Two phytoplankton species (T. pseudonana and Synechococcus sp. WH8102) were grown axenically in the laboratory under enriched Na213CO3 conditions in artificial seawater F/2 media without unlabeled inorganic carbon. The F/2 media was sparged overnight with N2, autoclave sterilized, and amended with algal vitamins (40 mg thiamine-HCl, 20 mg biotin, and 20 mg cobalamin per 250 mL). A total of 1 mL stock culture of each phytoplankton was transferred to a 250-mL culture flask of medium with either Na213CO3 or unlabeled Na2CO3 (used to monitor growth) added to the F/2 medium via stirring to 3-mM final concentration. Diatom and cyanobacterial cells were grown at 22 °C with the bottle sealed from the atmosphere under continuous light until late exponential phase (optical density at 600 nm of 0.2 and 0.1 after 150 and 200 h, respectively), whereupon the 13C-labeled cultures were centrifuged at 5,000 × g for 15 min. Supernatant was collected and sparged by bubbling with air, filter sterilized at 0.1 µm, and frozen at –80 °C (exudate). Cell pellets were rinsed and resuspended in sterile distilled and deionized water, freeze-thawed three times, sonicated, then centrifuged at 5,000 × g for 15 min. Supernatant was collected and frozen at –80 °C (lysate). Phytoplankton cells from the labeled cultures were confirmed to be >80% 13C using Lawrence Livermore National Lab’s nanoscale secondary ion mass spectrometry (nanoSIMS). In order to amend equal carbon mass to all treatments, DOM carbon content was measured by adding 250 to 750 µL exudate or lysate to 25-mm glass fiber filter membranes (baked for 4 h at 450 °C), allowing to dry overnight, then adding 250 µL 10% HCl to each filter and again drying overnight. Nutrient and elemental (CHN) analysis of DOM on filters was conducted by the Marine Science Institute (University of California, Santa Barbara). No measures of molecular weight or stoichiometric composition could be made on DOM, as the total exudate and lysate 13C-biomass was required to amend to mesocosms.