<i>Helicosphaera carteri</i> (Prymnesiophyceae) under high carbon dioxide: An experimental study

The coccolithophore Helicosphaera carteri is an understudied, yet ecologically and biogeochemically important, marine calcifier. Hence, its response to ocean acidification has implications for ecosystem function and the marine carbon cycle. Here we employed dilute batch cultures featuring a coupled C-system manipulation (295, 444, and 600 μatm CO₂) to analyze the response of H. carteri in terms of growth rate and particulate carbon production, two key eco-physiological and biogeochemical parameters. We highlight that both growth rate and organic carbon production are CO₂ limited at 295 µatm but are not proton inhibited at 600 µatm of CO₂. This finding, combined with the maintenance of a stable inorganic production rate, places H. carteri among the coccolithophores less sensitive to seawater acidification. In addition, we tested a widely applied assumption underpinning the determination of carbon production, namely the constancy of particulate carbon quotas over the course of a dilute batch culture. We determined that the assumption holds true, an important validation of a method used in many publications.