Visualizing Environmental Science

The Cycling of Matter in Ecosystems 111

PROCESS DIAGRAM

consume plant proteins and convert them to animal pro-

teins. In the ocean, certain marine algae release a com-

pound that bacteria convert to dimethyl sulfide (DMS).

DMS is released into the atmosphere, where it helps con-

dense water into droplets in clouds and may affect weather

and climate. Atmospheric DMS is converted to sulfate,

most of which is deposited into the ocean.

As in the nitrogen cycle, bacteria drive the sulfur cy-

cle. In freshwater wetlands, tidal flats, and flooded soils,

which are oxygen deficient, certain bacteria convert sul-

fates to hydrogen sulfide gas, which is released into the

atmosphere, or to metallic sulfides, which are deposited

as rock. In the absence of oxygen, other bacteria perform

a type of photosynthesis that uses hydrogen sulfide in-

stead of water. Where oxygen is present, different bacte-

ria oxidize sulfur compounds to sulfates.

Coal, and to a lesser extent oil, contain sulfur. Sulfur

dioxide, a major cause of acid deposition, is released into

the atmosphere when these fuels are burned and during

the smelting of sulfur-containing ores of such metals as

copper, lead, and zinc.

The Phosphorus Cycle

Unlike the biogeochemical cycles just discussed, the phos-

phorus cycle doesn’t have an atmospheric component.

Phosphorus cycles from the land into living organisms, then

Hydrogen sulfide (H 2 S)

+ O 2

Sulfuric acid (H 2 SO 4 )

Wet and dry deposition

270

Human-produced

gases (air pollution)

90

Ocean

Rock weathering

and erosion

Sulfate salts (SO 4 )

move from soil and water into food chain

Metallic sulfides

deposited as rock

Sulfur oxides (SOx)

+ H 2 O

2–

Decay organisms in soil,

wetlands, and ocean

Dimethyl

sulfide (DMS)

15

The largest sources of sulfur on Earth are sedimentary rock and the ocean. In the sulfur

cycle, sulfur compounds are incorporated into organisms and move among them, the

atmosphere, the ocean, and land. The values shown in the figure for the global sulfur

budget are expressed in units of 10^12 g of sulfur per year. For example, human-produced

gases (air pollution) emit an estimated 90 x 10^12 g of sulfur per year into the atmosphere.

Values are from Schlesinger, W. H.

Biogeochemistry: An Analysis of Global Change

, 2nd edition.

Academic Press, San Diego (1997) and based on several sources.

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✓✓THE PLANNER

Why does

a tidal marsh sometimes smell

like rotten eggs?

Think Critically