1. Numbers. Bacteria inhabit effectively every place suitable for the existence of life. Mother told you, after all, that bacterial "germs" require constant vigilance to combat their ubiquity in every breath and every mouthful—and the vast majority of bacteria are benign or irrelevant to us, not harmful agents of disease. One fact will suffice: during the course of life, the number of E. coli in the gut of each human being far exceeds the total number of people that now live and have ever inhabited the earth. (And E. coli is only one species in the normal gut "flora" of all humans.)
Numerical estimates, admittedly imprecise, are a stock in trade of all popular writing on bacteria. The Encyclopaedia Britannica tells us that bacteria live by "billions in a gram of rich garden soil and millions in one drop of saliva." Sagan and Margulis (1988, page 4) write that "human skin harbors some 100,000 microbes per square centimeter" ("microbes" includes nonbacterial unicells, but the overwhelming majority of "microbes" are bacteria); and that "one spoonful of high quality soil contains about 10 trillion bacteria." I was particularly impressed with this statement about our colonial status (Margulis and Sagan, 1986): "Fully ten percent of our own dry body weight consists of bacteria, some of which, although they are not a congenital part of our bodies, we can’t live without."
2. Places. Since the temperature tolerance and metabolic ranges of bacteria so far exceed the scope of all other organisms, bacteria live in all habi
-
tats accessible to any form of life, while the edges of life’s toleration are almost exclusively bacterial—from the coldest puddles on glaciers, to the hot springs of Yellowstone Park, to oceanic vents where water issues from the earth’s interior at 480°F (still below the boiling point at the high pressures of oceanic bottoms). At temperatures greater than 160°F, all life is bacterial. I shall say more in the following pages about new information on bacteria of the open oceans and the earth’s interior, but even conventional data from terrestrial environments prove the point. Thermophila acidophilum thrives at 140°F, and at a pH of 1 or 2, the acidity of concentrated sulfuric acid. This species, found on the surface of burning coals, and in the hot springs of Yellowstone Park, freezes to death below 100°F.
UTILITY. Importance for human life forms the most parochial of criteria for assessing the role of any organism in the history and constitution of life—though the conventional case for bacteria proceeds largely in this mode. I will therefore expand a bit toward utility (or at least "intrinsic-ness") for all of life, and even for the earth.
1. Historical. Oxygen, the most essential constituent of the atmosphere for human needs, now maintains itself primarily through release by multicellular plants in the process of photosynthesis. The earth’s original atmosphere apparently contained little or no free oxygen, and this otherwise unlikely element both arose historically, and is now maintained, by the action of organisms. Plants may provide the major input today, but oxygen started to accumulate in the atmosphere about 2 billion years ago, substantially before the evolution of multicellular plant life. Bacterial photosynthesis supplied the atmosphere’s original oxygen (and, in concert with multicellular plants, continues to act as a major source of resupply today).
But even if plants release most of today’s oxygen, the source of resupply remains, ultimately and evolutionarily, bacterial. The photosynthetic organelle of the eukaryotic cell-the chloroplast—is, by ancestry, a photosynthesizing bacterium. According to an elegant and persuasive notion—the endosymbiotic theory for the origin of the eukaryotic cell—sev— eral organelles of eukaryotes arose by greater coordination and integration of an original symbiotic assemblage of prokaryotic cells. In this sense, the eukaryotic cell began as a colony, and each unit of our own body can be traced to such a cooperative beginning.
The case has been made persuasively only for the mitochondrion— the "energy factory" of all cells—and the chloroplast—the photosynthetic organelle—though some proponents extend the argument more generally to cilia (seen as descendants of spirochete bacteria) and other parts of cells. The evidence seems entirely convincing for mitochondria and chloroplasts: both are about the same size as bacteria (prokaryotes are substantially smaller than eukaryotes, so several bacterial cells easily fit inside a eukaryote); they look and function like bacteria; they have their own DNA programs (small because most genetic material has, through evolutionary time, been transferred to the nucleus)—all indicating ancestral status as independent organisms. Thus, even today, atmospheric oxygen is a bacterial product—released either directly by bacterial photosynthesis, or by bacterial descendants in eukaryotic cells.
