Hypothesis: Ostreobium spp. secrete sulfated polysaccharides that raise the pH of the diffuse boundary layer surrounding coral tissues, thereby mitigating the deleterious effects of ocean acidification on calcification.

Rationale: Ostreobium colonizes coral recruits within a week of settlement and resides in the endolithic zone where it can supply photosynthates during Symbiodiniaceae loss【https://pmc.ncbi.nlm.nih.gov/articles/PMC12591052/】. Endolithic algae are known to exude polysaccharides that modify microenvironments【https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2025.1635356/full】. Under low pH, many marine algae increase polysaccharide production as a stress response, which can bind protons and elevate local pH. Thus Ostreobium may function as a dynamic pH‑buffering partner that complements its metabolic backup role.

Predictions: (1) In acidified seawater (pH ≈ 7.6), corals harboring intact Ostreobium populations will maintain a higher pH at the tissue–seawater interface (measured with microsensors) than corals where Ostreobium has been experimentally reduced or absent. (2) Ostreobium polysaccharide output, quantified by alcian blue staining or phenol‑sulfuric acid assay, will rise significantly under reduced pH conditions. (3) Removing Ostreobium (e.g., via targeted bleaching or low‑dose antibiotics that suppress endolithic algae without affecting Symbiodiniaceae) will decrease calcification rates under acidification, whereas adding purified Ostreobium polysaccharides to the surrounding water will restore calcification toward control levels.

Experimental approach: Generate nubbins of a common reef‑building coral (e.g., Acropora hyacinthus) and split them into four treatments: (a) control seawater (pH ≈ 8.1), (b) acidified seawater (pH ≈ 7.6), (c) acidified seawater with Ostreobium depleted, (d) acidified seawater with Ostreobium depleted plus exogenous Ostreobium polysaccharides. Monitor Ostreobium abundance via qPCR of the 18S rRNA gene and chlorophyll fluorescence. Use pH microsensors to profile the diffusive boundary layer at the coral surface. Measure calcification using the buoyant weight technique and total alkalinity anomaly. Assess polysaccharide concentration in coral mucus and surrounding water. Run the experiment for 30 days to capture acclimation responses.

Potential implications: If Ostreobium‑derived polysaccharides indeed elevate extracellular pH, this reveals a microbiome‑mediated mechanism for acid tolerance that operates independently of symbiont photosynthesis. It would suggest that probiotic or seeding strategies aimed at enhancing Ostreobium colonization could buy time for reefs facing accelerating acidification. Furthermore, models predicting reef resilience should incorporate endolithic algae as active modulators of the chemical microenvironment, not merely as passive inhabitants.