Acidogenic Bacteria Dominate Brood Pollen Provisions Across Diverse Bee Taxa.

Bacteria and fungi play vital roles in bee health and development, yet microbial communities associated with pollen provisions remain largely uncharacterized for most bee species, particularly ground-nesting taxa. Microbial communities may prevent pollen provision spoilage, liberate nutrients from pollen, supply direct nutrition and/or suppress pathogens, though these functions appear context-dependent and variable among hosts. To determine whether bee evolutionary history and ecological traits structure pollen provision microbiomes, we analyzed 345 'live' brood cell pollen provisions representing 39 species, 18 tribes and six bee families spanning diverse life histories, diet breadths, floral resource use and nesting substrates. Microbial composition was determined by amplicon sequencing of bacterial 16S rRNA (V5-V7) and fungal ITS (1 and 2) regions. Bacterial richness and community composition were strongly structured by bee taxonomy and were also associated with several ecological traits, while community composition was significantly associated with host phylogeny. Fungal richness also varied among bee taxa and ecological groups, whereas fungal community composition showed greater variability and weaker, though detectable, taxonomic structure, with comparatively small ecological-trait effects. Across phylogenetically distant bee tribes, pollen provisions were often dominated by lactic- or acetic acid-producing bacteria, indicating a strategy of microbial fermentation that reduces bacterial richness and may enhance food preservation. Together, these results reveal broad evolutionary and ecological patterns underlying the microbiome of solitary bee brood cells and suggest that acid-producing bacteria play a stabilizing role in the preservation and function of pollen provisions.