THE EFFECTS OF SEQUENTIAL AND SUBLETHAL STRESS ON MUSSEL METABOLISM AND ANTIOXIDANT CAPACITY

Andrea Kristine Delgado

doi:10.7273/000007238

The ecosystem functions provided by intertidal mussel beds may be susceptible to increasingly frequent atmospheric heat waves and coastal deoxygenation. Intertidal species’ responses to sublethal heat and hypoxic stress have been studied in isolation, but the interactive effects of these variables are less understood. Here, we address the consequences of sequential exposure to high temperature and low oxygen in the foundation species, Mytilus californianus. Aerial, sublethal heat stress at low tide leads to reactive oxygen species (ROS) creation, causing macromolecular damage that is often repaired during high tide periods upon reoxygenation. However, if the surrounding water is hypoxic, mussels may be forced to rely on less efficient anaerobic metabolism, threatening their recovery and performance. Mussels collected from Starfish Point, Washington in the late spring/summer of 2023 were subjected to a fully crossed set of tidal cycle scenarios wherein air temperature during the daytime low tide (DTLTT; 18, 25, or 32°C) and dissolved oxygen (DO) during high tide periods (1.12, 2.0 or 6.0 mg L-1) were manipulated for 1 or 5 days. Adductor muscle and gill tissues’ aerobic (citrate synthase; CS), anaerobic (phosphoenolpyruvate carboxykinase; PEPCK), and antioxidant (catalase; CAT) enzyme activities were assayed. Exposure to sublethal hypoxia or heat stress, alone or in combination, did not affect CS or CAT in either tissue. Mean gill PEPCK generally increased under low DO conditions, while also increasing at the highest DTLTT under 6.0 mg L-1 and 2.0 mg L-1 DO. However, no rise in PEPCK activity was observed when severe hypoxia during high tide coincided with the highest DTLTT. Intertidal mussels often utilize ‘preparatory’ strategies to accommodate for the dynamic lifestyle of the intertidal. Our results suggest M. californianus lacks the ability to increase anaerobic capacity under exposure to sublethal aerial heat stress when followed by severe hypoxia in the intertidal zone, as anaerobic capacity reached levels similar to control conditions of 18°C at DTLT and followed by 6.0 mg L-1 DO (normoxia). No mortality was observed after exposure to 24 hours of any experimental treatment, whereas 18% of mussels in the most extreme treatment group experienced mortality in the 5-day exposure. As aerobic metabolic capacity was not seen to be affected by acute heat stress or oxygen availability, the return of anaerobic capacity under our most extreme scenario to that of control conditions suggests that prolonged exposure to high tides under low DO conditions could be stressful to intertidal mussels. This is likely a function of the inability to compensate for ‘oxygen debts’ incurred at low tide, where lower clearance and ventilation rates under low DO scenarios decrease food and oxygen intake and limit deposition into energy stores. High tides with low DO likely do not provide sufficient oxygen for processing anaerobic end-products or allow for sufficient feeding or digestion to increase energy stores in preparation for subsequent exposure to aerial heat stress.

THE EFFECTS OF SEQUENTIAL AND SUBLETHAL STRESS ON MUSSEL METABOLISM AND ANTIOXIDANT CAPACITY

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