Double whammy: Nitrate pollution heightens susceptibility to both hypoxia and heat in a freshwater salmonid

The vulnerability of sharks, skates, and rays to ocean deoxygenation: Physiological mechanisms, behavioral responses, and ecological impacts

Levels of dissolved oxygen in open ocean and coastal waters are decreasing (ocean deoxygenation), with poorly understood effects on marine megafauna. All of the more than 1000 species of elasmobranchs (sharks, skates, and rays) are obligate water breathers, with a variety of life-history strategies and oxygen requirements. This review demonstrates that although many elasmobranchs typically avoid hypoxic water, they also appear capable of withstanding mild to moderate hypoxia with changes in activity, ventilatory responses, alterations to circulatory and hematological parameters, and morphological alterations to gill structures. However, such strategies may be insufficient to withstand severe, progressive, or prolonged hypoxia or anoxia where anaerobic metabolic pathways may be used for limited periods. As water temperatures increase with climate warming, ectothermic elasmobranchs will exhibit elevated metabolic rates and are likely to be less able to tolerate the effects of even mild hypoxia associated with deoxygenation. As a result, sustained hypoxic conditions in warmer coastal or surface-pelagic waters are likely to lead to shifts in elasmobranch distributions. Mass mortalities of elasmobranchs linked directly to deoxygenation have only rarely been observed but are likely underreported. One key concern is how reductions in habitat volume as a result of expanding hypoxia resulting from deoxygenation will influence interactions between elasmobranchs and industrial fisheries. Catch per unit of effort of threatened pelagic sharks by longline fisheries, for instance, has been shown to be higher above oxygen minimum zones compared to adjacent, normoxic regions, and attributed to vertical habitat compression of sharks overlapping with increased fishing effort. How a compound stressor such as marine heatwaves alters vulnerability to deoxygenation remains an open question. With over a third of elasmobranch species listed as endangered, a priority for conservation and management now lies in understanding and mitigating ocean deoxygenation effects in addition to population declines already occurring from overfishing.

Impacts of sequential salinity and heat stress are recovery time-specific in freshwater crustacean, Daphnia pulicaria

Stressors can interact to affect animal fitness, but we have limited knowledge about how temporal variation in stressors may impact their combined effect. This limits our ability to predict the outcomes of pollutants and future dynamic environmental changes. Elevated salinity in freshwater ecosystems has been observed worldwide. Meanwhile, heatwaves have become more frequent and intensified as an outcome of climate change. These two stressors can jointly affect organisms; however, their interaction has rarely been explored in the context of freshwater ecosystems. We conducted lab experiments using Daphnia pulicaria, a key species in lakes, to investigate how elevated salinity and heatwave conditions collectively affect freshwater organisms. We also monitored the impacts of various recovery times between the two stressors. Daphnia physiological conditions (metabolic rate, Na+-K+-ATPase (NKA) activity, and lipid peroxidation level) and life history traits (survival, fecundity, and growth) in response to salt stress as well as mortality in heat treatment were examined. We found that Daphnia responded to elevated salinity by upregulating NKA activity and increasing metabolic rate, causing a high lipid peroxidation level. Survival, fecundity, and growth were all negatively affected by this stressor. These impacts on physiological conditions and life history traits persisted for a few days after the end of the exposure. Heat treatments caused mortality in Daphnia, which increased with rising temperature. Results also showed that individuals that experienced salt exposure were more susceptible to subsequent heat stress, but this effect decreased with increasing recovery time between stressors. Findings from this work suggest that the legacy effects from a previous stressor can reduce individual resistance to a subsequent stressor, adding great difficulties to the prediction of outcomes of multiple stressors. Our work also demonstrates that cross-tolerance/susceptibility and the associated mechanisms remain unclear, necessitating further investigation.

