Biological control of toxic cyanobacteria by mixotrophic predators: an experimental test of intraguild predation theory

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.

Dynamics of autotroph-mixotroph interactions with the intraguild predation structure

A mathematical model with the intraguild predation structure is proposed to describe the interactions of autotrophs and mixotrophs containing light and nutrients in a well-mixed aquatic ecosystem. The dissipation, existence and stability of equilibria of the model are proved, and the ecological reproductive indexes for the extinction, survival and coexistence of autotrophs and mixotrophs are established. We also consider the influence of Holling type functional responses and abiotic factors on the coexistence and biomass of autotrophs and mixotrophs. It is shown that the intraguild predation structure is beneficial to phytoplankton biodiversity and provides an explanation for the phytoplankton paradox.

Elevated temperature mitigates the prolonged effect of high nitrogen on Microcystis aeruginosa removal through mixotrophic Ochromonas gloeopara grazing

Cyanobacterial blooms are increasingly threatening the aquatic ecosystem functioning as a result of the global warming and eutrophication. The "top-down" control of cyanobacteria from consumers like the protozoans shows great potential because of the effectiveness and environment-friendliness. To reveal how the nutrition availability and elevated temperature affect the cyanobacteria removal through protozoans grazing, we grew the toxic Microcystis aeruginosa and the mixotrophic Ochromonas gloeopara in monocultures and cocultures at environmentally relevant nitrogen levels (0.5-8.0 mg L^-1) under 25 °C and 30 °C, respectively. The growth of M. aeruginosa in monocultures was significantly enhanced as nitrogen concentration and temperature rose, partially benefitting from the promoted photosynthesis. By contrast, nitrogen availability affected neither the photoautotrophic growth nor the feeding on Microcystis of the mixotrophic O. gloeopara, but high temperature induced the mixotroph to be more heterotrophic as evidenced by the suppressed photosynthesis but strengthened feeding activity. Accordingly, the M. aeruginosa removal through O. gloeopara grazing in cocultures was delayed with increasing nitrogen, which, however, was sharply accelerated by elevated temperature. Based on the Gaussian models fitting, the theoretical time that the Microcystis was removed at 25 °C was prolonged from about 7.5 days to 10 days with increased nitrogen, but it was reduced to less than 4.6 days in all groups at 30 °C. While the intensity of Microcystis blooms is strongly positively correlated to the nutrition availability and temperature, the present study provided references for the practical application of Microcystis removal through grazing outdoors.

Dynamics of Stoichiometric Autotroph-Mixotroph-Bacteria Interactions in the Epilimnion

Autotrophs, mixotrophs and bacteria exhibit complex interrelationships containing multiple ecological mechanisms. A mathematical model based on ecological stoichiometry is proposed to describe the interactions among them. Some dynamic analysis and numerical simulations of this model are presented. The roles of autotrophs and mixotrophs in controlling bacterioplankton are explored to examine the experiments and hypotheses of Medina-Sánchez, Villar-Argaiz and Carrillo for La Caldera Lake. Our results show that the dual control (bottom-up control and top-down control) of bacteria by mixotrophs is a key reason for the ratio of bacterial and phytoplankton biomass in La Caldera Lake to deviate from the general tendency. The numerical bifurcation diagrams suggest that the competition between phytoplankton and bacteria for nutrients can also be an important factor for the decrease of the bacterial biomass in an oligotrophic lake.

Mixotrophic Ochromonas Addition Improves the Harmful Microcystis-Dominated Phytoplankton Community in In Situ Microcosms

Driven by global warming and eutrophication, outbreaks of cyanobacterial blooms have severely impacted ecosystem stability and water safety. Of the organisms used to control cyanobacteria, protozoa can highly resist cyanotoxins, efficiently control cyanobacterial populations, and show considerably different feeding strategies from those of metazoans. Thus, protozoa have great potential to control harmful cyanobacteria and improve phytoplankton composition in eutrophic waters. To evaluate the actual effects of protozoa in controlling cyanobacteria and improving the phytoplankton community structure in the field, an in situ microcosm study was performed using a flagellate Ochromonas gloeopara that ingests Microcystis. Results showed that adding Ochromonas reduced the cyanobacterial populations and increased the chlorophyte and diatom proportions. Furthermore, the species richness and diversity of the phytoplankton community were enhanced in microcosms with Ochromonas. Additionally, there was a gradual increase in the chlorophyte population in the unicellular Microcystis control, while Ochromonas addition significantly accelerated the replacement of dominant species. This study was the first to show the practical effects of protozoa on controlling cyanobacteria in the field, highlighting that a reduction in in situ cyanobacteria via protozoa can improve the phytoplankton community structure, dredge the toxic cyanobacteria-dominated microbial food web, and mitigate harmful cyanobacteria risks in fresh waters.

