Assessing pollen nutrient content: a unifying approach for the study of nutritional ecology

Authors: Lau, Pierre; LESNE, PIERRE - TEXAS A&M UNIVERSITY; GREBENOK, ROBERT - CANISIUS COLLEGE; RANGEL, JULIANA - TEXAS A&M UNIVERSITY; BEHMER, SPENCER - TEXAS A&M UNIVERSITY

Submitted to: Philosophical Transactions of the Royal Society B
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2022
Publication Date: 5/2/2022
Citation: Lau, P.W., Lesne, P., Grebenok, R.J., Rangel, J., Behmer, S.T. 2022. Assessing pollen nutrient content: a unifying approach for the study of nutritional ecology. Philosophical Transactions of the Royal Society B. https://doi.org/10.6084/m9.figshare.c.5923123.
DOI: https://doi.org/10.6084/m9.figshare.c.5923123

Interpretive Summary: Flowering plants vary in the quantity and quality of resources they provide to pollinators. The resources contained in pollen from certain plant species can be more effective than others in improving colony growth and mitigating stressors to bee health. Therefore, assessing the nutritional content of bee-collected pollen is a priority to inform decision-making processes related to selecting pollinator-attracting plants for land management, as well as for creating artificial pollen substitutes for supplementary feeding. Current literature on the macronutrients in pollen indicate that pollen contains ~10% to 60% protein and 2% to 20% lipid. These large ranges are likely due to the unique physical nature of pollen, the sheer diversity of pollen types, and the different methods used to analyze the proteins and lipids contained in pollen. In this study, we review the benefits and limitations of some of the commonly used methods to assess proteins and lipids in pollen. We also analyzed the nutritional content (i.e., amounts of proteins, lipids, carbohydrates, fatty acids and elements) in Brassica and Rosa pollen and compared how different methods affected protein and lipid estimation. Because pollen is covered by an exine that protects the grain’s proteins and lipids within the cytoplasm, we found that pollen fracturation enables the complete extraction of proteins, lipids and fatty acids contained within Brassica pollen, but not Rosa pollen. We also show differences in the protein estimates obtained through the Bradford and the Dumas assays. We discuss the implications of these findings and make recommendations for how bee researchers should move forward with respect to analyzing and interpreting pollen protein and lipid content to improve our understanding of the nutritional needs for pollinators.

Technical Abstract: Poor nutrition and landscape changes are two top factors regularly cited for causing the decline of pollinator populations. However, what constitutes “poor nutrition” currently remains inadequately defined. Several key pollinators including most bees are true palynivores: the broad suite of macro- and micronutrients they require comes only from pollen. However, the nutritional content of different pollen types varies, which in turn impacts pollinator foraging behavior and nutrient regulation. Thus, the characterization of the multidimensional nutrient content of pollen is a critical first step to better understand pollinator health. However, the use of a wide range of analytical approaches to assess pollen nutritional content has complicated between-studies comparisons and blurs our understanding of pollinator nutrition. In the current study we first reviewed different methods used to estimate pollen protein and lipid content, two important macronutrients for pollinator health. We uncovered significant inconsistencies between methods and experimentally revealed these biases using Brassica and Rosa pollen. Second, we performed a carbohydrate assay and an elemental analysis on these pollen types to broadly characterize pollen nutrient content. We use our collective data to propose a unifying protocol for the analysis of pollen nutritional content for the study of pollinator health.