Allies and Enemies: How the World Depends on Bacteria (25 page)

decades. Microbiologists now build modified columns to emphasize a

certain type of metabolism. For example, columns containing sediments from iron bogs or iron springs have more iron metabolism than a standard Winogradsky column.

Feedback and ecosystem maintenance

Beijerinck and Winogradsky made a pivotal decision to study mixed

populations of bacteria as they are in nature. By doing so, they helped define the concept of an ecosystem. The Winogradsky column contains numerous interdependencies between bacteria, yet it is a simple example of larger natural ecosystems.

A properly working ecosystem does not remain static but rather

evolves in a process called succession. On a large scale, deforested land offers the best visible illustration of succession. New life takes hold on denuded land when cyanobacteria begin to grow in numbers.

Some of the cyanobacteria team with new fungi entering the environment to form lichens that begin to cover the nutrient-scarce land.

Mosses follow, and in turn small plants follow them. Over a period of

months, higher plants such as bushes become established. Small trees

and progressively larger, longer-lived trees establish over the next years. As this succession progresses, some species disappear as new more complex species emerge. Microscopic ecosystems follow a similar type of succession.

When bacteria enter a pristine habitat, nutrients may be plentiful

and competition low. (Nature has no completely pristine habitats, but

some natural events like floods and fires can create habitats that have lost a lot of their life and are ripe for recolonization.) The first bacteria to colonize the habitat are usually microbes that start out in the highest numbers or grow faster than the other microbes. Equally important, these bacteria have already adapted to the environmental

Chapter 6 · the invisible universe

139

conditions. They begin to change the habitat according to their specific type of metabolism. Some bacteria alter the pH, others remove all the oxygen, and some excrete simple organic compounds.

The altered conditions might favor another group of bacteria over

the original species. For example, an acid-producing bacterium eventually chokes under the buildup of acids. But another species that uses organic acids as a carbon source sees the habitat as a nutrient-rich place to colonize. In rare instances, the original colonizer alters the environment so much that no other organism can live there. For example, areas exposed to mine drainage become increasingly acidic

when T. ferrooxidans grows there and produces sulfuric acid as a by-product of the iron-sulfur compound pyrite. An ecosystem of diverse life cannot develop in situations like this, and the area turns into an extreme environment where only acid-loving extremophiles can live.

In the development of a healthy ecosystem, bacteria provide the

foundation for food chains. Increasingly complex organisms become

established. In healthy ecosystems, the new food chains develop associations that link them horizontally as well as vertically. In other words, a food web develops.

The more complex an ecosystem, the better it withstands changes

in the environment. In simple ecosystems with few food chains all of

the members depend on a relatively few species. If one or two species

disappear, the entire ecosystem collapses. By contrast, complex

ecosystems with many alternate paths for energy-and nutrient—

sharing are versatile and can adjust to change. Rich biodiversity benefits all life, and this biodiversity extends all the way to microscopic life.

Ecosystems are hardwired to control the number and variety of

species they contain. Two types of control processes operate: bottom-up and top-down. Bottom-up control uses microbes as the primary determinant of ecosystem health. If bacteria begin to disappear, the foundation of the ecosystem’s food chains also disappears. Top-down

ecosystem control theorizes that predators control the health of an ecosystem. By regulating the size of its prey, each member of an ecosystem prevents an exploding population of another member.

Nature rarely follows hard and fast rules, so ecosystems tend to utilize a mixture of both control mechanisms.

140

allies and enemies

In any ecosystem, organisms depend on feedback to help them

regulate their activities. The simplest feedback mechanism to understand is food supply; when a person is full, he stops eating (hopefully).

Being highly attuned to their environment, bacteria constantly interpret the immediate surroundings and respond by using feedback systems. For example, under starvation conditions, Bacillus turns into an endospore and myxobacteria produce fruiting bodies. When an ecosystem undergoes dramatic changes, even feedback may not be

sufficient to save all of the system’s members.

Microbial blooms are an example of an ecosystem gone out of

balance. A bloom is a rapid overgrowth of microbes that drastically

change an environment to the harm of other species. Blooms are caused by a sudden increase in numbers of aquatic algae, protozoa, or bacteria. Cyanobacteria and purple sulfur-metabolizing bacteria create most bacterial blooms, but bacteria play a part in algal blooms, too. Cyanobacteria and algae bloom in fresh and marine water when large, sudden influxes of nitrogen and/or phosphorus enter the environment. Runoff carrying fertilizer or manure from farmland acts as

the main cause of blooms. Nitrogen and phosphorus wash into waters

that usually contain low levels of these nutrients. The sudden bounty

of nutrients causes an equally sudden explosion of microbes. As the

microbial population grows increasingly dense, the cells release oxygen into the water as well as nutrients in the form of dead cells.

Heterotrophic bacteria (bacteria that use sugars, fibers, amino acids,

and fats) begin feasting, and they create a second bloom.

Bacteria in the second bloom are not photosynthetic so do not produce oxygen. Instead the fast-growing heterotrophs suck up all the oxygen from water around them. The oxygenless conditions soon cause other life to disappear; fish, crustaceans, and small invertebrates suffocate. Nutrient influx followed by ecosystem imbalance is called eutrophication. Cyanobacteria Anabaena and Nostoc are two common causes of blooms.

