Environmental Conditions Determine the Course and Outcome of Phytoplankton Chytridiomycosis

Widely used herbicide metolachlor can promote harmful bloom formation by stimulating cyanobacterial growth and driving detrimental effects on their chytrid parasites

Metolachlor (MET) is a widely used herbicide that can adversely affect phytoplanktonic non-target organisms, such as cyanobacteria. Chytrids are zoosporic fungi ubiquitous in aquatic environments that parasitize cyanobacteria and can keep their proliferation in check. However, the influence of organic pollutants on the interaction between species, including parasitism, and the associated ecological processes remain poorly understood. Using the host-parasite system consisting of the toxigenic cyanobacterium Planktothrix agardhii and its chytrid parasite Rhizophydium megarrhizum, we investigated the effects of environmentally relevant concentrations of MET on host-parasite interactions under i) continuous exposure of chytrids and cyanobacteria, and ii) pre-exposure of chytrids. During a continuous exposure, the infection prevalence and intensity were not affected, but chytrid reproductive structures were smaller at the highest tested MET concentration. In the parasite's absence, MET promoted cyanobacteria growth possibly due to a hormesis effect. In the pre-exposure assay, MET caused multi- and transgenerational detrimental effects on parasite fitness. Chytrids pre-exposed to MET showed reduced infectivity, intensity, and prevalence of the infection, and their sporangia size was reduced. Thus, pre-exposure of the parasite to MET resulted in a delayed decline of the cyanobacterial cultures upon infection. After several parasite generations without MET exposure, the parasite recovered its initial fitness, indicating that detrimental effects are transient. This study demonstrates that widely used herbicides, such as MET, could favor cyanobacterial bloom formation both directly, by promoting cyanobacteria growth, and indirectly, by inhibiting their chytrid parasites, which are known to play a key role as top-down regulators of cyanobacteria. In addition, we evidence the relevance of addressing multi-organism systems, such as host-parasite interactions, in toxicity assays. This approach offers a more comprehensive understanding of the effects of pollutants on aquatic ecosystems.

What makes a cyanobacterial bloom disappear? A review of the abiotic and biotic cyanobacterial bloom loss factors

Cyanobacterial blooms present substantial challenges to managers and threaten ecological and public health. Although the majority of cyanobacterial bloom research and management focuses on factors that control bloom initiation, duration, toxicity, and geographical extent, relatively little research focuses on the role of loss processes in blooms and how these processes are regulated. Here, we define a loss process in terms of population dynamics as any process that removes cells from a population, thereby decelerating or reducing the development and extent of blooms. We review abiotic (e.g., hydraulic flushing and oxidative stress/UV light) and biotic factors (e.g., allelopathic compounds, infections, grazing, and resting cells/programmed cell death) known to govern bloom loss. We found that the dominant loss processes depend on several system specific factors including cyanobacterial genera-specific traits, in situ physicochemical conditions, and the microbial, phytoplankton, and consumer community composition. We also address loss processes in the context of bloom management and discuss perspectives and challenges in predicting how a changing climate may directly and indirectly affect loss processes on blooms. A deeper understanding of bloom loss processes and their underlying mechanisms may help to mitigate the negative consequences of cyanobacterial blooms and improve current management strategies.

Effects of water movement and temperature on Rhizophydium infection of Planktothrix in a shallow hypereutrophic lake

Grand Lake St. Marys (GLSM) is a popular recreational lake located in western Ohio, United States, generating nearly $150 million in annual revenue. However, recurring algal blooms dominated by Planktothrix agardhii, which can produce harmful microcystin toxins, have raised concerns about water safety and negatively impacted the local economy. Planktothrix agardhii is host to a number of parasites and pathogens, including an obligate fungal parasite in the Chytridiomycota (chytrids). In this study, we investigated the potential of these chytrid (Rhizophydium sp.) to infect P. agardhii blooms in the environment by modifying certain environmental conditions thought to limit infection prevalence in the wild. With a focus on temperature and water mixing, mesocosms were designed to either increase or decrease water flow compared to the control (water outside the mesocosm). In the control and water circulation mesocosms, infections were found infrequently and were found on less than 0.75% of the Planktothrix population. On the other hand, by decreasing the water flow to stagnation, chytrid infections were more frequent (found in nearly 3x as many samples) and more prevalent, reaching a maximum infection rate of 4.12%. In addition, qPCR coupled with 16S-18S sequencing was utilized to confirm the genetic presence of both host and parasite, as well as to better understand the effect of water circulation on the community composition. Statistical analysis of the data confirmed that chytrid infection was dependent on water temperature, with infections predominantly occurring between 19°C and 23°C. Additionally, water turbulence can significantly reduce the infectivity of chytrids, as infections were mostly found in stagnant mesocosms. Further, decreasing the water circulation promoted the growth of the cyanobacterial population, while increasing water agitation promoted the growth of green algae (Chlorophyta). This study starts to explore the environmental factors that affect chytrid pathogenesis which can provide valuable insights into controlling measures to reduce the prevalence of harmful algal blooms and improve water quality in GLSM and similarly affected waterbodies.

