Geochemical evidence of temporal ecosystem photosynthetic plasticity within a pristine coral atoll

The impacts of ocean acidification on coral reef macroalgal community composition and metabolism have implications for the habitat supporting capacity of future reefs. In this pilot study, we use co-located semi-hourly measurements of total dissolved inorganic carbon (DIC), total alkalinity, and the stable carbon isotope composition of DIC (δ¹³C_DIC) over a 27 + h period from Tetiaroa Atoll, French Polynesia, to investigate the potential for reef carbonate chemistry to record information related to benthic photosynthetic community composition and response to natural gradients in ambient acidity and dissolved carbon dioxide. The results of this preliminary sampling and modeling exercise suggest that Tetiaroa’s macroalgal communities express plastic carbon-concentrating mechanisms (CCMs) over daily cycles of productivity but may potentially vary this expression as a function of ambient CO₂ and acidity within the ecosystem. Additional studies are, therefore, underway to investigate the implications of these observations for reef macroalgal compositional differences under rapidly acidifying oceans.