Negative Effects of Cyanotoxins and Adaptative Responses of Daphnia

Does Microcystis exposure history facilitate adaptation of Diaphanosoma dubium to expanding filamentous cyanobacterium Raphidiopsis raciborskii?

Diaphanosoma, so-called 'tropical Daphnia' are widely distributed in warm waterbodies with Microcystis blooms, and are supposed to have high level of tolerance to Microcystis. However, expanding filamentous cyanobacterium, Raphidiopsis raciborskii is increasingly replacing Microcystis, or becoming co-dominant in warm eutrophic waterbodies. Whether previous experience with Microcystis can facilitate adaptation of Diaphanosoma to R. raciborskii or a shift in cyanobacteria assemblage from Microcystis to R. raciborskii will reduce their tolerance to cyanobacteria is debated. We address this question by investigating the performance of fourteen clones from three Diaphanosoma dubium populations differing in cyanobacteria exposure history. They were fed pure diets of cylindrospermopsin-/non-cylindrospermopsin-producing R. raciborskii, microcystin-producing Microcystis aeruginosa and green alga Auxenochlorella pyrenoidosa. The three cyanobacteria all sustained the growth and reproduction of three D. dubium populations, and both somatic and population growth rates increased with dietary PUFA content for each clone, irrespective of manageability and toxicity. Clones with cyanobacteria exposure history have much better performance than those without exposure history on all food types. Moreover, clones with Microcystis exposure history for >40 years, performed better on all food types than clones with R. raciborskii exposure history for <10 years. The results suggest that D. dubium population with much longer bloom exposure history may adapt to being more tolerant of toxic cyanobacteria in the diet. We infer that previous Microcystis exposure experience might facilitate adaptation of D. dubium to R. raciborskii by enhancing their ability to cope with nutritional constraint and toxicity.

Changes in population fitness and gene co-expression networks reveal the boosted impact of toxic cyanobacteria on Daphnia magna through microplastic exposure

The concomitant prevalence of toxic cyanobacteria blooms and plastic pollution in aquatic ecosystems is emerging as a pressing global water pollution dilemma. While toxic cyanobacteria and microplastics (MPs) can each independently exert significant impacts on aquatic biota, the magnitude and trajectory of the combined interactions remains rudimentary. In this study, we evaluated how MPs influences cyanobacterial stress on keystone grazer Daphnia, focusing on population, individual, biochemical and toxicogenomic signatures. We found that toxic Microcystis (TM) adversely affected the fitness of Daphnia populations (intrinsic rate of population increase), and these adverse effects were amplified in the presence of MPs. Through detailed observation, it was ascertained that MPs promoted the ingestion of TM, culminating in enhanced microcystin bioaccumulation. Using the Eco-Evo model, we found that there was potential absence of correlation between the MPs toxicity and the effect size of MPs on the TM. Utilizing gene set enrichment analysis (GSEA), we further identified a marked suppression of molecular pathways and entities crucial to individual growth and development in the TM-MPs consortium compared to exposure to TM alone. The present study provides important insights about the influence of MPs on cyanobacteria toxicity and the prediction the risk of harmful algal blooms in aquatic ecosystems.

Nanoplastics as Gene and Epigenetic Modulators of Endocrine Functions: A Perspective

Nanoplastics (NPs) represent a major challenge in environmental contamination resulting from the physical, chemical, and biological degradation of plastics. Their characterization requires advanced and expensive methods, which limit routine analyses. The biological effects of NPs depend on their chemical and physical properties, which influence toxicity and interactions with biological systems. Studies in animal models, such as Daphnia magna and Danio rerio, show that NPs induce oxidative stress, inflammation, DNA damage, and metabolic alterations, often related to charge and particle size. NPs affect endocrine functions by acting as endocrine disruptors, interfering with thyroid and sex hormones and showing potential transgenerational effects through epigenetic modifications, including DNA hyper- and hypomethylation. Behavioral and neurofunctional alterations have been observed in Danio rerio and mouse models, suggesting a link between NP exposure and neurotransmitters such as dopamine and serotonin. Despite limited human studies, the presence of NPs in breast milk and placenta underscores the need for further investigation of health effects. Research focusing on genetic and epigenetic markers is encouraged to elucidate the molecular mechanisms and potential risks associated with chronic exposure.

