Understanding interactive inducible defenses of Daphnia and its phytoplankton prey

Control of Microcystis aeruginosa by Daphnia: Experimental evidence and identification of involved infochemicals

Infochemicals refer to chemicals responsible for information exchange between organisms. We evaluated the effects of Daphnia magna and Daphnia galeata infochemicals on Microcystis aeruginosa for 15d. The Daphnia infochemicals were obtained from spent medium after culturing Daphnia in Elendt M4 medium for 48 h. Both Daphnia infochemicals significantly increased (p < 0.05) the intracellular reactive oxygen species level and microcystin-LR concentration in M. aeruginosa. This cellular effect increased colony formation of M. aeruginosa, thereby inhibiting the growth of M. aeruginosa. D. galeata infochemicals provoked significantly greater (p < 0.05) adverse effects on M. aeruginosa than those of D. magna infochemicals, which were further exaggerated by pre-exposure of Daphnia to M. aeruginosa. This result seems to be related to the different compositions and concentrations of Daphnia infochemicals. Several Daphnia infochemicals, such as methyl ferulate, cyclohexanone, 3, 5-dimethyl, hexanedioic acid, and bis(2-ethylhexyl) ester, showed a high correlation with M. aeruginosa cell concentration (|r | >0.6), suggesting that they may play a key role in controlling harmful cyanobacteria. Additionally, pre-exposure of D. magna and D. galeata to M. aeruginosa produced oleic acid, methyl ester, and n-hexadecanoic acid, with a highly correlation with M. aeruginosa cell concentration (|r | >0.6). p-tolyl acetate and linoleic acid were detected only in the pre-exposed D. galeata infochemicals. These findings suggest that some of Daphnia infochemicals identified in this study can be a promising tool to control M. aeruginosa growth. However, further studies are required to verify the specific actions of these infochemicals against cyanobacteria.

Understanding host-microbiome-environment interactions: Insights from Daphnia as a model organism

Microbes perform a variety of vital functions that are essential for healthy ecosystems, ranging from nutrient recycling, antibiotic production and waste decomposition. In many animals, microbes become an integral part by establishing diverse communities collectively termed as "microbiome/s". Microbiomes defend their hosts against pathogens and provide essential nutrients necessary for their growth and reproduction. The microbiome is a polygenic trait that is dependent on host genotype and environmental variables. However, the alteration of microbiomes by stressful condition and their recovery is still poorly understood. Despite rapid growth in host-associated microbiome studies, very little is known about how they can shape ecological processes. Here, we review current knowledge on the microbiome of Daphnia, its role in fitness, alteration by different stressors, and the ecological and evolutionary aspects of host microbiome interactions. We further discuss how variation in Daphnia physiology, life history traits, and microbiome interactive responses to biotic and abiotic factors could impact patterns of microbial diversity in the total environment, which drives ecosystem function in many freshwater environments. Our literature review provides evidence that microbiome is essential for Daphnia growth, reproduction and tolerance against stressors. Though the core and flexible microbiome of Daphnia is still debatable, it is clear that the Daphnia microbiome is highly dependent on interactions among host genotype, diet and the environment. Different environmental factors alter the microbiome composition and diversity of Daphnia and reduce their fitness. These interactions could have important implications in shaping microbial patterns and their recycling as Daphnia are keystone species in freshwater ecosystem. This review provides a framework for studying these complex relationships to gain a better understanding of the ecological and evolutionary roles of the microbiome.

