Climate-driven epiphytic bacterial community along a latitudinal gradient and its implications for submerged macrophyte production.

Epiphytic bacterial communities play a regulatory role in biogeochemical carbon (C) and nitrogen (N) cycling and host plant growth. Although their biogeography has been well studied in terrestrial ecosystems, the latitudinal diversity patterns of epiphytic bacterial communities and their implications for cosmopolitan macrophyte production in freshwater ecosystems remain unclear. Here, we conducted a field survey to investigate the latitudinal patterns of bacterial communities colonising the leaf and root surfaces of Myriophyllum spicatum and a surface microbe sterilisation experiment using sediments from high and low latitude regions to verify their causal effects on plant biomass. Distinct diversity patterns were observed across plant compartments. Leaf bacterial community richness increased significantly and linearly with latitude, whereas root bacterial community richness exhibited no significant pattern. Leaf and root bacterial community β diversity significantly decreased with latitude. Mean annual precipitation and temperature emerged as the primary factors influencing the leaf and root community compositions, respectively. The total network edges, total network nodes, and node degree of the leaf bacterial community significantly increased with latitude. Moreover, the network stability peaked at mid-latitudes. The field survey revealed a significant unimodal pattern in host plant biomass along the latitudinal gradient. Significant pathways linking environmental nutrient and leaf bacterial N cycling to host biomass were observed. In the sterilisation experiment, epiphytic bacterial communities significantly increased host biomass in sediment from low latitudes, whereas an opposite pattern was observed in sediment from high latitudes. Sediment source and root bacterial N cycling were identified as significant direct drivers of host biomass. These findings demonstrate the biogeographic patterns of multifaceted epiphytic bacterial matrices (taxonomy, function, and network) of a cosmopolitan macrophyte and highlight the key roles of environmental nutrients and epiphytic bacterial communities in shaping primary production in freshwater ecosystems.