The enormous consequences of microbial interactions

“No man is an island, entire of itself; every man is a piece of a continent, a part of the main.” John Donne, Meditations XVII, 1624

Little did John Donne know, that those words expressing his sentiment on men’s interconnectedness would one day serve as a powerful metaphor for biological interactions — “No organism is an island.” Indeed, some biologists have long alluded to it in their treatises on various kinds of interactions (1, 2). 

Truly, no organism lives in isolation. An organism is an individual that is part of a population, interacting with members of its own kind. This population, in turn, interacts with populations of other kinds of organisms.

The statement is also true in a sense that a large organism is colonized and influenced by a small one. The smaller party, the microbe, is called by different names, depending on the situation: germ, pathogen, normal microflora, epiphyte, endosymbiont, and so on. This second context is the basis of the present article, which highlights the importance of beneficial microbial interactions in the maintenance of organisms and ecosystems.

“Great fleas have little fleas upon their backs to bite ‘em, and little fleas have lesser fleas, and so ad infinitum.” — Augustus de Morgan, A Budget of Paradoxes, 1872

No organism is naturally germ-free. From the time we were born, we became colonized internally and externally by a community of microorganisms that will more or less stay with us until we die. The relationship existing between us and our microbial partners may be permanent or shifting, and may or may not be species-specific, but these microbes, in one way or another, must have had a hand in shaping us into what we are through evolutionary time. The same may be true for all other animals, plants and perhaps many eukaryotic microorganisms.

What are the unique features of microbes, especially the prokaryotes, that they have permeated so deeply into the fabric of life? First, as any microbiology textbook will tell us, is their small size, which allows them to have fast growth rates and to be easily brought by physical forces to many places. Second is their ancient origins — they have been here on Earth longer than any other group of organisms. Given millions of years in which to co-exist with higher organisms, the resulting variety and depth of relationships is staggering.

Here are some examples of beneficial relationships in which the microbe plays a starring role:

• The zooxanthellae of reef-building corals can provide an additional source of nutrition for the animal and allow adaptation to changing environmental conditions. These zooxanthellae are actually dinoflagellates, or motile microscopic algae, that have developed a mutually beneficial relationship with corals by providing them with food through photosynthesis.

• The deep-sea hydrothermal vent invertebrates, including the remarkable tube worms, are nutritionally supported by symbiotic sulfur-oxidizing and methane-oxidizing bacteria. These bacteria are chemoautotrophs, deriving their energy from reduced inorganic compounds, in this case hydrogen sulfide and methane. They fix carbon dioxide much like plants and algae do, but without the need for sunlight. 

• Growth-promoting microbes in the root zone help supply certain plants with nutrients. Nitrogen-fixing bacteria, such as those found in legume roots, transform nitrogen gas to ammonia, which then gets assimilated into plant protein. The mycorrhizal fungi, which are associated with many root systems, can enhance plant absorption of various mineral nutrients including phosphorus. On the whole, these associations allow plants to successfully colonize nutrient-poor areas without the aid of chemical fertilizers.

• The trillions of bacteria in our intestines are not merely free riders. They lend us the capability to digest and derive energy from the different kinds of carbohydrates that we eat, including complex ones from plants such as amylose and pectin. Also, they protect the intestinal lining from pathogens, and produce important metabolites such as vitamin K, which we can’t synthesize. 

When some of these specific interactions are replicated many times, forming larger scale systems, as in coral reefs, forested lands, and yes, human societies, then microbial activities and interactions can be said to truly support not only organisms, but also ecosystems and civilizations. That is why Christon Hurst (1), a noted microbiologist, was very accurate in saying that “Microbes form the understructure which supports what we perceive as being the macrobial realm.” In a very big sense, microbes do rule our world. 

 “We shall not cease from exploration, and the end of all our exploring will be to arrive where we started, and know the place for the first time.” — T.S. Eliot, Little Gidding, 1942

Interaction is the theme that is at the heart of behavior, from the level of electromagnetic interactions between atoms, to the gravitational attraction among celestial bodies. Most importantly, interactions define life. The cell, after all, is an open system. The “biocomplex perspective” of Goldenfeld and Woese (3) suggests that life, in whatever form it takes, is to be definitively defined in terms of its constant dependence on “flux from the environment” as its source of energy, metabolic intermediates, and genetic material. On a broader scale, Paracer and Ahmadjian (4) earlier recognized the importance of biological interactions as a possible unifying theme in biology, with the view that “organisms function only in relation to others.” 

One of the long-standing challenges to ecologists is to uncover relationships, because it seems that a key to ecological homeostasis lies in how organisms, big and small, are linked. It is especially challenging to microbiologists, not only because of their subjects’ small size, but also because our view of microbial ecology is still only opening up, particularly where viruses are concerned. However, with new perspectives and tools, including modeling algorithms, applications of genomics and proteomics, and in-situ viewing of microbes, among others, we can be pretty sure that the field of microbial interactions will bring many surprising findings in the years to come.

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Citations

(1) Hurst C. 2002. Neighborhoods and community involvement: No microbe is an island. In: C. Hurst (Ed.)

Manual of Environmental Microbiology 2nd Edition. ASM Press, Washington D.C., pp. 6-18.

(2) Pennisi E. 2005. A mouthful of microbes. Science 307:1899-1900.

(3) Goldenfeld N. & C. Woese. 2007. Biology’s next revolution. Nature 445:369.

(4) Paracer S. & V. Ahmadjian. 2000. Symbiosis: An introduction to biological associations, 2nd Edition.

Oxford University Press, New York, p.13.

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Mary Ann G. Santos teaches microbiology at the College of Science, University of Santo Tomas. She is currently a Ph.D. student of the Marine Science Institute, UP Diliman. She wishes to thank R.V. Azanza, R.J. Calugay, T.E. dela Cruz and H.T. Yap for helpful comments on this essay. Her e-mail address is [email protected].