Chronic Thermal Acclimation Effects on Critical Thermal Maxima (CTmax) and Oxidative Stress Differences in White Epaxial Muscle between Surface and Cave Morphotypes of the Mexican Cavefish (Astyanax mexicanus).

AbstractIn the face of increasing environmental temperatures, operative differences between mitochondrial function and whole-animal phenotypic response to the environment are underrepresented in research, especially in subtemperate ectothermic vertebrates. A novel approach to exploring this connection is to examine model species that are genetically similar but that have different whole-animal phenotypes, each of which inhabits different environments. The blind Mexican cavefish (Astyanax mexicanus) has the following two morphotypes: a surface form found in aboveground rivers and an obligate cave-dwelling form. Each morphotype inhabits vastly different thermal and oxygen environments. Whole-animal and mitochondrial responses to thermal acclimation and oxidative stress, with respect to increasing temperatures, have not been previously determined in either morphotype of this species. Here, we chronically acclimated both morphotypes to three temperatures (14°C, 25°C, and 31°C) to establish potential for acclimation and critical thermal maxima (CTmax) for each morphotype of this species. After measuring CTmax in six cohorts, we additionally measured enzymatic antioxidant capacity (catalase, superoxide dismutase, and glutathione peroxidase activities), peroxyl scavenging capacity, and lipid peroxidation damage in white epaxial muscle for each individual. We found a significant effect of acclimation temperature on CTmax (F=29.57, P<0.001) but no effect of morphotype on CTmax (F=2.092, P=0.162). Additionally, we found that morphotype had a significant effect on glutathione peroxidase activity, with the surface morphotype having increased glutathione peroxidase activity compared with the cave morphotype (F=6.270, P=0.020). No other oxidative stress variable demonstrated significant differences. Increases in CTmax with chronic thermal acclimation to higher temperatures suggests that there is some degree of phenotypic plasticity in this species that nominally occupies thermally stable environments. The decreased glutathione peroxidase activity in the cave morphotype may be related to decreased environmental oxygen concentration and decreased metabolic rate in this environmentally constrained morphotype compared to in its surface-living counterparts.

White epaxial muscle aerobic and anaerobic potential and muscle fiber structure in surface and cave morphotypes of the Mexican cavefish (Astyanax mexicanus).

Proper muscle function and muscle fiber structures that match the environmental demands of organisms are imperative to their success in any ecosystem. The Mexican cavefish, Astyanax mexicanus, has two morphotypes: an obligate cave-dwelling form that lives in thermally insulated caves and an O2 poor environment, and a surface form that lives in a more thermally variable, but O2 rich river environment. As environment can determine physiological adaptations, it is of interest to compare the aerobic and anaerobic metabolic profiles of white muscle metabolism in both morphotypes of this species, as well as their muscle structures. Here, we used white muscle of both morphotypes of the Mexican cavefish to determine citrate synthase (CS) activity as a measure of aerobic potential, and lactate concentration as a measure of anaerobic potential at three different chronic acclimation temperatures (14°C, 25°C, and 31°C). By examining aerobic and anaerobic potential in both morphs, we sought to link environmental thermal flexibility to muscle metabolism. We found that the surface morphotype had higher CS activity and lower lactate concentration, suggesting an overall more efficient usage of aerobic metabolism; whereas the cave morphotype showed lower CS activity and higher lactate concentration, suggesting a stronger reliance on anaerobic pathways. We also measured white muscle histological variables that have been previously linked to whole-animal metabolism: fiber diameter, number of nuclei per mm of fiber and myonuclear domain (MND) of both morphotypes at 25°C to examine cell-level differences in muscle morphology. However, we found no differences in fiber diameter, number of nuclei per mm of fiber or MND between the two morphotypes. Thus, although the cellular morphology is similar in these species, the environmental differences in the evolution of the two morphs has led to differences in their metabolic profiles.

Enzymatic responses reveal different physiological strategies employed by eurytolerant fish during extreme hot and cold cycling acclimation temperatures.

Heat waves and cold snaps are projected to rise in magnitude, duration, interval, and harshness in the coming years. The current literature examining thermal impacts on the physiology of organisms rarely uses chronic, variable thermal acclimations despite the fact that climate change predictions project a more variable environment. If we are to determine species' susceptibility to climate change, chronic and variable lab acclimations should be prioritized. Here, we acclimated the eurytolerant sheepshead minnow (Cyprinodon variegatus) to two extreme cycling thermal regimes: one warm [resting 27 °C with a spike to 33 °C for 8 h daily], one cold [resting 6.5 °C with a spike to 12 °C for 8 h daily], and three chronically stable conditions (10, 22, and 30 °C) for comparison. We measured enzymatic antioxidants (catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx)), total antioxidant capacity, lipid peroxidation (LPO) damage, and citrate synthase (CS) activity in white epaxial muscle. Of particular note, we found significant increases in log CAT activity and SOD concentration in the warm cycling temperatures, and significant increases in GPx activity in the cold cycling temperatures. We found no significant accumulation of LPO damage in any of our thermal acclimation treatments. Thus, sheepshead minnows demonstrate two particularly different mechanisms towards dealing with thermal variation in low and high temperatures. The enzymatic differences between low and high cycling temperatures may define pathways of eurytolerant organisms and how they may survive predicted variability in thermal regimes.

