Prokaryotic cells have differing oxygen requirements.

OBLIGATE AEROBES

Use O2 for

respiration;

cannot grow

without it

FACULTATIVE ANAEROBES

Use O2 when

available;

ferment when

O2 isn’t

available

OBLIGATE ANAEROBES

Poisoned by O2; use

fermentation or live

by anaerobic

respiration. In

anaerobic respiration,

inorganic molecules

like SO4, NO3, and

Fe3+ are used instead

of oxygen.

A B C

BILL: For each sample, determine the oxygen requirements for the bacteria living within the bracketed area.

VITAL ROLES IN ECOSYSTEMS: Producers

VITAL ROLES IN ECOSYSTEMS: Decomposers

VITAL ROLES IN ECOSYSTEMS: Nitrogen Fixers in the Nitrogen Cycle

Nitrogen (N 2) in the atmosphere is converted into ammonia (NH3), making the nitrogen available to plants

and animals.

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The poisonous gas …Oxygen • The amount of free oxygen

in the atmosphere increased after the evolution of photosynthesis in prokaryotes

• O2 appeared from about 2 billion years ago and reached about 21% 1 billion years ago

• Obligate anaerobes either became extinct or found niches where oxygen is absent

Stromatolites, Shark Bay Australia

Oxygen atmosphere • Oxygen begins to accumulate 2.7 bya

• evidence in banded iron in rocks (rusting)

• makes aerobic respiration possible

Stromatolites Fossilized mats of prokaryotes resemble modern microbial colonies