Allies and Enemies: How the World Depends on Bacteria (27 page)

same claim.

Similar nonsunlit systems include E. coli hydrogenase with the enzyme carbon monoxide dehydrogenase (CMD) from Carboxydothermus hydrogenoformans. CMD splits carbon monoxide. The overall reaction is

carbon monoxide (CO) + water (H O) → carbon dioxide (CO )

2

2

+ hydrogen (H )

2

C. hydrogenoformans that catalyzes this reaction is an anaerobe isolated in 1991 from a freshwater hot spring on Kunashir Island in the Sea of Japan. The German Collection of Microorganisms in

Braunschweig owns one of the world’s few cultures of this obscure microbe.

Astute readers will notice that the preceding reaction gets rid of

the greenhouse gas carbon monoxide but produces another culprit in

global warming, carbon dioxide. Scientists have dreamed up various

ways to pull carbon dioxide from the air. Ideas include giant filters strewn across the landscape to suck in carbon dioxide and then pump it deep into the earth. Others have proposed seeding the oceans with

nutrients so that algae and cyanobacteria increase and thus consume

more carbon dioxide.

Carbon-dioxide consumers in the bacterial world occupy special

niches. Chemolithotrophs (growing only on inorganic salts and carbon

dioxide) and photolithotrophs (growing on sunlight and carbon dioxide) draw some of this gas from the atmosphere. The other important consumers of carbon dioxide live in dark places where they digest organic matter and prevent the Earth from becoming choked in waste.

How is a cow like a cockroach?

The methane gas emanating from sewage treatment plants, landfills,

and the muck submerged in swamps comes mainly from methane—

producing archaea. These so-called methanogens interact with bacteria in a way that allows both to thrive and keep ecosystems running.

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Methanogens sustain cattle, goats, sheep, deer, elephants, and all

other ruminants plus cockroaches, termites, beetles, and milli—

pedes—thousands of arthropod species in all. Their digestive tracts

contain a heterogeneous mixture of microbes from the three domains

of living things: Archaea, Bacteria, and Eukarya. The bacteria and archaea cling to the rumen wall and to feedstuffs entering the rumen while protozoa tend to stay in the liquid.

A cow’s four-part digestive organ—rumen, reticulum, omasum,

and abomasum—evolved for fermentation. Ruminant animals

scarcely chew their food after yanking it from soil; they masticate just enough to mix the grasses with saliva, and then send the bolus into the esophagus leading to the rumen. The cow rumen holds up to 20

gallons, the interior resembling a perpetual washing machine lined with small protuberances called papillae. These structures increase the rumen’s inner surface area to make absorption more efficient and to increase attachment sites for microbes. Rumen fluid ranges from

Kelly green from grass diets to olive-green when the cow gets mainly

a hay diet. Every minute or so the esophagus launches a bolus into

the mix like a torpedo. The fluid softens the bolus, and then the animal regurgitates and rechews it. After “chewing the cud,” the bolus goes back to the rumen where bacteria and protozoa continue the fiber digestion. As the rumen contents slosh about, the animal regurgitates larger pieces and sends smaller denser pieces to the intestines where a different population of bacteria continues the digestion.

A microscope slide holding a drop of rumen fluid or a speck from

a cockroach’s innards reveals a mob of microbial life. Cocci and rods

bob in the currents. Every second or so a spirillum twirls through the

microscope’s field; blink and you have missed it. Protozoa come and

go, looking massive next to the bacteria. These eukaryotes range from

20 to more than 100 times the volume of bacteria. Some flagellated

protozoa poke through the liquid in fits and starts while other protozoa blanketed in cilia whiz past.

Most of the digestive anaerobes differ from E. coli because they cannot tolerate even miniscule amounts of oxygen. These über—

anaerobes (obligate anaerobes to microbiologists) include Bacteroides, Butyrivibrio, Clostridium, Eubacterium, Lactobacillus, Peptostreptococcus, Ruminococcus, Selenomonas, Streptococcus, 152

allies and enemies

Succinimonas, Succinivibrio, and Veillonella. Cattle also harbor Lactobacillis, Clostridium, and E. coli—cattle is humans’ main source of the deadly E. coli O157 when farm waste contaminates food. At least 20 species of archaea and 50 species of protozoa also inhabit the digestive tract.

When fibers (cellulose, hemicelluloses) and polysaccharides enter

the rumen, bacteria degrade these large compounds into smaller sugars for energy. Protozoa subsist on sugars but also graze on the variety of bacteria and archaea. The archaeal methanogens use carbon dioxide plus vitamins and minerals available in the rumen fluid.

The cow uses relatively little of the nutrients in grass and grain directly. Ruminants live mostly on the volatile compounds emitted by the bacteria. These so-called volatile fatty acids (VFAs) named acetic

(two carbons), propionic (three), and butyric (four) acids pass through the animal’s gut lining and enter the bloodstream. The fat and flavor of fresh cow’s milk result from the mammary gland’s synthesis of long fats from the short VFAs. Goats produce a different array of fats from the same three VFAs, which results in distinctive flavors in products made

from goat’s milk.

Cows receive most of their amino acids and vitamins from bacteria that the animal’s digestive enzymes degrade. Unlike humans, ruminants survive on very poor quality protein, meaning the protein contains a limited variety of amino acids, because the bacteria improve the variety of amino acids available for absorption.

