osmoregulatory role as its cellular concentrations
have been found to vary with salinity. It is generally
thought that healthy algal cells do not leak either
DMSP or DMS, although mechanical release into the
surrounding sea water can lead to DMS production
during cell senescence and grazing by zooplankton or
as a consequence of viral attack on phytoplankton
cells. Oceanic regions dominated by prolific DMSPproducing phytoplankton tend to have high DMS
and DMSP concentrations.
Breakdown of DMSP, presumably after transfer
from the particulate algal (pDMSP) form to a dissolved (dDMSP) form in sea water, can proceed in
different ways, mostly depending on microbiological
conditions. One major pathway involves cleavage of
DMSP to DMS and acrylic acid. Bacterial metabolism of dDMSP may be a major mechanism for DMS
production in sea water, with acrylic acid residue
acting as a carbon source for heterotrophic growth.
Sulfonium compounds are vulnerable to attack by
hydroxide ion; the resulting chemical elimination
reaction occurs rapidly and quantitatively in strong
base but only slowly at the pH of sea water.
DMS in sea water has many potential fates. The
volatility of DMS and the concentration gradient
across the sea–air interface lead to the ocean being the
major source of DMS to the atmosphere. Estimates of
the annual sulfur release (as DMS) vary from 13–37
Tg S y
À1 (Kettle and Andreae, 1999). However,
whereas the absolute flux of DMS from sea to air may
be large on a global scale, sea–air exchange may
represent only a minor sink for seawater DMS. It has
been estimated that DMS loss to the atmosphere is
only a very small percentage of the DMS sink, but
this undoubtedly depends on the biogeochemical
conditions in the water column at the time. Photochemical oxidation of DMS, either to dimethylsulfoxide (DMSO) or to other products, occurs via
photosensitized reactions. The amount of photochemical decomposition depends on the amount of
light of appropriate wavelengths and the concentration of colored organic compounds in solution to
convert light energy into reactive radicals. Light declines exponentially with depth; the distribution of
colored dissolved organic materials exhibits depth
and seasonal variability. Microbial consumption of
DMS, although extremely variable in both time and
space in the ocean, appears to be a significant sink for
oceanic DMS. The residence time of DMS is probably
of the order of a day or two in most seawater systems.
Since the atmospheric residence time of DMS is
about a day or two, the atmospheric consequences of
DMS flux are mostly confined to the troposphere. In
the troposphere, DMS is oxidized primarily by hydroxyl radical. The main atmospheric oxidation
products are methane sulphonic acid, SO 2 and
DMSO.
Carbonyl Sulfide
Carbonyl sulfide (COS, OCS) is the major sulfur gas
in the atmosphere, present throughout the troposphere at 500 pptv. COS has a long atmospheric
residence time (B4 years). Because of its relative
inertness COS diffuses into the stratosphere where it
oxidizes to sulfate particles and contributes in reactions involving stratospheric ozone chemistry.
Unlike DMS which is photochemically oxidized in
the troposphere, the major sink for COS is terrestrial
vegetation and soils. COS is taken up by plants by
passing through the stomata and subsequently
hydrolyzing to CO 2 and H 2 S through the action of
carbonic anhydrase inside plant cells. There is no
apparent physiological significance to the process; it
appears to just occur accidentally to the normal
physiology of plants.
COS is produced in the ocean by photochemical
oxidation of organic sulfur compounds whereby
dissolved organic matter acts as a photosensitizer.
The aqueous concentration of COS manifests a
strong diel cycle, with the highest concentrations in
daytime (concentration range on the order of 0.03–
0.1 nmol l
À1
). COS hydrolyzes in water to H 2 S at
rates dependent on water temperature and pH. The
flux of oceanic COS to the atmosphere may represent
about one-third of the global COS flux.
Carbon Disulfide
Concentrations in surface water are around 10
À11
mol 1
À1 . Although a number of studies have indicated that the ocean forms an important source for
atmospheric CS 2 , the underlying biochemical cycles
still remain poorly understood. CS 2 is formed by
photochemical reactions (possibly involving precursors such as DMS, DMSP and isothiocyanates).
CS 2 formation has been observed to occur in bacteria
in anoxic aquatic environments and in cultures of
some marine algae species.
The residence time of CS 2 in the atmosphere is
relatively short (about one week). Although CS 2
might contribute directly to SO 2 in the troposphere,
its main significance is in the formation of COS via
photochemical oxidation which results in the production of one molecule each of SO 2 and COS per
molecule of CS 2 oxidized. The resulting COS may
contribute to the stratospheric aerosol formation.
