Allies and Enemies: How the World Depends on Bacteria (24 page)

global nitrogen recycling system. The nitrogen cycle is perhaps the most-studied nutrient cycle due to its importance to agriculture and the health of humans and other animals. Most people do not suffer from a lack of carbon in their diet. Nitrogen in the form of protein is another story. Nitrogen often occurs in limited supply in diverse environments, making the nitrogen cycle all the more crucial for Earth’s organisms.

The proteins in a steak dinner result from a global cycle of nitrogen use and reuse. Without exaggeration, the steak’s nitrogen may have come from cyanobacteria in a distant ocean or soils on another continent. Nitrogen gas makes up 78 percent of the atmosphere, almost four times as much as the next most abundant constituent, oxygen. Despite this apparent abundance of nitrogen, living things expend much more energy to get nitrogen into their bodies than they

do to absorb oxygen. Except for bacteria, no life takes nitrogen gas

directly into the body like oxygen enters. Bacteria, including

cyanobacteria, make nitrogen available for all other life by taking in

the gas in a process called nitrogen fixation and converting the nitrogen into a form usable by plants. Grazers such as rabbits convert the plant nitrogen (mainly in vitamins and nucleic acids such as DNA) to

animal nitrogen (mainly as protein in muscle).

Some species, such as Azotobacter and Beijerinckia, live independently in the soil and perform nitrogen fixation there. These bacteria convert the gas to ammonia by adding hydrogen atoms to each nitrogen atom. The bacteria Rhizobium (discovered by Beijerinck) and Bradyrhizobium also absorb nitrogen from the air, but they do so from inside bumps on the roots of plant cells, called root nodules.

Martinus Beijerinck discovered this bacteria-plant process in 1888.

The bacteria-plant relationship in nitrogen fixation represents symbiosis, which is the cooperative association of two organisms living in close proximity. The intestinal bacteria in humans, other animals, and insects also illustrate a symbiotic relationship.

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After nitrogen gas has been converted into ammonia, the nitrogen

passes through a sequence of reactions each carried out by the bacteria that Sergei Winogradsky discovered more than a century ago.

Nitrosomonas converts the ammonia to compounds called nitrites (two oxygens attached to a nitrogen), Nitrobacter turns nitrites into nitrates (three oxygens attached to nitrogen), and plant roots then absorb the nitrates for their own nitrogen needs. An entirely different group of

bacteria takes excess nitrates from the soil, turns it into the gas nitrous oxide, and releases this gas back into the atmosphere.

The nitrogen that ends up in plants is used by the plant to build

vitamins, nucleic acids, and proteins. Cattle grazing on grasses and clovers take in the plant nitrogen, and then rumen bacteria begin their task of building microbial proteins. Humans benefit from the entire process when they ingest proteins in beef. The environment also receives a share of nitrogen when plants die and decay (by the action of soil bacteria such as Bacillus), releasing nitrogen into the soil, and manure from cattle farms leaches into the soil and surface waters.

The nitrogen cycle, so essential for all life, takes up a lot of space.

Meat-producing cattle and sheep take up thousands of square miles

of land across the world. Countries with a large land area like the United States or Canada can manage this problem, but water-stressed and tropical regions find that meat production makes impossible demands on their environment. Meat animals compete with humans

for water in an increasingly large portion of the world. In the tropics, meanwhile, farmers are cutting down or burning jungle to clear land for cattle. As the tropics shrink so does biodiversity.

Many environmentalists feel that large animal meat production

threatens the environment, prompting scientists to investigate bacteria as a direct protein source. The nitrogen cycle will continue running, of course, but humans might put less demand on it by using alternate protein sources. The cyanobacterium Spirulina (see Figure 6.2) has drawn interest as a potential microbial protein source. Dried Spirulina powder serves as a vitamin and protein supplement.

Spirulina cells are up to 70 percent protein. Most other bacteria consist of about 50 percent protein. Furthermore, Spirulina protein is high-quality protein, meaning it contains all of the essential amino

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acids for humans. Spirulina resembles all other photosynthetic organisms by supplying a variety of vitamins and minerals. The enzymes that run photosynthesis require a constant input of vitamins and minerals that act as co-factors, supplementary molecules that participate in chemical reactions. Consider the following attributes of Spirulina as a food:

· More beta-carotene, which the body converts to vitamin A,

than carrots

· 28 times more iron than beef liver

· Higher concentration of vitamin B than any other food.

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Figure 6.2 Spirulina pacifica. This filamentous cyanobacterium has been used for centuries as food. Masses of growth are collected from water, sun-dried, and patted into flat cakes to cook or eat directly. (Courtesy of Dennis Kunkel Microscopy, Inc.) Does Spirulina have a future as a new protein source for under-nourished regions of the world? On a per-acre basis, the cyanobacterium supplies 200 times the protein yield of beef and consumes 315

times less water. NASA has experimented with the use of Spirulina,

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misidentified as algae, as a food for space flights. Spirulina farms have grown in number worldwide from Thailand, India, and to the United States. These farms contain large ponds for growing the bacteria in a continuous flow system of incoming nutrients and outgoing final product. Microbiologists maintain a narrow range of growth conditions to enhance cyanobacteria growth and inhibit the growth of contaminants.

The environment’s current precarious condition requires people

to make hard choices regarding the materials they consume.

Spirulina may become an important aspect of sustainability, but it has not yet arrived there.

