Nutritional and toxicity constraints of phytoplankton from a Brazilian reservoir to the fitness of cladoceran species

Chlorella pyrenoidosa mitigated the negative effect of cylindrospermopsin-producing and non-cylindrospermopsin-producing Raphidiopsis raciborskii on Daphnia magna as a dietary supplement

Feeding effects are crucial for evaluating the capacity of zooplankton to regulate phytoplankton populations within freshwater ecosystems. To examine the impact of the bloom-forming cyanobacteria Raphidiopsis raciborskii, which occurs in tropical and subtropical freshwaters, on the growth of zooplankton Daphnia in relation to toxins, filament length and fatty acid content, we fed D. magna with R. raciborskii only (cylindrospermopsin (CYN)-producing and non-CYN-producing, as the negative controls), Chlorella pyrenoidosa only (as the positive control) and a mixed diet containing R. raciborskii (CYN-producing and non-CYN-producing) and C. pyrenoidosa. Consequently, our findings revealed that the toxic effect of CYN-producing R. raciborskii strains on Daphnia was mitigated by the coexistence of C. pyrenoidosa containing stearidonic acid (SDA, C18:4 ω3) in mixed diets. This was evident in the elevated survival rate compared that from diets containing only R. raciborskii and a significantly higher reproduction and population intrinsic increase rate compared to diets consisting of only R. raciborskii or C. pyrenoidos. Additionally, a strong positive correlation was observed between arachidonic acid (ARA, 20:4ω6) and the population intrinsic increase rate of Daphnia; notably, R. raciborskii strains were found to be rich in the ω6 polyunsaturated fatty acid ARA. These outcomes reinforce the crucial role of polyunsaturated fatty acids in predicting the population increase of crustacean zooplankton, which has long been neglected. Furthermore, our results underscore the potential effectiveness of zooplankton, particularly in temperate lakes, in controlling CYN-producing R. raciborskii populations.

Saxitoxin-Producing Raphidiopsis raciborskii (Cyanobacteria) Constrains Daphnia Fitness and Feeding Rate despite High Nutritious Food Availability

Changes in food quality can dramatically impair zooplankton fitness, especially in eutrophic water bodies where cyanobacteria are usually predominant. Cyanobacteria are considered a food with low nutritional value, and some species can produce bioactive secondary metabolites reported as toxic to zooplankton. Considering that cyanobacteria can limit the survival, growth and reproduction of zooplankton, we hypothesized that the dietary exposure of neotropical Daphnia species (D. laevis and D. gessneri) to saxitoxin-producing cyanobacteria impairs Daphnia feeding rates and fitness regardless of a high availability of nutritious algae. Life table and grazing assays were conducted with different diets: (1) without nutritional restriction, where neonates were fed with diets at a constant green algae biomass (as a nutritious food source), and an increasing cyanobacterial concentration (toxic and poor food source), and (2) with diets consisting of different proportions of green algae (nutritious) and cyanobacteria (poor food) at a total biomass 1.0 mg C L-1. In general, the presence of high proportions of cyanobacteria promoted a decrease in Daphnia somatic growth, reproduction and the intrinsic rate of population increase (r) in both diets with more pronounced effects in the nutritionally restricted diet (90% R. raciborskii). A two-way ANOVA revealed the significant effects of species/clone and treatments in both assays, with significant interaction between those factors only in the second assay. Regarding the grazing assay, only D. laevis was negatively affected by increased cyanobacterial proportions in the diet. In the life table assay with constant nutritious food, a reduction in the reproduction and the intrinsic rate of the population increase (r) of all species were observed. In conclusion, we found adverse effects of the toxic cyanobacterial strain R. raciborskii on Daphnia fitness, regardless of the constant amount of nutritious food available, proving the toxic effect of R. raciborskii and that the nutritional quality of the food has a greater influence on the fitness of these animals.

Recent Advances in the Ecology of Bloom-Forming Raphidiopsis (Cylindrospermopsis) raciborskii: Expansion in China, Intraspecific Heterogeneity and Critical Factors for Invasion

Water blooms caused by the invasive cyanobacterium Raphidiopsis raciborskii occur in many reservoirs in the tropical and subtropical regions of China. In recent decades, this species has spread rapidly to temperate regions. Phenotypic plasticity and climate warming are thought to promote the worldwide dispersion of R. raciborskii. However, investigations into the genetic and phenotypic diversities of this species have revealed significant intraspecific heterogeneity. In particular, competition between R. raciborskii and Microcystis aeruginosa was highly strain dependent. Although the concept of an ecotype was proposed to explain the heterogeneity of R. raciborskii strains with different geographic origins, microevolution is more reasonable for understanding the coexistence of different phenotypes and genotypes in the same environment. It has been suggested that intraspecific heterogeneity derived from microevolution is a strong driving force for the expansion of R. raciborskii. Additionally, temperature, nutrient fluctuations, and grazer disturbance are critical environmental factors that affect the population establishment of R. raciborskii in new environments. The present review provides new insights into the ecological mechanisms underlying the invasion of R. raciborskii in Chinese freshwater ecosystems.

