Allies and Enemies: How the World Depends on Bacteria (23 page)

Certain bacteria in nature prey on other bacteria each in their unique way. Bdellovibrio lives in a broad range of habitats from soil to fresh and salt waters and in sewage. This gram-negative genus preys on other gram-negatives by attaching to a cell and secreting enzymes

that bore a hole in the cell wall. The predator then squeezes into the

space between the prey’s cell wall and membrane. The prey cell dies

but the Bdellovibrio stays and wears it like a coat that somehow resists any new predators.

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The aptly named Vampirococcus attaches to its prey but does not penetrate the bacterium. It excretes enzymes to partially degrade the prey cell, preferably photosynthetic species. After sucking out all of

the prey’s cytoplasm, Vampirococcus leaves behind an empty cell wall.

Myxobacteria have their own distinctive type of predation. Motile

myxobacteria form “wolf packs” of a few dozen to hundreds of cells

that glide through the soil in search of prey. The long, thin rods line up in parallel with a few leader cells extending a bit in front of the pack. Myxobacteria packs gracefully patrol the waters for food. After

devouring all bacteria in an area, the myxobacteria cells aggregate into a huge funguslike structure called a fruiting body that grows up to 75 millimeters in height. This body, like nothing else in the bacterial world, contains pigments that color the colonies red, orange, yellow, or brown. The fruiting body’s stalk raises a sac of cells above the soil’s surface. Wind or rain liberates the myxobacteria and carries them to a new location. If conditions at the new site look good, the myxobacteria begin a new life cycle. Fruiting bodies are easy to spot

on decaying organic matter, particularly beech and elder trees.

Microbial ecologists have not determined the role of predation in

the microbial world. Predation certainly benefits predators in places

with low nutrient supply. The predator lets the prey do the work of

absorbing and concentrating nutrients, and then gulps the entire meal. Some predators take in bacteria but do not digest them. In the termite gut for instance, bacteria inside protozoa inside the insect digest the woody fibers that termites ingest.

Three hallmarks of bacteria contribute to their versatility. First, the huge size of bacterial populations increases the chance of developing mutants with one or more new, favorable traits. Second, short generation times help a species make the new trait part of its genetic makeup.

Third, because bacteria are compact, they have developed enzymes that can do more than one function. For example, enzymes that degrade common organic compounds in nature might also decompose pollutants. The principle behind bioremediation is to use microbes that prefer decomposing a pollutant even when other foods are available.

Large numbers of organisms with different nutrient needs,

energy generation, and adaptations would be expected to create a

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diverse population such as found among microbes. Microbial diversity dwarfs that of other life forms, and microbial ecologists suspect that the diversity is highest in the belt that circles the globe near and at the equator. Biodiversity of higher organisms, plant and animal, in the equatorial tropics exceeds that in other regions on Earth. In this

region, an abundance of sunlight might lead to higher numbers of photosynthetic bacteria, which give rise to food chains on land and in water. Because of the tropics’ higher overall biodiversity, the region

is environmentally stable. This allows countless small and specialized

populations to exist. Diverse populations in turn offer bacteria more

options for creating symbiotic relationships. Finally, a stable tropical climate compared with the seasonal changes of temperate regions gives bacteria better opportunities to evolve and develop useful adaptations.

Microbial ecology challenges microbiologists because the bacteria

studied in laboratories are not necessarily the most abundant. This is

due to VBNC, meaning viable but not culturable. Craig Venter’s genetic analysis of marine microbes supported the idea long suspected

by biologists, that microbial diversity is far greater than even the highest estimates. VBNC bacteria either do not grow in lab conditions or microbiologists have yet to discover the things these species need. As a result, microbiology must base most of its theories on how a small minority of the world’s bacteria behave in a laboratory. Genetic testing, such as Venter’s, will help solve this problem because whole bacteria need no longer be the focus of experimental study. By analyzing gene

diversity, microbiologists will learn more about microbial diversity.

Cyanobacteria

No single bacterium can be thought of as more important than any

other, but if pressed to select one above others, I would pick cyanobacteria. These microbes that biologists originally misidentified as blue-green algae almost single-handedly symbolize bacterial diversity.

The bacteria that began providing the Earth with oxygen three

and a half billion years ago show wonderful versatility that spans terrestrial and aquatic environments, freshwater and marine. Cyanobacteria (see Figure 6.1) have few constraints on where they live other than needing sunlight for photosynthesis. On land, cyanobacteria

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often pair with fungi to form lichens that live on inorganic surfaces.

The cyanobacterium Anabaena forms a similar relationship in water with Azolla, a small floating fern. In this association, the plant supplies about 90 percent of the photosynthesis, and Anabaena takes responsi-bility for pulling nitrogen from the air to supply itself and Azolla.

