Consummate imperialists, species of bacteria have adapted to survive in virtually every habitat on the planet, from the deepest abysses of the ocean floor to the thin air of the stratosphere. For example, one strain of pseudomonad bacteria live, grow, and reproduce themselves on the surface of snow, where they appear as a coating of red dust or pink film visible to the naked eye. This kind of geographic and climactic versatility is matched by an equally fluid exchange of genes amongst different kinds of bacteria, such that it is virtually impossible isolate individual species. The challenge to traditional taxonomy is further compounded by the proclivity of bacteria to commingle and functionally interact in symbiotic living situations. If these traits resist structural and genealogical forms of classification, they also suggest another approach, that of description along the axes of functional interaction. For it is these dynamic inter-relationships that structure bacteriological communities themselves.
One example of such a complex system are the layered bacterial communities known as "microbial mats." Studied extensively by biologist Lynn Margulis, she describes microbial mats as "sticky, textile like expanses of mud, usually found in intertidal regions where the strong wave motion of the open ocean is minimized by a barrier of dunes or rocks. Mats are often spectacularly colored purple and green as a result of massive populations* of photosynthetic bacteria.... Mats usually flourish in areas that are too salty or stagnant for larger organisms." The uppermost layers of the mat contain blue-green bacteria which are capable of photosynthesis; that is, they use the energy of light to split hydrogen molecules from water and hook them onto carbon dioxide molecules. In this way they manufacture organic compounds needed for their bodies, as well as produce oxygen. Immediately below this food-producing layer are several distinct strata of oxygen-tolerant bacteria, which consume the food and oxygen produced above them. Below and protected by this layer from the blue-greens are bacteria which require light but which are poisoned by oxygen.
Such communities, through symbiotic production of food and micro habitat, can survive and grow for hundreds of years. Fossil evidence indicates mat colonies dating back as early as 3.5 billion years, and contemporary examples still flourish in locations including the Caribbean Islands, the southeastern United States, and Baja California.
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"Billions and billions," the numerical image attributed to her recently deceased first husband, Carl Sagan, may have seemed provincial to Margulis, whose son and co-author Dorion Sagan estimates that a spoonful of high quality soil may contain as many as 10 trillion bacteria (source: Garden of Microbial Delights, Dorion Sagan and Lynn Margulis, Harcourt Brace Jovanovich, Publishers, Boston, San Diego, New York, 1988, p. 105.).