Deoxygenation lowers the thermal threshold of coral bleaching

Abstract

Exposure to deoxygenation from climate warming and pollution is emerging as a contributing factor of coral bleaching and mortality. However, the combined effects of heating and deoxygenation on bleaching susceptibility remain unknown. Here, we employed short-term thermal stress assays to show that deoxygenated seawater can lower the thermal limit of an Acropora coral by as much as 1 °C or 0.4 °C based on bleaching index scores or dark-acclimated photosynthetic efficiencies, respectively. Using RNA-Seq, we show similar stress responses to heat with and without deoxygenated seawater, both activating putative key genes of the hypoxia-inducible factor response system indicative of cellular hypoxia. We also detect distinct deoxygenation responses, including a disruption of O2-dependent photo-reception/-protection, redox status, and activation of an immune response prior to the onset of bleaching. Thus, corals are even more vulnerable when faced with heat stress in deoxygenated waters. This highlights the need to integrate dissolved O2 measurements into global monitoring programs of coral reefs.

Transcriptional responses indicate acclimation to prolonged deoxygenation in the coral Stylophora pistillata

“…Moreover, in a coral (Acropora sp.) exposed to short-term deoxygenation combined with thermal stress, deoxygenation was reported to lower the thermal threshhold of coral bleaching (Alderdice et al, 2022b). Since S. pistillata, reported to be thermally tolerant, bleaches under this stress (Kvitt et al, 2011) but not under hypoxia stress (Figure 1D), the correlation between thermal stress, bleaching and deoxygenation should be investigated in additional coral species.…”

Maximal coral thermal tolerance is found at intermediate diel temperature variability

“…Encouragingly, the range in coral thermotolerance across geomorphological zones within a single reef system (< 5 km) were as large as differences observed across vast latitudinal gradients (>300 km), and future studies could investigate the mechanisms (e.g., physiological plasticity, constitutive upregulation of stress-response genes, and/or epigenetic modifications) enabling these corals to develop resistance to acute heat stress. Further, while co-occurring environmental conditions (e.g., pCO 2 , oxygen, and irradiance) were not quantified in this study, pCO 2 fluctuations and irradiance are known to differ across these same habitats (Brown & Mello-Athayde, 2022), and the combined effect of ocean warming and deoxygenation can lower the thermal threshold of some corals (Alderdice et al, 2022). As such, more research is needed to understand the interactions between physicochemical conditions that co-occur within thermally variable habitats and their influence on coral thermal tolerance.…”

The Coral Bleaching Automated Stress System (CBASS): A low‐cost, portable system for standardized empirical assessments of coral thermal limits

“…Alternatively, red (R), green (G), and blue (B) pixel intensities from an RGB photograph can be extracted to infer loss of chlorphyll density as inferred from an increase in pixel intensity of the red channel (Voolstra et al 2020). Last, coral pigmentation/coloration can be scored against a color scale reference for broad implementation and consistency across scorers and studies (Alderdice et al 2022).…”

“…The flow-through nature of the system allows for continuous water exchange from a water supply manifold. This means that additional water manipulations (e.g., pH, salinity, dissolved oxygen, or nutrients) are possible with addition/ control of multiple pretreated water sources for each individual aquarium (Alderdice et al 2022). Herein, we provide full design schematics of this system (Figs.…”