biostudies-literatureUnknown8(1)Noonan SHCOcean acidification from increasing atmospheric CO₂ is progressively affecting seawater chemistry, but predicting ongoing and near-future consequences for marine ecosystems is challenging without empirical field data. Here we quantify tropical coral reef benthic communities at 37 stations with varying exposure to submarine volcanic CO₂ seeping, and determine the aragonite saturation state (Ω_Ar) where significant changes occur in situ. With declining Ω_Ar, reef communities displayed progressive retractions of most reef-building taxa and a proliferation in the biomass and cover of non-calcareous brown and red algae, without clear tipping points. The percent cover of all complex habitat-forming corals, crustose coralline algae (CCA) and articulate coCommunications biology1518https://www.ebi.ac.uk/biostudies/studies/S-EPMC12644485biostudies-literatureProgressive changes in coral reef communities with increasing ocean acidification.PMC12644485Fisher RFabricius KEBirrell CNoonan SHCfalseProgressive changes in coral reef communities with increasing ocean acidification.Ocean acidification from increasing atmospheric CO₂ is progressively affecting seawater chemistry, but predicting ongoing and near-future consequences for marine ecosystems is challenging without empirical field data. Here we quantify tropical coral reef benthic communities at 37 stations with varying exposure to submarine volcanic CO₂ seeping, and determine the aragonite saturation state (Ω_Ar) where significant changes occur in situ. With declining Ω_Ar, reef communities displayed progressive retractions of most reef-building taxa and a proliferation in the biomass and cover of non-calcareous brown and red algae, without clear tipping points. The percent cover of all complex habitat-forming corals, crustose coralline algae (CCA) and articulate co2025-01-01T00:00:00Z2025 Nov2026-06-05T19:16:52.529Z2026-05-20T03:14:33.575ZS-EPMC126444854128636310.1038/s42003-025-08889-w