Efficacy of SARS-CoV-2 wastewater surveillance for detection of COVID-19 at a residential private college.

Many colleges and universities utilized wastewater surveillance testing for SARS-CoV-2 RNA as a tool to help monitor and mitigate the COVID-19 pandemic on campuses across the USA during the 2020-2021 academic year. We sought to assess the efficacy of one such program by analyzing data on relative wastewater RNA levels from residential buildings in relation to SARS-CoV-2 cases identified through individual surveillance testing, conducted largely independent of wastewater results. Almost 80% of the cases on campus were associated with positive wastewater tests, resulting in an overall positive predictive value of 79% (Chi square 48.1, Df = 1, P < 0.001). However, half of the positive wastewater samples occurred in the two weeks following the return of a student to the residence hall following the 10-day isolation period, and therefore were not useful in predicting new infections. When these samples were excluded, the positive predictive value of a positive wastewater sample was 54%. Overall, we conclude that the continued shedding of viral RNA by patients past the time of potential transmission confounds the identification of new cases using wastewater surveillance, and decreases its effectiveness in managing SARS-CoV-2 infections on a residential college campus.

Thermal stability vs. variability: Insights in oxidative stress from a eurytolerant fish.

Climate change will increase the frequency, intensity, and duration of heatwaves. This thermal volatility will challenge to the oxidative homeostasis of aquatic ectotherms through many temperature-dependent environmental factors. In this study, we examined the effects of chronic exposure of sheepshead minnows (Cyprinodon variegatus) to multiple thermal regimes on the oxidative physiology of white muscle in these eurytolerant fish. The thermal treatments included stable (15 °C and 30 °C) and cycling regimes (between 21 and 29 °C at 6, 8 and 10-h intervals). The effect of these thermal treatments on oxidative stress during an acute thermal challenge (12 h at 32 °C) was also examined. Enzymatic activity of catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx), scavenging capacities of hydroxyl and peroxyl radicals, and lipid peroxidation (LPO) damage were quantified. We found no differences between or across treatments in any of the enzymatic antioxidants or LPO damage. We found that peroxyl radical scavenging was greatest at the peak of the 8- and 10-h thermal cycles. Peroxyl scavenging after an acute thermal challenge was greater than before the challenge for the steady 15 °C and 8-h cycle treatments, greater before the acute challenge for the steady 30 °C and 6-h cycle, and equivalent in the 10-h cycle. These findings demonstrate that even the most tolerant of marine ectotherms must engage oxidative defenses when presented with thermal variability and heighten concerns about the impact of climate change on less tolerant species.

How does chronic temperature exposure affect hypoxia tolerance in sheepshead minnows' (Cyprinodon variegatus variegatus) ability to tolerate oxidative stress?

Estuarine environments are characterized by cyclical fluctuations in tides, with tidal shifts drastically, frequently, and acutely altering temperature, dissolved oxygen, and salinity. Despite these ecological challenges, the sheepshead minnow, Cyprinodon variegatus, seems to dominate estuarine landscapes. Here, we held sheepshead minnows to four temperature treatment groups for 1 month. We then tested whether temperature exposure had an effect on acute hypoxia tolerance via oxidative stress. We measured superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activities, as well as total antioxidant capacity and lipid peroxidation (LPO) damage in white muscle. We found that exposure to increasing temperatures (15, 20, 25, and 30 °C) for 1 month led to significantly higher CTmax in sheepshead minnows. We also found that CAT activity significantly increased in the 20 and 25 °C temperature treatment groups, whereas it did not change between control and hypoxia trials. SOD activity was significantly higher in control groups of the 15 and 30 °C temperature treatment groups compared with hypoxia groups of these same temperature treatments. GPx activity was significantly lower in the 30 °C temperature treatment group regardless of control or hypoxia trials. Hydroxyl scavenging capacity varied across temperature treatment and control/hypoxia groups. Peroxyl scavenging capacity and LPO damage showed no significant differences across temperature treatment groups or between control and hypoxia trials.