Hive-stored pollen of honey bees: Many lines of evidence are consistent with pollen preservation, not nutrient conversion
Mol Ecol. 2014 Oct 15
Honey bee hives are filled with stored pollen, honey, tree resins, and wax, all antimicrobial to differing degrees. Stored pollen is the nutritionally rich currency used for colony growth, and consists of 40-50% simple sugars.
Many studies speculate that prior to consumption by bees, stored pollen undergoes long-term nutrient conversion, becoming more nutritious "bee bread" as microbes pre-digest the pollen. We quantified both structural and functional aspects associated with this hypothesis using behavioral assays, bacterial plate counts, microscopy, and 454 amplicon sequencing of the 16S rRNA gene from both newly-collected and hive-stored pollen.
We found that bees preferentially consume fresh pollen stored for less than three days. Newly-collected pollen contained few bacteria, values which decreased significantly as pollen was stored >96 hours. The estimated microbe to pollen grain surface area ratio was 1:1,000,000 indicating a negligible effect of microbial metabolism on hive-stored pollen. Consistent with these findings, hive-stored pollen grains did not appear compromised according to microscopy. Based on year round 454 amplicon sequencing, bacterial communities of newly-collected and hive-stored pollen did not differ, indicating the lack of an emergent microbial community co-evolved to digest stored pollen.
In accord with previous culturing and 16S cloning, acid resistant and osmotolerant bacteria like Lactobacillus kunkeei were found in greatest abundance in stored pollen, consistent with the harsh character of this microenvironment.
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 of pollen itself.