Nectar Yeasts Research Articles

Yeasts in nectar of an early‐blooming herb: sought by bumble bees, detrimental to plant fecundity

Through their effects on physicochemical features of floral nectar, nectar-dwelling yeasts can alter pollinator behavior, but the effect of such changes on pollination success and plant reproduction is unknown. We present results of experiments testing the effects of nectar yeasts on foraging patterns of captive and free-ranging bumble bees, and also on pollination success and fecundity of the early-blooming, bumble bee-pollinated Helleborus foetidus (Ranunculaceae). Under controlled experimental conditions, inexperienced Bombus terrestris workers responded positively to the presence of yeasts in artificial sugar solutions mimicking floral nectar by visiting proportionally more yeast-containing artificial flowers. Free-ranging bumble bees also preferred yeast-containing nectar in the field. Experiments conducted in two different years consistently showed that natural and artificial nectars containing yeasts were more thoroughly removed than nectars without yeasts. Experimental yeast inoculation of the nectar of H. foetidus flowers was significantly associated with reductions in number of pollen tubes in the style, fruit set, seed set, and mass of individual seeds produced. These results provide the first direct evidence to date that nectar yeasts can modify pollinator foraging patterns, pollination success, and the quantity and quality of seeds produced by insect-pollinated plants.

Correction for Phylogenetic relatedness predicts priority effects in nectar yeast communities

[ Proc. R. Soc. B 279 , 749–758 (25 September 2012; Published online 20 July 2011) (doi:[10.1098/rspb.2011.1230][2])][2] On page 753, the formula for the invasion score metric used to measure the relative strength of priority effects is given incorrectly. The correct formula should be ( y i

Micro-organisms behind the pollination scenes: microbial imprint on floral nectar sugar variation in a tropical plant community

Variation in the composition of floral nectar reflects intrinsic plant characteristics as well as the action of extrinsic factors. Micro-organisms, particularly yeasts, represent one extrinsic factor that inhabit the nectar of animal-pollinated flowers worldwide. In this study a 'microbial imprint hypothesis' is formulated and tested, in which it is proposed that natural community-wide variation in nectar sugar composition will partly depend on the presence of yeasts in flowers. Occurrence and density of yeasts were studied microscopically in single-flower nectar samples of 22 animal-pollinated species from coastal xeric and sub-humid tropical habitats of the Yucatán Peninsula, Mexico. Nectar sugar concentration and composition were concurrently determined on the same samples using high-performance liquid chromatography (HPLC) methods. Microscopical examination of nectar samples revealed the presence of yeasts in nearly all plant species (21 out of 22 species) and in about half of the samples examined (51·8 % of total, all species combined). Plant species and individuals differed significantly in nectar sugar concentration and composition, and also in the incidence of nectar yeasts. After statistically controlling for differences between plant species and individuals, nectar yeasts still accounted for a significant fraction of community-wide variance in all nectar sugar parameters considered. Significant yeast × species interactions on sugar parameters revealed that plant species differed in the nectar sugar correlates of variation in yeast incidence. The results support the hypothesis that nectar yeasts impose a detectable imprint on community-wide variation in nectar sugar composition and concentration. Since nectar sugar features influence pollinator attraction and plant reproduction, future nectar studies should control for yeast presence and examine the extent to which microbial signatures on nectar characteristics ultimately have some influence on pollination services in plant communities.

Nectar yeasts of two southern Spanish plants: the roles of immigration and physiological traits in community assembly

Recent studies have shown that dense yeast populations often occurring in floral nectar are numerically dominated by a few species from the flower-insect interface specialized genus Metschnikowia, while generalist yeast species commonly occurring on leaf surfaces, soil, freshwater, and air were rarely isolated from nectar samples. This study was designed to understand the main factors responsible for the assembly of nectar yeast communities, by combining field experiments with laboratory tests characterizing the physiological abilities of all yeast species forming the pool of potential colonizers for two Spanish flowering plants (Digitalis obscura and Atropa baetica). Yeast frequency and species richness were assessed in external sources (bee glossae, air, plant phylloplane) as well as in pollinator rewards (pollen, nectar). Yeasts were most frequent in external sources (air, flower-visiting insects), less so in the proximate floral environment (phylloplane), and least in pollen and nectar. Nectar communities appeared to be considerably impoverished versions of those in insect glossae and phylloplane. Nectar, pollen, and insect yeast assemblages differed in physiological characteristics from those in other substrates. Nectarivorous Metschnikowia were not more resistant than other yeast species to plant secondary compounds and high sugar concentrations typical of nectar, but their higher growth rates may be decisive for their dominance in ephemeral nectar communities.

Phylogenetic relatedness predicts priority effects in nectar yeast communities.

