Tiny but mighty? Overview of a decade of research on nectar bacteria

Abstract

An emerging focus of research at the intersection of botany, zoology, and microbiology is the study of floral nectar as a microbial habitat, referred to as the nectar microbiome, which can alter plant-pollinator interactions. Studies on these microbial communities have primarily focused on yeasts, and it was only about a decade ago that bacteria began to be studied as widespread inhabitants of floral nectar. This review aims to give an overview of the current knowledge on nectar bacteria, with emphasis on evolutionary origin, dispersal mode, effects on nectar chemistry and plant-animal interactions, community assembly, agricultural applications, and their use as model systems in ecological research. We further outline gaps in our understanding of the ecological significance of these microorganisms, their response to environmental changes, and the potential cascading effects.

Keywords: bacteria; floral nectar; plant–microbe–animal interaction; pollination; yeast.

Fig. 1

Genome size comparisons of nectar specialist bacteria of the Acinetobacter nectaris/Acinetobacter boissieri (ANAB) clade (left) and genus Rosenbergiella (right) with their closest phylogenetic relatives (other Acinetobacter species and other genera of the Erwiniaceae family, respectively). These graphs were produced from the data included in Supporting Information Tables S2 and S3 using R v.4.3.1 (R Core Team, 2023) and the R library ggplot2 v.3.5.1 (Wickham, 2016).

Fig. 2

Nectar microbes as ‘ecosystem engineers’. As a result of their metabolic activity, nectar‐dwelling bacteria and yeasts can alter floral nectar's physicochemical conditions in multiple ways. For example, these microbes can reduce the overall sugar content of nectar (a), alter its sucrose : hexose ratio (b), and consume the amino acids and other nitrogen sources (c). However, some nectar microbes including the members of the Acinetobacter nectaris/Acinetobacter boissieri clade may increase the nutrient content of nectar, for example by bursting the pollen grains that fall into nectaries (d). Additionally, nectar microbes typically acidify nectar (e), reduce the concentration of some toxic metabolites of plant origin, and might produce secondary metabolites that are toxic to other (micro)organisms (f). Finally, nectar microbes can alter the floral scent profile by releasing a wide variety of volatile organic compounds that elicit behavioral responses in pollinators and other plant‐visiting animals (g). Created in BioRender: Álvarez‐Pérez S. 2024. BioRender.com/s75h142.

Fig. 3

Nectar microbiome as a model system in ecological research. Floral nectar microbial communities are powerful systems for ecological research due to the short generation times of most of their members, their relative simplicity compared with other natural microbiomes (e.g. rhizosphere and phyllosphere), and their organization in a well‐defined hierarchical structure of increasing complexity (nectaries within flowers, flowers within individual plants, plants within populations, etc.), thus allowing multi‐scale approaches. Initially sterile, floral nectar often receives various species of bacteria and yeasts via flower‐visiting animals and by other means (a, b). The harsh physicochemical conditions of nectar (e.g. high osmotic pressure, scarcity of nitrogen sources, and presence of defensive compounds of plant origin) act as a filter of the incoming microbial community brought by pollinators and other floral visitors (c). Competition for nectar nutrients between nectar microbes (d), which can result in priority effects (e), and microbe‐mediated changes in nectar chemistry that can affect the growth of other members of the community through niche modification (f) are some of the processes that may determine community assembly in the nectar microbiome. Created in BioRender: Álvarez‐Pérez S. 2024. BioRender.com/p98m725.