Final Report Summary - PACE (Precedents for Algal Adaptation to Atmospheric CO2: New indicators for eukaryotic algal response to the last 60 million years of CO2 variation) The PACE project has investigated how marine algae living in the ocean have adapted to long term natural changes in atmospheric CO2 during the geological past. Marine algae are highly sensitive to CO2 because it is an essential resource for their photosynthesis. We have investigated how the adaptations made by algae in the past might be encoded into the fossilized remains of the algae, especially the tiny shells of calcite and opal used by coccolithophorid algae and diatoms to cover their cells, respectively. We have discovered that as CO2 diminishes, coccolithophorid algae shift their allocation of cellular resources like bicarbonate from the process of calcification (shell formation) to the process of photosynthesis. We find that this shift, a major adaptation to cope with limitations in CO2, can be tracked by measurements of the two isotopes of carbon found in the calcite shells, or coccoliths, made by the algae. Measurements of these isotopes in fossil coccoliths shows that this adaptation became prevalent within the last 10 million years. With new measurements from the chemistry of marine diatoms and organic molecules produced by coccolithophorid algae, we have confirmed that atmospheric CO2 has declined significantly over the last 11 million years. This decline in atmospheric CO2 is confirmed to be the major forcing of climate responsible for the cooling observed over this time interval. Coccolithophorids have adapted to this CO2 decline by reducing the amount of carbon used to make shells (in the case of small celled coccolithophorids) or by modifying the mineral forming process in order to continue with a high degree of calcification despite less bicarbonate resource to the process (for large-celled coccolithophorids). The decrease in abundance of large celled coccoliths suggests that the former strategy was advantageous. The finding of the thickest, most heavily calcified coccolithophorid cells during the middle Miocene, when CO2 was highest and seawater pH was lowest, suggests that on long timescales there is no precedent for ocean acidification jeopardizing coccolithophorid calcification.