Novel high throughput sequencing - fluorometric approach demonstrates Microcystis blooms across western Lake Erie are promoted by grazing resistance and nutrient enhanced growth

Cyanobacterial harmful algal blooms (CHABs) are a global public health threat. While CHABs are often promoted by nutrients, an important and often overlooked influence on bloom dynamics is zooplankton grazing. In the present study, zooplankton grazing and nutrient enrichment experiments were combined with next generation sequencing and fluorometric analyses to quantify differential grazing and nutrient effects on specific cyanobacterial genera across the western basin of Lake Erie. Grazing by two different sized daphnids, Daphnia magna and Daphnia pulex, was compared to protozooplankton grazing effects assessed via a dilution approach at sites within the Maumee and Sandusky Bays where Planktothrix, Microcystis, Synechococcus, and Dolichospermum were the dominant genera. Daphnid grazing significantly reduced Synechococcus net growth rates at most sites as well as Planktothrix net growth in Sandusky Bay and Dolichospermum in Maumee Bay. Dilution resulted in significant growth increase of Synechococcus at half of the sites and Planktothrix at most sites evidencing substantial grazing pressure by the protozooplankton community on these genera. In contrast, Microcystis populations were largely unaffected by daphnids and protozooplankton grazing but benefitted from nutrient enrichment more than other CHAB genera. When diatoms were present in moderate abundance, grazing rates by daphnids on diatoms were significantly greater than grazing rates on cyanobacteria. The novel approach used in this study established differences in grazing pressure and nutrient effects on differing taxa and revealed that, while many taxa were grazed by multiple classes of zooplankton (e.g. Planktothrix, Synechococcus, Dolichospermum, diatoms), the lack of grazing pressure on Microcystis coupled with nutrient-enhanced growth in western Lake Erie promotes the occurrence of CHABs of this genus.

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.

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.

Elevated temperature and toxic Microcystis reduce Daphnia fitness and modulate gut microbiota

The gut microbiota has been increasingly recognized to regulate host fitness, which in turn is dependent on stability of community structure and composition. Many biotic and abiotic factors have been demonstrated to shape gut microbiota of cladocerans. However, the interactive effects of these variables on cladocerans fitness due to alteration of gut microbiota and their linkage with life history parameters are poorly understood. Here, we investigated the responses of Daphnia magna gut microbiota to the combined effects of toxic Microcystis aeruginosa and high temperature and its associations with fitness. We found that under good food regime, the temperature has no effect on the composition of the gut microbiota, whereas under high proportion of toxic M. aeruginosa and high temperature conditions, D. magna lost their symbionts. High proportion of toxic M. aeruginosa and high temperature had synergistically negative effects on D. magna performance due to altered gut microbiota. The high abundance of symbiotic Comamonadaceae and good food increased D. magna fitness. The present study illustrates that understanding life history strategies in response to multiple stressors related to changes in the gut microbiota diversity and composition requires integrated approaches that incorporate multiple linked traits and tether them to one another.

Effects of Harmful Blooms of Large-Sized and Colonial Cyanobacteria on Aquatic Food Webs

Cyanobacterial blooms are the most important and best studied type of harmful algal blooms in fresh waters and brackish coastal seas. We here review how and to which extent they resist grazing by zooplankton, how zooplankton responds to cyanobacterial blooms and how these effects are further transmitted to fish. Size, toxicity and poor nutritional value are widespread mechanisms of grazing defense by cyanobacteria. In some cases, defenses are inducible, in some they are obligate. However, to some extent zooplankton overcome grazing resistance, partly after evolutionary adaptation. Cyanotoxins are also harmful to fish and may cause fish kills. However, some fish species feed on Cyanobacteria, are able to reduce their abundance, and grow on a cyanobacterial diet. While reduced edibility for crustacean zooplankton tends to elongate the food chain from primary producers to fish, direct feeding by fish tends to shorten it. The few available comparative studies relating fish yield to nutrients or phytoplankton provide no indication that cyanobacteria should reduce the ratio fish production: primary production.