p38 MAPK determines the sensitivity of the aquatic keystone species Moina macrocopa to toxic Microcystis: Insights into potential biomarker applications

Harmful cyanobacterial blooms, particularly those caused by Microcystis species, pose significant ecological threats to freshwater environments by negatively impacting zooplankton populations, essential components of aquatic food webs. Understanding the molecular mechanisms underlying zooplankton responses to these toxic blooms is crucial for assessing and mitigating these impacts. The mitogen-activated protein kinase (MAPK) pathway, known for its critical role in stress response signaling, offers a promising area of study to elucidate these mechanisms. However, the specific involvement of MAPK in zooplankton responses to cyanobacterial stress remains unclear. In this study, we identify and characterize the p38 MAPK gene (MmMAPK) from the zooplankton Moina macrocopa. The gene contains conserved structural elements typical of MAPKs, including a Thr-Gly-Tyr (TGY) motif and a substrate-binding site, Ala-Thr-Arg-Trp (ATRW), indicating its potential functional relevance in stress signaling pathways. Expression analysis reveals a significant upregulation of MmMAPK in M. macrocopa upon exposure to toxic Microcystis, suggesting its role in mediating the organism's stress response. Furthermore, RNA interference (RNAi) experiments demonstrate that knockdown of MmMAPK results in reduced survival and decreased body size, particularly under cyanobacterial stress, underscoring its importance in maintaining stress sensitivity. These findings provide new insights into the molecular mechanisms by which M. macrocopa responds to harmful algal blooms and highlight the potential of MmMAPK as a biomarker for ecological risk assessment and management of cyanobacterial pollution in freshwater ecosystems.

Inhibition of gut digestive proteases by cyanobacterial diets decreases infection in a Daphnia host-parasite system

Secondary metabolites produced by primary producers have a wide range of functions as well as indirect effects outside the scope of their direct target. Research suggests that protease inhibitors produced by cyanobacteria influence grazing by herbivores and may also protect against parasites of cyanobacteria. In this study, we asked whether those same protease inhibitors produced by cyanobacteria could also influence the interactions of herbivores with their parasites. We used the Daphnia-Metschnikowia zooplankton host-fungal parasite system to address this question because it is well documented that cyanobacteria protease inhibitors suppress trypsin and chymotrypsin in the gut of Daphnia, and because it is known that Metschnikowia infects via the gut. We tested the hypothesis that Daphnia gut proteases are necessary for Metschnikowia spores to be released from their asci. We then also tested whether diets that decrease trypsin and chymotrypsin activity in the guts of Daphnia lead to lower levels of infection. Our results show that chymotrypsin promotes the release of the fungal spores from their asci. Moreover, a diet that strongly inhibited chymotrypsin activity in Daphnia decreased infection levels, particularly in the most susceptible Daphnia clones. Our results support the growing literature that cyanobacterial diets can be beneficial to zooplankton hosts when challenged by parasites and uncover a mechanism that contributes to the protective effect of cyanobacterial diets. Specifically, we demonstrate that host chymotrypsin enzymes promote the dehiscence of Metschnikowia spores; when cyanobacteria inhibit the activity of chymotrypsin in hosts, this most likely traps the spore inside the ascus, preventing the parasite from puncturing the gut and beginning the infection process. This study illustrates how secondary metabolites of phytoplankton can protect herbivores against their own enemies.

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.

Experimental Studies on Zooplankton-Toxic Cyanobacteria Interactions: A Review

Cyanobacterial blooms have been recognized as a problem in fresh water for about 150 years. Over the past 50 years, experimental studies on the subject have gained importance considering the increasing need to control toxic cyanobacterial blooms. This article presents information on the different lines of research that have been undertaken on zooplankton-cyanobacteria interactions over the past 50 years. These include information on filtering/ingestion rates and phytoplankton preferences of small and large rotifers, cladocerans, and copepods; growth rates of zooplankton on cyanobacterial diets; feeding rates of other freshwater invertebrates on cyanobacteria; role of zooplankton in top-down biomanipulation efforts; effect of cyanotoxins on zooplankton; bioaccumulation of cyanotoxins; and physical and chemical control of cyanobacterial blooms. We also highlight measures that have led to successful lake management and improvement of water quality in selected waterbodies.