Acclimation response type shapes temporal stability of cyanobacterial communities under thermal fluctuations

Thermal fluctuations increasingly take the form of recurring heatwaves, yet how the acclimation responses of community members shape collective temporal stability remains untested. Using communities of the marine pico-cyanobacterium Synechococcus sp. assembled in microcosms to span a broad range of mean response and response diversity, we tracked total cell density and per-cell chlorophyll a through a 16-day warming-cooling fluctuation regime. Temporal stability of total density was predicted by community mean response and response diversity, but only when computed from acute acclimation responses, not when computed from chronic responses at sustained conditions. Higher acute response diversity increased stability, while a higher mean acute response reduced it, independently of intraspecific richness. Community growth was sub-additive at warming transitions but matched the additive prediction at cooling, indicating that inter-strain interactions suppress community growth specifically as the community enters the warm state. Acclimation response is thus a determinant of both the predictability and the realized dynamics of microbial communities under recurring thermal stress.