The Relations Between Predatory Fungus and Its Rotifer Preys as a Noteworthy Example of Intraguild Predation (IGP)

Intraguild predation (IGP) is a widespread interaction combining predation and competition. We investigated a unique IGP example among predacious fungus Zoophagus sp. and two rotifers, the predacious Cephalodella gibba and the common prey Lecane inermis. We checked the influence of the fungus on its competitor C. gibba and their joint influence on shared prey L. inermis, and the impact of the competitive predator on the growth of predacious fungus. The experiment on grown mycelium showed that Zoophagus strongly, negatively influences the growth of C. gibba (intermediate consumer) whose number did not increase throughout the experiment. The intermediate consumer was also trapped by Zoophagus and become extinct when it was its only prey, whereas in the absence of the fungus and with unlimited access to prey, its number grew quickly. As only few C. gibba were trapped by fungi when common preys were present, competition for food seems to have stronger effect on intermediate consumer population than predation. The experiment with conidia of the fungus showed that intermediate consumer significantly limits the growth of Zoophagus by reducing the number of available prey. It was observed that although the fungus can trap C. gibba, the latter does not support its growth. Trapping the intermediate consumer might serve to eliminate a competitor rather than to find a source of food. The chances of survival for L. inermis under the pressure of the two competing predators are scarce. It is the first example of IGP involving representatives of two kingdoms: Fungi and Animalia.

Intraguild predation enables coexistence of competing phytoplankton in a well-mixed water column

Resource competition theory predicts that when two species compete for a single, finite resource, the better competitor should exclude the other. However, in some cases, weaker competitors can persist through intraguild predation, that is, by eating their stronger competitor. Mixotrophs, species that meet their carbon demand by combining photosynthesis and phagotrophic heterotrophy, may function as intraguild predators when they consume the phototrophs with which they compete for light. Thus, theory predicts that mixotrophy may allow for coexistence of two species on a single limiting resource. We tested this prediction by developing a new mathematical model for a unicellular mixotroph and phytoplankter that compete for light, and comparing the model's predictions with a laboratory experimental system. We find that, like other intraguild predators, mixotrophs can persist when an ecosystem is sufficiently productive (i.e., the supply of the limiting resource, light, is relatively high), or when species interactions are strong (i.e., attack rates and conversion efficiencies are high). Both our mathematical and laboratory models show that, depending upon the environment and species traits, a variety of equilibrium outcomes, ranging from competitive exclusion to coexistence, are possible.

Production of Cyanotoxins by Microcystis aeruginosa Mediates Interactions with the Mixotrophic Flagellate Cryptomonas

Eutrophication of inland waters is expected to increase the frequency and severity of harmful algal blooms (HABs). Toxin-production associated with HABs has negative effects on human health and aquatic ecosystem functioning. Despite evidence that flagellates can ingest toxin-producing cyanobacteria, interactions between members of the microbial loop are underestimated in our understanding of the food web and algal bloom dynamics. Physical and allelopathic interactions between a mixotrophic flagellate (Cryptomonas sp.) and two strains of a cyanobacteria (Microcystis aeruginosa) were investigated in a full-factorial experiment in culture. The maximum population growth rate of the mixotroph (0.25 day⁻¹) occurred during incubation with filtrate from toxic M. aeruginosa. Cryptomonas was able to ingest toxic and non-toxic M. aeruginosa at maximal rates of 0.5 and 0.3 cells day⁻¹, respectively. The results establish that although Cryptomonas does not derive benefits from co-incubation with M. aeruginosa, it may obtain nutritional supplement from filtrate. We also provide evidence of a reduction in cyanotoxin concentration (microcystin-LR) when toxic M. aeruginosa is incubated with the mixotroph. Our work has implications for “trophic upgrading” within the microbial food web, where cyanobacterivory by nanoflagellates may improve food quality for higher trophic levels and detoxify secondary compounds.

