Hypothesis

Corals that maintain a robust circadian rhythm of calcifying fluid pH elevation (pH_cf) can buffer the deleterious effects of ocean acidification on skeleton thickening, but this protective gating is disrupted when low‑abundance, heat‑sensitive Symbiodiniaceae variants (e.g., C15h) elevate nocturnal reactive oxygen species (ROS) production, uncoupling the circadian repair cycle from pH_cf regulation. Consequently, under combined thermal and acidification stress, corals harboring these variants show accelerated loss of skeletal density and increased breakage susceptibility, independent of bleaching severity.

Mechanistic Reasoning

1. Circadian pH_cf gating – Daytime photosynthesis by symbionts drives ATP production that fuels H⁺‑ATPase activity, raising pH_cf and promoting calcification (see Energy‑intensive pH regulation allows 100x faster calcification). At night, corals enter a sleep‑like state to repair ROS‑induced DNA damage (Corals exhibit sleep‑like rest at night to repair DNA damage). This temporal separation ensures that pH_cf elevation is not compromised by oxidative stress.

2. Low‑abundance symbiont variants as ROS amplifiers – Variants such as C15h, though rare, have been linked to lower heat tolerance and earlier bleaching (Low‑abundance variants correlate with reduced heat tolerance in hotspots). We propose that these variants possess a less efficient photoprotective antenna system, leading to excess excitation energy and heightened ROS generation during daylight hours. The ROS persist into the night, overwhelming the coral’s repair mechanisms and impairing the nocturnal downregulation of H⁺‑ATPase activity, thereby causing a maladaptive drop in pH_cf.

3. Feedback to skeleton thickening – Ocean acidification primarily reduces skeleton density by inhibiting the thickening phase (OA impedes skeleton thickening, reducing density and breakage resistance). A nocturnal decline in pH_cf directly reduces the carbonate ion concentration ([CO₃^2‑]) in the calcifying fluid, slowing the precipitation of dense aragonite layers. Thus, the interaction of variant‑driven ROS and circadian pH_cf dysregulation specifically threatens skeletal thickening, not linear extension.

Testable Predictions

• Prediction 1: Corals dominated by C15h (or similar low‑abundance, heat‑sensitive variants) will exhibit a significantly lower nocturnal pH_cf (measured via microelectrodes or pH‑sensitive dyes) than conspecifics from thermal refugia, even when daytime pH_cf is comparable.
• Prediction 2: Experimental addition of ROS scavengers (e.g., N‑acetylcysteine) during the night will restore nocturnal pH_cf elevation in C15h‑dominated corals and rescue skeleton density under combined OA and heat stress.
• Prediction 3: Knock‑down of coral circadian clock genes (e.g., Clock, Cycle) will abolish the protective daytime pH_cf elevation, making all corals equally vulnerable to OA‑induced thinning, regardless of symbiont composition.
• Prediction 4: Long‑term monitoring of reefs with known symbiont community profiles will show that sites with higher prevalence of low‑abundance, heat‑sensitive variants experience faster declines in skeletal density (via CT‑derived density bands) than sites dominated by tolerant Cladocopium types, independent of bleaching frequency.

Experimental Approach

1. Field sampling: Collect nubbins from paired hotspot and refugia sites across the Indo‑Pacific. Quantify symbiont ITS2 diversity via amplicon sequencing to determine relative abundance of C15h and other low‑abundance variants.
2. In situ pH_cf profiling: Use pH‑sensitive microelectrodes or the fluorescent probe SNARF‑1 attached to the calcifying epithelium to sample pH_cf every 3 h over a 48‑h cycle under natural light/temperature regimes.
3. Laboratory stress assays: Maintain nubbins in factorial tanks (ambient vs. elevated pCO₂; ambient vs. +2 °C) for 4 weeks. Apply nocturnal ROS scavenger or vehicle control. Measure skeletal density changes via micro‑CT and calculate thickening rates.
4. Genetic/pharmacological perturbations: Use CRISPR‑Cas9 or morpholino knock‑down of core circadian genes in Acropora larvae; confirm loss of rhythm via luciferase reporters. Assess pH_cf and density outcomes under stress.
5. Statistical analysis: Mixed‑effects models with site, symbiont variant abundance, time of day, and treatment as fixed effects; colony ID as random effect. Interaction terms will test whether variant abundance modifies the effect of time of day on pH_cf and whether this mediates density loss.

Falsifiability

If observations show that (a) nocturnal pH_cf does not differ between C15h‑dominated and refugia corals, (b) ROS scavenging fails to rescue pH_cf or skeletal density, or (c) circadian gene knock‑down does not abolish daytime pH_cf elevation, the core mechanistic link between variant‑driven ROS, circadian pH_cf gating, and skeleton thickening would be refuted. Conversely, consistent support across these experiments would validate the hypothesis and highlight a previously unappreciated avenue for predicting multi‑stressor reef resilience.

Implications

Understanding that the timing of pH_cf regulation—not just its magnitude—can be sabotaged by specific symbiont genotypes reframes bleaching‑centric models. It suggests that probiotic or microbiome‑management strategies aimed at suppressing low‑abundance, ROS‑producing variants could preserve the circadian calcification rhythm, thereby maintaining reef structural integrity even as thermal and acidification pressures mount.