Bacterial symbiosis—with bacteria remaining as coherent creatures, taxonomically independent if ecologically dependent, and not fully incorporated like mitochondria and chloroplasts—is a vital and potent phenomenon in many of life’s central processes and balances. We could not digest and absorb food properly without our gut "flora." Grazing animals, cattle and their relatives, depend upon bacteria in their complex, quadripartite stomachs to digest grasses in the process of rumination. About 30 percent of atmospheric methane can be traced to the action of methanogenic bacteria in the guts of ruminants, largely released into the atmosphere—how else to say it—by belches and farts. (The most cultured and distinguished British ecologist, G. Evelyn Hutchinson, once published a famous calculation on the substantial contributions to atmospheric methane made by the flatulence of domestic cattle. Sagan and Margulis [1988, page 113] advance the "semiserious suggestion that the primary function served by large mammals is the equitable distribution of methane gas throughout the biosphere.")
In another symbiosis essential to human agriculture, plants need nitrogen as an essential soil nutrient, but cannot use the ubiquitous free nitrogen of our atmosphere. This nitrogen is "fixed," or chemically converted into usable form, by the action of bacteria like Rhizobium, living symbiotically in bulbous growths on the roots of leguminous plants.
Some symbioses are eerie in their complexity and almost gory precision. Nealson (1991) documents the story of a nematode (a tiny round-worm) parasitic upon insects and potentially useful as a biological control upon pests. The nematode enters the insect’s mouth, anus, or spiracle (breathing organ) and migrates into the hemocoel (or blood cavity). There the nematode ejects millions of bacterial symbionts from its own intestine into the insect’s circulatory system. These bacteria, though harmless to the nematode, kill the insect within hours. (Bacteria need the nematode to feast upon the insect because bacteria entering by themselves never reach the hemocoel and therefore do not attack the insect.) The dead insect becomes bioluminescent (another consequence of bacterial action) and darkly pigmented, but does not putrefy (perhaps because the nematode also releases antibiotics that kill other bacteria but leave their own symbionts harmless). The pigment and glow then attract other nematodes to the insectan feast. The nematodes grow and reproduce by eating the insect; they also take on the helpful bacteria as symbionts. This source can yield up to 500,000 nematodes per gram of infected insects.
The recent discovery of the remarkable deep-sea "vent faunas," at zones of effusion for hot, mineral-laden waters from the earth’s interior to the ocean floor, has provided another striking case of bacterial necessity and symbiosis. An old saw of biological pedagogy (I well remember the phrase emblazoned on the chapter heading of my junior high school textbook) proclaims, "All energy for biological processes comes ultimately from the sun." (I remember the pains that teachers took to trace even the most indirect pathways to a solar source—worms on the sea bottom eating decomposed bodies of fishes, which had fed on other fishes in shallow waters, with the little fishes eating shrimp, shrimp eating copepods, copepods ingesting algal cells, and algal cells growing by photosynthesis from that ultimate solar source.)
The vent faunas provide the first exception to this venerable rule, for their ultimate source of energy comes from the heat of the earth’s interior (which warms the emerging waters, contributes to the solubility of minerals, and so on). Bacteria form the base of this unique and independent food chain—mostly sulfur-oxidizing forms that can convert the minerals of emerging waters into metabolically useful form. Some rift organisms form amazing symbiotic associations with these bacteria. The largest animal of this fauna, the vestimentiferan worm Riftia pachyptila, grows to several feet in length, but has no mouth, gut, or anus. This creature is so morphologically simplified that taxonomists have still not been able to determine its zoological affinity with confidence (current opinion favors a status within a small group of marine worms, the phylum Pogonophora). Riftia does contain a large and highly vascularized organ called the trophosome, filled with specialized cells (bacteriocytes) that house the symbiotic sulfur bacteria. Up to 35 percent of the trophosome’s weight consists of these bacteria (Vetter, 1991).
2. Current. As discussed above, bacteria produced our atmospheric oxygen, fix nitrogen in our soil, facilitate the rumination of grazing animals, and build the food web of the only nonsolar ecosystem on our planet. We could also compile a long list of more parochial uses for particularly human needs and pleasures: the degradation of sewage to nutrients suitable for plant growth; the possible dispersion of oceanic oil spills; the production of cheeses, buttermilk, and yogurt by fermentation (we make most alcoholic drinks by fermentation of eukaryotic yeasts); the bacterial production of vinegar from alcohol, and of MSG from sugars.