Mixotrophy in nanoflagellates across environmental gradients in the ocean

Mixotrophy, the combination of autotrophic and heterotrophic nutrition, is a common trophic strategy among unicellular eukaryotes in the ocean. There are a number of hypotheses about the conditions that select for mixotrophy, and field studies have documented the prevalence of mixotrophy in a range of environments. However, there is currently little evidence for how mixotrophy varies across environmental gradients, and whether empirical patterns support theoretical predictions. Here I synthesize experiments that have quantified the abundance of phototrophic, mixotrophic, and heterotrophic nanoflagellates, to ask whether there are broad patterns in the prevalence of mixotrophy (relative to pure autotrophy and heterotrophy), and to ask whether observed patterns are consistent with a trait-based model of trophic strategies. The data suggest that mixotrophs increase in abundance at lower latitudes, while autotrophs and heterotrophs do not, and that this may be driven by increased light availability. Both mixotrophs and autotrophs increase greatly in productive coastal environments, while heterotrophs increase only slightly. These patterns are consistent with a model of resource competition in which nutrients and carbon can both limit growth and mixotrophs experience a trade-off in allocating biomass to phagotrophy vs. autotrophic functions. Importantly, mixotrophy is selected for under a range of conditions even when mixotrophs experience a penalty for using a generalist trophic strategy, due to the synergy between photosynthetically derived carbon and prey-derived nutrients. For this reason mixotrophy is favored relative to specialist strategies by increased irradiance, while at the same time increased nutrient supply increases the competitive ability of mixotrophs against heterotrophs.

Influence of cyanobacteria, mixotrophic flagellates, and virioplankton size fraction on transcription of microcystin synthesis genes in the toxic cyanobacterium Microcystis aeruginosa

Toxic cyanobacteria such as Microcystis aeruginosa are a worldwide concern in freshwater reservoirs. Problems associated with their mass occurrence are predicted to increase in the future due to global warming. The hepatotoxic secondary metabolite microcystin is of particular concern in this context. This study aimed to determine whether co-occurring microorganisms influence the expression of microcystin biosynthesis genes. To this end, we performed cocultivation experiments and measured mcyB and mcyD transcripts in M. aeruginosa using RT-qPCR. We utilized representatives from three different plankton groups: the picocyanobacterium Synechococcus elongatus, the unicellular flagellate grazer Ochromonas danica, and virioplankton from two different lakes. The presence of S. elongatus significantly increased mcyB and mcyD transcription in M. aeruginosa. Cocultivation with the mixotrophic chrysophyte O. danica did not increase the transcription of mcyB and mcyD; in fact, mcyD transcripts decreased significantly. The virioplankton size fraction of environmental water samples induced a significant increase in mcyB and mcyD transcription when obtained from lakes with cyanobacterial blooms. Our results show that co-occurring microorganisms influence the expression of microcystin biosynthesis genes in M. aeruginosa.

Seasonal dynamics of microcystin-degrading bacteria and toxic cyanobacterial blooms: Interaction and influence of abiotic factors

Massive proliferations of cyanobacteria coexist and have different interactions with other microorganisms, including microcystin (MC)-degrading bacteria. Despite their relevance in the environment for the removal of MCs, this bacterial community has been scarcely studied. The influence of physicochemical factors and the seasonal dynamics of toxic cyanobacteria on the relative abundance and seasonal dynamics of the MC-degrading bacterial community with mlr genes (mlr⁺) were investigated during a two-year study at a water reservoir in central Spain. The capacity of the total bacterial community on the degradation of MCs during the whole period of study was also evaluated. The results showed that the relative abundance of mlr⁺ bacteria started to increase after the increase in the relative abundance of toxic cyanobacteria and MC concentrations in the water, indicating a related seasonal dynamic and an important interaction between the two communities. The correspondence of several peaks of mlr⁺ bacteria with decreases in the relative abundance of toxic cyanobacteria and vice versa may also suggest a possible antagonistic relationship that deserves an in-depth study. The lack of a significant relationship between the physicochemical factors and the temporal shifts of both MC producers and degraders also supports the notion that the interaction of the two communities is an important driver of their seasonal dynamics in nature. Regarding the capacity of the total bacterial community for the degradation of MCs, this capacity was only observed during the toxic cyanobacterial bloom episodes, highlighting the importance of the pre-exposure to MCs in the reservoir for triggering the MC biodegradation process.

