Ocean acidification alters the calcareous microstructure of the green macro-alga Halimeda opuntia

Decreases in seawater pH and carbonate saturation state ( Ω ) following the continuous increase in atmospheric CO 2 represent a process termed ocean acidification, which is predicted to become a main threat to marine calcifiers in the near future. Segmented, tropical, marine green macro-algae of the genus Halimeda form a calcareous skeleton that involves biotically initiated and induced calcification processes influenced by cell physiology. As Halimeda is an important habitat provider and major carbonate sediment producer in tropical shallow areas, alterations of these processes due to ocean acidification may cause changes in the skeletal microstructure that have major consequences for the alga and its environment, but related knowledge is scarce. This study used scanning electron microscopy to examine changes of the CaCO 3 segment microstructure of Halimeda opuntia specimens that had been exposed to artificially elevated seawater pCO 2 of ~650 µatm for 45 d. In spite of elevated seawater pCO 2 , the calcification of needles, located at the former utricle walls, was not reduced as frequent initiation of new needle-shaped crystals was observed. Abundance of the needles was ~22 % µm −2 higher and needle crystal dimensions ~14 % longer. However, those needles were ~42 % thinner compared with the control treatment. Moreover, lifetime cementation of the segments decreased under elevated seawater pCO 2 due to a loss in micro-anhedral carbonate as indicated by significantly thinner calcified rims of central utricles (35–173 % compared with the control treatment). Decreased micro-anhedral carbonate suggests that seawater within the inter-utricular space becomes CaCO 3 undersaturated ( Ω