Active modulation of the calcifying fluid carbonate chemistry (δ¹¹B, B/Ca) and seasonally invariant coral calcification at sub-Tropical limits

Coral calcification is dependent on both the supply of dissolved inorganic carbon (DIC) and the up-regulation of pH in the calcifying fluid (cf). Using geochemical proxies (δ¹¹B, B/Ca, Sr/Ca, Li/Mg), we show seasonal changes in the pH_cf and DIC_cf for Acropora yongei and Pocillopora damicornis growing in-situ at Rottnest Island (32°S) in Western Australia. Changes in pH_cf range from 8.38 in summer to 8.60 in winter, while DIC_cf is 25 to 30% higher during summer compared to winter (×1.5 to ×2 seawater). Thus, both variables are up-regulated well above seawater values and are seasonally out of phase with one another. The net effect of this counter-cyclical behaviour between DIC_cf and pH_cf is that the aragonite saturation state of the calcifying fluid (Ω_cf) is elevated ~4 times above seawater values and is ~25 to 40% higher during winter compared to summer. Thus, these corals control the chemical composition of the calcifying fluid to help sustain near-constant year-round calcification rates, despite a seasonal seawater temperature range from just ~19° to 24 °C. The ability of corals to up-regulate Ω_cf is a key mechanism to optimise biomineralization, and is thus critical for the future of coral calcification under high CO₂ conditions.