Center for Computational and Theoretical Biology

The Plant-Pollinator-Microbe Triangle


Despite the major ecological and agricultural importance of plant-pollinator associations, the microbiota of the anthosphere and of wild pollinators are not well understood. I started to work on this line of research during the final phase of my doctoral studies with first floral microbiome assessments, and now supplement this as a young investigator groupleader with the bee-microbiota association perspective. I adress fundamental questions about diversity and community structure, but also aim to understand the ecological functions and the molecular mechanisms. The major research objectives for the near future are:

  • Assessing diversity and community structure of microbiota for plant reproductive organs (KE1743/5-1) and wild pollinators (KE1743/4-1)
  • Connecting flower-microbe ecology with visiting pollinators, land-use-intensity, floral carbon and nitrogen resources as well as secondary metabolites (DFG KE1743/5-1)
  • Relating pollinator-microbe ecology to plant diversity, foraging patterns, immunity and land-use (DFG KE1743/6-1)
  • Microbial genomics, transcriptomics and bioassays to identify ecological functions and understand the molecular mechanisms (DFG KE1743/5-1, KE1743/6-1)
  • Identifying co-evolutionary patterns of microbes with respect to host physiology and ecology (DFG E1743/4-1)
  • Finding and understanding similarities and discrepancies in microbial taxonomic and genetic diversity and corresponding community structuring rules (DFG KE1743/5-1)
  • Addressing impacts of cross-transferred microbiota on pollination services, health and life-history of plants and pollinators (DFG KE1743/5-1, KE1743/6-1) 


Bee Microbiota

Bees are important agents in ecological and agricultural systems, mostly through their incommensurable pollination services. It is however not only the abundance of bees that regulates ecosystem benefits, but also a diversity of pollinators is required to maintain effective pollination. Microbial assessments are rare beyond honey- and bumble-bees, although it is of great ecological importance to address these to understand life history and threats. There is not only a lack of knowledge about pathogens, but bee-microbe associations in general. Mutualists are important for host health as well as nutrition uptake, but their co-evolution with bees species completely unclear. Also in how far foraging strategies, immunity and the chemical composition of body surfaces are linked to such. My group performed initial studies in this system to address these gaps. The project starts with microbial diversity assessments throughout the Apiformes, alongside determination of foraging patterns and pathogens. Secondly, we aim to determine biochemical capabilities of isolated bacteria through bioassays for pollen fermentation and antimycotic activity as well as screening of secondary metabolites. Lastly, the project will assemble genomes of isolates to infer the evolution of biochemically relevant genes and pathways important to hosts, their phylogenetic history as well as ecology.

Plant Microbiota

Together with Robert R. Junker from the University of Salzburg we aim to identify patters in microbiota of flower communities and their effects on pollination.

Land-use effects

Within the framework of the Biodiversity Exploratories Initiative, we also conduct experiments and assessments how land-use and diversity affects the microbial compositions in plants (with. R.R. Junker) and bees (with S. Leonhardt and M. Schloter). Both projects aim to understand the interactions on a taxonomic, functional as well as genomic level alongside a gradient of different land-use intensities. Bacterial associates are capable of horizontal gene transfer, so that taxon diversity and genetic diversity of hosts and associates may thus be inconsistent and add complexity to analyses, but provide fast adaptation potential to co-evolutionary processes. The impact of such effects on community assembly, association with specific hosts and their respective ecology is currently completely unclear. We aim to analyse such effects in the long-term in experimental approaches and computer simulations.


