Symbioses

Symbiosis. To my horror I realized this morning that after decades working as a biologist I still regularly misspell the word. My poor spelling is likely inexcusable, but certainly the meaning of symbiosis has been a slippery one. Taken literally, the word simply means “living together.” Back in 1877 Albert Bernhard Frank used symbiosis to describe the mutually beneficial relationship of fungi and algae living together as lichens. But there are many ways for individuals of two species to dance, all with positive and negative outcomes on one or the other. So in 1879 mycologist Heinrich Anton de Bary loosened the strings on the definition to include any long-term interaction between species. Each of these symbiotic associations has its own catchy term. As nouns, they are mutualism, commensalism, parasitism, predation, and amensalism. Just replace the “-ism” with “-istic” when you are in need of an adjective.

My favorite type of symbiosis is mutualism, in which both interacting species benefit from the relationship. And my favorite example of mutualism hangs with me because I am a self-centered forager who always enjoys an edible outcome of evolution. In a coniferous forest, spores of the hedgehog mushroom (as well as many other fungi that aren’t nearly as cute) germinate and grow into an underground network (the mycelium), with hairs that reach out and become intertwined with the tree roots, feeding on the gift of surplus sugars produced by photosynthesizing needles in the sunlit canopy. Hedgehog mycelia benefit their tree hosts by increasing the ability of their roots to take up water and minerals from the soil. Simple. And the mushrooms make excellent omelets.

Dr. Lynn Margulis: http://en.wikipedia.org/wiki/Lynn_Margulis

Dr. Lynn Margulis: http://en.wikipedia.org/wiki/Lynn_Margulis

There is something that resonates with us about species coexisting and, over the vastness of evolutionary time, eventually cooperating in a mutualistic two-step, especially when the liaison becomes so tight that neither species can survive without the other. These interactions remind me of a good long-term marriage, and rightfully we are awed. I agree with the late Dr. Lynn Margulis, who discovered the ancient and now obligatory association between our own eukaryotic cells and their energy producing mitochondria (which were once free-living single-celled organisms), when she asserted that cooperation as an evolutionary force has received short shrift compared with other win-lose interactions such as competition and predation. But I also wonder how much of our infatuation with these mutualistic relationships is borne of rectitude, our own deep-seated notion that cooperation is the “right” thing to do. We like these win-win just-so stories, even while knowing that evolution is an amoral process that cares not a whit about righteousness. Mutualism evolves because individuals who cooperate send more of their genes into the next generation.

Recently the study of mycorrhyzal mutualism has revealed a level of complexity previously unimagined. Enter Dr. Suzanne Simard of the Department of Forest Sciences at the University of British Columbia. She and others have discovered that the mycorrhizal relationships between trees and fungi are actually a community-wide phenomenon. Although individual trees and their fungal symbionts do exchange carbon and nutrients, Simard and others have demonstrated an extensive set of mycorrhizal networks in which carbon and water can be transferred from tree to fungus and then to other trees in the forest. This nutrient and water exchange can even depend on the needs of individual trees and fungi in the network, with a disproportionate quantity being shared with those individuals that are stressed. Young seedlings grow faster when they are able to tap into a mycorrhizal network established by older trees, and the exchange of water and nutrients can occur among multiple species of both fungi and trees. Thus, individuals that are connected to mycorrhizal networks not only help themselves through cross-species reciprocal relationships, they also help other individuals of the same and different species. Suddenly, my limited vision of symbiotic mutualism has expanded from a simple case of two individuals in a “you help me, I help you” relationship to one in which the ebb and flow of life-giving materials in a community occurs among many individuals of a variety of species and age groups.

The metaphorical consequences of this network of species linked through subterranean relationships fascinate me. I thought that symbiosis was a term first used in biology that was then co-opted to describe any situation in which there is an association between two things, including people, interacting for mutual benefit. But the reverse is actually true. The first use of the word dates back to the 1620’s and was meant to depict different people living together for mutual benefit within a human community. So symbiosis began as a sociological concept for describing a community of cooperating individuals. Apparently Albert Bernhard Frank was the borrower of the term when he applied it to mutually beneficial interactions between two different biological species. The recent work on mycorrhizal networks by Dr. Simard and her colleagues, in which many individuals of several species interact beneficially, seems to return the concept of symbiosis, or at least symbiotic mutualism, back to the original intent of the word.

On April Fool’s day I bolted from work under a stalwart overcast. As I forked leaf compost into a wheelbarrow for a new potato bed in my front yard, darkness closed around the front of the house, hastened along by a gray brick and mortar sky. My next-door neighbor passed by on the sidewalk, flushed and sweaty from his run and wondering why running had become so hard in the last month. “Tree pollen” I replied. Down the street Doris rolled in from work and from her front yard began talking to her basset hound that was baying in anticipation from inside the house. Later she came by with the dog to see what I was planting. Directly across the street the squeals of the small, still wordless voice of our new nine-month-old neighbor reached me through their open kitchen window while her parents engaged her in “conversation” and worked on dinner. I smiled to myself and forked more leaves into the wheelbarrow. We really are a community, like trees and fungi, fungi and algae, algae and salamanders, all exchanging energy in the form of sounds and materials and goodwill from one person to another. We grow best in this mutualisic network of relationships. We are symbiotic. We are mutual.