Aquatic ecosystems are under increasing stress from global anthropogenic and natural changes, including climate change, eutrophication, ocean acidification, and pollution. In this critical review, we synthesize research on the microbiota of aquatic vertebrates and discuss the impact of emerging stressors on aquatic microbial communities using two case studies, that of toxic cyanobacteria and microplastics. Most studies to date are focused on host-associated microbiomes of individual organisms, however, few studies take an integrative approach to examine aquatic vertebrate microbiomes by considering both host-associated and free-living microbiota within an ecosystem. We highlight what is known about microbiota in aquatic ecosystems, with a focus on the interface between water, fish, and marine mammals. Though microbiomes in water vary with geography, temperature, depth, and other factors, core microbial functions such as primary production, nitrogen cycling, and nutrient metabolism are often conserved across aquatic environments. We outline knowledge on the composition and function of tissue-specific microbiomes in fish and marine mammals and discuss the environmental factors influencing their structure. The microbiota of aquatic mammals and fish are highly unique to species and a delicate balance between respiratory, skin, and gastrointestinal microbiota exists within the host. In aquatic vertebrates, water conditions and ecological niche are driving factors behind microbial composition and function. We also generate a comprehensive catalog of marine mammal and fish microbial genera, revealing commonalities in composition and function among aquatic species, and discuss the potential use of microbiomes as indicators of health and ecological status of aquatic ecosystems. We also discuss the importance of a focus on the functional relevance of microbial communities in relation to organism physiology and their ability to overcome stressors related to global change. Understanding the dynamic relationship between aquatic microbiota and the animals they colonize is critical for monitoring water quality and population health.

: The microbiome plays an essential role in the health and onset of diseases in all animals, including humans. The microbiome has emerged as a central theme in environmental toxicology because microbes interact with the host immune system in addition to its role in chemical detoxification. Pathophysiological changes in the gastrointestinal tissue caused by ingested chemicals and metabolites generated from microbial biodegradation can lead to systemic adverse effects. The present critical review dissects what we know about the impacts of environmental contaminants on the microbiome of aquatic species, with special emphasis on the gut microbiome. We highlight some of the known major gut epithelium proteins in vertebrate hosts that are targets for chemical perturbation, proteins that also directly cross-talk with the microbiome. These proteins may act as molecular initiators for altered gut function, and we propose a general framework for an adverse outcome pathway that considers gut dysbiosis as a major contributing factor to adverse apical endpoints. We present 2 case studies, nanomaterials and hydrocarbons, with special emphasis on the Deepwater Horizon oil spill, to illustrate how investigations into the microbiome can improve understanding of adverse outcomes. Lastly, we present strategies to functionally relate chemical-induced gut dysbiosis with adverse outcomes because this is required to demonstrate cause–effect relationships. Further investigations into the toxicant–microbiome relationship may prove to be a major breakthrough for improving animal and human health. Environ Toxicol Chem 2018;37:2758–2775. © 2018 SETAC

Among the benzotriazole ultraviolet stabilizers (BUVSs), UV-234 and UV-320 are frequently detected in aquatic ecosystem. Despite the fact that these chemicals are present in low ng/L levels in surface water, they show high bio-accumulation potential and pose exposure risks to aquatic organisms. However, there are limited toxicological data available in fish. In this study, zebrafish embryos were exposed to 0.01, 0.1 and 1 μM UV-234 or UV-320 for up to 6 days. Developmental toxicity as well as effects on mitochondrial bioenergetics, immune system responses, and locomotor activity in zebrafish were measured. After UV-234 treatment (0.1-1 μM), hatching time of embryos was increased compared to controls. There was also a ∼20-40% reduction in non-mitochondrial respiration and oligomycin-dependent mitochondrial respiration in embryos treated with 1 μM UV-234 for 24 and 48 h respectively; conversely basal respiration and non-mitochondrial respiration were increased ∼20-30% in embryos treated with 1 μM UV-320 at 48 h. Transcript levels of sod1 were down-regulated with BUVSs while sod2 mRNA was highly up-regulated with both UV-234 and UV-320, suggesting an oxidative damage response. Considering that mitochondrial signaling regulates innate immune pathways, we measured the expression of immune related transcripts (tlr5a, tlr5b, mmp9, il8, tnfa, cxcl-C1c, nfkb1, and ifng). Of these, only il8 and cxcl-C1c mRNA were decreased in response to 0.1 μM UV-320. To associate early molecular events with behavior, locomotor activity was assessed. UV-234 reduced larval activity in a dark photokinesis assay by ∼15%, however behavioral responses at environmentally-relevant concentrations of BUVSs were not consistent across experiments nor BUVSs. These data suggest that BUVSs can perturb mitochondrial bioenergetics, embryonic development, and locomotor activity of zebrafish, but these responses appear to be dose-, time- and BUVSs dependent, suggesting these chemicals may have unique modes of action.

Bisphenol A alternatives are manufactured as potentially less harmful substitutes of bisphenol A (BPA) that offer similar functionality. These alternatives are already in the market, entering the environment and thus raising ecological concerns. However, it can be expected that levels of BPA alternatives will dominate in the future, they are limited information on their environmental safety. The EU PARC project highlights BPA alternatives as priority chemicals and consolidates information on BPA alternatives, with a focus on environmental relevance and on the identification of the research gaps. The review highlighted aspects and future perspectives. In brief, an extension of environmental monitoring is crucial, extending it to cover BPA alternatives to track their levels and facilitate the timely implementation of mitigation measures. The biological activity has been studied for BPA alternatives, but in a non-systematic way and prioritized a limited number of chemicals. For several BPA alternatives, the data has already provided substantial evidence regarding their potential harm to the environment. We stress the importance of conducting more comprehensive assessments that go beyond the traditional reproductive studies and focus on overlooked relevant endpoints. Future research should also consider mixture effects, realistic environmental concentrations, and the long-term consequences on biota and ecosystems.