The diatom Thalassiosira gravida under Arctic change: algal exudates and their microbial transformation under contrasting temperatures and photoperiods.

Climate change threatens polar ecosystems through rising temperatures and changing light regimes. Phytoplankton release of dissolved organic carbon (DOC) is a major carbon-cycle component, yet it remains unclear how temperature, light, and associated bacteria jointly affect DOC and dissolved organic matter (DOM) composition. Here, we analyzed axenic and xenic cultures of the Arctic diatom Thalassiosira gravida at 9°C and 13.5°C under 16:8 h and 24:0 h light:dark cycles. Extracellular DOC was quantified, and molecular composition characterized by untargeted ultrahigh-resolution mass spectrometry in original culture filtrates. In axenic cultures, growth rates were 266% higher at 13.5°C 24:0 h, and cell-normalized DOC concentrations 52% lower compared to 9°C 16:8 h, suggesting a shift in carbon allocation. Xenic cultures contained 50-80% lower cell-normalized DOC concentrations than axenic cultures and converged at 11.7 ± 0.9 pmol DOC cell-1. Despite similar DOC concentrations in xenic cultures, the 13.5°C 24:0 h treatment showed lower chemodiversity (-8%), H/C ratios (-7%), and higher oxidation (+24%) than 9°C 16:8 h, consistent with more strongly processed DOM. At 9°C, axenic 24:0 h cultures showed higher chemodiversity (+14%) and H/C ratios (+7%) compared to 16:8 h, indicating that prolonged photoperiods altered DOM composition toward signatures associated with greater putative bioavailability. Contrasting axenic-xenic patterns suggest that bacterial presence weakened the treatment-dependent DOM responses observed in axenic cultures. Overall, temperature and photoperiods altered algal DOC quantity and quality, and its microbial transformation, with implications for carbon cycling.IMPORTANCEThe Arctic Ocean is warming rapidly, and changing sea-ice conditions alter light availability. Phytoplankton release dissolved organic carbon (DOC), which can be transformed by associated bacteria, thus playing an essential role in the marine carbon cycle. Still, the fate of such DOC transformations remains difficult to predict in the scope of future climate change scenarios. Using axenic and xenic cultures of the Arctic diatom Thalassiosira gravida as a model, our findings emphasize that algal extracellular release and its microbial transformation must be evaluated not only in the framework of bulk DOC dynamics but also with respect to molecular diversity and composition. Therefore, physiological responses of Arctic phytoplankton to higher temperatures and prolonged photoperiods may affect algal carbon exudation as well as the diversity and composition of microbially processed DOM in a warming Arctic Ocean.