2022[ to top ]
  • Plants, pollinators and their interactions under global ecological change: The role of pollen DNA metabarcoding. Bell, Karen L.; Turo, Katherine J.; Lowe, Abigail; Nota, Kevin; Keller, Alexander; Encinas-Viso, Francisco; Parducci, Laura; Richardson, Rodney T.; Leggett, Richard M.; Brosi, Berry J.; Burgess, Kevin S.; Suyama, Yoshihisa; de Vere, Natasha. In Molecular Ecology, n/a(n/a). 2022.
  • Wild bee larval food composition in five European cities. Casanelles Abella, Joan; Keller, Alexander; Müller, Stefanie; Aleixo, Cristiana; Alós Orti, Marta; {Chiron, François andand Laanisto}; Pinho, Pedro; Samson, Roeland; Tryjanowski, Piotr; Van Mensel, Anskje; Villarroya-Villalba, Lucia; Pellissier, Loic; Moretti, Marco. In Ecology, bl in press. 2022.
  • Critical links between biodiversity, nutrition and health in wild bee conservation. Parreno, M.A.; Alaux, C.; Brunet, J.-L.; Butschkau, S.; Buydens, L.; Filipiak, M.; Henry, M.; Keller, Alexander; Klein, A.-M.; Kuhlmann, M; Leroy, C.; Meeus, I; Palmer-Young, E; Piot, N.; Requier, F.; Ruedenauer, F.; Smagghe, G.; Stevenson, P.C; Leonhardt, S.D. In Trends in Ecology and Evolution, bl in press. 2022.
  • Diets maintained in a changing world: Does land-use intensification alter wild bee communities by selecting for flexible generalists?. Peters, Birte; Keller, Alexander; Leonhardt, Sara D. In Ecology and Evolution, bl in press. 2022.
  • Do amino and fatty acid profiles of pollen provisions correlate with bacterial microbiomes in the mason bee Osmia bicornis?. Leonhardt, Sara D.; Peters, Birte; Keller, Alexander. In Philosophical Transactions of the Royal Society B, bl in press. 2022.
  • Forest landscapes increase diversity of honeybee diets in the tropics. Cannizzaro, Chris; Keller, Alexander; Wilson, Rachele S.; Elliott, Brittany; Newis, Ryan; Ovah, Raywin; Inae, Kelly; Kerlin, Douglas H.; Bar, Ido; Kämper, Wiebke; Shapcott, Alison; Wallace, Helen M. In Forest Ecology and Management, bl in press. 2022.
2021[ to top ]
  • (More than) Hitchhikers through the network: The shared microbiome of bees and flowers. Keller, Alexander; McFrederick, Quinn S.; Dharampal, Prarthana; Steffan, Shawn; Danforth, Bryan N.; Leonhardt, Sara D. In Current Opinion in Insect Science. 2021.
  • An integrative environmental pollen diversity assessment and its importance for the Sustainable Development Goals. Hornick, Thomas; Bastl, M; Bohlmann, S; Bonn, A; Bumberger, J; Dietrich, P; Gemeinholzer, B.; Grote, R; Harpole, W. S.; Heinold, B; Keller, Alexander; Luttkus, M. L.; Mäder, P.; Motivans, E.; Passonneau, S.; Punyasena, S.W.; Rakosy, Demetra; Richter, A.; Richter, R.; Sickel, W.; Steffan-Dewenter, I.; Theodorou, P.; Treudler, R.; Werchan, B.; Werchan, M.; Wolke, R.; Dunker, S. In Plants, People, Planet, bl in press. 2021.
  • Pollen diets and niche overlap of honey bees and native bees in protected areas. Elliott, Brittany; Wilson, Rachele; Shapcott, Alison; Keller, Alexander; Newis, Ryan; Cannizzaro, Chris; Burwell, Chris; Smith, Tobias; Leonhardt, Sara D.; Kämper, Wiebke; Wallace, Helen. In Basic and Applied Ecology, Special issue: Sequence based molecular ecology. 2021.
  • Many small rather than few large sources identified in long-term bee pollen diets in agroecosystems. Wilson, Rachele; Keller, Alexander; Shapcott, Alison; Leonhardt, Sara D.; Sickel, Wiebke; Hardwick, J. L.; Heard, Tim; Kaluza, Benjamin F; and Wallace, Helen. In Agriculture, Ecosystems and Environment. 2021.
2020[ to top ]