How to build an ecosystem

A pond, meadow, or tidal pool is an example of an ecosystem. Each

ecosystem contains a network of interactions between multicellular plants and animals, tiny invertebrates, microbes, and inanimate

objects such as soil, water, rocks, and air. Bacteria operate in ecosystems by interacting with other microbes as well as with the immediate microscopic environment composed of liquid and solid surfaces.

In liquids, bacteria contend with the positively or negatively

charged substances dissolved or suspended in the microenvironment.

Some cells turn on their chemotaxis mechanism to swim toward favorable conditions or away from harmful conditions. Other bacteria float in the milieu and absorb any nutrients they meet, or these cells are

swept toward communities of mixed species and settle down there.

In liquid environments with high content of organic matter, bacteria aggregate into a thin film that covers the surface. In this position, cells take in oxygen from the air and absorb nutrients from below.

Bacteria in surface films must regulate the surface tension of the air—

water interface to stabilize the film’s structure and reduce breakup.

Some bacteria, such as Pseudomonas, secrete surfactants to help regulate surface tension. These detergentlike substances allow

hydrophobic compounds that land on the film to become miscible with the water, and this provides the film with a potential new nutrient source. Surfactants also help nutrients enter microbial cells, thus helping the cells stay in the fragile surface film. Surfactants play a similar role for bacteria in soil on root surfaces.

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Soil bacteria live in a microenvironment made mainly of silicon,

the Earth’s most abundant element. Soil also has substantial amounts

of aluminum, iron, calcium, sodium, potassium, and magnesium.

Most of these elements occur in a positively charged form that influences the cells’ attachment to soil particles. Sometimes bacteria live in a community attached to the inanimate particles, but in other instances they inhabit moist micropores varying from micrometers to

millimeters in size. These tiny microenvironments are often the places where nutrients begin cycling through the Earth, atmosphere, oceans, and to every living thing.

The sulfur cycle consists of more chemical conversions than any

other known nutrient cycle. Sulfur as sulfur dioxide gas enters the atmosphere when released from volcanic activity, including hot sulfur springs. Fossil fuel combustion also adds large amounts to the atmosphere. Most of the Earth’s sulfur is held in the planet’s core with lesser amounts in biological matter. The Earth’s crust contains almost 2 x 1016 tons of sulfur; the terrestrial and marine biological matter holds about 1 x 1010 tons.

Two groups of bacteria nicknamed the green sulfurs and purple

sulfurs for the type of pigments they contain convert elemental sulfur

to sulfate compounds. Elemental sulfur—this is pure sulfur unattached

to any other element—is a solid that sticks to soil particles as well as the surface of bacterial cells. Yellowish sulfur granules cover these bacteria, which secrete enzymes to convert the granules to more soluble sulfate compounds. A wide variety of soil microbes then use the sulfates.

Still ponds or swamps that give off a rotten egg smell, characteristic of hydrogen sulfide, provide evidence of active sulfur cycling taking place underground. Because this cycle depends on anaerobic bacteria, it occurs in sediments and the deepest waters lacking oxygen. If a light were to be lowered into a swamp and clicked on, the sulfur bacteria would be green and pinkish-purple.

Aside from his studies on nitrogen and sulfur bacteria, Winogradsky examined the bacteria that generate energy by oxidizing or reducing iron. The iron cycle takes place in waters that drain from slow-moving bodies such as swamps and ponds and involves continual conversions of the element’s chemical form by releasing or accepting

electrons from other atoms. In soils suspected of having high iron

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levels, orange to reddish soil indicates that more oxidation (releasing electrons) is occurring than reduction (accepting electrons). Three prevalent bacteria that carry out this step are Thiobacillus ferrooxidans, which works in acidic conditions, Gallionella that prefers neutral conditions, and Sulfolobus, which grows best in acidic and high-temperature conditions. Microbiologists find T. ferrooxidans in areas that contain drainage from mining operations and Sulfolobus in sulfur hot springs. High-iron soils that are dark green or black contain more reduction than oxidation. The anaerobic Geobacter, Desulfuromonas, and Ferribacterium perform this reaction.

Winogradsky’s legacy has been captured in a simple experiment

that pulls together all of these metabolisms and mimics bacterial activities in nature. The “Winogradsky column” is a tall cylinder or jar filled with wet mud from a pond, lake, or ocean shore and topped with water. The amateur scientist adds shredded and chopped newspaper (as carbon source) and egg yolk (sulfur), and then puts the glass in a well-lighted place. After six weeks, bacteria from the mud settle into layers defined by oxygen levels with anaerobic mud below and

aerated water above. The bacterial numbers start out low, but the appearance of colored striations in the column indicate the populations have grown to high densities. The colors give clues to the organization of bacteria in the column: · Blue-green cyanobacteria receiving sunlight at the top

· Sulfide-using bacteria Beggiatoa and Thiobacillus in a light brown layer

· Photosynthetic Rhodospirillum in a large, nutrient-rich, rust-colored layer · Red Chromatium in a low-oxygen layer using filtered light for photosynthesis · Green Chlorobium absorbing hydrogen sulfide gas rising from the mud

· Brown anaerobic mud filled with hydrogen sulfide-producing

Desulfovibrio and cellulose (newspaper)-degrading Clostridium Iron-reducing bacteria, if present, live in the anaerobic sediment

at the bottom of the column, and iron-oxidizing bacteria develop a rusty-red zone above the sediment.

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In a single glass container, a person can watch the real activities of

bacteria in nature. The Winogradsky column also creates a simplified

microcosm of evolution; anaerobic actions beginning life in an oxygenless environment, and then progressing to photosynthesis and oxygen-respiring organisms.

Winogradsky columns can metabolize for months to years to