Toxic effects of combined exposure to cadmium and nitrate on intestinal morphology, immune response, and microbiota in juvenile Japanese flounder (Paralichthys olivaceus)

Cadmium (Cd2+) and nitrate (NO3-) are important environmental pollutants in the offshore marine ecological environment. However, limited research has explored their combined effects, particularly regarding their impact on the microbiota and intestinal health of marine fish. In this study, juvenile Japanese flounders (P. olivaceus) were immersed in seawater samples with different combinations of Cd2+ (0, 0.2, and 2 mg/L) and NO3- (0 and 80 mg/L NO3N) for 30 days to explore their toxic impacts on intestinal morphology, tight junction (TJ) barrier, immune response, and microbiota. Our results showed that Cd2+ or NO3- exposure alone led to histopathological damage of the gut, while their co-exposure aggravated intestinal damage. Moreover, co-exposure substantially decreased TJ-related gene expression, including occludin, claudin-10, and ZO-2, suggesting increased TJ permeability in the gut. Regarding the immune response, we observed upregulated expression of immune-related markers such as HSP40, IL-1β, TNF-α, and MT, suggesting the onset of intestinal inflammation. Furthermore, Cd2+ and NO3- exposure led to changes in intestinal microflora, characterized by decreased the abundance of Sediminibacterium and NS3a_marine_group while increasing the prevalence of pathogens or opportunistic pathogens such as Ralstonia, Proteus, and Staphylococcus. This alteration in microbiota composition increased network complexity and α-diversity, ultimately causing dysbiosis in the fish gut. Additionally, combined exposure resulted in metabolic disorders that affected the predicted functions of the intestinal microbiota. Overall, our study demonstrates that Cd2+-NO3- co-exposure amplifies the deleterious effects compared to single exposure. These findings enhance our understanding of the ecological risks posed by Cd2+-NO3- co-exposure in marine ecosystems.

Multi-omics provide mechanistic insight into the Pb-induced changes in tadpole fitness-related traits and environmental water quality

Water pollution from lead/Pb²⁺ poses a significant threat to aquatic ecosystems, and its repercussions on aquatic animals have received considerable attention. Although Pb²⁺ has been found to affect numerous aspects of animals, including individual fitness, metabolic status, and symbiotic microbiota, few studies have focused on the associations between Pb²⁺-induced variations in fitness, metabolome, symbiotic microbiome, and environmental parameters in the same system, limiting a comprehensive understanding of ecotoxicological mechanisms from a holistic perspective. Moreover, most ecotoxicological studies neglected the potential contributions of anions to the consequences generated by inorganic lead compounds. We investigated the effects of Pb(NO₃)₂ at environmentally relevant concentrations on the Rana omeimontis tadpoles and the water quality around them, using blank and NaNO₃-treated groups as control. Results showed that Pb(NO₃)₂ not only induced a rise in water nitrite level, but exposure to this chemical also impaired tadpole fitness-related traits (e.g., growth and development). The impacts on tadpoles were most likely a combination of Pb²⁺ and NO₃^-. Tissue metabolomics revealed that Pb(NO₃)₂ exposure influenced animal substrate (i.e., carbohydrate, lipid, and amino acid) and prostaglandin metabolism. Pb(NO₃)₂ produced profound shifts in gut microbiota, with increased Proteobacteria impairing Firmicutes, resulting in higher aerobic and possibly pathogenic bacteria. NaNO₃ also influenced tadpole metabolome and gut microbiome, in a manner different to that of Pb(NO₃)₂. The presence of NO₃^- seemed to counteract some changes caused by Pb²⁺, particularly on the microbiota. Piecewise structural equation model and correlation analyses demonstrated connections between tissue metabolome and gut microbiome, and the variations in tadpole phenotypic traits and water quality were linked to changes in tissue metabolome and gut microbiome. These findings emphasized the important roles of gut microbiome in mediating the effects of toxin on aquatic ecosystem. Moreover, it is suggested to consider the influences of anions in the risk assessment of heavy metal pollutions.

Exposure to nitrate induces alterations in blood parameter responses, liver immunity, and lipid metabolism in juvenile turbot (Scophthalmus maximus)

