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

Abstract Dissolved organic matter (DOM) comprises one of the most abundant forms of carbon in aquatic environments and plays a central role in the global carbon cycle. Phytoplankton is an important 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 cyanobacterium 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, cell counts by flow cytometry and DOM characterization by fluorescence spectroscopy. Short-term (non-acclimated) warming almost doubled DOC release rates in both species, but returned to baseline levels in M. aeruginosa cultures with long-term (acclimated) warming, while for Cyclotella sp. acclimated cultures these rates varied from baseline to increased values, probably reflecting the known negative effects of sustained temperature increase on many diatom species. Bioavailability proxies consistently increased with warming in M. aeruginosa exudates. In contrast, Cyclotella sp. showed a decline in bioavailability proxies with rising temperature. Coefficients of variation for fluorescent DOM in response to temperature and growth stage are similar, revealing that both factors may contribute equally for the observed shifts in DOMp. These findings indicate that warming can alter the quantity and composition of DOMp, a key resource with potential consequences for community structure and functioning under changing climate scenarios.