By Vilhelmiina Haavisto
When you think of pollination, you probably think of bees and butterflies flocking to colorful and delightfully scented flowers, transporting pollen from one to another in exchange for a sugary nectar reward. However, there is more to this picture than meets the eye — especially as the third party cannot be seen without a microscope.
Like most corners of the Earth, floral nectar is also inhabited by microbes; the sweet liquid houses different species of bacteria and yeasts that feed on the sugars it contains. These microbial tenants have the power to change the nectar’s chemical characteristics. How, you might ask? They produce small molecules collectively known as metabolites, including volatile organic compounds, or VOCs. These are often produced as byproducts when the microbes break down sugars to produce energy. However, VOCs also appear to be important for influencing the attractiveness of the nectar to pollinators.
You’ve got a friend in yeast
Different species of yeasts are common nectar-inhabitants and can modify the nectar in different ways. For example, yeasts ferment the sugars found in nectar, which changes the abundance of different sugars and other carbon-containing compounds. In this case, the effect appears to be positive: ethanol, an alcohol and a key fermentation product, has almost double the caloric value of sugars, so visiting yeast-inhabited flowers can give pollinators a lot of energy.
One species of yeast, Metschnikowia reukaufii, is specialized to live in the nectar environment, and many pollinators appear to respond positively to its presence. For example, bumblebees trained to associate the yeast’s unique nectar profile with a specific floral color (yes, bees can be trained!) actively sought out flowers of that color when given a choice of many. They also appear to spend more time on yeast-inoculated flowers than at flowers without yeasts.
So what makes Metschnikowia-nectar so tempting? One factor may be that it produces a particular blend of VOCs that appears to be highly attractive to pollinators such as the European honey bee, who are far more interested in VOC mixes that mimic Metschnikowia’s specific blend than those of other bacteria and yeasts. Bumblebees also exhibit a preference; when given a choice of feeding on nectar inoculated with different microbes, they often choose nectar containing Metschnikowia over nectar containing the bacterium Asaia.
One bad bacteria spoils the lot
Unlike yeasts, many bacteria seem to make nectar less tasty; the presence of some species, such as Asaia astilbes and Apilactobacillus kunkeei, can strongly deter honey bees. Asaia is known to acidify its environment by producing metabolites like acetic acid, the main component of vinegar, which might produce a taste in the nectar that many pollinators would rather avoid.
Bacteria also seem to cause bigger changes in nectar sugar content than yeasts, which can be unattractive to pollinators. For example, when a species of Gluconobacter is present in the nectar of a hummingbird-pollinated shrub’s flowers, pollination is significantly reduced, presumably because the mutualism between plant and pollinator is disturbed. However, when the nectar is instead inoculated with our old friend Metschnikowia, the mutualism is not disrupted and pollination continues as usual.
Nectar microbes out in the field
The ecological impacts that nectar microbes have on pollination in nature is still being studied. In some cases, nectar microbes appear to benefit pollinators. Foraging on nectar containing specific yeast species, including a species of Metschnikowia, improves bumblebee colony growth, and can inhibit the growth of a common bumblebee gut pathogen. However, how this works, as well as its relevance in natural settings, is still unclear.
Beyond influencing pollinators, nectar metabolites may also be a tactic employed by the microbes themselves to increase their spread from flower to flower by hitchhiking on the pollinator mouthparts. This kind of exploitation of an established mutualism is not uncommon, and it may be that yeasts are more dependent on pollinator-hitchhiking than bacteria, as evidenced by the way their presence appears to appeal to pollinators.
The VOCs that pollinators detect might also provide them with cues about the nectar reward’s quality. For example, they could learn that yeast-produced VOCs signal rewards with high ethanol or protein content — the yeasts themselves can be eaten as sources of the latter. This could help pollinators improve their foraging efficiency, by getting the maximum ‘bang for their buck’ when visiting flowers. However, VOCs are just one of many cues that together form a complex signal to attract the right pollinators to a flower, and their relative importance is still not well understood.
Pollination in the face of climate change
A significant proportion of the Earth’s biodiversity depends on the activity of animal pollinators, which is why it is so worrying that pollinators are in decline. The reasons are manifold, and include pollution, anthropogenic climate change, and loss of plant habitats.
What’s more, the latest research suggests that nectar microbes can also be affected by climate change. In an experiment where natural nectar-associated communities were kept at different temperatures, it became apparent that warmer temperatures promote the growth of bacteria. Their increased abundance in the nectar reduces its sugar content, which in turn deters bumblebees. However, these bees still preferred microbially colonized nectar to sterile nectar, indicating that there is some ‘sweet spot’ of microbial abundance.
The tight coupling of plant-pollinator interactions means that both parties must adapt in tandem if the relationship is to survive climate change, but nectar microbes might add another layer of complexity. If plants change their nectar chemistry in response to warming, it could favor different microbial inhabitants that would in turn change pollinator activity. Moreover, the slow evolution of pollinator preferences compared to the rapid escalation of climate change means that these foundational mutualisms are even more likely to be disturbed.
Plant-microbe-pollinator interactions are definitely interesting in their own right, considering just how influential these minute third parties can be. However, we also stand to learn a lot about how plants, microbes, and pollinators will all adapt together under climate change by studying the interactions that unite them. Altogether, nectar-inhabiting microbes add a fascinating dimension to the well-known, and already very complex, concept of plant-pollinator interactions.
Vilhelmiina Haavisto is a M
Sc student in Microbiology and Immunology at ETH Zürich in Switzerland. When not working with marine microbes and microbial communities, she can be found trying her hand at many different arts and crafts, reading, and exploring the great Swiss outdoors.
Featured image from Nathalie Hausser.
