This dataset comprises dissolved organic carbon (DOC) concentrations from axenic and xenic cultures of Thalassiosira gravida that were cultivated at the Alfred-Wegener-Institute (Bremerhaven, Germany) in March, 2023. After a cell density of ~ 15.000 cells * mL-1 was reached, cultures were filtered through a 0.2 µm polycarbonate (PC) filter (Whatman) that was cleaned by soaking in 10 % hydrochloric acid (HCl, Merck suprapure) for at least 12 h and subsequently rinsing with ultrapure water (Merck Millipore MilliQ). DOC was quantified by high temperature catalytic oxidation with a Shimadzu TOC analyzer (VCPN-TOC, Shimadzu) according to Garzón-Cardona et al. 2024; doi: 10.1016/J.JMARSYS.2023.103893. Cultures were grown under two temperatures (9 °C, 13.5 °C) and two photoperiods (16:8 h, 24:0 h light:dark). The aim was to investigate responses of algal extracellular release and bacterial DOC transformation to marine heatwave-like conditions.