Hive-stored pollen of honey bees: many lines of evidence are consistent with pollen preservation, not nutrient conversion

Authors: Anderson, Kirk; Carroll, Mark; Sheehan, Timothy; Mott, Brendon; MAES, PATRICK - University Of Arizona; Corby-Harris, Vanessa

Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2014
Publication Date: 11/5/2014
Citation: Anderson, K.E., Carroll, M.J., Sheehan, T.H., Mott, B.M., Maes, P., Corby-Harris, V.L. 2014. Hive-stored pollen of honey bees: many lines of evidence are consistent with pollen preservation, not nutrient conversion. Molecular Ecology. doi: 10.1111/mec.12966.

Interpretive Summary: Honey bees consume pollen stores to produce food for their developing young and are fed food supplements during times of pollen dearth. A clear understanding of natural vs. artificial nutrition in the honey bee is critical to the long term management of migratory operations, overwintering success and pollination services. To better model natural food storage we examined the structure and potential function of stored pollen in honey bee colonies using next generation sequencing technology and bacterial plate counts to identify and enumerate the bacteria found in this microenvironment. We also tested the preference of bees for older vs. younger food stores that had aged naturally inside the hive. Our results indicate that stored pollen is not evolved for microbially mediated nutrient conversion, but is a preservative environment due primarily to added honey, nectar, bee secretions, properties inherent to the pollen grain, and perhaps acidic contributions from bacteria that can endure water-poor conditions. Our findings have important implications for the improvement of natural food storage, artificial food supplements, and water balance in the hive especially during overwintering.

Technical Abstract: The honey bee colony is a complex homeostatic unit, filled with nutritionally rich resources and a broad spectrum of microbial microenvironments. The hive contains tree resins, wax, honey and pollen, all substances known to sterilize or inhibit microbes. Stored pollen provides the bulk of proteins, vitamins and lipids for colony growth. Stored pollen is also warm, moist and sugar rich, a reservoir for unknown pathogens and potentially degradative microorganisms. Studies suggest that stored pollen is distinguished from “bee bread” because microbes ferment its simple sugars and/or convert the nutrients present in the pollen portion. To explore the bacterial community structure of newly collected and stored pollen, we determined their associated bacterial communities throughout the year using 454 amplicon sequencing of the 16S rRNA gene. Communities differed significantly by sampling period, but colony origin and pollen state (newly-collected vs. stored) did not differ. While community structure showed some constancy concerning microbial presence, prevailing conditions throughout the hive or pollination environment may play a major role in structuring the microbial communities of both newly-collected and stored pollen. Bacteria capable of digesting complex plant polymers were present in all pollen samples. Despite the presence of these groups, microscopy revealed the conspicuous absence of digested or compromised pollen grains in all samples regardless of season. Bacterial OTU quantification of pollen stored in the hive for 1-5 days revealed a significant negative association between storage time and bacterial number, and an estimated microbe to pollen grain surface area ratio of 1:1,000,000 for pollen aged >96 hours. Consistent with these results, the time required for nutritional alteration of pollen grains prior to consumption was lacking. We conclude that stored pollen is not evolved for microbially mediated nutrient conversion, but is a preservative environment due primarily to added honey, nectar, bee secretions and properties inherent to the pollen grain itself.