Interactions between chytrids cause variable infection strategies on harmful algal bloom forming species

Cyanobacteria have a great diversity of natural enemies, such as herbivores and pathogens, including fungal pathogens within the Chytridiomycota (chytrids). While these pathogens have been previously described on a select number of cyanobacterial hosts and are suspected to play a significant ecological role, little is understood about species interactions and how competition between parasites can affect epidemic development and bloom formation. Here, three Planktothrix agardhii isolates from Sandusky Bay, Lake Erie (OH, USA) were challenged in monoculture and polyculture against infection by three isolates (C1, C2, C10) of their obligate chytrid fungal pathogen, Rhizophydiales sp. The chytrid isolates were inoculated as single isolates or a mixture of up to three different isolates. In monoculture, host isolates were characterized as highly susceptible (P. agardhii 1030), moderately susceptible (P. agardhii 1808) or mostly resistant (P. agardhii 1801). Co-infection of chytrid isolates on the highly susceptible host isolate had an additive effect on chytrid prevalence, leading to a culture crash where 2 or 3 chytrid isolates were present. Co-infection of chytrid isolates on the moderately susceptible and mostly resistant isolates had no effect on chytrid infection outcome or prevalence compared to infection with a single isolate. In polyculture, the effect on host growth was most significant in the single chytrid isolate treatment, which was attenuated with the addition of mixed chytrid treatments. Genetic analysis of the resulting population after the experimental period showed a tendency for the chytrid isolate C1 and P. agardhii 1801 to dominate in mixed population samples. Two different interspecific interactions seem to be in play; varied parasite infection strategies allow for the amplification of infection prevalence due to mixed chytrids in a susceptible monoculture, or competition allows for the dominance of a single chytrid isolate in monoculture and the reduction of infection prevalence in a host polyculture. This work thus highlights how interactions between chytrid infections can change the course of epidemic development and harmful algal bloom formation.

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.

Complex effects of chytrid parasites on the growth of the cyanobacterium Planktothrix rubescens across interacting temperature and light gradients

Chytrids are important drivers of aquatic ecosystems as phytoplankton parasites. The interaction between these parasites and their hosts are shaped by abiotic factors such as temperature and light. Here, we performed a full-factorial experiment to study how temperature and light interact to affect the dynamics of the bloom-forming toxic cyanobacterium Planktothrix rubescens and its chytrid parasite. We used a dynamic host-parasite model to explore how temperature and light affect long term dynamics. At low temperatures, chytrids do not survive. Higher light and temperature levels stimulated both phytoplankton and chytrid growth, with complex effects on their dynamics. Model exploration indicates that increasing temperature and light shifts equilibrium outcomes from P. rubescens persisting alone to stable coexistence and then to limit cycles. This provides an alternative biological explanation for why P. rubescens is mainly found in the relatively cold and dark lake metalimnion - it may enable avoidance of its parasite. Our study emphasizes the importance of investigating how abiotic factors interact with biotic interactions to drive complex outcomes.

Seasonality of parasitic and saprotrophic zoosporic fungi: linking sequence data to ecological traits

Zoosporic fungi of the phylum Chytridiomycota (chytrids) regularly dominate pelagic fungal communities in freshwater and marine environments. Their lifestyles range from obligate parasites to saprophytes. Yet, linking the scarce available sequence data to specific ecological traits or their host ranges constitutes currently a major challenge. We combined 28 S rRNA gene amplicon sequencing with targeted isolation and sequencing approaches, along with cross-infection assays and analysis of chytrid infection prevalence to obtain new insights into chytrid diversity, ecology, and seasonal dynamics in a temperate lake. Parasitic phytoplankton-chytrid and saprotrophic pollen-chytrid interactions made up the majority of zoosporic fungal reads. We explicitly demonstrate the recurrent dominance of parasitic chytrids during frequent diatom blooms and saprotrophic chytrids during pollen rains. Distinct temporal dynamics of diatom-specific parasitic clades suggest mechanisms of coexistence based on niche differentiation and competitive strategies. The molecular and ecological information on chytrids generated in this study will aid further exploration of their spatial and temporal distribution patterns worldwide. To fully exploit the power of environmental sequencing for studies on chytrid ecology and evolution, we emphasize the need to intensify current isolation efforts of chytrids and integrate taxonomic and autecological data into long-term studies and experiments.