Cattle spend one-third of their time eating, one-third ruminating

or chewing the cud, and one-third resting. During rest, bacterial activity reaches its peak: Large molecules decompose in fermentation to VFAs, carbon dioxide, and a little hydrogen. These reactions would

soon stop if carbon dioxide built up in the gut. Methanogenic archaea

play the vital role of absorbing carbon dioxide as it appears and turning it into methane:

CO + H → CH

2

2

4

A dairy cow with a 15-gallon rumen belches 65 to 130 gallons or

5,370 to 10,740 cubic feet of methane a day. The world’s domesticated

and wild ruminants produce about 22 percent of the atmosphere’s methane, one million tons of methane put into the atmosphere a year.

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Since methane exerts more than 20 times the atmosphere-warming

effect of carbon dioxide, ruminants contribute to global warming.

When the media make coy references to ruminant flatulence as a major cause of global warming they really should blame belching.

Microbiologists study the goings on inside cow rumens by using

fistulated animals. A fistula is an opening about the diameter of an orange leading from the outside of the animal to the inside of the rumen. The left wall of a cow’s rumen lies against the animal’s left side, making the distance from outside to inside less than 3 inches.

After a veterinarian surgically fistulates the left side of the animal, the patient recovers quickly and begins eating again within the first few hours after surgery. (Humans can last days without food, but a ruminant cannot go 24 hours without food before becoming deathly ill.) The fistula, like a rubberized doughnut, can be closed with a tight—fitting plastic plug. When opening a fistula plug, a rush of methane

bursts from within.

Cockroaches use processes similar to ruminants but with a more

active role by protozoa. Bacteria and archaea living inside the protozoa

that live inside the insect’s gut carry out the chemical reactions of digestion. Protozoa presumably take in nutrients that sustain the prokaryotes and protect them from predation by other protozoa. As a result, 80 percent of the (American) cockroach’s methane emissions comes from its protozoa.

The protozoa inside ruminants, cockroaches, and termites live in

mutualistic symbiosis with the host. Termites contain symbionts

within symbionts. The insect lacks fiber-degrading enzymes, so it depends on gut protozoa to digest the wood fibers. But the protozoa, such as Trichonympha sphaerica, also make little progress in digesting wood. T. sphaerica relies on spirochete (spiral-shaped) bacteria living inside it. The bacteria produce the enzyme cellulase that decomposes the cellulose so that the insect, the protozoa, and the bacteria all benefit.

A second group of bacteria live on the outside of termite protozoa. Some spirochetes and other rod-shaped cells line up in precise

rows in grooves between the protozoan’s cilia. Electron microscopy

has revealed that the curvy spirochetes line up end-to-end and undu—

late in unison. The protozoan moves by the combined action of its

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allies and enemies

cilia and the coordinated beating of thousands of spirochete flagella,

creating a smooth wave of propulsion. No one has yet figured out if

the protozoa tell the bacteria where to swim or if the bacteria control where protozoa go. Regardless of the answer, protozoa need their bacteria; if the bacteria disappear, the protozoan stops dead in the water.

Microscopic power plants

In the 1990s, Al Gore’s tireless campaign to address global warming

prompted scientists to identify the world’s main sources of methane.

Twenty times more active in warming the atmosphere than carbon dioxide, methane became a strategic target in the global warming campaign. The scientists estimated that enteric fermentations of ruminants and insects account for almost 25 percent of the atmosphere’s methane. Cattle manure accounts for another 7.5 percent.

More than half of the methane from human-made structures

such as landfills and wastewater treatment already goes into systems

that use it as an energy source. The methane from swamps, stagnant

ponds, manure piles, and domesticated and wild ruminants goes lost

to the atmosphere. An adult cow produces about 27 pounds of solid

waste daily and the 100 million cattle in the United States add close to 14,000 tons of manure to waste piles every day. Central Vermont Public Service offers manure-derived methane, or “cow power,” to more than 3,000 homes and businesses. The state’s dairy farms supply the

manure that produces the biogas, and the utility converts the gaseous

energy to electrical energy and distributes the electricity.

Bacteria in nature or in test tubes always take the most efficient

path for finding, absorbing, and metabolizing nutrients. Heterotrophs

prefer sugars, fibers, amino acids, and fats for energy and building new cells. Other bacteria called autotrophs thrive on a less heterogeneous variety of nutrients, namely water and carbon dioxide for cell—building and sunlight or metal for energy. Autotrophs (also called lithotrophs) grow on a chunk of rock devoid of organic matter or in the nutrient-empty ultrapure water used in semiconductor manufacturing. The bacteria being discovered in subsurface microbiology are all autotrophs. They get small bits of energy from chemical reactions

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between water and basalt, and scavenge nitrogen and sulfur from tiny

pockets of air.

Heterotroph and autotroph energy production happens in the

cell membrane, a multilayered covering that lies just inside the cell

wall. Energy generation in bacteria resembles that of humans in that

it uses a stepwise transfer of electrons from compound to compound.

Each transfer produces small spurts of energy. Humans use membrane-bound proteins called cytochromes to perform most of the

electron transfers. Bacteria depend on pigments. The blue-green

hues of ocean and freshwater cyanobacteria, the striking colors in hot

springs from sulfur and iron metabolizers, and the green and purple

intertidal flats populated by photosynthetic bacteria all give evidence that bacteria are hard at work.

Bacteria can be harnessed to produce energy directly rather than

energy in the form of fuel. University of Massachusetts microbiologists Derek Lovley and Gemma Reguera have showed that biofilms grow tiny filaments between cells. These filaments act as “nano-wires” to transmit electrical current, which the cell consortium amplifies about