochrome calcofluor white (which binds to beta-1,3 and beta-1,4 polysaccharides) for diagnosing, identifying, and counting chitinaceous fungal parasites (i.e., the sporangia of chytrids) of phytoplankton. The protocol was applied to freshwater samples collected during different seasons (spring and summer/autumn) in two lakes whose trophic statuses varied. Because few samples were collected (i.e., two dates per site), the findings are considered preliminary and mainly a "proof of concept" rather than a valid comparison of sites versus seasons. The results from the proposed protocol indicate higher diversity of infected host and parasite communities than in previous studies. Chytrid epidemics were omnipresent, infecting diverse phytoplankton host communities, primarily diatoms, chlorophytes, and colonial and filamentous cyanobacteria. The diversity and numerical abundance of sporangia and of hosts, and the prevalence of infection (range, <1 to 24% of total host cells) as well, increased from the oligotrophic Lake Pavin to the eutrophic Lake Aydat, while the temporal changes in parasites were apparently more influenced by the host community composition. We conclude that the proposed protocol offers a valid method for the quantitative ecology of chytrid epidemics in aquatic ecosystems and food web dynamics.

AMPHIBIAN POPULATION DECLINES AT SAVANNAH RIVER SITE ARE LINKED TO CLIMATE, NOT CHYTRIDIOMYCOSIS

Amphibian populations at the Savannah River Site (SRS), South Carolina, USA, have been censused consistently for 35 years, and this provides a time series to examine the causes of population fluctuations. We examined archived museum specimens of 15 anuran species collected at wetlands on the SRS for the presence of the causative agent (Batrachochytrium dendrobatidis ) of chytridiomycosis, an emerging disease asso- ciated with population declines elsewhere. Infections were present in three out of 137 (2.18%) individuals; the pathogen was detected in two Rana catesbeianaand a single Rana sphenocephala, all collected between 1978 and 1981. Lesions were not consistent with the later stages of fatal chytridiomycosis. Analysis of population trajectories of nine amphibian species over 26 years at SRS showed that four species declined significantly over this period, including R. sphenocephala. However, we demonstrate that these declines are more likely caused by an increase in the number of years with insufficient rainfall and a shortened hydroperiod at the breeding site than by chytrid epidemics. This pattern appears to be linked to a drying trend at SRS through the 1990s, although it is unclear whether this was caused by climate change. This study demonstrates that the presence of B. dendrobatidis in am- phibian communities where some species are declining does not always implicate chytrids as a cause of the decline. Like many other emerging pathogens, the outcome of infection can vary among individuals and populations, depending on life history traits, environmental conditions, and virulence factors of the pathogen. Our report also demonstrates the use- fulness of archived museum specimens and long-term population monitoring in studying the host-parasite ecology of emerging diseases.

HOST PARASITE INTERACTIONS BETWEEN FRESHWATER PHYTOPLANKTON AND CHYTRID FUNGI (CHYTRIDIOMYCOTA)1

Some chytrids are host‐specific parasiticfungithat may have a considerable impact on phytoplankton dynamics. The phylum Chytridiomycota contains one class, the Chytridiomycetes, and is composed of five different orders. Molecular studies now firmly place the Chytridiomycota within the fungal kingdom. Chytrids are characterized by having zoospores, a motile stage in their life cycle. Zoospores are attracted to the host cell by specific signals. No single physical–chemical factor has been found that fully explains the dynamics of chytrid epidemics in the field. Fungal periodicity was primarily related to host cell density. The absence of aggregated distributions of chytrids on their hosts suggested that their hosts did not vary in their susceptibility to infection. A parasite can only become epidemic when it grows faster than the host. Therefore, it has been suggested that epidemics in phytoplankton populations arise when growth conditions for the host are unfavorable. No support for such a generalization was found, however. Growth of the parasitic fungus Rhizophydium planktonicum Canter emend, parasitic on the diatom Asterionella formosa Hassal, was reduced under stringent nutrient limitation,because production and infectivity of zoospores were affected negatively. A moderate phosphorous or light limitation favored epidemic development, however. Chytrid infections have been shown to affect competition between their algal hosts and in this way altered phytoplankton succession. There is potential for coevolution between Asterionella and the chytrid Zygorhizidium planktonicum Canter based on clear reciprocal fitness costs, absence of overall infective parasite strains, and possibly a genetic basis for host susceptibility and parasite infectivity.