Parasites do not adapt to elevated temperature, as evidenced from experimental evolution of a phytoplankton-fungus system

Global warming is predicted to impact the prevalence and severity of infectious diseases. However, empirical data supporting this statement usually stem from experiments in which parasite fitness and disease outcome are measured directly after temperature increase. This might exclude the possibility of parasite adaptation. To incorporate the adaptive response of parasites into predictions of disease severity in a warmer world, we undertook an experimental evolution assay in which a fungal parasite of phytoplankton was maintained at elevated or control temperatures for six months, corresponding to 100–200 parasite generations. Host cultures were maintained at the respective temperatures and provided as substrate, but were not under parasite pressure. A reciprocal infection experiment conducted after six-month serial passages revealed no evidence of parasite adaptation. In fact, parasite fitness at elevated temperatures was inferior in parasite populations reared at elevated temperatures compared with those maintained under control temperature. However, this effect was reversed after parasites were returned to control temperatures for a few (approx. 10) generations. The absence of parasite adaptation to elevated temperatures suggests that, in phytoplankton–fungus systems, disease outcome under global warming will be largely determined by both host and parasite thermal ecology.

Environmental factors affecting chytrid (Chytridiomycota) infection rates on Planktothrix agardhii

Planktothrix agardhii dominates the cyanobacterial harmful algal bloom biomass in Sandusky Bay, Lake Erie (USA) from May until September. This filamentous cyanobacterium known parasites including the chytrid fungal species Rhizophydium sp. C02, which was previously isolated from this region. The purpose of our work has been to establish how parasitic interactions affect Planktothrix population dynamics during a bloom event. Samples analyzed from the 2015 to 2019 bloom seasons using quantitative PCR investigate the spatial and temporal prevalence of chytrid infections. Abiotic factors examined in lab include manipulating temperature (17-31°C), conductivity (0.226-1.225 mS/cm) and turbulence. Planktothrix-specific chytrids are present throughout the bloom period and are occasionally at high enough densities to exert parasitic pressure on their hosts. Temperatures above 27.1°C in lab can inhibit chytrid infection, indicating the presence of a possible upper thermal refuge for the host. Data suggest that chytrids can survive conductivity spikes in lab at levels three-fold above Sandusky Bay waters if given sufficient time (7-12 days), whereas increased turbulence in lab severely inhibits chytrid infections, perhaps due to disruption of chemical signaling. Overall, these data provide insights into the environmental conditions that inhibit chytrid infections during Planktothrix-dominated blooms in temperate waters.

Isolation and Characterization of Rhizophydiales sp. (Chytridiomycota), Obligate Parasite of Planktothrix agardhii in a Laurentian Great Lakes Embayment

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 preference for different host isolates, suggesting possible ecological partitioning even within the same sample population. Examining mechanisms of chytrid pathogenesis, 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 media type, highlighting the importance of media choice on 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 aimed at determining environmental factors favoring loss mechanisms for Planktothrix agardhii-dominated blooms.IMPORTANCE Whereas many studies have focused on the factors contributing to the establishment and persistence of cyanobacterial harmful algal blooms (cHABs), few studies have examined bloom pathogenesis. Chytrid fungi infect cyanobacteria and stimulate food web interactions through manipulation of previously hard to digest filaments and the release of nutrients to support heterotrophic microbes. Specifically, chytrids infective on filamentous Planktothrix agardhii exhibit a species-specific infection that fragments trichomes into shorter units that can be consumed more easily by grazers. Chytrid zoospores also serve as a high-quality food source for the lower food web. Understanding host-pathogen relationships and mechanisms of pathogenesis on cyanobacteria will be necessary to effectively model the ecology of cHABs.