Priority effects, in which the outcome of species interactions depends on the order of their arrival, are a key component of many models of community assembly. Yet, much remains unknown about how priority effects vary in strength among species in a community and what factors explain this variation. We experimented with a model natural community in laboratory microcosms that allowed us to quantify the strength of priority effects for most of the yeast species found in the floral nectar of a hummingbird-pollinated shrub at a biological preserve in northern California. We found that priority effects were widespread, with late-arriving species experiencing strong negative effects from early-arriving species. However, the magnitude of priority effects varied across species pairs. This variation was phylogenetically non-random, with priority effects stronger between closer relatives. Analysis of carbon and amino acid consumption profiles indicated that competition between closer relatives was more intense owing to higher ecological similarity, consistent with Darwin's naturalization hypothesis. These results suggest that phylogenetic relatedness between potential colonists may explain the strength of priority effects and, as a consequence, the degree to which community assembly is historically contingent.

Species Richness of Yeast Communities in Floral Nectar of Southern Spanish Plants

Floral nectar of insect-pollinated plants often contains dense yeast populations, yet little quantitative information exists on patterns and magnitude of species richness of nectar-dwelling yeasts in natural plant communities. This study evaluates yeast species richness at both the plant community and plant species levels in a montane forest area in southern Spain, and also explores possible correlations between the incidence of different yeast species in nectar and their reported tolerance to high sugar concentrations, and between yeast diversity and pollinator composition. Yeast species occurring in a total of 128 field-collected nectar samples from 24 plant species were identified by sequencing the D1/D2 domain of the large subunit rDNA, and rarefaction-based analyses were used to estimate yeast species richness at the plant community and plant species levels, using nectar drops as elemental sampling units. Individual nectar samples were generally characterized by very low species richness (1.2 yeast species/sample, on average), with the ascomycetous Metschnikowia reukaufii and Metschnikowia gruessii accounting altogether for 84.7% of the 216 isolates identified. Other yeasts recorded included species in the genera Aureobasidium, Rhodotorula, Cryptococcus, Sporobolomyces, and Lecythophora. The shapes and slopes of observed richness accumulation curves were quite similar for the nectar drop and plant species approaches, but the two approaches yielded different expected richness estimates. Expected richness was higher for plant species-based than for nectar drop-based analyses, showing that the coverage of nectar yeast species occurring in the region would be improved by sampling additional host plant species. A significant correlation was found between incidence of yeast species in nectar and their reported ability to grow in a medium containing 50% glucose. Neither diversity nor incidence of yeasts was correlated with pollinator composition across plant species.

Nectar yeasts warm the flowers of a winter-blooming plant

Yeasts are ubiquitous in terrestrial and aquatic microbiota, yet their ecological functionality remains relatively unexplored in comparison with other micro-organisms. This paper formulates and tests the novel hypothesis that heat produced by the sugar catabolism of yeast populations inhabiting floral nectar can increase the temperature of floral nectar and, more generally, modify the within-flower thermal microenvironment. Two field experiments were designed to test this hypothesis for the winter-blooming herb Helleborus foetidus (Ranunculaceae). In experiment 1, the effect of yeasts on the within-flower thermal environment was tested by excluding them from flowers, while in experiment 2 the test involved artificial inoculation of virgin flowers with yeasts. Nectary temperature (T(nect)), within-flower air temperature (T(flow)) and external air temperature (T(air)) were measured on experimental and control flowers in both experiments. Experimental exclusion of yeasts from the nectaries significantly reduced, and experimental addition of yeasts significantly increased, the temperature excess of nectaries (DeltaT(nect) = T(nect) - T(air)) and the air space inside flowers in relation to the air just outside the flowers. In non-experimental flowers exposed to natural pollinator visitation, DeltaT(nect) was linearly related to log yeast cell density in nectar, and reached +6 degrees C in nectaries with the densest yeast populations. The warming effect of nectar-dwelling yeasts documented in this study suggests novel ecological mechanisms potentially linking nectarivorous microbes with winter-blooming plants and their insect pollinators.

Presence of yeasts in floral nectar is consistent with the hypothesis of microbial-mediated signaling in plant-pollinator interactions

Olfactory floral signals are significant factors in plant-pollinator mutualisms. Recently, unusual fermentation odours have been described in the nectar and flowers of some species. Since yeasts are common inhabitants of many angiosperms nectars, this raises the possibility that nectar yeasts may act as causal agents of fermentation odours in flowers and, therefore, as possible intermediate agents in plant signaling to pollinators. A recent field study has reported that nectar yeasts were quite frequent in floral nectar across three different regions in Europe and America, where they reached high densities (up to 105 cells/mm3). Yeast incidence in floral nectar differed widely across plant host species in all sampling sites. A detailed study currently in progress on one of the species surveyed in that study (Helleborus foetidus, Ranunculaceae) has detected that, in addition to interespecific differences in yeast incidence, there is also a strong component of variance in yeast abundance that takes place at the subindividual level (among flowers of the same plant, among nectaries of the same flower). If yeast metabolism is eventually proved to contribute significantly to floral scent, then multilevel patchiness in the distribution of nectar yeasts (among species, among individuals within species, and among flowers and nectaries of the same individual) might contribute to concomitant multilevel variation in plant signaling and, eventually, also in pollination success, pollen flow and plant fitness.

