Symbiosis

Steffen Krejberg Knudsen

Symbiosis is simply the living together in physical contact of organisms of different species.

Symbiosis is the close, long-term living together of different species. In the biological sciences, it is a fundamental phenomenon at all scales of life, from the microbial to the planetary, and it is a concept that is at the heart of recent reworkings of the foundational thinking of biology.

Symbiosis poses a fundamental challenge to traditional conceptions of the biological individual. Practically no lifeform is merely an individual, living a truly independent life. Rather, it’s a host or a symbiont, always already caught up in symbiosis with other species. In symbiosis, the co-existence of host and symbiont is both a condition in the present and one that extends back throughout the evolutionary past in which species have evolved together, developing wide ranges of dependencies and reciprocities, from shared metabolisms relying on the presence of friends to immune systems expecting to fend off old enemies.

This ever-present intimacy of strangers suggests a new kind of biocentrism in which the human individual is displaced from the centre of things, conceived instead as part of a biological whole – a holobiont, as is the term for the ecological assemblage of a host organism and its symbionts (Scott 2017). A human with its gut microbes, a tree with its mycorrhiza root-fungi symbiosis, or the coral reef and its photosynthesizing algae are all holobionts that have evolved to depend deeply on each other for essential functions. The scientific and philosophical ramifications of this paradigm shift only keep unfolding, with the holobiont theory at the current frontier of rethinking the ecology, evolution, and development of life. In a commonplace understanding, symbiosis is often taken to suggest the peaceful cohabitation of species. But going beyond this meaning can offer a more precise description of the types of connectedness between species – ranging from mutualism over commensalism to parasitism. In biology today, symbiosis is regarded as a continuum of possible relations, referred to as the mutualism-parasitism-continuum or extended even further to include competition (Andersen 2011; Krejberg Knudsen 2022).

At the one end of the continuum, mutualism is a symbiosis which is of reciprocal use to both species, as seen with flowering plants and their insect pollinators, the animal-aided seed dispersal of a tree, or the mycorrhiza symbiosis of roots and fungi. In between the extremes, there are a of possibilities – for instance, the commensalism of one species benefitting and the other being neither harmed nor benefitting from the contact, or the amensalism of one species taking harm from another without the latter benefitting. At the other end of the continuum, parasitism occurs when contact with a benefitting parasite is harmful to the host – with the worms specialized in living in the innards of animals being one of the most widespread examples. From a symbiosis perspective, competition is yet another form of long-term ecological association where two organisms are both harmed, often while competing for resources, with the tendency to result in the exclusion of the weaker organism from an ecological niche.

A brief history of symbiosis – from discerning lichens to holobiont theory

Since the origin of the concept, symbiosis has been observed at different levels and scales of life according to the state and the advances in the biological sciences.

In 1869, Swiss botanist Simon Schwendener was the first to suggest that lichen were, in fact, two separate organisms – a fungus and an alga – living in ever so close cooperation as to resemble a single, unitary species, and in 1877 and 1878 German botanists Albert Bernhard Frank and Anton de Bary coined the terms ‘Symbiotismus’ and ‘Symbiose’, respectively, to describe the living together of these different organisms (Sapp 1994, 4–7).

Curiously, the concept is a close contemporary of Herbert Spencer’s social Darwinian idea of ‘the survival of the fittest’ which, when first expressed in The Principles of Biology in 1864 (Spencer 1867), posited competition as the central paradigm of evolution and the social organization of life in both biology and popular perception. Symbiosis – with its connotations of interspecies collaboration – remained in the background for a long time, regarded more as a curious exception in a world of independent individuals than a rule of necessary and potentially benign co-existence.

It has taken more than a century for symbiosis to enter into the foreground of biological thought. From its humble beginnings in German-speaking botany, it enjoyed an avantgarde development in Russian biology when in 1905 and 1910 Konstantin Mereschkowsky described a phenomenon later to be termed symbiogenesis: It suggests that multicellular life forms developed by way of symbiotic bacteria merging, a single-celled organism engulfing and incorporating another. Over the coming decades, French physiologist Paul Portier and American biologist Ivan Wallin further developed the endosymbiosis theory stating that the organelles inside cells of multicellular life forms such as plants, fungi, and animals are a testament to this integration. (Sapp 1994; Margulis 1998)

