Bacterial DMSP Metabolism Modulates Host Calcifying Fluid pH to Couple Thermal Tolerance with Ocean Acidification Resilience in Corals

Hypothesis\n\nSpecific probiotic bacteria that enhance dimethylsulfoniopropionate (DMSP) metabolism in the coral surface mucus layer can elevate the pH of the coral's internal calcifying fluid by producing alkalinizing metabolites (e.g., ammonia, acetate) and by stimulating host V-type H+-ATPase activity, thereby coupling thermal tolerance mechanisms with resistance to ocean acidification.\n\n## Mechanistic Basis\n\n1. DMSP‑derived acrylate and methanethiol act as signaling molecules that upregulate host carbonic anhydrase and proton‑pumping genes, increasing intracellular pH at the site of calcification.\n2. Bacterial excretion of ammonia raises local alkalinity, directly buffering protons generated during CaCO3 precipitation.\n3. Exopolysaccharide matrices produced by these bacteria bind free protons, further stabilizing the calcifying microenvironment.\n\nThese processes complement known microbiome benefits such as vitamin biosynthesis [1] and Mameliella‑mediated algal growth [1], but they add a direct physico‑chemical link to the coral’s ability to maintain aragonite saturation under simultaneous warming and acidification stress.\n\n## Testable Predictions\n\n- Corals inoculated with DMSP‑active bacteria will show higher calcifying fluid pH (measured with pH‑sensitive dyes) under elevated temperature (+2 °C) and reduced aragonite saturation (Ωar ≈ 2.5) than uninoculated controls.\n- The pH elevation will correlate with increased calcification rates and preserved symbiont density relative to controls.\n- If the bacterial strain lacks DMSP lyase activity, the protective effect will disappear, falsifying the proposed mechanism.\n\n## Experimental Approach\n\n1. Collect nubbins of a branching coral species (e.g., Acropora cervicornis) and maintain them in factorial aquaria: temperature (ambient vs. +2 °C) × Ωar (ambient vs. reduced) × inoculation (DMSP‑active probiotic vs. heat‑killed control vs. no bacteria).\n2. After 4 weeks, sample the calcifying fluid via micro‑extraction and determine pH using fluorescent probes; quantify calcification via buoyant weight; assess Symbiodiniaceae density via chlorophyll fluorescence; profile mucus bacterial communities with 16S rRNA sequencing.\n3. Include a mutant bacterial strain knocked out for DMSP lyase to test mechanistic specificity.\n\n## Potential Outcomes and Falsifiability\n\n- Support: Inoculated corals exhibit significantly higher calcifying fluid pH, calcification, and symbiont retention under combined stress, while the mutant strain fails to confer these benefits.\n- Refute: No difference in pH or calcification between inoculated and control groups, or the mutant strain provides equal protection, indicating that DMSP metabolism is not the causal link.\n\nThis hypothesis directly ties microbiome‑mediated chemical modulation to the coral’s biogeochemical resilience, offering a clear, falsifiable route to prioritize probiotic strains that function under realistic multi‑stressor scenarios.\n\n## References\n\n[1] https://academic.oup.com/femsre/article/47/2/fuad005/7071893\n[2] https://eos.org/articles/coral-diversity-drops-as-ocean-acidifies\n[3] https://pmc.ncbi.nlm.nih.gov/articles/PMC12644485/\n[4] https://news-oceanacidification-icc.org/2026/03/02/persistence-of-coral-reef-structures-into-the-twenty-first-century/\n[5] https://www.nationalacademies.org/news/coral-reefs-resilience-and-the-stories-we-tell\n[6] https://www.nsf.gov/news/coral-microbiome-key-surviving-climate-change-new\n[7] https://www.colorado.edu/today/2025/09/30/corals-might-be-adapting-climate-change\n[8] https://news.miami.edu/rosenstiel/stories/2026/03/university-of-miami-launches-new-research-on-governance-of-floridas-coral-reef.html\n[9] https://www.sciencealert.com/2026-may-be-the-year-coral-reefs-around-the-world-finally-collapse