110 CHAPTER 5 How Ecosystems Worksuch as proteins. Animals assimilate nitrogen when they
consume plants or other animals and convert the pro-
teins into animal proteins.
Ammonification occurs when organisms produce
nitrogen-containing waste products such as urine. These
substances, plus the nitrogen compounds that occur in
dead organisms, are decomposed, releasing the nitrogen
into the abiotic environment as ammonia. The bacteria
that perform this process are called ammonifying bacte-
ria. Other bacteria perform denitrification, in which nitrate
is converted back to nitrogen gas. Denitrifying bacteria
typically live and grow where there is little or no free oxy-
gen. For example, they are found deep in the soil near the
water table, an environment that is nearly oxygen free.
Human activities have disturbed the balance of the
global nitrogen cycle. Nitrogen in fertilizers washes into
rivers, lakes, and coastal areas, where it stimulates the
growth of algae. As these algae die, their decomposition
by bacteria robs the water of dissolved oxygen, which in
turn causes many fishes and other aquatic organisms to die
of suffocation. These no-oxygen conditions have formed
large dead zones in about 150 coastal areas around the world
(see the Chapter 11 case study). Nitrogen compounds are
also released into the atmosphere as air pollutants when
fossil fuels are burned, contributing to photochemical smog
(see Chapter 8) and acid deposition (see Chapter 9).The Sulfur Cycle
Scientists are still piecing together how the global sulfur cy-
cle works. Most sulfur is underground in sedimentary rocks
and minerals, which over time erode to release sulfur-con-
taining compounds into the ocean ( Figure 5.13). Sulfur
gases enter the atmosphere from natural sources in both
the ocean and land. Sea spray delivers sulfates (SO 4 2–)
into the air, as do forest fires and dust storms. Vol-
canoes release both hydrogen sulfide (H 2 S), a
poisonous gas that smells like rotten eggs, and
sulfur oxides (SOx). Hydrogen sulfide reacts with
oxygen to form sulfur oxides, and sulfur oxides
react with water to form sulfuric acid (H 2 SO 4 ).
Although sulfur gases make up a minor part of
the atmosphere, the total movement of sulfur to
and from the atmosphere is substantial.
A tiny fraction of global sulfur is present in living
organisms, where it is an essential component of pro-
teins. Plant roots absorb sulfate and incorporate the sulfur
into plant proteins. Animals assimilate sulfur when theyBacteria are the only organisms involved in each of these
steps except assimilation.
Nitrogen-fixing bacteria carry out nitrogen fixation in
soil and aquatic environments. The process gets its name
from the fact that nitrogen is fixed into a form that orga-
nisms can use, ammonia (NH 3 ). Volcanic activity, light-
ning, and human activities—combustion and industrial
processes—also fix considerable nitrogen because all sup-
ply enough energy to break apart atmospheric nitrogen.
Nitrogen-fixing bacteria split atmospheric nitrogen
and combine the resulting nitrogen atoms with hydrogen.
Some nitrogen-fixing bacteria, Rhizobium, live inside swell-
ings, or nodules, on the roots of legumes such as beans or
peas and some woody plants (}ÕÀiÊx°£Ó>). In moist en-
vironments, photosynthetic bacteria called cyanobacteria
perform most of the nitrogen fixation (}ÕÀiÊx°£ÓL).
During nitrification, soil bacteria convert ammonia to
nitrate (NO 3 – ). The process of nitrification furnishes these
bacteria, called nitrifying bacteria, with energy. In assimi-
lation, plants absorb ammonia or nitrate through their
roots and convert the nitrogen into plant compoundsÌÀ}iÊwÝ>ÌÊUÊ}ÕÀiÊx°£ÓÊSTEPHEN SHARNOFF/NG
Image CollectionDr Jeremy Burgess/Science Source Imagesb. Nostoc, a
cyanobacterium
that fixes nitrogen,
grows here on a
mossy bank. This
particular species
forms colonies that
range in size from a
pinhead to a potato.a. Bacteria carry out
nitrogen fixation in
the nodules of a pea
plant’s roots.