Hypothesis

Ocean acidification (OA) modifies the biochemical composition and physical properties of coral surface mucus, reducing its viscosity and altering the diffusion of signaling metabolites (e.g., DMSP, vitamins B1/B6) between Symbiodiniaceae and associated bacteria. This mucus‑mediated decoupling weakens the bacterial support for symbiont shuffling under thermal stress, thereby lowering holobiont heat tolerance independent of direct effects on calcification.

Mechanistic Rationale

1. OA‑induced mucus remodeling – Under lowered pH, corals upregulate bicarbonate transporters to maintain calcifying fluid pH, diverting cellular energy from glycoprotein synthesis. This yields a mucus layer with lower sulfated polysaccharide content and higher water content, decreasing viscosity (as shown in related marine invertebrates under OA) [4][5].
2. Metabolite diffusion barrier – Beneficial bacteria such as Mameliella alba rely on mucus‑bound DMSP and vitamin exchanges to stimulate algal growth and support symbiont shuffling [1][2]. A less viscous mucus increases diffusional loss of these compounds to the surrounding seawater, lowering their local concentration at the microbe‑alga interface.
3. Consequences for symbiont shuffling – Heat‑shuffling requires rapid bacterial‑mediated vitamin B1/B6 provision to sustain nascent symbiont populations [2]. If OA‑driven mucus thinning reduces vitamin retention, the bacterial boost to shuffling is attenuated, leading to slower or incomplete symbiont turnover during bleaching events.
4. Feedback on holobiont resilience – Impaired shuffling prolongs photosystem II damage, exacerbating bleaching severity and reducing the coral’s capacity to sustain positive CaCO₃ production despite any compensatory calcification mechanisms [5][6].

Testable Predictions

• Prediction 1: Corals exposed to OA‑simulated conditions (pH ≈ 7.8) will produce mucus with measurably lower viscosity and sulfate content compared to ambient pH controls.
• Prediction 2: In OA‑treated mucus, the half‑life of added DMSP or vitamin B6 will be significantly shorter due to increased diffusive efflux.
• Prediction 3: Under a combined heat‑OA stress experiment, corals with OA‑altered mucus will exhibit delayed or reduced symbiont shuffling (quantified via qPCR of clade‑specific Symbiodiniaceae markers) relative to heat‑only controls, despite identical bacterial inoculations.
• Prediction 4: Supplementing OA‑treated mucus with exogenous high‑molecular‑weight sulfated polysaccharides (e.g., purified carrageenan) will restore viscosity, retain metabolites, and rescue shuffling rates to heat‑only levels.

Experimental Approach

1. Mucus characterization: Collect mucus from nubbins of Acropora spp. maintained at ambient (pH 8.1) and OA (pH 7.8) conditions for 4 weeks. Measure viscosity (rotational rheometry), sulfate assay, and glycoprotein concentration.
2. Metabolite turnover: Spike mucus aliquots with fluorescently labeled DMSP or vitamin B6; monitor fluorescence decay in sealed chambers over time to calculate efflux rates.
3. Symbiont shuffling assay: Fragment nubbins, inoculate with a standardized bacterial consortium containing Mameliella alba, then expose to a heat ramp (30 → 32 °C) under ambient or OA pH. Track symbiont clade ratios daily using ITS2 sequencing.
4. Rescue test: Parallel OA‑treated groups receive mucus‑viscosity restoring polysaccharide additions; repeat heat‑OA exposure and assess shuffling.

Falsifiability

If OA does not alter mucus viscosity or metabolite retention, or if changes in mucus properties fail to predict differences in symbiont shuffling rates between OA and control groups, the hypothesis would be falsified. Conversely, observing the predicted microbiome‑mediated mechanistic link would support the hypothesis and highlight a non‑calcificatory pathway by which OA undermines coral heat tolerance.