Nest ecology-associated impacts of wastewater on wild bee microbiomes

Anthropogenic pollution affects environments differently depending on proximity to the pollution source, exposure route, and species ecology. Thus, understanding an organism's ecological role and exposure route to contaminants is central to assessing pollution impact. Treated municipal wastewater releases contaminants into waterways and alters microbial communities. Plants absorb contaminants and expose animals through foraging and nest-building activities. Nesting ecology differences of ground and wood cavity-nesting bees offer insight into niche-specific susceptibility to pollution. We compared gut microbiomes of directly exposed soil-nesting Halictus ligatus and indirectly exposed wood-nesting Ceratina spp. upstream and downstream of wastewater. We collected bees, flowers, and soil and analyzed their bacterial microbiomes using 16S rRNA sequencing. The effect from wastewater explained a much larger proportion of the variance in the ground-nesting H. ligatus microbiome than wood cavity-nesting Ceratina adults, Ceratina larvae, and pollen provisions. Conversely, soil and flower microbiomes remained stable, indicating higher stability. These findings suggest that wastewater exposure leads to strong host-associated microbiome disruption, with ground-nesting bees most affected.IMPORTANCEBecause contaminants bind strongly to soil, ground-nesting bees near wastewater are likely most impacted, while wood cavity-nesting bees are likely less impacted since plant uptake of contaminants is species dependent. These results suggest that exposure routes drive contaminant susceptibility, with bee-associated microbes most vulnerable. Because bees are important pollinators and biodiversity contributors, these disruptions point to broader ecological risks in increasingly contaminated landscapes.