Yeasts in floral nectar of some South African plants: Quantification and associations with pollinator type and sugar concentration

There is increasing evidence that nectarivorous yeasts are an important third player in plant–pollinator mutualisms, but their distribution and ecological effects remain poorly known. Here we provide a survey of the frequency and abundance of yeasts in floral nectar from 40 taxonomically diverse South African plant species, test whether they affect nectar properties, and investigate associations between yeast incidence and pollinator type. Microscopical observations of nectar samples revealed that yeasts are widespread in floral nectar of South African species, as revealed by the high percentage of plants (51.3%) and flowers (43.2%) containing those microbes, and that when present, they can reach high densities (up to 3.6 × 10 6 yeast cells/mm 3 in Moraea graminicola). Further, a significant negative correlation was found between yeast density and sugar content ( R s = − 0.463, P = 0.039) and yeast density and nectar concentration ( R s = − 0.470, P = 0.037) in a Watsonia species. Interestingly, variation in yeast incidence among plant species was related to differences in pollinator type, in such a way that the plant species pollinated by birds showed the highest proportion of plants and flowers with yeasts, while those visited only by Hymenoptera showed the lowest values. Our study confirms the ubiquity of nectarivorous yeasts in plant communities and identifies novel ways of approaching the study of nectar characteristics and exciting new perspectives on the role of yeasts in plant–pollinator relationships.

Yeasts in floral nectar: a quantitative survey

One peculiarity of floral nectar that remains relatively unexplored from an ecological perspective is its role as a natural habitat for micro-organisms. This study assesses the frequency of occurrence and abundance of yeast cells in floral nectar of insect-pollinated plants from three contrasting plant communities on two continents. Possible correlations between interspecific differences in yeast incidence and pollinator composition are also explored. The study was conducted at three widely separated areas, two in the Iberian Peninsula (Spain) and one in the Yucatán Peninsula (Mexico). Floral nectar samples from 130 species (37-63 species per region) in 44 families were examined microscopically for the presence of yeast cells. For one of the Spanish sites, the relationship across species between incidence of yeasts in nectar and the proportion of flowers visited by each of five major pollinator categories was also investigated. Yeasts occurred regularly in the floral nectar of many species, where they sometimes reached extraordinary densities (up to 4 x 10(5) cells mm(-3)). Depending on the region, between 32 and 44 % of all nectar samples contained yeasts. Yeast cell densities in the order of 10(4) cells mm(-3) were commonplace, and densities >10(5) cells mm(-3) were not rare. About one-fifth of species at each site had mean yeast cell densities >10(4) cells mm(-3). Across species, yeast frequency and abundance were directly correlated with the proportion of floral visits by bumble-bees, and inversely with the proportion of visits by solitary bees. Incorporating nectar yeasts into the scenario of plant-pollinator interactions opens up a number of intriguing avenues for research. In addition, with yeasts being as ubiquitous and abundant in floral nectars as revealed by this study, and given their astounding metabolic versatility, studies focusing on nectar chemical features should carefully control for the presence of yeasts in nectar samples.

Ecology of yeasts in plant–bumblebee mutualism in Central Europe

Yeast community involved in plant–bumblebee mutualism was investigated in three successive years. Yeasts were isolated from floral nectar, bumblebee queens after hibernation, bumblebee workers, and the honey provisions in nests. From the distribution of yeast species in the various microhabitats in the course of the year their ecology was assessed. Nectar of numerous plant species belonging to various plant families was analyzed in order to uncover possible impacts on the yeasts present in the nectar. Only ascomycetous yeasts were autochthonous members of the communities in the plant–bumblebee mutualism. Species in the Metschnikowia clade, the Starmarella clade, and the genera Debaryomyces and Zygosaccharomyces were associated with the mutualism. Some species appeared highly specialized, whereas others had a broader distribution. While physical and chemical properties of nectar had only limited influence on the abundance of nectar yeasts, the attractiveness of plants to the flower-visiting insects appears to have had a greater impact on the abundance and frequency of yeasts in the nectar of different plant species.

Foliar Herbivory Affects Floral Characters and Plant Attractiveness to Pollinators: Implications for Male and Female Plant Fitness

Foliar Herbivory Affects Floral Characters and Plant Attractiveness to Pollinators: Implications for Male and Female Plant Fitness