The sulfur cycle describes the movement of sulfur through the environment. Sulfur enters the atmosphere from volcanic eruptions, decomposition of organic matter, and the burning of fossil fuels. Microbes and algae also produce sulfur compounds. Sulfur dioxide and hydrogen sulfide gases react to form sulfuric acid which falls as acid rain. Plants absorb sulfate from the soil and it becomes incorporated into proteins. Animals obtain sulfur by eating plants or other animals. Decomposition returns sulfur to the environment, completing the cycle. Human activities like burning fossil fuels have increased the rate of acid deposition.

1. SULFUR
CYCLE
B Y S U N I T A S H E K H A W A T
M . S C . F I N A L Z O O L O G Y

2. BIOGEOCHEMICAL CYCLE
Bio = Life
Geo = Earth
Chemical= Elements ( C, N, O, P, S, Fe)
DEFINATION:
 Chemical cycles/ interactions that exist between the earth systems( i.e.
hydrosphere, lithosphere & biosphere)
 Chemical elements or nutrients flow from the non-livings to the living & back to
the non-living components of the ecosystem in a more or less cyclic path
Biogeochemical cycle have two basic types:
1. Gaseous Cycle: N, O, C
2. Sedimentary Cycle: S, P

3. SULFUR
Yellow, brittle, tasteless, odourless non-metal
Components of protein
Cycles in both a gas & sedimentary cycle
Source is lithosphere (Earth’s crust- underground rocks & minerals)
Released by precipitation, weathering of rocks & geothermal vents
Released in atmosphere as H2S (Hydrogen sulfide) by burning of fossil fuels, volcanic
activities & decomposition of organic molecules, etc.
SO2 & water vapor makes H2SO4 ( weak sulfuric acid) & carried to earth in rainfall
Plants directly absorb S-containing amino acids i.e. methionine, cystine, & cysteine
& then travels through food chain & eventually released through decomposition

4. SULFUR CYCLE
DEFINATION:
Cyclic movement of Sulfur in different chemical forms from the environment to the
organism & then back to the environment
PROCESS:
Sulfur is released by weathering of rocks
Then comes in contact with air & convert into sulfates
Sulfates taken up by plants & microbes & convert into organic forms
Organic form of Sulfur consumed by the animals through food & moves in food
chain
When animal die, some of the Sulfur released by decomposition while some enter
the tissues of microbes
Some natural sources such as volcanic eruptions, evaporation of water, &
breakdown of organic matter in swamps, that release Sulfur directly into
& this Sulfur falls on earth with rainfall

8. Hydrogen sulfide (H2S) released into the atmosphere by volcanic eruptions, hot springs
& anaerobic decay of sulfur containing biological material in swamps, tidal flats, etc.
Certain marine algae produce DMS (Dimethyl sulfide, CH3SCH3) a volatile compound
that enters the atmosphere as tiny particles
Sulfur dioxide (SO2) gas is released into the atmosphere by volcanoes & also forms
when DMS reacts with oxygen gas
Burning of fossil fuels such as coal & oil, releases SO2 into the atmosphere
SO2 reacts with water droplets to form H2SO4 (Sulfuric acid)
SO3 (Sulfur trioxide) also reacts with ammonia to produce ammonium salts
Wind carry droplets of H2SO4 & particles of sulfate salts, which then fall to the earth in
the acid deposition
By adding sulfur compounds to the air, the use of fossil fuels increases the rate of acid
deposition
All living things require sulfur to make proteins
Plants get sulfur by taking up ions of sulfate salts (𝑆𝑂4
−2
)
Animals get sulfur by eating plants & all living things release sulfur compounds when
the decay

9. Deposition releases sulfate salts, which can be taken up by plants as well as gaseous
H2S
Some H2S enters the atmosphere
But when decay occurs in an oxygen free environment, anaerobic bacteria break down
H2S & release sulfur gas
Oxygen requiring bacteria can incorporate sulfur into sulfate salts, which can taken up
by plants & enter the food chain once again
S cycle includes both gases & solids
Gas- H2S, SO2 & other are solids or sedimentary forms

10. HUMAN INTERVENTION IN THE
SULFUR CYCLE
Burning S-containing coal & oil to produce electric power
Refine S-containing petroleum to make gasoline, heating oil, & other useful products
Convert S-containing metallic mineral ores into free metals such as Cu, Pb , Zn
Emissions from these, react with chemicals in the atmosphere ( Sulfate salts ACID RAIN )
damage the natural environment ( affects both plants & animals) as well as man-made
environments ( weathering/corrosion of buildings)
Bleached look to
deciduous leaves

11. EFFECTS ON HUMAN
Irritation in respiratory tract
High concentration cause air pollution disasters
Bronchial asthma