DIC measurement: an aliquot of the slurry was filtered with a 0.2-µm filter to remove cells and other particles. After filtration, 1 mL of samples were acidified with 100 μL 45% phosphoric acid overnight in an Exetainer® vial pre-purged with CO2-free air before analysis. All samples were measured with a Delta Ray Isotope Ratio Infrared Spectrometer (IRIS) with URI Connect and autosampler (Thermo Fisher Scientific, Germany) with an analytical error of ±1‰. All isotopic values are reported in the delta notation as δ13C relative to the Vienna PeeDee Belemnite (VPDB) standard. Deviations of triplicate isotopic measurement of sample DIC were between ± 1‰ and ± 1,000‰ (for DIC with label uptake of >10,000‰). DIC concentration was calculated from the released amount of CO2 by calibration with sodium hydrogen carbonate solution.