Parasitic Chytrids Upgrade and Convey Primary Produced Carbon During Inedible Algae Proliferation

Microbial parasites have only recently been included in planktonic food web studies, but their functional role in conveying dietary energy still remains to be elucidated. Parasitic fungi (chytrids) infecting phytoplankton may constitute an alternative trophic link and promote organic matter transfer through the production of dissemination zoospores. Particularly, during proliferation of inedible or toxic algal species, such as large Cyanobacteria fostered by global warming, parasites can constitute an alternative trophic link providing essential dietary nutrients that support somatic growth and reproduction of consumers. Using phytoplankton-parasites associations grown under laboratory controlled conditions we assessed the fatty acids and biochemical composition of species with different nutritional quality and followed the metabolic pathway from the algal host and their parasites zoospores using compound-specific stable isotope analysis. This study demonstrated that chytrids are trophic upgraders able to retain essential nutrients that can be transferred to upper trophic levels both in terms of organic matter quantity and nutritional quality. Through the production of zoospores, nutritionally important long-chain polyunsaturated fatty acids that can be consequently assimilated by consumers. We conclude that parasitism at the base of aquatic food webs may represent a crucial trophic link for dietary nutrients and essential biomolecules alternative to herbivory or bacterivory, which can be particularly crucial during the proliferation of inedible or nutritionally inadequate algal species fostered by climate change.

Light intensity and spectral distribution affect chytrid infection of cyanobacteria via modulation of host fitness

Light gradients are an inherent feature in aquatic ecosystems and play a key role in shaping the biology of phytoplankton. Parasitism by chytrid fungi is gaining increasing attention as a major control agent of phytoplankton due to its previously overlooked ubiquity, and profound ecological and evolutionary consequences. Despite this interest, if and how light conditions modulate phytoplankton chytridiomycosis remains poorly studied. We investigated life-history traits of a chytrid parasite, Rhizophydium megarrhizum, under different light intensities and spectral compositions when infecting two closely related planktonic cyanobacteria with different light-harvesting strategies: Planktothrix rubescens and P. agardhii. In general, parasite transmission was highest under light conditions (both intensity and quality) that maximized growth rates for uninfected cyanobacteria. Chytrid encystment on hosts was significantly affected by light intensity and host strain identity. This likely resulted from higher irradiances stimulating the increased discharge of photosynthetic by-products, which drive parasite chemotaxis, and from strain-specific differences at the cell-surface. Comparisons of parasite transmission and host growth rates under different light conditions suggest the potential for epidemic development at higher irradiances, whereas host and parasite could coexist without epidemic outbreaks at lower light levels. These results illustrate the close relationship between parasite transmission and host fitness, which is ultimately modulated by the external environment.

Low temperatures can promote cyanobacterial bloom formation by providing refuge from microbial antagonists

Freshwater cyanobacteria are prone to a wide range of highly potent microbial antagonists. Most of these exploit their prey in a frequency-dependent manner and are therefore particularly well suited to prevent any accumulation of cyanobacteria. Mass developments of cyanobacteria, the so-called blooms, should therefore be rare events, which is in striking contrast to what we actually see in nature. Laboratory experiments of the present study showed that the temperature range 5.8–10 °C forms a thermal refuge, inside which the cyanobacterium Planktothrix can grow without being exploited by two otherwise highly potent microbial antagonists. In nature, access of Planktothrix to this refuge was associated with positive net growth and a high probability of bloom formation, confirming that refuge temperatures indeed allow Planktothrix to grow with a minimum of biomass loss caused by microbial antagonists. Contact to higher temperatures, in contrast, was associated with decreases in net growth rate and in probability of bloom formation, with population collapses and with the occurrence of parasite infection. This is in agreement with the finding of laboratory experiments that above 10 °C exploitation of Planktothrix by multiple microbes increases in a temperature-dependent manner. Taken together, above findings suggest that temperature modulates the microbial control of natural Planktothrix populations. Low temperatures form a thermal refuge that may promote Planktothrix bloom formation by shielding the cyanobacterium from otherwise highly potent microbial antagonists.

Fitness and eco-physiological response of a chytrid fungal parasite infecting planktonic cyanobacteria to thermal and host genotype variation

Understanding how individual parasite traits contribute to overall fitness, and how they are modulated by both external and host environment, is crucial for predicting disease outcome. Fungal (chytrid) parasites of phytoplankton are important yet poorly studied pathogens with the potential to modulate the abundance and composition of phytoplankton communities and to drive their evolution. Here, we studied life-history traits of a chytrid parasite infecting the planktonic, bloom-forming cyanobacterium Planktothrix spp. under host genotype and thermal variation. When expressing parasite fitness in terms of transmission success, disease outcome was largely modulated by temperature alone. Yet, a closer examination of individual parasite traits linked to different infection phases, such as (i) the establishment of the infection (i.e. intensity of infection) and (ii) the exploitation of host resources (i.e. size of reproductive structures and propagules), revealed differential host genotype and temperature × host genotype modulation, respectively. This illustrates how parasite fitness results from the interplay of individual parasite traits that are differentially controlled by host and external environment, and stresses the importance of combining multiple traits to gain insights into underlying infection mechanisms.