Cross talk: Two way allelopathic interactions between toxic Microcystis and Daphnia

Due to eutrophication, freshwater ecosystems frequently experience cyanobacterial blooms, many of which produce bioactive metabolites that can affect vertebrates and invertebrates life traits. Zooplankton are able to develop tolerance as a physiological response to cyanobacteria and their bioactive compounds, however, this comes with energetic cost that in turn influence Daphnia life traits and may impair populations. Vice versa, it has been suggested that Daphnia are able to reduce cyanobacterial dominance until a certain cyanobacterial density; it remains unclear whether Daphnia metabolites alone influence the physiological state and bioactive metabolites production of cyanobacteria. Hence, this study investigates mutual physiological reactions of toxic Microcystis aeruginosa PCC7806 and Daphnia magna. We hypothesize that a) the presence of D. magna will negatively affect growth, increase stress response and metabolites production in M. aeruginosa PCC7806 and b) the presence of M. aeruginosa PCC7806 will negatively affect physiological responses and life traits in D. magna. In order to test these hypotheses experiments were conducted in a specially designed co-culture chamber that allows exchange of the metabolites without direct contact. A clear mutual impact was evidenced. Cyanobacterial metabolites reduced survival of D. magna and decreased oxidative stress enzyme activity. Simultaneously, presence of D. magna did not affect photosynthetic activity. However, ROS increase and tendencies in cell density decrease were observed on the same day, suggesting possible energy allocation towards anti-oxidative stress enzymes, or other protection mechanisms against Daphnia infochemicals, as the strain managed to recover. Elevated concentration of intracellular and overall extracellular microcystin MC-LR, as well as intracellular concentrations of aerucyclamide A and D in the presence of Daphnia, indicating a potential protective or anti-grazing function. However, more research is needed to confirm these findings.

Changes in the life history traits of Daphnia magna are associated with the gut microbiota composition shaped by diet and antibiotics

The gut microbiota has a crucial role in host physiology and fitness. Host-microbiota relationships can be disrupted by environmental stressors, which further affect host growth and survival. However, the link between host performance and the gut microbiota composition shaped by increasing antibiotic pollution under different food conditions is not clearly understood. In the present study, we used Daphnia magna as a model organism to investigate the interactive effects of diets (Chlorella with or without Microcystis) and antibiotics on its life history traits, gut microbiota alterations, and their relationship. The results showed that poor diet consumption by D. magna at low and high antibiotic concentrations reduced reproduction and survival. Under good diet conditions, the fitness was reduced only at a high antibiotic concentration. Under good diet conditions, high concentration of antibiotics reduced the abundance of Comamonadaceae and increased the abundance of Pseudomonadaceae, whereas under poor diet conditions, both low and high concentrations of antibiotics increased the abundance of Pseudomonadaceae. Performances of life history traits were positively correlated with an increased abundance of Comamonadaceae but were negatively correlated with increased Pseudomonadaceae abundance. The results of this study revealed the interactive effects of diet and antibiotics on D. magna fitness and correlations between bacterial abundance and life history traits, which has important implications for understanding the effects of pollutants on host-microbiota interactions through changes in phenotypes.

Effects of Microcystis aeruginosa on the life history traits and SOD activity of Daphnia similoides sinensis

With water eutrophication and global warming, cyanobacteria blooms have occurred frequently, and the interaction between M. aeruginosa and Daphnia has been widely paid attention by researchers. However, the effects of toxic M. aeruginosa on the SOD activity of Daphnia are poorly known. Six D. similoides sinensis clones collected from Lake Junshan and the offspring of two clones were employed. The effects of toxic M. aeruginosa on the life history traits and SOD activities of D. similoides sinensis in the mother and their offspring were studied. Toxic M. aeruginosa could significantly inhibit the life history traits (e.g., body lengths, offspring numbers at first reproduction, cumulative offspring numbers, and the intrinsic rate of population) and induce higher SOD activities of D. similoides sinensis. Compared with the mother, the effects of toxic M. aeruginosa on the life history traits and SOD activities of D. similoides sinensis in the offspring showed obvious differences. Moreover, the adaptability of the offspring to M. aeruginosa indicated also the differences between two clones. Our results suggested that the mother exposed to toxic M. aeruginosa could enhance the fitness of their offspring to Microcystis by maternal effect and was also affected by the D. similoides sinensis genotypes.