Shallower Living Depth Instead of Higher Seawater Alkalinity Enhanced Calcification in Bloom-Forming Coccolithophores During Their Pleistocene Acme Event

Abstract

Coccolithophores are a group of marine phytoplankton precipitating about 50% of total calcite carbonate in the surface ocean. During the Pleistocene, coccolithophores experienced several periodic high-abundance and dominance intervals (acmes) that significantly altered the ocean carbon cycle by increasing the production of carbonate in the ocean. However, the reason for these episodes of enhanced calcification is still unclear. Here, we focus on one of the most significant dominance intervals, the Gephyrocapsa caribbeanica acme event, that lasted between ∼500 and 300 thousand years ago. We find that the variations of seawater alkalinity made only a minor contribution to the increased calcification rates during coccolithophore blooms. Rather, coccolithophore carbon isotopic fractionation indicates that coccolithophores employed a stronger bicarbonate pumping to increase intracellular carbon availability. Greater nutrient availability and shallower living depth likely facilitated higher bicarbonate pumping rates. The upregulation of bicarbonate pumping indicates the vital role of nutrients and light, and not only the ocean carbonate system, in the evolution of marine phytoplankton. Models of future coccolithophore calcification response to changing ocean carbon chemistry would, therefore, benefit from a more comprehensive consideration of how light and nutrient availability affect cellular energy budgets and drive carbon uptake.