Characterizing the effects of temperature variation on metabolic capacity in darter fish (Etheostoma spp.)

Abstract

With progressive increases in global temperatures from climate change, aquatic ectotherms are particularly at risk, considering they are incapable of maintaining their internal body temperatures. As environmental temperatures increase, metabolic rate rises in ectothermic poikilotherms, with thermal extremes presenting potentially lethal implications. By gaining an understanding of the metabolic functioning of local species, we can draw conclusions on the implications of thermal stress on biochemical responses. Here, we focused on understanding the effect of increased temperatures on enzyme activity in the brain, heart, and muscle tissues of three closely related darter species found in the Grand River of Southern Ontario: Johnny (Etheostoma nigrum: JD), Fantail (Etheostoma flabellare: FTD) and Rainbow darter fish (Etheostoma caeruleum: RBD) to determine whether energetic enzymes are a potential limiting factor in thermal tolerance. Analysis of enzymatic activity of four key metabolic enzymes including lactate dehydrogenase (LDH), pyruvate kinase (PK), malate dehydrogenase (MDH) and citrate synthase (CS) were conducted via enzyme assays through a temperature profile, ranging from 20C to 34C, just past the predetermined critical thermal maximum (CTmax), or upper thermal tolerance, at 30.7C ± 0.9, 31.9C ± 0.6 and 33.3C ± 0.8 for RBD, JD and FTD, respectively (Weber & Craig, 2025). Through modelling enzymatic activity in a segmented regression, breakpoint estimates were found for brain at 22.0C ± 0.8, heart at 22.0C ± 5.0 and muscle at 26.0C ± 4.7 indicating decreases in enzymatic activity at higher temperatures. These results indicate that enzymatic activity in brain and heart tissue is most impacted by increased temperatures as evident by the lower trending breakpoints, suggesting that these tissues may be implicated in defining thermal tolerance limits. Muscle tissue enzymatic activity also decreased at higher temperatures but had a higher trending breakpoint, suggesting compensatory mechanisms. Moreover, breakpoints also occurred far before previously determined CTmax values for these fish, indicating that biochemical processes decline in performance prior to ecological death. Overall, these findings enhance our understanding of the biochemical processes that limit thermal tolerance in these local species, with potential implications for conservational efforts in defining temperatures of concern.