In nature, the rarest ingredient can be the one that holds everything together. In a new article in the Cell Press journal Trends in Ecology & Evolution, Dr Erika C. Freeman of the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin argues that a handful of scarce molecules can shape entire living communities, far out of proportion to how little of them is present. These are what ecologists call “keystone molecules”.
In her article, Dr Erika Freeman points to examples such as the defensive compounds, known as alderenes, produced by the sea slug Alderia harvardiensis, which significantly alter the composition of animal communities in the tidal flats. Another example is serotonin, known as a neurotransmitter in the human brain, which influences the soil microbiome in the roots of switchgrass.
“These two examples from recent studies reframe how chemistry shapes the living world, and suggest that science has been overlooking rare-but-powerful compounds in favour of the abundant ones”, said Erika Freeman, head of the IGB research group “Aquatic Biogeochemistry of Carbon”. In her article she also hands researchers a testable framework to hunt for these molecules across systems, from oceans to soils to freshwaters.
Keystone molecules as the chemical counterpart to keystone species in biology
The name keystone molecule is a deliberate nod to one of ecology’s most productive ideas. In 1969, the ecologist Robert Paine showed that a single, uncommon predator, a starfish, held an entire rocky shoreline together; remove it, and the community collapsed. Ecologists have relied on this “keystone species” concept ever since. Freeman’s point is that the chemistry itself can play the same role: the rare molecules that carry these outsized effects can be the operative agents, not merely bystanders.
The idea of a keystone molecule is not itself new. The neuroecologists Richard Zimmer and Ryan Ferrer named it in 2007, setting out its logic with specific examples. What had not followed, Freeman argues, was systematic evidence that such molecules operate in real communities, together with a practical way to identify them. Her Forum article makes the case that recent findings now supply both.
Two new studies, from completely different corners of the living world, point the same way. In the estuarine mudflats of California, sea slugs (Alderia harvardiensis) produce defensive compounds called alderenes that make up only about 0.1 per cent of the animals’ body weight. Yet these trace molecules reorganise the whole mudflat community across four animal phyla, repelling some species and drawing in others. In the roots of switchgrass under nitrogen stress, two rare metabolites, serotonin (yes, the same molecule linked to mood in animals) and ectoine, act as keystone signals that reshape the plant’s root microbiome.
To help researchers tell a genuine keystone molecule from an ordinary one, the paper sets out four practical tests: the molecule is rare; its effect on the community is large; that effect spreads beyond a single target; and no other molecule can readily stand in for it. Freeman also points to the tools that now make such molecules findable, including network analysis of high-resolution mass-spectrometry data, experiments with simplified synthetic communities, and machine learning.
“For a long time we assumed the molecules present in the largest amounts were the ones doing the most work”, said Erika Freeman. “What these studies show is that a molecule can be vanishingly rare and still hold a whole community together, in the same way one uncommon predator can hold a shoreline together. Once you start looking for those few powerful molecules, you begin to see the natural world a little differently.”