Phaeocystisblooms caused carbon drawdown during the Antarctic Cold Reversal from sedimentary ancient DNA

Abstract

AbstractThe Southern Ocean plays a crucial role in the global carbon budget. Modeling studies propose that the atmospheric CO2plateau during the Antarctic Cold Reversal (ACR; 14,700 to 12,700 calibrated years before present (cal yr BP)) is related to increased marine productivity. However, proxy evidence relating environmental conditions as well as primary community composition and productivity to carbon drawdown is missing. Our ancient DNA shotgun metagenomic analysis of marine sediments revealedPhaeocystis antarctica(haptophyte) as a key element of the primary producer community. Independent proxy evidence (blooming-related bacteria, Ba/Fe ratio) from the same sediment record point to high productivity in response to enhanced sea-ice seasonality caused by ACR cooling. Post ACR, abruptPhaeocystiscommunity loss shows how sensitive this ecosystem is to warming, potentially representing a key tipping element that may be further enhanced by thePhaeocystis-related sulfur cycle–climate feedback. As an analogy for present warming, it highlights the importance of regions with high seasonal sea-ice variability andPhaeocystis-dominance, such as the Ross Sea, for stabilizing atmospheric CO2content. Additionally, our shotgun metagenomic data portray complex Holocene ecosystem establishment including key Antarctic taxa such as penguins, whales, and Antarctic fishes with implications for ongoing conservation efforts.