Figure 6.1 Cyanobacteria. Cyanobacteria contain a diversity of species and activities, all having photosynthesis in common. This string of Anabaena cells contains larger cystlike cells that fix nitrogen. (Courtesy of Dennis Kunkel Microscopy, Inc.) Cyanobacteria dominate microbial mats and attach to terrestrial

surfaces (the periphyton form). They live in the greatest abundance,

however, as free cells in aquatic habitats and make up a large portion

of marine plankton as shown on Table 6.1. At normal ocean concentrations of about 100,000 cells per milliliter, the microbes are invisible, but in larger, denser populations called blooms, cyanobacteria can turn the waters red, perhaps the inspiration for naming the Red Sea. The oceans receive ample sunlight at the surface, but the light

penetrates no deeper than about 320 feet. For this reason, cyanobacteria and all of the world’s marine photosynthesis occurs in this layer called the photic zone.

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Table 6.1 Main constituents of marine plankton

Organism

Cells per milliliter of seawater

Krill

Less than one

Algae

3,000

Protozoa

4,000

Photosynthetic bacteria

100,000

Heterotrophic bacteria

1,000,000

Viruses

10,000,000

Green plants, algae, and cyanobacteria act as the Earth’s main con—

duits for converting the sun’s energy into usable energy for animals.

The oceans contribute the major share of this process. Just as cyanobacteria provide the energy that powers the metabolism of microbial mats, they play a similar vital role as the foundation of marine food chains.

From the marine or freshwater cyanobacteria, energy transfers to small

organisms and to progressively larger animals until the food chain reaches the top predator, referred to as the “top of the food chain.”

As Earth’s oldest bacteria still in existence, cyanobacteria played a

part in the rise of algae, primitive plants, and today’s higher plants. The oldest known fossils are of cyanobacteria from the Archaean period before oxygen began accumulating in the atmosphere. These fossils

dated to 3.5 billion years are almost as old as the oldest rocks, dated to 3.8 billion years.

During the Archaean and Proterozoic Eras, cyanobacterial photosynthesis changed the atmosphere’s composition from oxygenless to

oxygenated. Sometime during the transition from the Proterozoic to

the Cambrian Era, some of the large cells dependent on oxygen engulfed a few cyanobacterial cells. A portion of the engulfed cyanobacteria managed to resist being digested inside the predator, and they evolved with succeeding predator generations to become an organelle of the host cell. Plant life descending from these primitive

cells evolved into more complex structures, and the vestiges of ancient cyanobacteria would become chloroplasts, the sites in plant cells that convert sunlight energy to chemical energy in the form of

sugars.

Cyanobacteria grow slower than most other microbes, doubling

about once per day, but they nevertheless compete well against other

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bacteria because of their durability and the capacity to exist on almost no nutrients; cyanobacteria need only sunlight for energy, carbon dioxide for carbon, and miniscule amounts of salts. These bacteria have larger than normal cell size that reaches several
ì
m in diameter, and they possess a more complex internal structure than other bacteria.

The cytoplasm contains a network of membranes that support the enzymes and pigments that run photosynthesis. Cyanobacteria cell shapes also present a unique collection of blocks, chains, and long filaments that microscopically resemble algae more than bacteria.

In the 16th century, Swiss physician Paracelsus—his birth name

was Aureolus Phillipus Theostratus Bombastus von Hohenheim!—

made one of the earliest observations on cyanobacteria. He noted the

mucuslike colonies growing on plants and named the growth Nostoc, a term generally meaning nasal discharge. If Nostoc was the first cyanobacterium studied, the marine species Prochlorococcus marinus is one of the newest. Discovered in 1986, P. marinus is possibly the most abundant organism of any type on the planet. P. marinus is also the smallest cyanobacterium and one of the smallest known bacteria

at 0.6
ì
m in diameter. The species acts as a photosynthesis machine with few other ecologically important activities. It contains only 1,716

genes. Because the ocean conditions change slowly, P. marinus can survive with only a few genes to help it respond to its environment.

The best places to see cyanobacteria in nature are rocky shorelines and on seashells. Microbial ecologist Betsey Dexter Dyer

described cyanobacteria on shorelines as a slippery, brown-black, velvety coating on rocks. In aquatic environments, the microbe is evident by its blue-green, green, yellow-red, orange, or violet pigments.

Cyanobacteria in all forms serve the Earth as a tremendous storage site for carbon and nitrogen. Photosynthesis converts carbon dioxide to sugar, which the plant uses to make structural fibers and starches. All bacteria that decompose organic matter on land or in the

sea add to the large stores of the Earth’s carbon.

Bacterial protein factories

Ruminant animals and to a lesser extent humans and other single-stomach animals (monogastric animals) get a large portion of their amino acid requirements from bacteria. Enzymes in the intestines 132

allies and enemies

digest bacteria, and then specific enzymes called proteases break down the bacterial proteins to liberate individual amino acids, which the animal absorbs. These amino acids or the nitrogen they contain

serve as the basis for the animal’s own protein synthesis.

This process, complicated though it may seem, is but one step in a