Dissolved inorganic carbon driven dynamics of calcite shell formation in 12 strains of the freshwater algae Phacotus lenticularis (Chlorophyta)

This article explores the close relationship between dissolved inorganic carbonate ion concentration (DIC) and the calcification state of Phacotus lenticularis, a globally abundant freshwater phytoplankton that is responsible for a significant part of modern lake carbonate production during bloom formation. We cultured 12 freshly isolated P. lenticularis strains under an ecologically relevant range of DIC (0.2 to 12 mmol l-1 total scale) for 14 days. From this experiment we gained information on responses in shell formation and growth that highlight strong lower boundaries in morphometric calcite shell variables with regards to DIC. All P. lenticularis strains showed reduced shell thickness by up to 60 % and dissolved calcite crystals structures at declining DIC < 4 mmol l-1. Increasing DIC > 4 mmol l-1 had no significant effect on shell thickness and crystal length in the culture experiments. We found a significant preadaptation of all 12 strains to ambient DIC concentrations measured in their lake of origin, but no dependence of growth rates up to a lethal DIC of > 10 mmol l-1. The simulation experiments illustrate the close relationship between shell function and dissolved inorganic carbonate ion concentration in lakes and highlight the need of continued research of important roles in biogenic carbon transformation and storage in a future world.