Boron isotope sensitivity to seawater pH change in a species of Neogoniolithon coralline red alga

The increase in atmospheric carbon dioxide (CO2) observed since the industrial revolution has reduced surface ocean pH by similar to 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 (pH(sw)) such as this due to the associated changes in the oceanic carbonate system. The boron isotopic composition (delta(11) B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pH(cf)) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pH(sw) on this process. Motivated by these investigations, this study examines the delta(11) B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pH(cf), and investigates how this taxon's pH(cf) is impacted by ocean acidification. delta B-11 was measured in multiple algal replicates (n = 4-5) cultured at four different pCO(2) scenarios-averaging (+/- 1 sigma) 409 (+/- 6), 606 (+/- 7), 903 (+/- 12) and 2856 (+/- 54) mu atm, corresponding to average pH(sw) (+/- 1 sigma) of 8.19 (+/- 0.03), 8.05 (+/- 0.06), 7.91 (+/- 0.03) and 7.49 (+/- 0.02) respectively. Results show that skeletal delta(11) B is elevated relative to the delta(11) B of seawater borate at all pH(sw) treatments by up to 18%. Although substantial variability in delta(11) B exists between replicate samples cultured at a given pH(sw) (smallest range = 2.32% at pHsw 8.19, largest range = 6.08% at pH(sw) 7.91), strong correlations are identified between delta(11) B and pH(sw) (R-2 = 0.72, p < 0.0001, n = 16) and between delta(11) B and B/Ca (R-2 = 0.72, p < 0.0001, n = 16). Assuming that skeletal delta(11) B reflects pH(cf) as previously observed for scleractinian corals, the average pH(cf) across all experiments was 1.20 pH units (0.79 to 1.56) higher than pH(sw), with the magnitude of this offset varying parabolically with decreasing pH(sw), with a maximum difference between pH(sw) and pH(cf) at a pH(sw) of 7.91. Observed relationships between pH(sw) and calcification rate, and between pH(sw) and pH(cf), suggest that coralline algae exhibit some resilience to moderate ocean acidification via increase of pH(cf) relative to pH(sw) in a similar manner to scleractinian corals. However, these results also indicate that pH(cf) cannot be sufficiently increased by algae exposed to a larger reduction in pH(sw), adversely impacting calcification rates of coralline red algae. (C) 2017 Elsevier Ltd. All rights reserved.