Aquatic organisms rely on microbial symbionts for coping with various challenges they encounter during stress and for defending themselves against predators, pathogens and parasites. Microbial symbionts are also often indispensable for the host's development or life cycle completion. Many aquatic ecosystems are currently under pressure due to diverse human activities that have a profound impact on ecosystem functioning. These human activities are also ex pected to alter interactions between aquatic hosts and their associated microbes. This can directly impact the host's health and -given the importance and widespread occurrence of microbial symbiosis in aquatic systems -the ecosystem at large. In this review, we provide an overview of the importance of microbial symbionts for aquatic organisms, and we consider how the beneficial services provided by microbial symbionts can be affected by human activities. The scarcity of available studies that assess the functional consequences of human impacts on aquatic microbial symbioses shows that our knowledge on this topic is currently limited, making it difficult to draw general conclusions and predict future changes in microbial symbiont−host relationships in a changing world. To address this important knowledge gap, we provide an overview of ap proaches that can be used to assess the impact of human disturbances on the functioning of aquatic microbial symbioses.
The depletion of oxygen as a result of increased stratification and decreased oxygen solubility is one of the most significant chemical changes occurring in aquatic ecosystems as a result of global environmental change. Hence, more aquatic organisms will be exposed to hypoxic conditions over time. Deciphering the effects of hypoxia on strong ecological interactors in this ecosystem's food web is critical for predicting how aquatic communities can respond to such an environmental disturbance. Here, (sub-)lethal effects of hypoxia and whether these are genotype specific in Daphnia, a keystone species of freshwater ecosystems, are studied. This is especially relevant upon studying genetic responses with respect to phenotypic switches (G x E interactions) upon environmental stress. Further, we investigated the effect of hypoxia on the Daphnia microbial community to test if the microbiome plays a role in the phenotypic switch and tolerance to hypoxia. For this, two Daphnia genotypes were exposed for two weeks to either hypoxia or normoxia and host performance was monitored together with changes in the host associated and free-living microbial community after this period. We found G x E interactions for some of the tested Daphnia performance traits. The microbial community responded to hypoxia stress with responses in the bacterioplankton and in the Daphnia associated microbial community with respect to species richness and community composition and structure. The latter response was different for the two genotypes suggesting that the microbiome plays an important role in G x E interactions with respect to hypoxia tolerance in Daphnia, but further testing (e.g. through microbiome transplants) is needed to confirm this.
Organisms are increasingly facing multiple, potentially interacting stressors in natural populations. The ability of populations coping with combined stressors depends on their tolerance to individual stressors and how stressors interact, which may not be correctly captured in controlled laboratory settings. One largely unexplored reason for this is that the microbial communities in laboratory settings often differ from the natural environment, which could result in different stressor responses and interaction patterns. In this study, we investigated the impact of single and combined exposure to a toxic cyanobacterium and an oomycete-like parasite on the performance of three Daphnia magna genotypes. Daphnia individuals were first sterilized and then experimentally given a natural or a laboratory-derived microbial inoculum. Survival, reproduction and body size were monitored for three weeks and gut microbiomes were sampled and characterized at the end of the experiment. Our study confirmed that natural and laboratory microbial inocula and gut microbiomes are differently structured with natural microbiomes being more diverse than laboratory microbiomes. Our results showed that exposure to the stressors reduced D. magna performance compared to the controlD. magna not exposed to any stressor. An antagonistic interaction between the two biotic stressors was revealed with respect to D. magna survival when Daphnia individuals were exposed to the laboratory microbial inoculum. This effect was consistent across all three genotypes. In Daphnia exposed to a natural microbial inoculum this antagonistic interaction could not be detected and the genotype x exposure interaction was genotype-dependent. Our results indicate that host-stressor interactions depend on the microbial inoculum and that the gut microbiome potentially has a strong role in this, thereby providing a largely unexplored dimension to multiple-stressor research.
The depletion of oxygen as a result of increased stratification and decreased oxygen solubility is one of the most significant chemical changes occurring in aquatic ecosystems as a result of global environmental change. Hence, more aquatic organisms will be exposed to hypoxic conditions over time. Deciphering the effects of hypoxia on strong ecological interactors in this ecosystem’s food web is critical for predicting how aquatic communities can respond to such an environmental disturbance. Here (sub-)lethal effects of hypoxia and whether these are genotype specific in Daphnia, a keystone species of freshwater ecosystems, are studied. This is especially relevant upon studying genetic responses with respect to phenotypic switches upon environmental stress. Further, we investigated the effect of hypoxia on the Daphnia microbial community to test if the microbiome plays a role in the phenotypic switch and tolerance to hypoxia. For this, two Daphnia genotypes were exposed for two weeks to either hypoxia or normoxia and host performance was monitored together with changes in the host associated and free-living microbial community after this period. We detected phenotypic plasticity for some of the tested Daphnia performance traits. The microbial community of the bacterioplankton and Daphnia associated microbial community responded via changes in species richness and community composition and structure. The latter response was different for the two genotypes suggesting that the microbiome plays an important role in phenotypic plasticity with respect to hypoxia tolerance in Daphnia, but further testing (e.g., through microbiome transplants) is needed to confirm this.
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