Cadmium and Selenate Exposure Affects the Honey Bee Microbiome and Metabolome, and Bee-Associated Bacteria Show Potential for Bioaccumulation

Abstract

Honey bees are important insect pollinators used heavily in agriculture and can be found in diverse environments. Bees may encounter toxicants such as cadmium and selenate by foraging on plants growing in contaminated areas, which can result in negative health effects. Honey bees are known to have a simple and consistent microbiome that conveys many benefits to the host, and toxicant exposure may impact this symbiotic microbial community. We used 16S rRNA gene sequencing to assay the effects that sublethal cadmium and selenate treatments had over 7 days and found that both treatments significantly but subtly altered the composition of the bee microbiome. Next, we exposed bees to cadmium and selenate and then used untargeted liquid chromatography-mass spectrometry (LC-MS) metabolomics to show that chemical exposure changed the bees' metabolite profiles and that compounds which may be involved in detoxification, proteolysis, and lipolysis were more abundant in treatments. Finally, we exposed several strains of bee-associated bacteria in liquid culture and found that each strain removed cadmium from its medium but that only Lactobacillus Firm-5 microbes assimilated selenate, indicating the possibility that these microbes may reduce the metal and metalloid burden on their host. Overall, our report shows that metal and metalloid exposure can affect the honey bee microbiome and metabolome and that strains of bee-associated bacteria can bioaccumulate these toxicants. IMPORTANCE Bees are important insect pollinators that may encounter environmental pollution when foraging upon plants grown in contaminated areas. Despite the pervasiveness of pollution, little is known about the effects of these toxicants on honey bee metabolism and their symbiotic microbiomes. Here, we investigated the impact of selenate and cadmium exposure on the gut microbiome and metabolome of honey bees. We found that exposure to these chemicals subtly altered the overall composition of the bees' microbiome and metabolome and that exposure to toxicants may negatively impact both host and microbe. As the microbiome of animals can reduce mortality upon metal or metalloid challenge, we grew bee-associated bacteria in media spiked with selenate or cadmium. We show that some bacteria can remove these toxicants from their media in vitro and suggest that bacteria may reduce metal burden in their hosts.

The direct and indirect effects of environmental toxicants on the health of bumble bees and their microbiomes

“…We used a DNA extraction protocol based on Engel et al 2013 (58), Pennington et al 2017 (59), and Rothman et al 2019 (60). We first surface sterilized individual bees using a 0.1% sodium hypochlorite wash followed by three rinses with ultrapure water.…”

“…We pooled 5 μL of each normalized library and performed a final clean up with a single column PureLink PCR Purification Kit, then sequenced the multiplexed libraries using a V3 Reagent Kit at 2 × 300 cycles on an Illumina MiSeq Sequencer in the UC Riverside Genomics Core Facility. Raw sequencing data are available on the NCBI Sequence Read Archive (SRA) under accession numbers SRR6788889 - SRR6789022, and microbiome data of selenate versus control treatments were previously published in Rothman et al 2019 (53).…”

“…It is unknown what effects field-realistic doses of heavy metals or oxidizers such as hydrogen peroxide may have on foraging bees, particularly if consumed over a period of days as would naturally occur. In addition to this direct lethality, increasing evidence suggests that exposure to low doses can cause sublethal damage to bee health (22). Sublethal effects on bee health are known to often have cascading and negative impacts on the overall vigor of bee colonies and populations, and consequently are of significant importance.…”

“…Cadmium (42,43), selenium (44), copper (45), imidacloprid (46), and hydrogen peroxide (as measured as ROS (47)) have been shown to alter host microbiomes in non-bee study systems. Outside of our work, little is known about how these toxicants affect bee microbiota, with field-realistic selenium and cadmium exposure altering proportions of microbes in honey bees (22), and imidacloprid not affecting the microbiome of honey bees (48). While the effects of toxicants on host-associated microbes have been studied in many systems, a new paradigm is emerging in which the microbiome potentially protects its host from metal(loid) toxicity.…”

“…For example, host-associated bacteria have been shown to detoxify chromium and lead (49), copper (50), arsenic (51), and selenite (52), mainly through accumulation or respiration. Specifically in bees, the microbiome has been shown to reduce selenate-induced mortality (53), and bee-associated microbes removed cadmium from their environment (22), although the mechanisms for microbe-mediated protection against toxicants in bees remains largely unknown. As bacterial genes often encode metal(loid) transporters and detoxification pathways, we may be able to understand toxicant-protection mechanisms by annotating the pathways in symbiont genomes.…”