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Editorial: Uncovering physiological mechanisms of aquatic organisms under environmental stress in a changing climate
Summary
This collection of studies shows that ocean warming, acidification, pollution, and even bad diets are stressing marine and freshwater animals, from tuna to whales to crabs, in ways that often cause hidden damage long before any visible signs of harm appear, sometimes predicting death or reproductive failure. It also finds that combined stressors (like warmer water plus nanoplastics) tend to make things worse together than either would alone, a "Trojan horse" effect, which matters for humans too, since these same waterways supply our seafood and drinking water, and pollutants that harm fish physiology may behave similarly in our own bodies. The
Marine systems dominate this collection, with five experimental studies spanning contrasting taxa and stressors. Liu et al. examined acute cold stress in juvenile yellowfin tuna (Thunnus albacares), uncovering divergent antioxidant, metabolic and transcriptional trajectories in red and white muscle: antioxidant activities peaked early and then declined, while red muscle sustained induction of hspa1b and acadm and white muscle responded faster but proved more vulnerable to damage. Spencer et al. contrasted 8-hour and 88-day transcriptomic responses of juvenile snow crab (Chionoecetes opilio) to ocean acidification, separating short-term tolerance from chronic stress. The early response mobilized mitochondrial repair, cuticle maintenance and immune modulation; after 88 days, however, sustained upregulation of damage-mitigation pathways under severe acidification revealed a chronic stress state that preceded any outward physiological effect and anticipated later mortality. Their identification of a putative carbonic anhydrase 7 gene as a candidate biomarker illustrates the practical payoff of such designs.Two further contributions show that climate impacts often reach organisms indirectly, and that ontogeny governs who is exposed to the greatest risk. Silva et al. mapped critical thermal maxima across the life cycle of the stenophagous nudibranch Berghia stephanieae, finding juveniles to be the least tolerant stage, thermal tolerance plasticity to be minimal, and embryos to fail under prolonged heatwave conditions. In a companion study, Silva et al. showed that a marine heatwave combined with a diet of bleached prey impaired the cellular stress response, reduced egg output and lowered survival; a performance breakdown driven as much by what the animal ate as by temperature itself. Specialized consumers, rarely included in climate vulnerability assessments, may therefore pay a disproportionate toll.Finally, Mantor et al. confront a persistent constraint: the species of greatest conservation concern are often the least tractable experimentally. Using precision-cut adipose tissue slices from humpback whale blubber, they showed that ex vivo cortisol exposure, alone or combined with epinephrine, upregulated PPARG and downregulated TNF and TLR4, indicating that stress hormones drive blubber towards an anti-inflammatory state with potential immune consequences. Beyond the finding itself, the approach offers a nonlethal, reduction-aligned platform for probing multiple stressors in free-living cetaceans.The two freshwater contributions are reviews, and both converge on the same diagnosis from different starting points: single-stressor designs no longer describe the conditions organisms actually face. Ale et al. synthesized nanoplastic ecotoxicity under pollutant coexposure and climate-derived stressors, finding synergism (frequently mediated by a Trojan horse mechanism) to be the dominant reported interaction, and rising temperature to amplify nanoplastic toxicity across taxonomic groups. The available evidence, however, rests heavily on Chlorella spp., Daphnia magna and Danio rerio, leaving molluscs and macrocrustaceans as critical blind spots under climate change scenarios. Wickramasinghe and Ruvinda reach a parallel conclusion regionally, reviewing morphological, physiological and behavioral biomarker responses in Sri Lankan lotic fishes exposed to complex pollutant mixtures, and identifying the interplay between emerging contaminants (nanoparticles, microplastics, pharmaceuticals and endocrine disruptors) and climate-related physicochemical variation as the principal gap for indigenous species. Both reviews underscore that inland waters concentrate contaminant loads while offering organisms fewer thermal refuges than open marine systems, so interactive effects are likely to be both stronger and less easily escaped.Read together, these contributions point to three priorities. First, temporal design deserves explicit justification. Tolerance measured over hours is not tolerance: both Liu et al. and Spencer et al. show that conclusions invert depending on the exposure window, and studies should state the timescales over which their inferences hold. Second, early-warning biomarkers need cross-taxon validation. Several contributions independently converge on molecular signatures that precede organismal failure; linking these to survival and reproductive endpoints would convert them from descriptive readouts into usable tools for risk assessment. Third, taxonomic and geographic representation must be widened. Model species dominate, ecologically relevant taxa remain largely untested under realistic coexposure, and tropical and Southern Hemisphere freshwater systems are conspicuously underrepresented.Meeting these challenges will require multi-stressor experimental frameworks built on environmentally realistic concentrations and climate scenarios, coupled with predictive approaches capable of anticipating interaction outcomes rather than documenting them case by case. The physiological mechanisms explored here are, ultimately, the currency in which climate change is paid. We thank all authors and reviewers whose work made this Research Topic possible.