A holobiont is a host organism together with all the microorganisms that live on and inside it, functioning as a single biological unit. The term covers everything from a coral and its algae to a human and the trillions of bacteria in the gut. What makes the concept powerful, and controversial, is the claim that this composite entity is not just a convenient label but something natural selection can actually act on as a whole.
Where the Idea Came From
Lynn Margulis is widely credited with coining “holobiont” in the early 1990s, when she defined symbiosis as an association sustained through a significant portion of the life history of the organisms involved. But the concept predates her by decades. The German theoretical biologist Adolf Meyer-Abich introduced “holobiont” in 1943, nearly fifty years before Margulis, and spent the 1940s through the 1960s developing a comprehensive theory of evolutionary change through what he called “holobiosis.”1PubMed. The holobiont concept before Margulis Meyer-Abich’s work was largely forgotten, and the modern usage of “holobiont” traces back through Margulis’s framing, which linked symbiosis to her earlier and more famous work on the endosymbiotic origins of mitochondria and chloroplasts.
The concept gained real traction in the 2000s, when advances in DNA sequencing made it possible to catalog the full microbial communities living inside organisms. Researchers studying corals, sponges, squid, sea cucumbers, and insects had long known these animals depended on their microbes, but now they could see just how many species were involved and how consistently they showed up across host generations. That shift turned “holobiont” from a philosophical notion into a working framework for studying biology.
The Hologenome Theory and Why It Sparked a Fight
The most ambitious extension of the holobiont idea is the hologenome theory of evolution, which proposes that the holobiont is a unit of selection in evolution. Under this view, the combined genetic material of the host and all its associated microorganisms forms a “hologenome,” and natural selection operates on the whole package, not just on the host’s genome alone.2FEMS Microbiology Reviews. Role of microorganisms in the evolution of animals and plants: the hologenome theory of evolution After a decade of development, proponents have argued that the holobiont can function as a distinct biological entity, an additional organizational level on which selection acts, beyond cells, organisms, and populations.3PubMed Central. The hologenome concept of evolution after 10 years
This claim has been controversial within the microbiome community.4PubMed Central. What is the hologenome concept of evolution? Critics argue that calling a host and its entire microbiome a single cooperative unit of selection is based on overly restrictive assumptions. The hologenome framework requires high fidelity among partners across generations. Even when a particular host-microbe pair does get passed faithfully from parent to offspring, the argument goes, that level of fidelity is unlikely to hold for the entire microbiome. In practice, host-associated microbial communities include relationships ranging from mutualistic to parasitic, and different microbes are transmitted in different ways. Selective interests among members of the community often diverge, with both cooperative and antagonistic dynamics at play.5PubMed Central. Holes in the Hologenome: Why Host-Microbe Symbioses Are Not Holobionts
The debate matters because it shapes how researchers design experiments and interpret results. If the holobiont genuinely is a unit of selection, then you would expect to find coordinated adaptations across the host and its microbiome, and disrupting the microbiome would be akin to losing organs. If it is better understood as an ecological community, then the same host-microbe interactions can be explained by each partner pursuing its own fitness interests, sometimes cooperating and sometimes competing. Most researchers today accept the descriptive usefulness of “holobiont” as a label while remaining cautious about the stronger evolutionary claims.
Coral Bleaching as Holobiont Breakdown
Corals are the poster organisms for holobiont research. A coral animal hosts photosynthetic algae inside its tissues, along with bacteria, archaea, fungi, and viruses. The algae provide energy through photosynthesis; the coral provides shelter and nutrients. When ocean temperatures rise, the symbiosis between the coral host and its algae breaks down in a process known as bleaching, one of the most devastating outcomes for reef ecosystems.6PubMed Central. Defining Coral Bleaching as a Microbial Dysbiosis within the Coral Holobiont
The process is more nuanced than “the algae leave.” Recent work shows that even early stages of heat stress shift the entire holobiont’s metabolism. As temperatures climb, the increased metabolic demand pushes the coral into a state where the algae can no longer supply enough energy to meet the host’s needs, reducing overall holobiont performance.7PubMed Central. Destabilization of mutualistic interactions shapes the early heat stress response of the coral holobiont The broader microbial community also shifts under heat stress, and the specific composition of a coral’s microbiome is closely related to its susceptibility to thermal bleaching. Different coral species hosting different microbial assemblages bleach at different rates even under the same conditions.8PubMed Central. The microbiome dynamics and interaction of endosymbiotic Symbiodiniaceae and fungi are associated with thermal bleaching susceptibility of coral holobionts
This makes coral bleaching more than a story about heat killing algae. It is a collapse of an entire integrated system, where the fates of every microbial partner are tied together. Framing corals as holobionts has shifted conservation thinking toward the microbial community, not just the coral animal or its algae.
