In any ecosystem, certain species exert an influence greater than their numbers suggest. For example, sea otters sustain kelp forests by controlling sea urchin populations. This keystone species concept extends to the microscopic world, where specific bacteria act as organizers of their microbial communities. These “keystone bacteria” are defined by their disproportionately large impact on the structure and function of ecosystems from the human gut to the oceans.
Characteristics of Keystone Bacteria
A keystone bacterium is distinguished by its influence, not its abundance. Unlike dominant bacteria that shape environments through sheer numbers, keystones often have a low population size. Their significance stems from an outsized effect on the microbial community’s composition and stability. The removal of a keystone bacterium can trigger substantial shifts, often leading to a decrease in microbial diversity.
Scientists identify these microbes by analyzing their high degree of connectivity within microbial networks, meaning they interact with many other species. This impact upon removal is used as a primary criterion for their identification, underscoring their importance in maintaining a balanced microbial habitat.
How Keystone Bacteria Shape Their Environments
Keystone bacteria wield influence through several mechanisms. One method is producing unique metabolites, such as vitamins or enzymes, that other microbes in the community rely on. This dependency helps structure the community around the keystone.
These microbes also act as microscopic ecosystem engineers by altering local physical and chemical conditions. For example, some can change the pH of their microenvironment or degrade toxins that would otherwise inhibit other species. Others are instrumental in forming biofilms, complex structures that create protected microhabitats for diverse organisms.
Keystone bacteria also facilitate complex food webs through cross-feeding. They may break down complex substances into simpler compounds that other community members use as energy sources. This metabolic process is fundamental to nutrient cycling and supports a greater diversity of microbial life.
Significant Keystone Bacteria and Their Niches
The human gut provides well-studied examples of keystone bacteria. Faecalibacterium prausnitzii produces butyrate, an anti-inflammatory fatty acid that serves as a primary energy source for colon cells. Another example is Akkermansia muciniphila, which degrades mucin, helping to maintain the gut barrier’s integrity and influencing host metabolism.
In terrestrial ecosystems, certain nitrogen-fixing bacteria from the genus Rhizobium are keystones. They convert atmospheric nitrogen into ammonia, a form usable by plants, making this nutrient available to the entire ecosystem.
Marine environments also host keystone bacteria. Cyanobacteria perform a significant portion of oceanic photosynthesis, forming the base of the marine food web. In the soil around plant roots, bacteria like Pseudomonas can act as keystones by producing compounds that suppress pathogens and promote plant growth.
Consequences of Keystone Bacteria Imbalance
The decline or loss of keystone bacteria can lead to detrimental consequences. In the human microbiome, this disruption is a hallmark of dysbiosis, a microbial imbalance associated with health problems. The reduction of butyrate-producers like Faecalibacterium prausnitzii is linked to inflammatory bowel disease (IBD).
When keystone populations are disturbed, the community can become less resilient and more susceptible to invasion by pathogens. For example, antibiotic use can eliminate keystones, creating an opportunity for harmful microbes like Clostridium difficile to proliferate. Drastic dietary changes can also shift the gut environment in ways that no longer favor important keystone species.
Beyond human health, the loss of keystone bacteria in broader contexts can destabilize ecosystems. In soil, the disappearance of nitrogen-fixing or decomposing bacteria can impair nutrient cycling and reduce fertility. In aquatic systems, a decline in keystone primary producers can trigger a collapse of the food web, reducing overall biodiversity.