Grazer-Induced Chemical Defense in a Microcystin-Producing Microcystis aeruginosa (Cyanobacteria) Exposed to Daphnia gessneri Infochemicals

Cyanobacteria are photosynthetic microorganisms that compose phytoplankton and therefore have a trophic relationship with zooplankton, which represent an important link for energy flux in aquatic food webs. Several species can form blooms and produce bioactive metabolites known as cyanotoxins. However, the ecological and adaptative role of these toxins are still under debate. Many studies have addressed the cyanotoxins' function in defense against herbivory when grazing pressure by zooplankton plays a role in phytoplankton top-down control. Thus, the present study evaluated the ecophysiological responses of the cyanobacterial strain Microcystis aeruginosa NPLJ-4 underlying the chemical induced defense against the cladoceran Daphnia gessneri. Exposure to predator infochemicals consisted of cultures established in ASM-1 medium prepared in a filtrate from a culture of adults of D. gessneri at an environmentally relevant density. Daphnia infochemicals promoted a significant increase in toxin production by M. aeruginosa. However, no differences in growth were observed, despite a significant increase in both maximum photosynthetic efficiency and electron transport rate in response to zooplankton. Additionally, there was no significant variation in the production of exopolysaccharides. Overall, although a grazer-induced defense response was demonstrated, there were no effects on M. aeruginosa fitness, which maintained its growth in the presence of Daphnia alarm cues.

Reviewing Interspecies Interactions as a Driving Force Affecting the Community Structure in Lakes via Cyanotoxins

The escalating occurrence of toxic cyanobacterial blooms worldwide is a matter of concern. Global warming and eutrophication play a major role in the regularity of cyanobacterial blooms, which has noticeably shifted towards the predomination of toxic populations. Therefore, understanding the effects of cyanobacterial toxins in aquatic ecosystems and their advantages to the producers are of growing interest. In this paper, the current literature is critically reviewed to provide further insights into the ecological contribution of cyanotoxins in the variation of the lake community diversity and structure through interspecies interplay. The most commonly detected and studied cyanobacterial toxins, namely the microcystins, anatoxins, saxitoxins, cylindrospermopsins and β-N-methylamino-L-alanine, and their ecotoxicity on various trophic levels are discussed. This work addresses the environmental characterization of pure toxins, toxin-containing crude extracts and filtrates of single and mixed cultures in interspecies interactions by inducing different physiological and metabolic responses. More data on these interactions under natural conditions and laboratory-based studies using direct co-cultivation approaches will provide more substantial information on the consequences of cyanotoxins in the natural ecosystem. This review is beneficial for understanding cyanotoxin-mediated interspecies interactions, developing bloom mitigation technologies and robustly assessing the hazards posed by toxin-producing cyanobacteria to humans and other organisms.

Ecophysiological Aspects and sxt Genes Expression Underlying Induced Chemical Defense in STX-Producing Raphidiopsis raciborskii (Cyanobacteria) against the Zooplankter Daphnia gessneri

Cyanobacteria stand out among phytoplankton when they form massive blooms and produce toxins. Because cyanotoxin genes date to the origin of metazoans, the hypothesis that cyanotoxins function as a defense against herbivory is still debated. Although their primary cellular function might vary, these metabolites could have evolved as an anti-predator response. Here we evaluated the physiological and molecular responses of a saxitoxin-producing Raphidiopsis raciborskii to infochemicals released by the grazer Daphnia gessneri. Induced chemical defenses were evidenced in R. raciborskii as a significant increase in the transcription level of sxt genes, followed by an increase in saxitoxin content when exposed to predator cues. Moreover, cyanobacterial growth decreased, and no significant effects on photosynthesis or morphology were observed. Overall, the induced defense response was accompanied by a trade-off between toxin production and growth. These results shed light on the mechanisms underlying zooplankton-cyanobacteria interactions in aquatic food webs. The widespread occurrence of the cyanobacterium R. raciborskii in freshwater bodies has been attributed to its phenotypic plasticity. Assessing the potential of this species to thrive over interaction filters such as zooplankton grazing pressure can enhance our understanding of its adaptive success.

Can small-bodied Daphnia control Raphidiopsis raciborskii in eutrophic tropical lakes? A mesocosm experiment

Raphidiopsis raciborskii is being considered an expanding, invasive species all over the world. It is a potentially toxin producer cyanobacterium and form blooms specially in (sub)tropical lakes, causing concern to public health. Thus, controlling such phenomena are of vital importance. To test the hypothesis that a tropical clone of Daphnia laevis is able to reduce the biomass of R. raciborskii, we performed a mesocosm experiment simulating a bloom of this cyanobacterium in field conditions and exposing it to ecologically relevant densities of daphniids. In addition, we tested the hypothesis that omnivorous fish would be able to exert a top-down effect on Daphnia, decreasing the effectiveness of this control. We used treatments with (10 and 20 Daphnia L^-1) or without Daphnia and fish (3 per mesocosm). Daphnia was able to significantly reduce the biomass of R. raciborskii only at the highest density tested. Fish had low effect on Daphnia biomass, but it is suggested that nutrient recycling by fish might have contributed to the higher R. raciborskii biomass in fish treatments. This is the first evidence of Daphnia control over saxitoxin-producing cyanobacteria in a tropical ecosystem.