  • Tracking beekeepers in floristic regions of Iran: employing DNA metabarcoding to determine the geographical origin of honey. Khansaritoreh, Elmira; Salamaki, Y; Ramezani, E; Akbari-Azirani, T; Keller, Alexander; Neumann, Katrin; Alizadeh, K; Zarre, S; Beckh, G; Behling, H. In Heliyon. 2020.
2019[ to top ]
  • Bacterial community structure and succession in nests of two megachilid bee genera. Voulgari-Kokota, Anna; Grimmer, Gudrun; Steffan-Dewenter, Ingolf; Keller, Alexander. In FEMS Microbiology Ecology, 95(1), bl fiy218. 2019.
2018[ to top ]
  • Wild bees and their nests host Paenibacillus bacteria with functional potential of avail. Keller, Alexander; Brandel, Annette; Becker, Mira C.; Balles, Rebecca; Abdelmohlsen, Usama R.; Ankenbrand, Markus J.; Sickel, Wiebke. In Microbiome, (6), bl 229. 2018.
2017[ to top ]
  • Honey bee foraging ecology: Season but not landscape diversity shapes the amount and diversity of collected pollen. Danner, Nadja; Keller, Alexander; H{ä}rtel, Stephan; Steffan-Dewenter, Ingolf. In PloS one, 12(8), bl e0183716. Public Library of Science, 2017.
  • Generalist social bees maximize diversity intake in plant species-rich and resource-abundant environments. Kaluza, Benjamin F.; Wallace, Helen; Keller, Alexander; Heard, Tim A.; Jeffers, Bradley; Drescher, Nora; Blüthgen, Nico; Leonhardt, Sara D. In Ecosphere, 8(3), bl e01758-n/a. 2017.
2016[ to top ]
  • Laboratory rearing of solitary bees and wasps. Becker, Mira; Keller, Alexander. In Insect Science, 23, bl 918. 2016.
  • Pollen DNA barcoding: current applications and future prospects. Bell, Karen; De Vere, Natasha; Keller, Alexander; Richardson, Rodney; Gous, Annemarie; Burgess, Kevin; Brosi, Berry. In Genome, 59, bll 629–640. 2016.
2015[ to top ]
  • Functional and phylogenetic diversities of plant communities differently affect the structure of flower-visitor interactions and reveal convergences in floral traits. Junker, Robert R.; Blüthgen, Nico; Keller, Alexander. In Evolutionary Ecology, 29, bll 437–450. 2015.
  • Evaluating multiplexed next-generation sequencing as a method in palynology for mixed pollen samples. Keller, Alexander; Danner, Nadja; Grimmer, Gudrun; Ankenbrand, Markus; {von der Ohe}, Katharina; {von der Ohe}, Werner; Rost, Simone; Härtel, Stephan; Steffan-Dewenter, Ingolf. In Plant Biology, 17, bll 558–566. 2015.
  • Increased efficiency in identifying mixed pollen samples by meta-barcoding with a dual-indexing approach. Sickel, Wiebke; Ankenbrand, Markus; Grimmer, Gudrun; Holzschuh, Andrea; Härtel, Stephan; Lanzen, Jonathan; Steffan-Dewenter, Ingolf; Keller, Alexander. In BMC Ecology, 15, bl 20. 2015.
  • Microhabitat heterogeneity across leaves and flower organs promotes bacterial diversity. Junker, Robert R; Keller, Alexander. In FEMS Microbiology Ecology, 91(1), bl fiv09. 2015.
2014[ to top ]
  • Density-dependent responses by bumblebees to flower dwelling bacteria. Junker, Robert R; Romeike, Tanja; Keller, Alexander; Langen, Daniela. In Apidologie, 45, bll 467–477. 2014.
  • Pollen DNA barcoding using next-generation sequencing. Keller, Alexander; Härtel, Stephan; Steffan-Dewenter, Ingolf. In Barcode Bulletin, 5(3), bl 8. 2014.
2013[ to top ]
  • Diverse microbiota identified in whole intact nest chambers of the red mason bee Osmia bicornis (Linnaeus 1758). Keller, Alexander; Grimmer, Gudrun; Steffan-Dewenter, Ingolf. In PloS one, 8(10), bl e78296. Public Library of Science, 2013.
2011[ to top ]
  • Composition of epiphytic bacterial communities differs on petals and leaves. Junker, Robert R; Loewel, Christina; Gross, Roy; D{ö}tterl, Stefan; Keller, Alexander; Bl{ü}thgen, Nico. In Plant Biology, 13(6), bll 918–924. Wiley Online Library, 2011.