Concentrations around 14 pmol l
À1 of carbon disulfide in the mid-Atlantic Ocean were first observed
AIR–SEA TRANSFER 585
have been found to vary with salinity. It is generally
thought that healthy algal cells do not leak either
DMSP or DMS, although mechanical release into the
surrounding sea water can lead to DMS production
during cell senescence and grazing by zooplankton or
as a consequence of viral attack on phytoplankton
cells. Oceanic regions dominated by prolific DMSPproducing phytoplankton tend to have high DMS
and DMSP concentrations.
Breakdown of DMSP, presumably after transfer
from the particulate algal (pDMSP) form to a dissolved (dDMSP) form in sea water, can proceed in
different ways, mostly depending on microbiological
conditions. One major pathway involves cleavage of
DMSP to DMS and acrylic acid. Bacterial metabolism of dDMSP may be a major mechanism for DMS
production in sea water, with acrylic acid residue
acting as a carbon source for heterotrophic growth.
Sulfonium compounds are vulnerable to attack by
hydroxide ion; the resulting chemical elimination
reaction occurs rapidly and quantitatively in strong
base but only slowly at the pH of sea water.
DMS in sea water has many potential fates. The
volatility of DMS and the concentration gradient
across the sea–air interface lead to the ocean being the
major source of DMS to the atmosphere. Estimates of
the annual sulfur release (as DMS) vary from 13–37
Tg S y
À1 (Kettle and Andreae, 1999). However,
whereas the absolute flux of DMS from sea to air may
be large on a global scale, sea–air exchange may
represent only a minor sink for seawater DMS. It has
been estimated that DMS loss to the atmosphere is
only a very small percentage of the DMS sink, but
this undoubtedly depends on the biogeochemical
conditions in the water column at the time. Photochemical oxidation of DMS, either to dimethylsulfoxide (DMSO) or to other products, occurs via
photosensitized reactions. The amount of photochemical decomposition depends on the amount of
light of appropriate wavelengths and the concentration of colored organic compounds in solution to
convert light energy into reactive radicals. Light declines exponentially with depth; the distribution of
colored dissolved organic materials exhibits depth
and seasonal variability. Microbial consumption of
DMS, although extremely variable in both time and
space in the ocean, appears to be a significant sink for
oceanic DMS. The residence time of DMS is probably
of the order of a day or two in most seawater systems.
Since the atmospheric residence time of DMS is
about a day or two, the atmospheric consequences of
DMS flux are mostly confined to the troposphere. In
the troposphere, DMS is oxidized primarily by hydroxyl radical. The main atmospheric oxidation
products are methane sulphonic acid, SO 2 and
DMSO.
Carbonyl Sulfide
Carbonyl sulfide (COS, OCS) is the major sulfur gas
in the atmosphere, present throughout the troposphere at 500 pptv. COS has a long atmospheric
residence time (B4 years). Because of its relative
inertness COS diffuses into the stratosphere where it
oxidizes to sulfate particles and contributes in reactions involving stratospheric ozone chemistry.
Unlike DMS which is photochemically oxidized in
the troposphere, the major sink for COS is terrestrial
vegetation and soils. COS is taken up by plants by
passing through the stomata and subsequently
hydrolyzing to CO 2 and H 2 S through the action of
carbonic anhydrase inside plant cells. There is no
apparent physiological significance to the process; it
appears to just occur accidentally to the normal
physiology of plants.
COS is produced in the ocean by photochemical
oxidation of organic sulfur compounds whereby
dissolved organic matter acts as a photosensitizer.
The aqueous concentration of COS manifests a
strong diel cycle, with the highest concentrations in
daytime (concentration range on the order of 0.03–
0.1 nmol l
À1
). COS hydrolyzes in water to H 2 S at
rates dependent on water temperature and pH. The
flux of oceanic COS to the atmosphere may represent
about one-third of the global COS flux.
Carbon Disulfide
Concentrations in surface water are around 10
À11
mol 1
À1 . Although a number of studies have indicated that the ocean forms an important source for
atmospheric CS 2 , the underlying biochemical cycles
still remain poorly understood. CS 2 is formed by
photochemical reactions (possibly involving precursors such as DMS, DMSP and isothiocyanates).
CS 2 formation has been observed to occur in bacteria
in anoxic aquatic environments and in cultures of
some marine algae species.
The residence time of CS 2 in the atmosphere is
relatively short (about one week). Although CS 2
might contribute directly to SO 2 in the troposphere,
its main significance is in the formation of COS via
photochemical oxidation which results in the production of one molecule each of SO 2 and COS per
molecule of CS 2 oxidized. The resulting COS may
contribute to the stratospheric aerosol formation.
Concentrations around 14 pmol l
À1 of carbon disulfide in the mid-Atlantic Ocean were first observed
AIR–SEA TRANSFER 585