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. The biogenic sulfur compounds dimethyl sulfide (DMS), dimethylsulfoniopropionate (DMSP) and dimethyl sulfoxide (DMSO) are produced andtransformed by diverse populations of marine microorganisms and havesubstantial physiological, ecological and biogeochemical importance spanningorganism to global scales. Understanding the production and transformationdynamics of these compounds under shifting environmental conditions isimportant for predicting their roles in a changing ocean. Here, we reportthe physiological and biochemical response of a robust strain ofAlexandrium minutum, a dinoflagellate with the highest reported intracellular DMSP content,exposed to a 6  d increase in temperature mimicking mild and extremecoastal marine heatwave conditions ( +4 and +12 ∘C ). Under mild temperature increases ( +4 ∘C ), A. minutum growth wasenhanced, with no measurable physiological stress response. However, under avery acute increase in temperature ( +12 ∘C ) triggering thermalstress, A. minutum growth declined, photosynthetic efficiency ( FV∕FM ) wasimpaired, and enhanced oxidative stress was observed. These physiologicalresponses indicative of thermal stress were accompanied by increased DMS andDMSO concentrations followed by decreased DMSP concentration. At thistemperature extreme, we observed a cascading stress response in A. minutum, which wasinitiated 6 h after the start of the experiment by a spike in DMS and DMSOconcentrations and a rapid decrease in FV∕FM . This was followed byan increase in reactive oxygen species (ROS) and an abrupt decline in DMSand DMSO on day 2 of the experiment. A subsequent decrease in DMSP coupledwith a decline in the growth rate of both A. minutum and its associated totalbacterial assemblage coincided with a shift in the composition of the A. minutummicrobiome. Specifically, an increase in the relative abundance of the operational taxonomic units (OTUs)matching Oceanicaulis (17.0 %), Phycisphaeraceae SM1A02 (8.8 %) and Balneola (4.9 %) as well as adecreased relative abundance of Maribacter (24.4 %), Marinoscillum (4.7 %) and Seohaeicola (2.7 %) wereprimarily responsible for differences in microbiome structure observedbetween temperature treatments. These shifts in microbiome structure arelikely to have been driven by either the temperature itself, the changingphysiological state of A. minutum cells, shifts in biogenic sulfur concentrations, thepresence of other solutes, or a combination of all. Nevertheless, we suggestthat these results point to the significant effect of extreme heatwaves onthe physiology, growth and microbiome composition of the red-tide causingdinoflagellate A. minutum, as well as potential implications for biogenic sulfur cyclingprocesses and marine DMS emissions.

biogeosciences

Shifts in dimethylated sulfur concentrations and microbiome composition in the red-tide causing dinoflagellate &amp;lt;i&amp;gt;Alexandrium minutum&amp;lt;/i&amp;gt; during a simulated marine heatwave