More generally, bacteria (along with fungi) are the main reducers of dead organic matter, and thus act as one of the two major links in the fundamental ecological cycle of production (plant photosynthesis and, come to think of it, bacterial photosynthesis as well) and reduction to useful form for renewed production. (The ingesting animals are just a little blip upon this basic cycle; the biosphere could do very well without them.) Sagan and Margulis write in conclusion (1988, pages 4—5):
All of the elements crucial to global life—oxygen, nitrogen, phosphorus, sulfur, carbon—return to a usable form through the intervention of microbes.... Ecology is based on the restorative decomposition of microbes and molds, acting on plants and animals after they have died to return their valuable chemical nutrients to the total living system of life on earth.
NEW DATA ON BACTERIAL BIOMASS. This range of bacterial habitation and necessary activity certainly makes a good case for domination of life by the modal bacter. But one claim, formerly regarded as wildly improbable but now quite plausible, if still unproven, would really clinch the argument. We may grant bacteria all the above, but surely the main weight of life rests upon eukaryotes, particularly upon the wood of our forests. Another truism in biology has long proclaimed that the highest percentage of the earth’s biomass—pure weight of organically produced matter— must lie in the wood of plants. Bacteria may be ubiquitous and present in nearly uncountable numbers, but they are awfully light, and you need several gazillion to equal the weight of even a small tree. So how could bacterial biomass even come close to that of the displacing and superseding eukaryotes? But new discoveries in the open oceans and the earth’s interior have now made a plausible case for bacterial domination in biomass as well.
As Ariel, in The Tempest, proclaimed his ubiquity in all manifestations of life—"where the bee sucks, there suck I / In a cowslip’s bell I lie"—so, in this world, do bacteria dwell in virtually every spot that can sustain any form of life. And we have underestimated their global number because we, as members of a kingdom far more restricted in potential habitation, never appreciated the full range of places that might be searched.
For example, the ubiquity and role of bacteria in the open oceans have been documented only in the past twenty years. Conventional methods of analysis missed up to 99 percent of these organisms (Fuhrman, McCallum, and Davis, 1992) because we could identify only what could be cultured from a water sample—and most species don’t grow on most culture media. Now, with methods of genomic sequencing and other techniques, we can assess taxonomic diversity without growing a large, pure culture of each species.
Scientists had long known that the photosynthesizing Cyanobacteria ("blue-green algae" of older terminology) played a prominent role in the oceanic plankton, but the great abundance of heterotrophic bacteria (non-photosynthesizers that ingest nutrients from external sources) had not been appreciated. In coastal waters, these heterotrophs constitute from 5 to 20 percent of microbial biomass and can consume an amount of carbon equal to 20 to 60 percent of total "primary production" (that is, organic material made by photosynthesis)—giving them a major role near the base of oceanic food chains. But Jed A. Fuhrman and his colleagues then studied the biomass of heterotrophic bacteria in open oceans (that is, needless to say, by far the largest habitat on earth by area) and found that they dominate in these environments. In the Sargasso Sea, for example (Fuhrman et al., 1989), heterotrophic bacteria contribute 70 to 80 percent of microbial carbon and nitrogen, and form more than 90 percent of biological surface area.
When I visited Jed Fuhrman’s lab at the University of Southern California, I asked him if he could estimate the earth’s total bacterial biomass relative to contributions from the other kingdoms of life. These "back of the envelope" calculations have a long and honorable history in biological barroom discussions—and no one would want to grant them any more technical or firmer status. They must, of necessity, be based on a large number of assumptions and "best estimates" that may be wildly wrong for lack of better available data (average number of bacteria per milliliter of sea water for all the world’s oceans, for example). Still, such calculations serve a useful function in defining ballparks. Fuhrman made his best estimate for me, and came up with an oceanic bacterial biomass equal to about one-fiftieth of the entire terrestrial biota, including wood. This may not sound impressive, but whenever such a calculation gets you within an "order of magnitude" or two of a key number, then you are "in the same ballpark." (An order of magnitude—the standard measure of comparison for such rough calculations—is a multiple often. Thus, 1/50 is between one [1/10] and two [1/100] orders of magnitude from the terrestrial figure—and definitely in the same ballpark.) This figure is even more impressive when you realize (1) that all traditional estimates have granted domination to the multicells by orders of magnitude because the biomass of wood must be so high; (2) that Fuhrman has not included terrestrial bacteria of soil, gut floras, nodules of leguminous plants, etc.; and (3) that an even greater potential source of biomass from a "new" environment— the earth’s interior—has been similarly excluded. If we then turn to some stunning, and controversial, data on the earth’s interior, we may really be in for a surprise.