Long term warming increased richness in a nutritional mutualism between bacteria and microalga. Bacteria associated to microalga

Microalgae, the primary producer of the oceans, have large capacity for environmental adaptation but consequences for the species interactions are rarely approached depending on coevolutionary responses. In a long-term experiment (two years) we adapted the marine diatom Chaetoceros tenuissmus to warming temperature and examined responses of the asymmetric mutualistic association with bacteria. The algae adapted to warming increased temperature optimum and maximum growth rate the associated bacteria strongly increasing maximum growth rate but not the temperature optimum. One Rhodobacteria genus dominated the associated bacterial community showing no breakdowns when coevolving to warming temperature. Extinctions occurred in the low proportion genus and were compensated with the appearance of new partners after long-term warming, representing a gain in richness. The new warming consortium was stable when transferred to the ambient temperature indicating a strong association after long-term coevolution. Time exposure to temperature appeared to be a relevant aspect in determining the responses in the association, highlighting the consequences of short warming events. Our results agree with the prediction that long-term evolution of asymmetric mutualistic associations increases strength and diversity, occurring particularly under warming.