Nitrate (NO₃^-) pollution of waterbodies has attracted significant global attention as it poses a serious threat to aquatic organisms and human beings. This study aimed to evaluate the role of NO₃^-, an end product of biological nitrification processes, in immune status and lipid metabolism to have a comprehensive understanding of its toxic effects on fishes. Therefore, in this work, juvenile turbot (Scophthalmus maximus) were subjected to four nominal concentrations of NO₃^- (i.e., 0, 50, 200, 400 mg/L of NO₃^--N) for a 60-day period. The results indicated that increased exposure to NO₃^- (200 and/or 400 mg/L) enhanced the concentrations of plasma heat shock protein concentrations (HSP70), complement component 3 (C3), complement component 4 (C4), immunoglobulin M (IgM) and lysozyme (LYS), which meant that NO₃^-caused fluctuations in the plasma immune system. Higher exposure to NO₃^- (200 and/or 400 mg/L) also caused significant enhancements in plasma glutamic pyruvic transaminase (GPT), as well as glutamic oxaloacetic transaminase (GOT) activity. Furthermore, NO₃^- exposure resulted in upregulation of liver TNF-α, IL-1β, HSP70, HSP90, and LYS. Additionally, the results suggested that NO₃^-exposure caused a certain degree of histological damage and inflammation in the liver and activated the immune defense processes of juvenile turbot. Furthermore, the mRNA expression levels of certain genes associated with lipid metabolism (peroxisome proliferator-activated receptor-alpha [PPAR-α], carnitine palmitoyltransferase 1[CPT1], liver X receptor [LXR] together with sterol regulatory element binding protein-1 [SREBP-1]) increased significantly within fish liver exposed to 200/400 mg/L NO₃^--N treatments. Finally, the results obtained from the analysis of the integrated biological responses version 2 (IBRv2) also confirmed the toxic effects of NO₃^- on juvenile turbot. According to these findings, it can be found that NO₃^- emission in the aquatic environment needs to be strictly controlled, as it may cause immune and lipid metabolism disorders in fish.

Exercise training does not affect heat tolerance in Chinook salmon (Oncorhynchus tshawytscha)

The progression of climate warming will expose ectotherms to transient heatwave events and temperatures above their tolerance range at increased frequencies. It is therefore pivotal that we understand species' physiological limits and the capacity for various controls to plastically alter these thresholds. Exercise training could have beneficial impacts on organismal heat tolerance through improvements in cardio-respiratory capacity, but this remains unexplored. Using juvenile Chinook salmon (Oncorhynchus tshawytscha), we tested the hypothesis that exercise training improves heat tolerance through enhancements in oxygen-carrying capacity. Fish were trained once daily at 60% of their maximum sustainable swim speed, U_CRIT, for 60 min. Tolerance to acute warming was assessed following three weeks of exercise training, measured as the critical thermal maximum (CT_MAX). CT_MAX measurements were coupled with examinations of the oxygen carrying capacity (haematocrit, haemoglobin concentration, relative ventricle size, and relative splenic mass) as critical components of the oxygen transport cascade in fish. Contrary to our hypothesis, we found that exercise training did not raise the CT_MAX of juvenile Chinook salmon with a mean CT_MAX increase of just 0.35 °C compared to unexercised control fish. Training also failed to improve the oxygen carrying capacity of fish. Exercise training remains a novel strategy against acute warming that requires substantial fine-tuning before it can be applied to the management of commercial and wild fishes.

Fire and rain: A systematic review of the impacts of wildfire and associated runoff on aquatic fauna

Climate and land-use changes are expected to increase the future occurrence of wildfires, with potentially devastating consequences for freshwater species and ecosystems. Wildfires that burn in close proximity to freshwater systems can significantly alter the physicochemical properties of water. Following wildfires and heavy rain, freshwater species must contend with complex combinations of wildfire ash components (nutrients, polycyclic aromatic hydrocarbons, and metals), altered light and thermal regimes, and periods of low oxygen that together can lead to mass mortality events. However, the responses of aquatic fauna to wildfire disturbances are poorly understood. Here we provide a systematic review of available evidence on how aquatic animals respond to and recover from wildfire disturbance. Two databases (Web of Science and Scopus) were used to identify key literature. A total of 83 studies from across 11 countries were identified to have assessed the risk of wildfires on aquatic animals. We provide a summary of the main ecosystem-level changes associated with wildfires and the main responses of aquatic fauna to such disturbances. We pay special focus to physiological tools and biomarkers used to assess how wildfires impact aquatic animals. We conclude by providing an overview of how physiological biomarkers can further our understanding of wildfire-related impacts on aquatic fauna, and how different physiological tools can be incorporated into management and conservation plans and serve as early warning signs of wildfire disturbances.