Only with the pioneering microbiological research of evolutionary microbiologist Lynn Margulis in the 1960s did the evidence for such observations begin to mount. She developed the theoretical sketches of her predecessors into a more full-bodied view of evolution in which the symbiotic intimacy of strangers is seen as a main driving force in the development of everything from tissues and organs to new organisms and species. “The tendency of ‘independent’ life is,” as she writes, “to bind together and reemerge in a new wholeness at a higher, larger level of organization.” (Margulis 1998, 11)

Though Margulis’ theory of symbiosis was still a contested outsider position at her time, it shook the theoretical foundations of mainstream biology, and symbiosis began to constitute itself as a subfield of biological study. Margulis also pioneered the holobiont term (though already introduced by German biologist Adolf Meyer-Abich in 1943), leading the way towards the current paradigm of describing symbiosis as a source of evolutionary innovation.

In parallel to and related to her work on symbiogenesis, Margulis also engaged in a collaboration with the pioneering Earth System scientist James Lovelock on the so-called Gaia hypothesis. According to this, planet Earth can be considered a living system that operates to regulate its environment.

Though Margulis initially thought of these two endeavours as different lines of thought, she later wrote ‘Symbiotic Planet’ (1998) to tease out the connections between her microbiological research on symbiosis and the planetary vision proposed through the Gaia theory. In this book, she broadens her concept of symbiosis from a strict, local sense (of “species living together in physical contact”) to an explicit understanding of Gaia as symbiosis in a planetary sense. Pointing out her disinterest in any personification of Mother Gaia, she understood the Gaia metaphor to simply express “the idea that Earth is a network of ‘ecosystems’.” (Margulis 1998, 106) Gaia, she suggests, “is an emergent property of interaction among organisms, the spherical planet on which they reside, and an energy source, the sun.” (Margulis 1998, 119)

In this view, symbiotic relations come into view as a driving force in the forming of the Earth System insofar as the biosphere interacts with the spheres of ice, water, land, and air.

Following Margulis’ expanded view further, symbiosis also challenges our conception of what an environment is. “Gaia is just symbiosis as seen from space,” Margulis’ student Greg Hinkle succinctly put it, which to her meant to say that “all organisms are touching as all are bathed in the same air and the same flowing water.” (Margulis 1998, 2) Taking note of symbiosis brings to attention that any environment consists, on the one hand, of other living organisms who may be more or less persistent companions, ranging from fleeting chance encounters to old co-evolutionary allies. On the other hand, it is composed of abiotic material – which is in turn manipulated by other organisms, e.g., by the atmospheric oxygen that was first produced by photosynthesizing cyanobacteria about 3,5 billion years ago.

In this sense, a zone of indiscernibility where symbiosis and environment overlap seems to open up. Is, for instance, the gut microbiome also an interior environment? Are our intestines vice versa a habitat for bacteria? Is the bird that eats fruit and spreads the seed of a tree a symbiont of the tree – or just an organism passing by in its surroundings? And could perhaps the plants and algae that produce the air we breathe be considered – in an even more expanded sense – symbionts of ours?

Symbiosis and Social Theory

The associations of biology have often been taken to be suggestive for social theory, transferring concepts back and forth between the supposedly separated domains of nature and culture – thus creating metaphorical models that are at times poetical, at others imprecise.

Professor Aronax, the narrator in Jules Verne’s 1872 novel Twenty Thousand Leagues Under the Sea, took note of the ‘natural socialism’ of a coral reef, and where Bernard Mandeville once satirically described society by analogy with the beehive, mid-20th century sociobiology took its cues from the organization of bees and ants and exported the concept of ‘superorganism’ to sociology. Competition, surely, is a political-economical idea tangled up in evolutionary biology, and the success of such a concept is a testament to the weight with which the idea of ‘mutual dependence’ has been valorised negatively throughout the history of ideas as a form of ‘unfreedom’.

On the face of it, an implicit valorisation of negative freedom – freedom from this or that – might have played a big part in the ignorance of symbiosis. Such deeply rooted preferences for individualism can also be seen at work in the antibiotic paradigm of killing ‘germs’ that was instigated with Louis Pasteur’s discovery of microbes. Freedom from parasites and pathogens is of course still an important issue, but immunological research is beginning to recognize that – according to the so-called ‘old friends hypothesis’ – the absence of the gut worms we co-evolved with now tends to result in an underestimation of an entire branch of the human immune system which results in auto-immune illnesses in which the immune system acts upon itself.

