Cell-size-dependent responses of bacterial communities to warming in the alpine grasslands of the Qinghai-Tibet Plateau

Cell size is a fundamental determinant of bacterial ecology, influencing community dynamics, physiological processes, and ecological interactions. However, the impacts of climate change—especially warming—on bacteria of varying cell sizes remain poorly understood, particularly in sensitive ecosystems such as alpine grasslands. In this study, we conducted an open-top chamber warming experiment (+1.3 °C) to investigate how warming affects bacterial communities across four distinct cell size categories (<0.4 μm, 0.4–3.0 μm, 3.0–5.0 μm, and >5.0 μm) in the alpine grasslands of the Qinghai-Tibet Plateau. Our results show that warming differentially impacts bacterial communities depending on cell size. Small bacteria (<0.4 μm) were more sensitive to warming, with increased richness and diversity, while larger bacteria (3.0–5.0 μm) experienced a decline in both diversity and richness. These shifts were accompanied by compositional changes, particularly within the <0.4 μm fraction, where phyla such as β-Proteobacteria, δ-Proteobacteria, Bacteroidetes, and Thermotogae decreased, while Actinobacteria increased. Additionally, warming decoupled the interactions between large bacteria and soil or plant components, while enhancing plant-bacterial coupling for small bacteria. Co-occurrence network analysis revealed that warming reduced the complexity and connectivity of small bacterial communities, making them less stable and more influenced by deterministic processes. In contrast, warming promoted drift and heterogeneous selection in large bacterial communities, highlighting divergent ecological responses based on cell size. These findings underscore the critical role of cell size in determining bacterial vulnerability to climate change, offering new insights into microbial community dynamics in response to global warming.