Microbes, nectar, and ants–oh my!

Ants play crucial roles in the ecosystems they inhabit and can affect the microbial communities living in the nectar of flowering plants, from their compositions to the fitness of the plants themselves. Here, Leland Graber introduces us to the new review article “Ant relationships with floral and extrafloral nectar microbes” by Nelsen et al. published in Myrmecological News, which explores the relationships between ants, nectar-inhabiting microbes, and honeydew-producing insects.

Edit by Beatriz Portinha and Salvatore Brunetti

A review by Dr. Leland Graber

It always amazes me how ants have found their way into what seems to be an endless amount of associations with other organisms. Relationships between ants and microbes are particularly intriguing; ant-associated microbes have allowed for some of the most exceptional ant behaviors, like fungus farming, where ants cultivate fungi for food, and subsisting on nutritionally poor substrate. These relationships are also fascinating from the microbe point of view, as ants host a diversity of microbial taxa in their nests, on their cuticles, and inside their bodies.

In their recent Myrmecological News article, Nelsen, Philpott, and Mathis demonstrate that the ant-microbe relationship gets even more interesting when another player is added in the mix: nectar. Nectar is produced by floral and extrafloral (i.e., located on non-flowering parts of the plant) nectaries, the latter of which are thought to exist specifically for the purpose of attracting insects, like ants, to defend the plants against herbivory. Nectar, once thought to be inhospitable to microbes, actually hosts many different microorganisms, some of which appear to be specialists that are adapted to live in very sugary conditions. These nectar specialists can survive in high sugar concentrations and consume a variety of C (carbon) and N (nitrogen) sources; they also can produce antimicrobial proteins, potentially helping them with resource competition.

Since ants feed on nectar, especially nectar from extrafloral nectaries, it is no surprise that the presence of ants and their associated microbes can change the microbial composition of the nectar. The authors highlight five studies that have isolated and identified microbes vectored by ants in nectar. Most of the microbes studied were yeasts, and some of them had fitness impacts (both positive and negative) on the plants and ant vectors.

In addition to ant presence, other factors, like competition and availability of resources such as amino acids, determine the microbial community composition within nectar. Environmental factors, like temperature, humidity, and precipitation also impact microbial community composition. Even plant community composition can impact nectar-inhabiting microbes.

In addition to nectar, Nelsen et al. spend some time discussing honeydew producing sapsucking insects, like aphids and mealybugs, their relationship to ants, and their associated microbes. Ants will care for and protect phloem-feeding insects in exchange for their sugary secretions, known as honeydew. Further, honeydew producing insects rely on gut bacterial symbionts to supplement their nutritionally limited diet, and ants have been shown to acquire symbionts from the insects they tend. Ants that feed on nectar may also be feeding on honeydew, and could potentially be vectoring microbes between honeydew producers and nectaries, and vice versa.

Toward the end of the article, Nelsen, Philpott, and Mathis outline some pressing knowledge gaps related to nectar-inhabiting microbes and ants. The authors point out that little is known about the extent of viruses that may inhabit nectar, as well as what organisms these viruses may be associated with and their ecological impacts. Additionally, the authors acknowledge that most nectar-inhabiting microbe study has focused on the relationships between yeast and pollinators, and that more study is needed to understand nectar-inhabiting bacteria. Finally, the authors express a need for more study and experimentation to understand the complex web of interactions between ants, nectaries, pollinators, and honeydew producing insects, particularly in agroecosystems.

The article by Nelsen et al. left me with many of my own questions about ants and their many associates. I am amazed at the potential of ants being able to influence ecosystems so much just by vectoring microbes between nectaries! I hope some researchers in the myrmecological community will pursue the research questions outlined by Nelsen et al., as there is clearly a lot more to learn about this complex and fascinating system.