There is always biology in society – and, vice versa, ‘societal’ organization in biology. However how the overlaps and commonalities of such spheres are conceptualized matters significantly, and one should be wary of superimposing structures of one onto the other. important steps towards rethinking the interweaving of biological and social associations (Latour 2022). With her exposition of symbiotic anthropology, anthropologist Anna Tsing has taken symbiosis even further, suggesting a method to take account of, first, the biological symbiosis (of tree roots and fungi), second, the collaborations between the natural sciences and humanists (“in intertwined human and nonhuman histories”), and, third, “the emergence of landscapes of multispecies livability” (Tsing 2015, 13).

A dynamic view of biological relations

Overall, the concept of symbiosis might be translated from biology into the environmental sciences to offer a way of determining the biological relations of different species and what biological relations mean to more-than-biological assemblages.

Taken as a theoretical paradigm, the mutualism-parasitism-continuum suggests a dynamic view of symbiosis which encourages to pay attention to the interchangeable state of symbiotic relations: A one-time parasite can one day turn into a friendly neighbour, as was the case on the 22nd of July, 2022 when the microscopic mite Demodex folliculorum, which can only live on the skin of humans, was declared as no longer being a parasite but a commensal symbiont that no longer harms its host (Smith 2022). Especially between mutualism and parasitism, there can be shifts within the continuum, as species adapt to one another or the availability of resources in their shared environment change.

The dynamic view of symbiosis proposed here is complementary to what geographer Jamie Lorimer has called the ‘probiotic turn’, a practical tendency across disciplines such as nature conservation and immunotherapy towards “using life to manage life” (Lorimer 2020), turning ‘bad’, antibiotic relations (i.e., shooting migrating boars and wolves, spraying pesticides on crops, or ordaining antibiotic medicine to livestock) into probiotic ones where the ecological benefits of co-existence are favoured by design. Rewilding with megafauna such as wild horses, beavers, and lynx as well as DIY helminthic therapy (where patients themselves raise, exchange, and host gut worms) are Lorimer’s primary examples. This underlying view of life is suggestive of how knowledge of symbiosis can be of practical use in ecological restoration, biomedicine and far beyond.

All in all, paying attention to symbiosis can lead the way towards a world in which interactions between species are better understood and, possibly, play out in a more mutualistic manner.

References

Andersen, Sandra Breum. 2011. “Dynamics of ant-microbial interactions: coevolution along the parasitism-mutualism continuum.” PhD diss., Faculty of Science, University of Copenhagen.

Gilbert, Scott. 2017. “Holobiont by Birth – Multilineage Individuals as the Concretion of Cooperative Processes” In Arts of Living on a Damaged Planet, edited by Anna Tsing, Heather Swanson, Elaine Gan, and Nils Bubandt. Minneapolis: University of Minnesota Press.

Krejberg Knudsen, Steffen, host. 2022. “Ven eller fjende? – Symbiosens tidsalder.” Bloom – festival of nature and science. Podcast. https://www.bloom.ooo/explore/ven-eller-fjende

Latour, Bruno. 2022. “Gaia as a Problem of Social Theory.” In Writing Gaia: The Scientific Correspondences of James Lovelock and Lynn Margulis, edited by Bruce Clark and Sébastien Dutreuil, Cambridge: Cambridge University Press.

Lorimer, Jamie. 2020. The Probiotic Planet – Using Life to Manage Life. Minneapolis: University of Minnesota Press.

Margulis, Lynn. 1998. Symbiotic Planet. A New Look at Evolution. New York: Basic Books.

Margulis, Lynn and James Lovelock. 2022. Writing Gaia: The Scientific Correspondences of James Lovelock and Lynn Margulis. Cambridge: Cambridge University Press.

Sapp, Jan. 1994. Evolution by Association: A History of Symbiosis. Oxford: Oxford University Press.

Smith, Gilbert, Alejandro Manzano-Marín, Mariana Reyes-Prieto et al. 2022. “Human Follicular Mites: Ectoparasites Becoming Symbionts.” In Molecular Biology and Evolution 39 (6).

Spencer, Herbert. 1864–1867. The Principles of Biology (vol 1-2). London, Edinburgh: Williams and Norgate.

Tsing, Anna L. 2015. “In the Midst of Disturbance: Symbiosis, Coordination, History, Landscape”. ASA Firth Lecture, ASA Annual Conference 2015: Symbiotic Anthropologies: theoretical commensalities and methodological mutualisms.

Verne, Jules. 2019. “Twenty Thousand Leagues Under the Seas”. Translated by William Butcher. Oxford: Oxford University Press.