Symbiont Shuffling and Climate Adaptation
One of the more hopeful findings in coral biology is that holobionts are not static. After a bleaching event, some corals recover with a different mix of algal partners than they had before. The proportion of heat-tolerant algal symbionts can increase dramatically following severe bleaching, especially when recovery happens in warmer water. Even minor shifts in symbiont composition can measurably increase a coral’s resistance to future heat stress.9PubMed Central. Investigating the causes and consequences of symbiont shuffling in a multi-partner reef coral symbiosis under environmental change This process, called symbiont shuffling, acts as a rapid adjustment mechanism, potentially allowing corals to cope with greater temperature fluctuations in the future without waiting for the host’s own genes to evolve.10Ecological Indicators. Symbiont shuffling dynamics associated with photodamage during temperature stress in coral symbiosis
The effect may even cross generations. In photosynthetic sponges, a close relative system, parental exposure to future climate scenarios altered the microbial communities of offspring through both symbiont shuffling and symbiont switching within a vertically transmitted microbiome.11PubMed. Cross-generational effects of climate change on the microbiome of a photosynthetic sponge If microbiome changes in one generation can prepare the next for a warmer environment, the holobiont framework provides a route to adaptation that operates faster than conventional genetic evolution.
Insects That Cannot Eat Without Their Microbes
Some of the clearest holobionts on Earth are insects that depend absolutely on bacterial partners to survive on nutritionally poor diets. Pea aphids feed exclusively on plant phloem sap, which is deficient in essential amino acids and vitamins. They compensate through an obligate partnership with the intracellular bacterium Buchnera aphidicola, maintained over millions of years, in which the bacterium synthesizes the amino acids the aphid cannot get from its food.12PubMed. Quantifying supply and demand in the pea aphid-Buchnera symbiosis reveals the metabolic Achilles’ heels of this interaction
What makes this system remarkable is the intimacy of the metabolic collaboration. For several essential amino acids, neither the aphid nor the bacterium can complete the synthesis pathway alone. Instead, the two organisms split the work, with the aphid carrying out some enzymatic steps and the bacterium carrying out others within the same biosynthetic pathway. Genomic studies have revealed at least five cases of this within-pathway collaboration, including shared production of leucine, valine, isoleucine, methionine, and tyrosine.13PubMed Central. Genomic revelations of a mutualism: the pea aphid and its obligate bacterial symbiont Each aphid cell that houses Buchnera individually wraps the bacterium in a host-derived membrane, and specialized amino acid transporters move nutrients back and forth across this interface to enable the interleaved biochemistry.14PubMed Central. Trading amino acids at the aphid-Buchnera symbiotic interface
Termites offer a different flavor of the same principle. Wood is one of the most resistant organic materials to digestion, yet termites eat it efficiently, thanks to specialized gut microbial communities that break down the tough lignocellulose fibers.15PubMed Central. Uncovering the Potential of Termite Gut Microbiome for Lignocellulose Bioconversion in Anaerobic Batch Bioreactors These termite-specific gut microbiomes form complex metabolic networks that no single organism could replicate on its own.16PubMed Central. Species-wide Metabolic Interaction Network for Understanding Natural Lignocellulose Digestion in Termite Gut Microbiota In both cases, the holobiont concept feels almost literal: remove the microbes and the host starves.
Plants and Their Underground Partners
Plants are holobionts too, with microbial communities in and around their roots that influence how well they handle stress. When a plant experiences drought or other environmental challenges, its root metabolism shifts rapidly, changing the mix of organic acids, signaling compounds, and other molecules it releases into the soil. These changes selectively reshape the microbial community in the root zone, and the newly assembled community feeds back by regulating plant hormones, adjusting osmotic balance, and improving nutrient cycling.17PubMed. Holobiont Plasticity Under Abiotic Stress: A Systems Biology Perspective on Plant-Bacteria-Fungi Interactions
One recent study found that a specific plant-derived compound, trimethyllysine, is strongly associated with microbial populations that improve nutrient uptake, produce growth-promoting compounds, and help moderate stress responses.18PubMed Central. Interaction of plant-derived metabolites and rhizobiome functions enhances drought stress tolerance The plant is not passively hosting whatever microbes happen to be around; it is chemically recruiting the community it needs. This active recruitment is one reason that holobiont thinking in agriculture is moving beyond simply adding probiotics to soil and toward understanding the two-way chemical conversation between plant and microbiome.
