Meeting Abstract
Climate change and various human activities decrease the dissolved oxygen (DO) content of freshwater ecosystems. Historically, scientists have used the concept of the Pcrit (the DO level below which an animal can no longer oxyregulate) to infer hypoxia tolerance across species, primarily fish. The Pcrit concept has been recently challenged on several fronts and requires critical evaluation. Using cutting edge respirometry equipment, we tested the hypothesis that the Pcrit is positively correlated with temperature in the mayfly N. triangulifer. Pcrit estimates were taken from 96 individual larvae across 5 temperatures spanning 10°C. We found a modest (r= 0.45), but highly significant (p < 0.0001) association between temperature and Pcrit despite relatively large inter-individual variability (CV=25.9% ±8.35%). We next tested the concept that the Pcrit represents the tipping point between aerobic and anaerobic metabolism by quantifying the expression of hypoxia-responsive genes along a DO gradient at three different temperatures. Neither EGL-9 (an oxygen sensing gene and modulator of HIF-1a activity) nor LDH (a hypoxia indicator) were upregulated at oxygen levels above the temperature-specific Pcrit estimates. However, at or below the Pcrit estimates, expression of both genes was stimulated (e.g. 19.66- and 3.01-fold change for EGL-9 and LDH, respectively at 22°C). Our data provide modest support for the notion that the Pcrit represents a physiologically meaningful shift from aerobic to anaerobic metabolism in N. triangulifer. However, a high degree of inter-individual variation precludes the use of a single value to describe a species response to DO. Further, because hypoxia tolerance likely involves a complex suite of physiological attributes, the Pcrit alone may not be a suitable predictor of hypoxia tolerance.
