Abstract
To remain in equilibrium with rising concentrations of CO sub(2) in the atmosphere, the ocean CO sub(2) reservoir is absorbing increased quantities of CO sub(2), leading to increases in carbonic acid concentrations and decreases of pH levels. Open ocean pH values have decreased by 0.1 since the beginning of the Industrial Revolution to a modern surface ocean value of approximately 8.1 and are predicted to decrease by another 0.3 to 0.5 units by 2100. This process, known as ocean acidification, will likely modify the biogeochemical processes of calcification and carbonate sediment production. Previous studies have demonstrated that decreased levels of ocean pH affect the carbonate ultrastructure of marine calcifiers. Amphistegina gibbosa, a circumtropical calcareous symbiont-bearing rotaliid benthic foraminifer, has several important roles in geobiological research. Amphistegina gibbosa serves as a bioindicator of coral reef health, is useful in paleoecological reconstructions, and is an important producer of carbonate sediment. Specimens of living Amphistegina gibbosa were grown in two sealed aquarium tanks containing an aragonite substrate, air bubbler, and temperature and pH probes. The tanks were also connected to a CO sub(2) injection apparatus, which through a pH-stat system, controlled the levels of pCO2 in the water. The water in one tank was maintained at a pH of 8.1 (a control tank representing current surface ocean pH) and the other at pH 7.9 (representing projected future surface ocean pH). Alkalinity of the waters was allowed to vary, but was monitored at specific time intervals. Specimens were removed from the tanks after approximately four weeks. Scanning electron microscopy was used to image ultrastructural details of the individual shells, including surface pores, which have been shown to respond to environmental conditions. Preliminary analysis of more than 1,500 surface pores from specimens from each tank indicates that the mean pore width in specimens grown at pH 8.1 was significantly less than the pore width of specimens grown at pH 7.9. Further, the mean diameter of individual tests grown at pH 8.1 was 885 km, versus 817 km for those grown at pH 7.9. Future analyses will examine the process of ultrastructural change during calcification, the effects of ocean acidification on ontogenetic development, and changes across a range of pH levels.