Sponges and Chemical Defense
Marine sponges are ancient animals with notoriously rich microbiomes, and some of them illustrate a holobiont function that is easy to overlook: chemical warfare. Sponges harbor diverse bacterial communities with vast potential to produce secondary metabolites that may protect the host from predators and infections.19PubMed Central. Secondary Metabolite Production Potential in a Microbiome of the Freshwater Sponge Spongilla lacustris In one New Zealand sponge species, researchers discovered that multiple bacterial symbionts jointly generate chemical diversity, including pathways for three distinct drug-like compound families: microtubule inhibitors, contact poisons, and translation inhibitors. Rather than relying on one microbe for defense, the sponge’s moderately complex microbiome distributes chemical production across a broad range of bacteria.20PubMed Central. A multiproducer microbiome generates chemical diversity in the marine sponge Mycale hentscheli
This has pharmaceutical implications. Several sponge-derived compounds are in drug development pipelines, and understanding which bacterial partner actually produces the molecule is the first step toward scaling up production outside the sponge. The holobiont lens matters here practically: if you just look at the sponge, you see a chemical factory; if you look at the microbiome, you see who is actually running the machines.
Your Immune System as a Holobiont Product
Humans fit the holobiont framework as well, and the immune system is where the interaction runs deepest. Gut microbes play a major role in the development and maintenance of immune tolerance, starting in early life. They present an enormous repertoire of molecular signals and metabolites that shape how the immune system matures and calibrates itself between “attack” and “ignore.”21PubMed. The gut ecosystem and immune tolerance
Mouse experiments have shown that the specific composition of the gut microbiome during early life dictates the development of specific immune cell populations. In one study, mice colonized with only a pre-weaning microbial community developed markedly fewer of the regulatory immune cells (pTregs) that maintain intestinal peace, compared to mice with a normally maturing microbiome. Mice with a full adult community reached about 30% pTregs in the large intestine by four weeks of age, while those locked into a pre-weaning community reached only about 12%.22Cell Host & Microbe. A defined pre-weaning murine microbiota dictates structure of the immune system and response to enteric infection The implication is that the microbial transitions happening in your gut during the first years of life are not random side effects of changing diets. They are building blocks the immune system depends on.
Microbes Reaching the Brain
The gut-brain axis is one of the more surprising extensions of holobiont thinking. Gut microbes communicate with the central nervous system through several routes, including the vagus nerve, immune signaling molecules, and microbial metabolites that enter the bloodstream. Growing evidence suggests that the gut microbiota can influence host social behavior, stress responses, and performance in learning and memory tasks, with documented effects on brain gene expression and neurochemical metabolism across regions including the amygdala, hippocampus, and frontal cortex.23PubMed Central. The Microbiome and Host Behavior
Much of this work is still in animal models, and the field is honest about how much remains unknown. But the fact that microbes colonizing the gut can alter brain development and behavior across multiple host species fits squarely within the holobiont framework. It means the nervous system, something we tend to think of as quintessentially “the host’s own,” is partly sculpted by the microbial community.24PubMed Central. The role of the microbiome in the neurobiology of social behaviour
Microbes Rewriting the Host’s Gene Activity
One of the mechanisms connecting microbial metabolites to host physiology is epigenetics: changes in how genes are read without altering the DNA sequence itself. Gut microbes can influence patterns of gene activation in the host through at least two routes. Their metabolic byproducts, particularly short-chain fatty acids produced by fermenting dietary fiber, act as inhibitors of enzymes that normally keep certain genes turned off. By blocking those enzymes, short-chain fatty acids effectively unlock anti-inflammatory gene programs in host cells.25Gut. Interplay between gut microbiome, host genetic and epigenetic modifications in MASLD and MASLD-related hepatocellular carcinoma Gut bacteria also influence DNA methylation patterns, another layer of gene regulation that can alter inflammation and metabolic pathways.26PubMed Central. Implications of gut microbiota-mediated epigenetic modifications in intestinal diseases
In mouse studies, butyrate, one of the most studied short-chain fatty acids, has been shown to reduce weight gain in mice on high-fat diets by inhibiting a specific enzyme involved in gene regulation. When that enzyme was genetically removed, butyrate lost its protective effect, confirming the mechanism.27Gut. Interplay between gut microbiome, host genetic and epigenetic modifications in MASLD and MASLD-related hepatocellular carcinoma Short-chain fatty acids also act through host cell-surface receptors to modulate metabolic conditions like obesity and high blood pressure.28PubMed Central. Gut Microbiota-Derived Short-Chain Fatty Acids Facilitate Microbiota:Host Cross talk and Modulate Obesity and Hypertension What makes this epigenetic angle meaningful for the holobiont concept is that it provides a concrete molecular pathway through which microbial residents alter the host’s biology at the genetic-control level, not just by producing helpful vitamins or occupying space that pathogens might take.
