van der Zande, R. M. | bioRxiv Science

bioRxiv2026

Ocean acidification alters hypoxia sensitivity and oxyregulation in reef-building corals

Coastal marine ecosystems are increasingly threatened by multiple stressors such as ocean acidification and deoxygenation, but how these co-occurring stressors interact is often poorly understood. This is especially true for tropical coral reefs where deoxygenation is an emerging yet understudied threat. Using hypoxia response curves combined with rigorous pH control, we show that acidification alters hypoxia sensitivity and oxyregulation of reef-building corals in a species-specific manner: three species exhibited increased sensitivity to various degrees, while the fourth showed enhanced tolerance. Consequently, acidification pushes critical hypoxia thresholds into oxygen regimes already prevalent on reefs today, potentially driving shifts in community composition and accelerating risks to reef resilience as these stressors intensify in the future. Our findings challenge assumptions of uniform coral vulnerability under multi-faceted climate change, emphasizing the need for trait-based approaches and to account for stressor interactions in predictive models to better anticipate coral reef futures under rapid climate change.

ecology↗

bioRxiv2025

Compound hypoxia with heat or acidification stress induces synergistic and additive effects on coral physiology

As climate change accelerates, coastal marine ecosystems are increasingly exposed to co-occurring stressors whose combined effects are nonlinear and difficult to predict. Deoxygenation is a rapidly intensifying yet underrecognized threat to coral reefs that interacts with heat and acidification to alter coral physiology and stress resilience. However, the effects of hypoxia-related compound events on corals are largely unknown, underscoring the need for multi-stressor studies. Here, we conducted two extended-exposure experiments (12-17 days) across the coral species Porites furcata, Porites astreoides and Siderastrea siderea, to disentangle the individual and combined effects of low dissolved oxygen (hypoxia) with either heat or acidification. We measured eight phenotypic traits related to growth, metabolism, and symbiosis health to test whether hypoxia imposes energetic constraints or other physiological stress that amplify the effects of heat or acidification. Standardized effect size analysis across 24 stressor-trait combinations revealed 13 additive, 10 synergistic, and only one antagonistic response. Hypoxia consistently suppressed dark respiration by 37-49% across species and altered photophysiology in the two Porites species, whereas acidification alone had minimal effects, particularly in S. siderea. Heat stress caused the most pronounced declines across nearly all traits, and when combined with hypoxia, it produced the highest number of synergistic interactions. In contrast, the combination of hypoxia and acidification largely resulted in additive responses, suggesting that independent physiological mechanisms underlie these effects. All corals showed strong metabolic depression under hypoxia which is likely beneficial as a short-term adaptive response but may impose energetic constraints in the long-term. These findings highlight deoxygenation as critical yet often overlooked drivers of coral reef vulnerability. More multi-stressor experiments across a range of species are urgently needed to improve predictions of reef resilience under future ocean conditions, where compound stress events are expected to become more frequent and severe.

ecology↗