Antagonistic effects of hypersalinity on heat tolerance in a copepod emerge from nonadditive protein abundance...

Nonadditive interactions between environmental stressors, where biological responses to simultaneous stressors do not equal the sum of individual-stressor responses, commonly occur across organisms and environments. To enable predictions of organismal resilience to shifting patterns of environmental stressors, it is important to both identify these interactions and document the mechanisms underlying them. The supratidal copepod Tigriopus californicus demonstrates a nonadditive, antagonistic pattern of increased heat tolerance when simultaneously exposed to high salinities. We investigated salinity's chronic and acute effects on heat tolerance in a northern population and quantified responses in protein abundance to hypersalinity and heat stress, both alone and in combination. The overall proteomic response to multiple stressors was nonadditive and largely reflected that to high temperature. However, 42% of multistressor proteins were absent from either single-stressor response; we refer to these proteins that are only differentially abundant in the multistressor scenario as "emergent." Our results suggest that the increased heat tolerance of T. californicus conferred by hypersalinity may be driven by a combination of these emergent proteins, several proteins induced by hypersalinity in both single- and multistressor conditions that may contribute to cross-tolerance, and four proteins with additive abundance patterns (including a small heat shock protein). These candidate proteins play putative roles in several relevant processes including the heat shock response, protein folding, and regulation of metabolism and oxidative stress responses. Our results connect to prior whole organism findings and highlight promising pathways for future investigation in the context of heat tolerance and multistressor interactions.NEW & NOTEWORTHY Protein abundance responses to single and combined environmental stressors revealed several potential molecular mechanisms that could explain a nonadditive increase in heat tolerance of the intertidal copepod Tigriopus californicus when it is simultaneously exposed to high salinity. Protein abundance also follows a generally nonadditive pattern, with only a few exceptions. Furthermore, a number of proteins with putative roles in stress responses exhibit "emergent" abundance patterns, in which they only respond to the combined stressors.