Abstract
Climatic events driven by rising global temperature impact water quality and biota, in some cases leading to cyanobacterial blooms. Cyanobacteria and dinoflagellates can dominate phytoplankton communities, forming mixed or alternating harmful blooms whose shifts are influenced by seasonal temperature fluctuations. Microcystis aeruginosa , a microcystin-producing cyanobacterium, has formed dense blooms worldwide. Under certain conditions, M. aeruginosa alternates dominance with dinoflagellates, a phenomenon reported in several lakes. This study tested the hypotheses that increasing temperature would allow i) M. aeruginosa to outcompete Gymnodinium sp. and ii) stimulate production of temporary cysts by the dinoflagellate as a stress response. Batch cultures were established using mono- and co-cultures at 23 °C (control) and 30 °C (high temperature) for 14 days. Growth, photosynthetic activity, microcystin concentration, and cyst production were measured. The temperature increase alone did not influence the growth or photosynthetic performance of either species. However, in co-culture at 23 °C, M . aeruginosa showed enhanced growth and photosynthetic performance, whereas at 30 °C it exhibited lower microcystin concentration. For Gymnodinium sp., the combination of elevated temperature and interaction with M. aeruginosa had a negative impact, reducing growth, lowering photosynthetic efficiency, and increasing cyst formation. Under elevated temperature, M. aeruginosa gained a competitive advantage over the dinoflagellate. This, in turn, increased cyst formation as a stress response and survival strategy, favoring Microcystis through competition over resource exploitation rather than through direct physiological effects. These findings reinforce that warming may shift the phytoplankton structure toward cyanobacterial dominance, posing challenges for water quality and ecosystem management.