How to evaluate the potential occurrence of intraguild predation

Intraguild predation is the combination of exploitative competition and predation among potential competitors that use similar resources. It has the potential to shape population dynamics and community structure. Although there is much empirical evidence for the occurrence of intraguild predation in natural ecosystems, the study of its effects is mainly limited to short-term microcosm experiments. There is, therefore, certain skepticism about its actual significance in nature. A relevant concern is that there is no consensus regarding criteria to evaluate the possible occurrence of intraguild predation in short-term experiments, and methodological differences may therefore underlie apparent inconsistencies among studies. Our purpose here was to evaluate existing criteria to offer guidance for the design of experiments to determine whether two species may potentially engage in intraguild predation. The criteria are based on the condition that intraguild predators need to experience immediate energetic gains when feeding on the intraguild prey. Thus, a relevant experimental design must quantify predation but also fitness benefits of feeding on the other species, i.e. increases in reproduction, somatic growth, or survival.

The common bloom-forming cyanobacterium Microcystis is prone to a wide array of microbial antagonists

Many degraded waterbodies around the world are subject to strong proliferations of cyanobacteria - notorious for their toxicity, high biomass build-up and negative impacts on aquatic food webs - the presence of which puts serious limits on the human use of affected water bodies. Cyanobacterial blooms are largely regarded as trophic dead ends since they are a relatively poor food source for zooplankton. As a consequence, their population dynamics are generally attributed to changes in abiotic conditions (bottom-up control). Blooms however generally contain a vast and diverse community of micro-organisms of which some have shown devastating effects on cyanobacterial biomass. For Microcystis, one of the most common bloom-forming cyanobacteria worldwide, a high number of micro-organisms (about 120 taxa) including viruses, bacteria, microfungi, different groups of heterotrophic protists, other cyanobacteria and several eukaryotic microalgal groups are currently known to negatively affect its growth by infection and predation or by the production of allelopathic compounds. Although many of these specifically target Microcystis, sharp declines of Microcystis biomass in nature are only rarely assigned to these antagonistic microbiota. The commonly found strain specificity of their interactions may largely preclude strong antagonistic effects on Microcystis population levels but may however induce compositional shifts that can change ecological properties such as bloom toxicity. These highly specific interactions may form the basis of a continuous arms race (co-evolution) between Microcystis and its antagonists which potentially limits the possibilities for (micro)biological bloom control.

An evidence-based framework for predicting the impact of differing autotroph-heterotroph thermal sensitivities on consumer-prey dynamics

Increased temperature accelerates vital rates, influencing microbial population and wider ecosystem dynamics, for example, the predicted increases in cyanobacterial blooms associated with global warming. However, heterotrophic and mixotrophic protists, which are dominant grazers of microalgae, may be more thermally sensitive than autotrophs, and thus prey could be suppressed as temperature rises. Theoretical and meta-analyses have begun to address this issue, but an appropriate framework linking experimental data with theory is lacking. Using ecophysiological data to develop a novel model structure, we provide the first validation of this thermal sensitivity hypothesis: increased temperature improves the consumer's ability to control the autotrophic prey. Specifically, the model accounts for temperature effects on auto- and mixotrophs and ingestion, growth and mortality rates, using an ecologically and economically important system (cyanobacteria grazed by a mixotrophic flagellate). Once established, we show the model to be a good predictor of temperature impacts on consumer–prey dynamics by comparing simulations with microcosm observations. Then, through simulations, we indicate our conclusions remain valid, even with large changes in bottom-up factors (prey growth and carrying capacity). In conclusion, we show that rising temperature could, counterintuitively, reduce the propensity for microalgal blooms to occur and, critically, provide a novel model framework for needed, continued assessment.