Investigating the effects of ocean acidification and light chemistry on a reef-building crustose coralline alga from the tropical pacific at different depths

Ocean acidification (OA) negatively affects marine calcifying organisms, and can alter many chemical and physiological processes. Crustose coralline algae (CCA), such as Porolithon onkodes, are important structural calcifying components on coral reefs and they grow across a range of depths from shallow reef crests to deeper forereefs. The current study aimed to quantify the impacts of OA on this foundational reef-building species through measuring long term calcification and photosynthesis. This was done by investigating the interactive effects of light quantity, spectral quality and pCO₂ on P. onkodes to determine if CCA are impacted differentially by OA at depth. CCA samples were placed in mesocosms fitted with distinct and previously measured light filters, using both a PAR sensor and spectrometer, which were utilized to simulate the light characteristics that occur at shallow (2 m) and deeper (17 m) reefs in Moorea, French Polynesia in both ambient (400 μatm) and elevated (1000 μatm) pCO₂ treatments. Photosynthetic performance was measured weekly using Pulse Amplitude Fluorometry (PAM) and from rates of oxygen evolution in incubation chambers at the end of the 32 day experimental period. Results suggest that the rate of photosynthesis in OA conditions in high light environments is relatively unchanged and the difference in photosynthesis is significantly less in lower light environments at elevated pCO₂ levels when compared to ambient. Similarly, calcification rates over the course of the experiment were more impacted by light quantity than spectral quality. Overall, this study demonstrates that light quantity may partially mitigate the effects of OA in higher light environments. This potentially indicates that reefs of different depths may respond differently to future climate conditions such as OA thus resulting in differences among future reef structures.