A MULTI-LEVEL INVESTIGATION OF THE MECHANISMS UNDERLYING CROSS-TOLERANCE EFFECTS OF SALINITY ON HEAT TOLERANCE IN TIGRIOPUS CALIFORNICUS

Caroline Terry

Increasing multifaceted environmental variability under a changing climate is threatening marine and coastal ecosystems globally. Accurate forecasting of organismal and ecosystem outcomes under global change requires knowledge of how multiple stressors interact with one another to impact organismal functioning, as single-stressor approaches fail to capture their often non-additive and unpredictable effects. Furthermore, generating understanding of how stressors interact mechanistically at multiple physiological levels will generate causal models for these interactions, increasing predictive ability of multi-stressor interactions across biological systems. Because biological responses at molecular and sublethal physiological levels determine higher-level whole-organism, population, and ecosystem outcomes, these levels contribute greatly to a mechanistic understanding of multi-stressor interactions. Due to its tolerance of extreme and simultaneous changes in multiple environmental parameters, the supratidal copepod Tigriopus californicus is a useful model in which to study the mechanisms by which multiple stressors interact to produce unpredictable, non-additive biological outcomes. High salinity and heat interact in this system, resulting in a non-additive interaction at the level of survival. In this antagonistic interaction, copepods exposed to salinity increases have unexpectedly higher heat tolerance than under ambient salinity, but the mechanisms by which this interaction occurs are unclear. Here, we investigate mechanistic drivers of this antagonistic interaction at multiple physiological and molecular levels. At the sublethal physiological level, we measured whole-organism aerobic metabolic rate and locomotor activity under long-term and acute salinity changes alone and in combination with increased temperature. At the molecular level, we applied standard and novel techniques to measure changes in protein and metabolite abundance in response to acute changes in salinity and increased temperature. We show that non-additive responses at these levels contribute to the antagonistic response in survival in this copepod, and that the outcomes of multi-stressor exposure are highly unpredictable from single-stressor responses. Additionally, multi-stressor responses at any one level are unpredictable from those at other levels, but when considered together they lead to greater understanding of the mechanistic processes driving the interaction between salinity and heat. We propose that regulation of aerobic and anaerobic metabolism, oxidative stress responses, heat-protective proteins called chaperones, and possibly the accumulation of organic osmolytes contribute to the cross-protective effect of high salinity on heat tolerance in T. californicus. This research first highlights the importance of multi-stressor investigations for best understanding responses to environmental change in natural environments, as these interactions are unpredictable from single-stressor outcomes. It also demonstrates the utility of coordinated, multi-level investigations of these interactions’ mechanistic drivers, as lines of evidence from a single level of investigation were often insufficient to fully describe the physiological underpinnings of the interaction of interest.

A MULTI-LEVEL INVESTIGATION OF THE MECHANISMS UNDERLYING CROSS-TOLERANCE EFFECTS OF SALINITY ON HEAT TOLERANCE IN TIGRIOPUS CALIFORNICUS

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