Constant and variable warming differentially shape bacterial coexistence through phage-mediated interactions

Frequency-dependent predation helps maintain bacterial diversity, but its stabilizing role may be compromised under climate warming. Whereas theory suggests that warming can weaken predator-prey interactions and destabilize prey coexistence, it remains unclear whether constant and variable warming regimes differentially disrupt predation and alter coexistence outcomes in bacterial communities. Here, we experimentally tested how constant and variable warming (both +4{degrees}C above ambient, but negligible versus high thermal variance) affect the coexistence of two Pseudomonas species in the presence of their lytic phage. Phage predation increased competitive symmetry between the two bacterial species and promoted bacterial coexistence. Under constant warming (negligible variance), this phage-mediated frequency dependence buffered competitive asymmetries and often promoted persistence of the otherwise inferior P. putida, thereby magnifying coexistence. In contrast, variable warming (high variance) weakened phage control, shifted the advantage to P. protegens, and increased competitive exclusion events. The erosion of top-down control under variable warming was consistent with strong thermal sensitivity of phage infection rates, revealed by thermal performance curves. Our findings demonstrate that phage reduction of competitive asymmetry between bacterial hosts is amplified under constant warming but undermined under variable warming, conditions that are becoming increasingly frequent under climate change.