Nectar bacteria, but not yeast, weaken a plant–pollinator mutualism

Abstract

Mutualistic interactions are often subject to exploitation by species that are not directly involved in the mutualism. Understanding which organisms act as such 'third-party' species and how they do so is a major challenge in the current study of mutualistic interactions. Here, we show that even species that appear ecologically similar can have contrasting effects as third-party species. We experimentally compared the effects of nectar-inhabiting bacteria and yeasts on the strength of a mutualism between a hummingbird-pollinated shrub, Mimulus aurantiacus, and its pollinators. We found that the common bacterium Gluconobacter sp., but not the common yeast Metschnikowia reukaufii, reduced pollination success, seed set and nectar consumption by pollinators, thereby weakening the plant-pollinator mutualism. We also found that the bacteria reduced nectar pH and total sugar concentration more greatly than the yeasts did and that the bacteria decreased glucose concentration and increased fructose concentration whereas the yeasts affected neither. These distinct changes to nectar chemistry may underlie the microbes' contrasting effects on the mutualism. Our results suggest that it is necessary to understand the determinants of microbial species composition in nectar and their differential modification of floral rewards to explain the mutual benefits that plants and pollinators gain from each other.

Environmental variability counteracts priority effects to facilitate species coexistence: evidence from nectar microbes

“…After 4 days of plate incubation at 228C, we determined the species identity of colonies based on morphology and enumerated colony-forming units (CFU) of each species. Molecular sequencing of colonies, conducted as described by Belisle et al [22] for yeasts and by Vannette et al [31] for bacteria, confirmed that colony morphology could be used reliably to identify the four species used in our experiment. Previously, we confirmed that the number of CFU was correlated closely to the number of cells in solution for yeasts [30] and bacteria [31].…”

“…A field survey of M. aurantiacus nectar at JRBP indicated that species diversity was low, with on average about one yeast species per flower, most commonly Metschnikowia reukaufii [22]. Individuals belonging to the genera of acetic acid bacteria such as Gluconobacter were some of the most common bacterial species found in M. aurantiacus nectar [31]. Although less common, many other species have also been found in M. aurantiacus nectar at JRBP, including another yeast species, Starmerella bombicola [22], and a bacterial species, Asaia sp.…”

“…To each tube, we added 10 ml of artificial nectar, which was within the range of the amount of nectar contained by a single flower [30]. The artificial nectar was prepared by filter-sterilizing 15% w/v sucrose solution supplemented with 0.32 mM amino acids from digested casein, to mimic levels of sugar and amino acids of M. aurantiacus nectar in the field [31].…”

“…Evidence indicates the presence of inhibitive priority effects among some of these nectar-inhabiting species [30] and high sensitivity of these species to ambient temperature [29]. It has also been suggested that microbial species affect one another via pre-emption of resources (amino acids and sugars) in nectar [30], and that species vary in the extent to which they alter and tolerate nectar pH [31]. Based on these previous studies, we hypothesized that temperature variability would alter the strength of priority effects by influencing the extent of resource pre-emption and pH modification by different species.…”

Nectar thieves influence reproductive fitness by altering behaviour of nectar robbers and legitimate pollinators in Corydalis ambigua (Fumariaceae)

“…A change in nectarrobbing behaviour of an individual bumblebee can be perceived quite quickly by the other bumblebees (Leadbeater & Chittka 2008). It is also possible that differences in bacterial or yeast communities that develop from visits by robbers and thieves could influence each other's patterns of visitation (Herrera, Pozo & Medrano 2013;Vannette, Gauthier & Fukami 2013). Further research on insect behavioural ecology may provide more insights into this phenomenon.…”