Hypothesis\nChronic ocean acidification (OA) diminishes the coral host’s heterotrophic feeding capacity, which in turn restricts the energetic advantage conferred by heat‑tolerant symbionts during acute marine heatwaves, leading to a net loss of calcification despite symbiont community shifts.\n\n## Mechanistic basis\nIt's known that OA lowers seawater pH, increasing the energetic cost of proton pumping required for both calcification and phagocytic vesicle acidification {[https://pmc.ncbi.nlm.nih.gov/articles/PMC12619985/]}. This diverts ATP away from the host’s heterotrophic machinery, so we don't see the usual boost in prey capture and digestion rates. Simultaneously, heat‑tolerant symbionts such as Durusdinium provide photosynthetic carbon but can't fully compensate for the loss of heterotrophic nitrogen and lipids that support tissue repair and mucus production {[https://news.miami.edu/rosenstiel/stories/2025/05/heat-tolerant-symbionts-a-critical-key-to-protecting-floridas-elkhorn-coral-from-bleaching-during-marine-heatwaves.html]}. When heterotrophic input is curtailed, the host relies more on symbiont‑derived photosynthate, which under heat stress is often diverted to stress‑response pathways rather than growth. The resulting energetic deficit impairs calcification and weakens the mucus barrier, making the coral more susceptible to disease and limiting the durability of symbiont‑mediated bleaching resistance.\n\n## Predictions\n1. Corals pre‑exposed to elevated pCO2 (OA conditions) will show a significant reduction in Artemia nauplii uptake (heterotrophic feeding) compared with ambient‑pCO2 controls, even when symbiont communities are dominated by heat‑tolerant strains.\n2. Under a subsequent marine heatwave simulation, OA‑preconditioned corals will exhibit lower calcification rates and higher bleaching severity than non‑OA corals harboring the same symbiont proportion.\n3. Supplementing OA‑exposed corals with external organic nutrients (e.g., amino acids or lipids) will rescue heterotrophic feeding rates, partially restoring calcification and symbiont‑mediated thermal tolerance.\n4. The mucus layer of OA‑preconditioned corals will display altered carbohydrate composition and reduced antimicrobial peptide expression, correlating with increased pathogen proliferation post‑heatwave.\n\n## Experimental approach\n- Collect nubbins of Acropora palmata and Pocillopora grandis from field sites with low local disturbance to isolate OA effects {[https://coral.org/en/blog/turning-the-tide-for-coral-reefs-in-2025/]}.\n- We're going to maintain fragments in flow‑through tanks for 8 weeks at either ambient pCO2 (~400 µatm) or elevated pCO2 (~1000 µatm) while keeping temperature constant at 26°C.\n- Quantify heterotrophic feeding by fluorescently labeled Artemia ingestion and calculate clearance rates.\n- After acclimation, raise temperature to 32°C for 10 days to simulate a marine heatwave; monitor symbiont density (chlorophyll a), photochemical efficiency (Fv/Fm), and symbiont composition via qPCR of Durusdinium‑specific markers.\n- Measure calcification using the buoyant weight technique and alkalinity anomaly method.\n- Assess mucus composition via carbohydrate electrophoresis and antimicrobial peptide activity assays.\n- In a rescue sub‑experiment, add a mixture of essential amino acids and lipids to the seawater of OA‑treated fragments during the heatwave phase.\n\n## Implications\nIf OA‑induced feeding impairment limits the benefit of heat‑tolerant symbionts, then restoration strategies that focus solely on symbiont inoculation may fail in regions where acidification co‑occurs with warming. Effective conservation needs to pair local stressor reduction (e.g., watershed management to lower runoff‑derived CO2) with selective breeding or feeding enrichment to sustain heterotrophic capacity {[https://coral.org/en/blog/how-coral-reef-conservation-delivered-measurable-impact-in-2025-report/]} {[https://www.fisheries.noaa.gov/feature-story/mission-iconic-reefs-shares-strategic-priorities-2022-2025]}. This hypothesis directly addresses the highlighted knowledge gap on OA‑calcification energetics and offers a testable pathway to refine climate‑resilient reef networks {[https://www.wcs.org/our-work/species/coral/strategy]}