Biology Now, 2e

278 ■ CHAPTER 15 Bacteria and Archaea

BIODIVERSITY

iron ore, hydrogen sulfide, and ammonia, instead

of from sunlight.

In fact, the very first photoautotrophs on the

planet were prokaryotes. One type of aquatic

bacteria, the cyanobacteria, is believed to be

responsible for changing Earth’s chemistry by

evolving photosynthesis and producing oxygen

gas as a by-product. Oxygen gas accumulated in

the air and water, and the levels rose from next

to nothing to almost 10 percent about 2.1 billion

years ago. At that time in the fossil record, eukary-

otes appear, suggesting that the oxygen generated

by cyanobacteria may have facilitated the evolu-

tion of eukaryotes, especially multicellular forms.

To d ay, pr o k a r y o t e s c o nt i nu e t o a id e u k a r y-

otes. Bacteria directly help plants through a

process known as nitrogen fixation. Plants

need nitrogen in the form of ammonia or nitrate,

which they cannot make themselves. Bacteria,

however, can take nitrogen gas from the air—our

one of the warrior clans in our belly buttons and

other areas of the skin, producing antibiotics

that kill off other bacteria and even foot fungi,

says Dunn. Prokaryotes are either consumers,

called heterotrophs, or producers, called auto-

trophs (Figure 15.10). Autotrophs make food

on their own, but heterotrophs, like B. subtilis,

obtain energy by taking it from other sources.

Specifically, B. subtilis is a chemoheterotroph,

an organism that consumes organic molecules

to get energy (in the form of chemical bonds)

and carbon (in the form of carbon-containing

molecules). Some species of blue-green algae are

photoheterotrophs, which acquire carbon from

organic sources but their energy from sunlight.

Some autotrophs, called photoautotrophs,

absorb the energy of sunlight and take in carbon

dioxide to conduct photosynthesis. Others, called

chemoautotrophs, get their energy from inor-

ganic chemicals in their environment, including

Source of Energy

Light

Photoautotroph

Gloeocapsa

Chemicals

Chemoautotroph

Photoheterotroph Chemoheterotroph

Acidithiobacillus

Heliobacterium Escherichia

Carbon

dioxide

Source

of

Carbon

Organic

matter

Figure 15.10

How prokaryotes feed themselves

Prokaryotes have many more ways to feed themselves than do eukaryotes. They are categorized by

their energy source, and by the source of carbon they use.

Q1: What source of energy would you expect a cave-dwelling prokaryote to use?

Q2: In which of these categories would you place the bacteria responsible for nitrogen

fixing? Why?

Q3: In which of these categories do decomposers belong? Explain your reasoning.