Divergent morphological responses accompany the warming-induced competitive shift from Planktothrix (maladaptive...

Cyanobacterial blooms, particularly those dominated by Microcystis and Planktothrix, are becoming more frequent in eutrophic freshwaters worldwide. Although these two species often exhibit seasonal succession linked to rising temperatures, the role of morphological responses in shaping their temperature-dependent competitive dynamics remains largely unexplored. In this study, we conducted controlled laboratory experiments to examine how temperature (20, 26, and 32 °C) modulates the competitive interaction between Planktothrix agardhii and Microcystis aeruginosa under both mono-culture and co-culture conditions. Population growth and morphological traits of both species were tracked throughout the experiment. Key findings include: (1) At 20 °C, the two species coexisted with only weak mutual inhibition; at 32 °C, M. aeruginosa nearly outcompeted P. agardhii, and competitive displacement rates increased markedly along the temperature gradient. (2) P. agardhii showed strong morphological plasticity in response to competition, characterized by shortened filaments, increased cell width, and a reduced surface area to volume ratio, with the most severe morphological degradation coinciding with the strongest competitive suppression at 32 °C, a pattern consistent with a maladaptive stress response rather than beneficial acclimation; (3) M. aeruginosa maintained stable morphology across all temperatures when competing against P. agardhii. These results indicate that temperature, morphological strategy, and competitive outcome are closely associated, and that opposing morphological responses, maladaptive plasticity in P. agardhii versus a stable and functionally beneficial morphology in M. aeruginosa, accompany the temperature-driven shift in bloom composition. Notably, filament fragmentation in Planktothrix populations may serve as an early indicator of an impending transition toward Microcystis-dominated toxic blooms under climate warming.