Can oxygen supersaturation enhance thermal resilience in marine organisms?

Marine heatwaves (MHWs) represent a critical threat to coastal biodiversity, pushing organisms toward their upper lethal thermal limits. In productive coastal ecosystems, photosynthetic activity generates daytime oxygen supersaturation (hyperoxia), which has been hypothesized to mitigate thermal stress by compensating for increased metabolic oxygen demand. This study investigated the role of hyperoxia in enhancing the thermal resilience of diverse marine invertebrates collected from three sites in the Venice Lagoon characterized by distinct thermal regimes. Individuals from different taxa were exposed to acute thermal ramps under normoxic (90% DO) and hyperoxic (160% DO) conditions to estimate their upper lethal temperature thresholds (LT₅₀). The results reveal that the mitigating effect of hyperoxia is highly species-specific and site-dependent. The diversification of site-specific response was amplified under oxygen-enriched conditions, suggesting that population-specific responses to fluctuating environments shapes not only basal thermal limits but also the capacity for respiratory compensation.