Phylosymbiosis and the Mirroring of Evolutionary Trees
If hosts and their microbiomes really do form meaningful units, you might expect to see a pattern: closely related host species should harbor more similar microbial communities than distantly related hosts. This pattern has a name, phylosymbiosis, and it has been documented across a wide range of animals.29PubMed Central. An introduction to phylosymbiosis The microbial community tree, in other words, mirrors the host family tree.30PLOS Biology. Phylosymbiosis: Relationships and Functional Effects of Microbial Communities across Host Evolutionary History
The mechanisms driving this are still debated. One explanation is host filtering: each host’s immune system, gut chemistry, and diet create a selective environment that favors particular microbes, and because closely related hosts share similar bodies, they end up with similar microbial tenants. That explanation does not require the holobiont to evolve as a unified entity. It just requires that host evolution shapes the microbial neighborhood.31PubMed Central. Is Host Filtering the Main Driver of Phylosymbiosis across the Tree of Life? But if some microbial partnerships are vertically inherited and co-speciate with the host, that edges closer to the hologenome vision where the whole system evolves together.
Microbes as Matchmakers in Speciation
An even more provocative idea is that symbiotic microbes can contribute to the formation of new species. Wolbachia, a bacterium that infects a vast range of arthropods and is passed from mother to offspring, can induce cytoplasmic incompatibility: when an infected male mates with an uninfected female, the embryos die. This creates a reproductive barrier between populations that carry different Wolbachia strains, or between infected and uninfected populations.32PLoS ONE. Wolbachia-Induced Unidirectional Cytoplasmic Incompatibility and Speciation: Mainland-Island Model
In spider mites, researchers found that Wolbachia-induced incompatibilities and host-intrinsic reproductive barriers are mechanistically independent and additive. When both types of barrier operate in the same cross, they stack, making hybridization between populations even less likely. The implication is that Wolbachia may have a considerable effect between populations that are only partially isolated, potentially accelerating speciation processes already underway.33PubMed. Wolbachia and host intrinsic reproductive barriers contribute additively to postmating isolation in spider mites If a bacterium living inside your cells can determine who you can successfully reproduce with, the line between “the host” and “the holobiont” becomes genuinely blurry.
Fecal Transplants and Therapeutic Remodeling
The practical side of holobiont thinking is already reaching clinics. Fecal microbiota transplantation, where stool from a healthy donor is introduced into a patient’s gut, has been used to treat recurrent Clostridioides difficile infections and is being explored for metabolic disorders and other conditions associated with disrupted microbial communities.34PubMed Central. Fecal microbiota transplantation and next-generation therapies: A review on targeting dysbiosis in metabolic disorders and beyond The logic is explicitly holobiont-flavored: if the host’s health depends on the combined organism-plus-microbiome, then restoring a disrupted microbiome should restore some of that health.
Next-generation approaches are moving beyond whole-stool transplants toward defined microbial consortia, where specific bacterial species are selected, grown in the lab, and administered as a designed community. The long-term direction is to engineer or curate a holobiont, choosing which microbial partners a patient gets rather than transferring whatever happens to be in a donor’s gut. Whether that will work as cleanly as the concept promises remains to be seen, but the underlying science, from coral symbiont shuffling to aphid metabolic collaboration, provides the intellectual foundation for the attempt.
The Bobtail Squid’s Light Organ
Not every fascinating holobiont system involves nutrition or disease. The Hawaiian bobtail squid, Euprymna scolopes, has a specialized light organ that houses the bioluminescent bacterium Vibrio fischeri. The squid uses the bacterial light for counter-illumination camouflage, matching the moonlight from above so that predators looking up from below cannot see its shadow. What is striking is how deeply the bacterium is woven into the squid’s own developmental program. Genes involved in eye development are expressed in the light organ, and colonization by the symbiont triggers changes in the expression of these genes, including one gene whose down-regulation requires the bacteria to actually produce light.35PubMed Central. Eye-specification genes in the bacterial light organ of the bobtail squid Euprymna scolopes, and their expression in response to symbiont cues The squid’s body has, over evolutionary time, repurposed its own genetic toolkit to accommodate and respond to a bacterial partner. That is not coexistence; it is co-development.