Thalassiosira rotula culture experiments for the assessment of phytoplankton stable carbon isotopic composition as a tool to monitor anthropogenic CO2 submarine leakages

Three culture experiment on the diatom Thalassiosira rotula were conducted within two 2L-photobioreactors under controlled conditions of light and temperature (20 ± 0.02 °C). During each experiment, diatoms were simultaneously grown in the two bioreactors filled respectively with different media. In one of the two bioreactor microalgae were grown in a natural seawater (NAT) medium, whereas in the second one an artificial seawater medium (ASW) was used. ASW medium was artificially prepared, to reach a modified isotopic composition of DIC using an industrial compressed CO2, and at each experiment a different initial DIC concentration was tested to verify if carbon availability would affect phytoplankton algal metabolism. For NAT medium, a sterile-filtered Adriatic Sea water was used and a natural DIC concentration used. During the experiments, the evolution of T. rotula abundance was investigated, together with the variation of the isotopic composition of dissolved inorganic carbon, phytoplankton organic carbon (as particulate organic carbon) and prokaryotes organic carbon (as the fraction < 10 µm of particulate organic carbon). Carbonate system parameters (in situ pHNBS, total alkalinity, DIC and dissolved CO2 concentration), nutrient (dissolved inorganic nitrogen and phosphate) concentration and prokaryotes abundance were also analysed during diatom cultivation.