Cyanobacterial blooms now develop yearly in coastal areas and

specific rivers worldwide and cause a health threat beyond the harm

caused to aquatic life: cyanotoxins. Cyanotoxins are poisons released

by cyanobacteria and that remain in the water after the bacteria subside. A serious occurrence of cyanotoxin contamination took

Chapter 6 · the invisible universe

141

place in Brazil in 1993. Fifty hospitalized dialysis patients died because their therapy used a water source contaminated with

microcystin from the cyanobacterium Microcystis. (Water treatment technology has improved for removing pathogenic bacteria, but it remains poor at removing antibiotics, hormones, chemicals, and toxins.) Anaerobic blooms occur when purple sulfur-metabolizing bacteria Chromatium, Thiocapsa, or Thiospirillum grow out of control.

These blooms usually develop in oxygen-depleted waters in bogs and

lagoons, leaving a telltale pink-purple sheen over the water.

Many blooms disappear on their own when seasons change and

the hours of sunlight decrease, but several sites worldwide develop

annual cyanobacterial blooms. Problem blooms return every year to

the Great Lakes, the western United States, many Pacific islands, and

lakes and rivers in Europe.

Lake blooms can also come from the anaerobic purple bacteria

that live in darkness. Nutrient-rich lakes with a deep layer of bottom

sediments give rise to large anaerobic populations that support communities of Chromatium and Chlorobium just above the sediment layer. These two species possess the unusual ability to catch filtered sunlight that penetrates past the photic zone. As anaerobes prolifer—ate, they can turn the lake conditions unsuitable for other life.

In the 1970s Lake Císo in Spain became a topic for study because

it had developed an anaerobic bloom of sulfur-metabolizing bacteria.

The sediments emitted so much hydrogen sulfide that the gas filled

the lake’s entire water column, creating a rare type of anaerobic lake.

Sulfate-rich water now runs into the bottom of the Lake Císo and water dense with bacteria flow from the upper layer. Most other anaerobic lakes have an upper layer of cyanobacteria, on top of a Chromatium layer, that sits atop much darker, sulfur-saturated water.

Such an ecosystem is uninhabitable for fish and other animal life.

Macrobiology

In optimal conditions, ecosystems do not go out of balance. Whether

in lakes, soils, rumens, or insects, ecosystem members tend to self—

regulate their populations. These ecosystems receive extensive

142

allies and enemies

research and usually become study models for microbiology students.

Other ecosystems have offered few hints on how they work.

The luminescent bacterium Vibrio phosphoreum was discovered

in the 1970s in specialized glands of certain deep sea organisms (lantern fish, angler fish, and some jellyfish and eel), and they have since been found in Alaska salmon. Their role in water ecology has

puzzled scientists. The bacteria emit bluish-green light generated by

the pigment luciferin, the same compound that lights fireflies and creates the nighttime phosphorescence sailors see in their ship’s wake.

Does V. phosphoreum benefit the fish or does the host benefit the bacterium? Perhaps neither organism cares about the other even though they live as a pair, a relationship called neutralism.

Microbial ecologists have barely scratched the surface of the relationships between bacteria and global ecology. Their challenges

increase when considering almost inaccessible bacterial habitats deep

in the Earth’s mantle or miles under the ocean surface.

Only within the past decade or so have microbiologists extended

the reach of their studies to depths of about two miles into the earth or

approaching a mile into the polar ice sheets. The information ecologists draw upon to describe the functions of bacteria on Earth has come entirely from species close to or at the surface. Subsurface microbiology seeks to answer the questions of how bacteria of the deep contribute to life at the Earth’s surface. What do these bacteria eat in the darkness? How do they relate to the evolution of life at the surface? Do they have any connection at all to life on other planets?

The U.S. Department of Energy launched a program on subsurface microbiology in 1986. Wells drilled to aquifers about 700 feet deep reached a population of diverse and novel bacteria. With the help of geologists and hydrologists, researchers either drilled to the depths or gained access to the deep subsurface via existing mines. As

microbiologists probed deeper they found that bacteria became

increasingly dependent on inorganic materials for survival and less on

organic compounds.

Astrophysicist and NASA consultant Thomas Gold (who died in

2004) speculated in his book The Deep Hot Biosphere (1999) that the oceans’ food chains begin not with microscopic marine life in the water, but deep in the Earth’s lithosphere. These subsurface

Chapter 6 · the invisible universe

143

thermophiles, Gold proposed, exist on methane and hydrocarbons in

massive untapped oil reserves and represent the closest relatives to

life’s ultimate ancestors. A controversy rumbles, distant from the daily concerns of most microbiologists, as to whether life emerged on Earth’s surface or deep underground and grew outward to the surface. The bacteria that live in the deep-sea hydrothermal vents form the core of this argument because no one has as yet determined their

origin.

A plan exists for building a subsurface physics laboratory in South

Dakota’s Homestake gold mine one and a half miles down. Geomi—

crobiologists, who study the interactions of microbes with geological

formations, anxiously await. First, the construction must overcome issues of water purification, equipment installation, and the possibility of higher than normal radioactive bombardment. Then microbiologists will face the hurdle of growing these specialized bacteria under lab conditions.

Microbiologists continue to learn about the connection between