Floral nectar is a harsh environment that hosts a community of microbes that includes yeasts and bacteria. In this nitrogen-limited and osmotically-stressed environment, these microbes compete fiercely for resources. We surveyed a 200 km range of floral nectar from host plant sticky monkeyflower in California and assessed the distribution patterns of the nectar microbiome, in relation to guild membership and nectar chemistry. We further analyzed the prevalence of the dominant yeast Metschnikowia reukaufii, and identified genetic and physiological trade-offs that may play a role in facilitating coexistence in the nectar microbiome. Direct microcosm competition experiments against other species evaluated the competitive strength of M. reukaufii strains to explain the observed distribution patterns.
The nectar microbiome is a low diversity system, predominantly occupied by Metschnikowia yeasts and few bacterial species. Across the landscape, bacteria and yeasts were rarely found co-habiting the same flowers, such that flowers with high density of yeasts harbored fewer bacteria and vice versa. Population-level analysis of M. reukaufii revealed multiple local genotypes but two distinct phenotypic assignments. Variation in metabolic and physiological traits suggest that different M. reukaufii strains may occupy different niches in the nectar environment. Direct competition assays against other species further confirmed the disparity in competitive strength between the two M. reukaufii phenotypes and provided one explanation for its observed dominance in the nectar microbiome.