Better together: Cross-tolerance induced by warm acclimation and nitrate exposure improved the aerobic capacity and stress tolerance of common carp Cyprinus carpio

Climate warming is a threat of imminent concern that may exacerbate the impact of nitrate pollution on fish fitness. These stressors can individually affect the aerobic capacity and stress tolerance of fish. In combination, they may interact in unexpected ways where exposure to one stressor may heighten or reduce the resilience to another stressor and their interactive effects may not be uniform across species. Here, we examined how nitrate pollution under a warming scenario affects the aerobic scope (AS), and the hypoxia and heat stress susceptibility of a generally tolerant fish species, common carp Cyprinus carpio. We used a 3 × 2 factorial design, where fish were exposed to one of three ecologically relevant levels of nitrate (0, 50, or 200 mg NO₃^- L^-1) and one of two temperatures (18 °C or 26 °C) for 5 weeks. Warm acclimation increased the AS by 11% due to the maintained standard metabolic rate and increased maximum metabolic rate at higher temperature, and the AS improvement seemed greater at higher nitrate concentration. Warm-acclimated fish exposed to 200 mg NO₃^- L^-1 were less susceptible to acute hypoxia, and fish acclimated at higher temperature exhibited improved heat tolerance (critical thermal maxima, CTMax) by 5 °C. This cross-tolerance can be attributed to the hematological results including maintained haemoglobin and increased haematocrit levels that may have compensated for the initial surge in methaemoglobin at higher nitrate exposure.

Adding climate change to the mix: responses of aquatic ectotherms to the combined effects of eutrophication and warming

The threat of excessive nutrient enrichment, or eutrophication, is intensifying across the globe as climate change progresses, presenting a major management challenge. Alterations in precipitation patterns and increases in temperature are increasing nutrient loadings in aquatic habitats and creating conditions that promote the proliferation of cyanobacterial blooms. The exacerbating effects of climate warming on eutrophication are well established, but we lack an in-depth understanding of how aquatic ectotherms respond to eutrophication and warming in tandem. Here, I provide a brief overview and critique of studies exploring the cumulative impacts of eutrophication and warming on aquatic ectotherms, and provide forward direction using mechanistically focused, multi-threat experiments to disentangle complex interactions. Evidence to date suggests that rapid warming will exacerbate the negative effects of eutrophication on aquatic ectotherms, but gradual warming will induce physiological remodelling that provides protection against nutrients and hypoxia. Moving forward, research will benefit from a greater focus on unveiling cause and effect mechanisms behind interactions and designing treatments that better mimic threat dynamics in nature. This approach will enable robust predictions of species responses to ongoing eutrophication and climate warming and enable the integration of climate warming into eutrophication management policies.

The Atlantic salmon's stress- and immune-related transcriptional responses to moderate hypoxia, an incremental temperature increase, and these challenges combined

The marine environment is predicted to become warmer, and more hypoxic, and these conditions may negatively impact the health and survival of coastal fish species, including wild and farmed Atlantic salmon (Salmo salar). Thus, we examined how: (1) moderate hypoxia (∼70% air saturation) at 12°C for 3 weeks; (2) an incremental temperature increase from 12°C to 20°C (at 1°C week-1) followed by 4 weeks at 20°C; and (3) treatment "2" combined with moderate hypoxia affected transcript expression in the liver of post-smolts as compared to control conditions (normoxia, 12°C). Specifically, we assessed the expression of 45 genes related to the heat shock response, oxidative stress, apoptosis, metabolism and immunity using a high-throughput qPCR approach (Fluidigm Biomark™ HD). The expression profiles of 27 "stress"-related genes indicated that: (i) moderate hypoxia affected the expression of several stress genes at 12°C; (ii) their expression was impacted by 16°C under normoxic conditions, and this effect increased until 20°C; (iii) the effects of moderate hypoxia were not additive to those at temperatures above 16°C; and (iv) long-term (4 weeks) exposure to 20°C, with or without hypoxia, resulted in a limited acclimatory response. In contrast, the expression of 15 immune-related genes was not greatly affected until temperatures reached 20°C, and this effect was particularly evident in fish exposed to the added challenge of hypoxia. These results provide valuable information on how these two important environmental factors affect the "stress" physiology and immunology of Atlantic salmon, and we identify genes that may be useful as hypoxia and/or temperature biomarkers in salmonids and other fishes.