LAUSR

Abstract The increase in atmospheric carbon dioxide (CO2) observed since the industrial revolution has reduced surface ocean pH by ∼0.1 pH units, with further change in the oceanic system predicted in the coming decades. Calcareous organisms can be negatively affected by extreme changes in seawater pH (pHsw) such as this due to the associated changes in the oceanic carbonate system. The boron isotopic composition (δ11B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pHcf) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pHsw on this process. Motivated by these investigations, this study examines the δ11B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pHcf, and investigates how this taxon’s pHcf is impacted by ocean acidification. δ11B was measured in multiple algal replicates (n = 4–5) cultured at four different pCO2 scenarios – averaging (±1σ) 409 (±6), 606 (±7), 903 (±12) and 2856 (±54) μatm, corresponding to average pHsw (±1σ) of 8.19 (±0.03), 8.05 (±0.06), 7.91 (±0.03) and 7.49 (±0.02) respectively. Results show that skeletal δ11B is elevated relative to the δ11B of seawater borate at all pHsw treatments by up to 18‰. Although substantial variability in δ11B exists between replicate samples cultured at a given pHsw (smallest range = 2.32‰ at pHsw 8.19, largest range = 6.08‰ at pHsw 7.91), strong correlations are identified between δ11B and pHsw (R2 = 0.72, p