Assessing the response of coccolithophores and foraminifera to enhanced ocean alkalinity as a CO2 sequestration technique

Abstract

The alkalinity of seawater sets the overall capacity of the ocean to hold carbon dioxide in dissolved forms. Variations in past alkalinity, related to changing weathering or carbonate compensation, may have played an important role in moderating or controlling past variations of atmospheric p CO 2 . Future manipulation of ocean alkalinity by direct addition of suitable chemicals to seawater, or through enhanced weathering on land, has also been suggested as one possible route to intentionally draw CO 2 from the modern atmosphere and mitigate the impacts of future climate change [1]. Although we know an increasing amount about how biological species and ecosystems respond to changes in pH, we know much less about their response to changes in alkalinity. Calcifying plankton play a crucial role in modulating the surface ocean carbonate system and its buffering of alkalinity perturbations [2]. Here we investigate the growth and calcification response of both coccolithophores and foraminifera to elevated ocean alkalinity and potential CO 2 limitation [3] through a series of carefully designed batch culture laboratory experiments. Alkalinity is raised by two different methods during the experiments: by (i) addition of NaHCO 3 and (ii) addition of Na 2 CO 3 and CaCl 2 . The reason for two differing elevated alkalinity treatments is that they allow us to constrain how physiology and calcification respond to two different modes of alkalinity manipulation; both of which provide simple laboratory analogues for probable real-world scenarios.