UTS MARINE BIOLOGY FACT SHEET
Topic: Phytoplankton and Cloud Formation
1.The CLAW Hypothesis
Background: In 1987 four people (Charlson, Lovelock, Andrea, & Warren)
introduced a theory that a natural gas called Dimethylsulfide (DMS), produced
by microscopic plants in the ocean (phytoplankton), was a major contributor to
the formation of clouds in the atmosphere. This theory was named the CLAW
Hypothesis (from the first letter of each of their surnames).
Fast facts:

Phytoplankton produce DMSP (dimethylsulphoniopropionate), an organic
sulphur compound, which is converted to DMS in the ocean. The majority
of this DMS is consumed by bacteria but around 10% escapes into the
atmosphere.

When DMS is released into the air, a chemical reaction takes place
(called an oxidation reaction) and sulphate aerosols are formed – a gas
which acts as cloud condensation nuclei (CCN). This means it combines
with water droplets in the atmosphere to form clouds.

As a result, more clouds increase the reflectivity of the sun’s rays (earth
albedo) which decreases the amount of light reaching the earth’s
surface, and contributes to cooling the overall climate.

A decrease in light causes a decrease in phytoplankton productivity of
DMS. (Phytoplankton are primary producers which rely on light to
function).

This sequence of events is called a Negative Feedback Loop, because
phytoplankton increase DMS production but DMS forms clouds which
lowers the amount of light reaching the earth, resulting in less
phytoplankton and less DMS.

DMS emissions are a key step in the global sulphur cycle, which
circulates sulphur throughout the earth, oceans, and atmosphere. It is an
essential component in the growth of all living things.
Did you know…
DMS has a very strong smell which helps some marine animals and
microbes find their food
Diagram of the CLAW Hypothesis (adapted from Absolute
Astronomy <http://www.absoluteastronomy.com/topics/CLAW_
hypothesis>)
Learn more: Links/References
1. Charlson RJ, Lovelock JE, Andrea MO, Warren SG (1987) Oceanic
phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326:
655-661
2. Hill RW, Dacey JWH, Krupp DA (1995) Dimethylsulfoniopropionate in reef
corals. Bulletin of Marine Science 57: 489-494
3. Raina J-B, Dinsdale EA, Willis BL and Bourne DG (2009) Do the organic
sulfur compounds DMSP and DMS drive coral microbial associations?
Trends in Microbiology 18: 101-108
4. Schwartz SE (1988) Are global cloud albedo and climate controlled by
marine phytoplankton? Nature 336: 441-445
5. Chemoattraction to Dimethylsulfoniopropionate Throughout the Marine
Microbial Food Web,” by Justin R. Seymour, Rafel Simó, Tanvir Ahmed
and Roman Stocker. Science, July 16, 2010.