Warming-induced shifts in phytoplankton carbon release are species-dependent

Dissolved organic matter (DOM) is the most abundant labile form of carbon in aquatic environments, comprising a wide range of biologically-derived molecules. It plays a central role in the global carbon cycle by impacting water turbidity, thus primary production, and serving as the main energy source to bacteria. Phytoplankton is a major DOM source, however, a comprehensive understanding of how global warming impacts the quality, production and accumulation of phytoplankton-derived DOM (DOMp) is still a challenge. Here we subjected axenic cultures of the bloom-forming cyanobacteria Microcystis aeruginosa and the diatom Cyclotella sp. to warming assays (+ 4°C), with and without acclimation. Samples at different growth stages were analyzed for dissolved organic carbon (DOC) concentration, flow cytometry and fluorescence spectroscopy. Bioavailability and DOC release rates consistently increased with warming in M. aeruginosa exudates. In contrast, Cyclotella sp. showed a decline in bioavailability with rising temperature, while DOC release rates appeared to increase only in the short term (i.e., without the long acclimation exposure). These findings point to potential shifts in DOM quantity and composition that may affect the microbiome structure and functioning in freshwaters as a consequence of global warming. Acclimation had an effect on some parameters, illustrating the importance of this procedure in warming experiments. Also, the detection of humic-like components in exudates from axenic cultures highlights the need for caution when attributing a terrestrial origin to fluorescence data.