Climate change is increasing the frequency and severity of heatwaves, posing a significant threat to aquatic organisms globally. During an extended heatwave, aquatic organisms, like fish, may use phenotypic plasticity to help offset the effects of increased temperatures and associated low oxygen. The ability of a population to use plasticity to cope with heatwaves could be key for predicting species responses to climate change, especially in species of conservation concern like white sturgeon. We assessed the tolerance of juvenile white sturgeon from an endangered population to heatwave conditions and the effects of heatwave acclimation on subsequent stressors. We measured both thermal and hypoxia performance paired with underlying epigenetic and transcriptional mechanisms to assess the resilience of white sturgeon to heatwaves. Sturgeon exposed to simulated heatwave conditions had increased thermal tolerance and showed complete compensation for the effects of acute hypoxia. These changes were associated with increases in mRNA levels of genes associated with thermal and hypoxic stress. Global DNA methylation was sensitive to heatwave acclimation, changing in the gill and heart over the course of the heatwave exposure. Additionally, we observed rapid changes in DNA methylation during stressor trials over the course of an hour. These data demonstrate tremendous resilience in juvenile white sturgeon to heatwaves which was associated with improved cross-tolerance and responses in epigenetic and transcriptional mechanisms.

From the epigenome to performance: heatwave resilience through impressive thermal plasticity in white sturgeon

Atlantic killifish are found in estuarine habitats along the Atlantic coast of North America. In these habitats, fish are exposed to thermal variation at multiple scales - at timescales from days to seasons, and across latitudes, such that the northern subspecies is locally adapted to cold temperatures, and the southern subspecies to warm temperatures. The northern and southern subspecies differ substantially in mitochondrial properties, with mitochondria from northern fish having greater respiratory capacity, which likely helps to support their greater whole-organism aerobic metabolic rates. However, this pattern is only observed in mitochondria from the liver, and not in mitochondria from the heart or the brain, emphasizing the importance of examining mitochondrial properties across tissues. We also find differences in mitochondrial oxygen affinity between species (assayed in liver mitochondria), with mitochondria from southern fish having greater oxygen affinity, consistent with their superior whole organism hypoxia tolerance. In addition to the putatively adaptive variation, both subspecies demonstrate substantial plasticity in mitochondrial properties. Acclimation to high temperature results in a strong suppression of mitochondrial capacity in both subspecies, mediated upstream of cytochrome c oxidase, which may help to compensate for the effects of acute exposure to high temperature on reaction rates.  In contrast, acclimation to low temperatures causes relatively modest increases in mitochondrial capacity, but in the northern subspecies only, it also causes substantial increases in mitochondrial density. These data demonstrate the many ways that mitochondrial properties are shaped by both plasticity and adaptation in this small fish.

Plasticity and adaptation in mitochondrial function in killifish

Phenotypic plasticity allows species to cope with fluctuations in their environment. It is thus seen as the main route by which species may adjust to rapidly changing environmental conditions. However, there are still many unanswered questions regarding the capacity it provides for adjusting to anthropogenic stressors. For instance, why are some species and some traits more plastic than others, why are some stressors more likely to induce a plastic response than others, how do the mechanisms underlying intergenerational plasticity compare to those operating within an individual and why is there interspecific variation in the plasticity rate (i.e. the rate at which the phenotype changes)? Of particular relevance in a changing world is the plastic phenotypic response referred to as acclimation. The capacity of animals to cope with heat waves is likely to depend on their acclimation capacity but also their rate of acclimation. Despite frequent tests of acclimation (particularly in relation to thermal challenges), understudied aspects of acclimation remain. For example, what physiological mechanisms underlie acclimation? How long do animals require to fully acclimate? And what limits acclimation capacity? We are interested in showcasing the dynamics of acclimation and underlying mechanisms that allow animals to acclimate to various environments, as well as the factors which may affect how they acclimate. The session will first cover general phenotypic plasticity, with a more specific focus on the dynamics and mechanisms of thermal acclimation in the second session.

**Session Organisers:** Dr. Zara-Louise Cowan, Dr. Anna H. Andreassen, Jeremy De Bonville, Dr. Tim Burton, Prof. Neil B. Metcalfe and Dr. Jose C. Noguera

**Invited Speakers:** Dr. Madison Earhart, Ivan Gomez-Mestre, Dr. Jennifer Hellmann and Dr. Alex Little

This session has kindly been sponsored by 

**American Physiological Society** 

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**PyroScience**

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A18 - Phenotypic Plasticity and Acclimation Mechanisms in a Changing World

technical paper

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Mitochondrial metabolim is increasing recognized as a key mechanism that influences animal perfomance and adaptation to environmental conditions. Mitochondrial metabolic traits can be very plastic, but consistent differences still exist among individulas. This is probably due to the convoluted nature of mitchondrial proteins encoded by two different genomes but also to the ability of animals to respond to these environmental cues. Accounting for variation in mitochondrial genotype and phenotype is thus required to fully grasp the role of mitochandra in a braod range of organisms experiencing environmental changes and to explain the source of consistent variation both between and within populations. In this session , we will review the current knowledge of adjustments and adaptations allowing different organisms to cope with their environment at the mitochondrial level.

A14: Mitochondrial plasticity and adaptation: proximate determinants and ultimate consequences for adjustments to environmental conditions

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SEB Annual Conference 2022

2022 will find over 1500 experimental biologists from across the globe converging on the third-largest French city near the Mediterranean coast and the fastest-growing city in the country over the past 25 years - Montpellier, France

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From the epigenome to performance: heatwave resilience through impressive thermal plasticity in white sturgeon

Plasticity and adaptation in mitochondrial function in killifish

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