Hypothesis

Excess nitrogen fixation disrupts the coral‑Symbiodiniaceae nitrogen‑to‑phosphorus balance, triggering a cascade that remodels the resident bacterial community toward taxa that overproduce extracellular polymeric substances (EPS). This altered EPS matrix changes the microenvironment at the calcifying epithelium by sequestering protons and modulating local carbonate chemistry, thereby mitigating the depressive effect of ocean acidification on skeletal growth. Conversely, when nitrogen‑to‑phosphorus ratios are balanced, EPS production remains at baseline levels and acidification proceeds unchecked. The hypothesis predicts that manipulating ambient nitrogen and phosphorus concentrations will shift the EPS‑producing fraction of the microbiome, which can be quantified via carbohydrate staining and EPS‑specific metatranscriptomics, and that these shifts will correlate with measurable changes in calcification rates under controlled pCO2 conditions.

Mechanistic Rationale

1. Nutrient dysbiosis selects for EPS‑rich bacteria – Recent work shows that heat‑stress favors opportunistic Vibrio and alters Alpha‑ and Gammaproteobacteria dominance 1. Certain members of these groups, notably Endozoicomonas and Roseobacteraceae, are known to secrete polysaccharides that bind metal ions and influence biofilm viscosity 2. When excess nitrogen fuels their growth, EPS output rises disproportionately.

2. EPS as a proton buffer – Polysaccharide matrices possess carboxylate and sulfate groups that can bind H⁺, creating a microzone of elevated pH relative to bulk seawater. This local alkalinization favors the carbonate system (CO₃²⁻) over bicarbonate, raising the saturation state (Ωₐᵣₐ) at the site of calcification despite lowered bulk pH.

3. Feedback to the holobiont – Elevated Ωₐᵣₐ supports sustained CaCO₃ precipitation by the calicoblastic epithelium, preserving skeletal integrity and reducing energy diversion to acid‑base regulation. In turn, a healthier host can allocate more photosynthate to Symbiodiniaceae, reinforcing the symbiosis and limiting algal overgrowth that exacerbates nitrogen imbalance.

4. Testable predictions – (a) Coral fragments reared under high nitrate, low phosphate will show a ≥2‑fold increase in EPS‑associated gene expression (e.g., wza, ppsA) in the microbiome compared to balanced N:P conditions. (b) Microsensor measurements will reveal a pH elevation of 0.05–0.1 units within the EPS layer relative to ambient pH. (c) Calcification rates (buoyant weight or alkalinity anomaly) will be significantly higher in the high‑N, low‑P treatment when acidified (pCO₂ ≈ 800 µatm) than in the balanced N:P control, an effect abolished by enzymatic EPS degradation (e.g., added alginate lyase).

Experimental Outline

• Factorial design: Two nutrient regimes (high N:low P vs. balanced N:P) crossed with two pCO₂ levels (present‑day ~410 µatm, future ~800 µatm).
• Replicates: n = 6 per treatment, using genetically uniform fragments of Acropora hyacinthus.
• Measurements: 16S rRNA amplicon sequencing to track community shifts; metatranscriptomics for EPS biosynthesis genes; fluorescent lectin staining for EPS quantification; micro‑pH sensors for microenvironment; calcification via buoyant weight weekly.
• Controls: Antibiotic‑treated fragments to confirm microbiome dependence; EPS‑degrading enzyme addition to test causality.

If the data support the predicted relationships, we would demonstrate that nutrient‑driven microbiome remodeling can functionally buffer acidification impacts, offering a mechanistic link between nitrogen pollution, microbial ecology, and reef resilience under climate stress.