Stable carbon isotopes of phytoplankton as a tool to monitor anthropogenic CO2 submarine leakages

This study aims to validate the stable carbon isotopic composition (d13C) of phytoplankton as a tool for detecting submarine leakages of anthropogenic CO2(g), since it is characterised by d13C values significantly lower than the natural CO2 dissolved in oceans. Three culture experiments were carried out to investigate the changes in d13C of the diatom Thalassiosira rotula during growth in an artificially modified medium (ASW). Three different dissolved inorganic carbon (DIC) concentrations were tested to verify if carbon availability affects phytoplankton d13C. Simultaneously, at each experiment, T. rotula was cultured under natural DIC isotopic composition (d13C DIC) and carbonate system conditions. The available DIC pool for diatoms grown in ASW was characterised by d13C DIC values (-44.2 ± 0.9‰) significantly lower than the typical marine range. Through photosynthetic DIC uptake, microalgae d13C rapidly changed, reaching significantly low values (until -43.4‰). Moreover, the different DIC concentrations did not affect the diatom d13C, exhibiting the same trend in d13C values in the three ASW experiments. The experiments prove that phytoplankton isotopic composition quickly responds to changes in the d13C of the medium, making this approach a promising and low-impact tool for detecting CO2(g) submarine leakages from CO2(g) deposits