Photochemical Production and
Consumption of LMW Organic
Compounds and Trace Gases
The photolysis of CDOM produces a suite of LMW
organic compounds and a number of trace gases. The
production of these species presumably occurs
through radical and fragmentation reactions arising
from the net oxidative flow of electrons from CDOM
to O 2 (see above), although the exact mechanism(s)
have yet to be established. Most LMW organic
compounds produced contain three or fewer carbon
atoms and include such species as acetaldehyde,
acetate, acetone, formaldehyde, formate, glyoxal,
glyoxalate, methylglyoxal, propanal, and pyruvate.
The F values for the production of individual compounds are low, B0.001–0.0001%, with wavelengths in the UV-B the most effective; efficiencies
decrease rapidly with increasing wavelength. Available evidence indicates that the F for O 2
À and H 2 O 2
production are about one to two orders of magnitude
larger than those for the LMW organic compounds,
so it appears that the sum of the production rates for
the known LMW organic compounds is small with
respect to the flux of photochemical equivalents from
CDOM to O 2 .
Most, if not all, of these products are rapidly taken
up and respired by bacteria to CO 2 . Numerous investigators have presented evidence supporting enhanced microbial activity in waters exposed to
sunlight, with bacterial activities increasing from
1.5- to almost 6-fold depending presumably on the
length and type of light exposure, and the concentration and source of the CDOM. Recently, biologically labile nitrogen-containing compounds such
as ammonia and amino acids have also been reported
to be produced photochemically from CDOM. Because CDOM is normally considered to be biologically refractive, this recent work highlights the
important role that abiotic photochemistry plays in
the degradation of CDOM, not only through direct
photoreactions, but also through the formation of
biologically available products that can be respired
to CO 2 or used as nutrients by biota. A recent estimate suggests that the utilization of biologically labile photoproducts could account for as much as
21% of the bacterial production in some near-surface
waters.
Carbon dioxide and carbon monoxide are major
products of the direct photolysis of CDOM (Figure 3). Quantum yields for CO production are about
an order of magnitude smaller than those for O 2
À and
H 2 O 2 production, ranging from B0.01% at 300 nm
to B0.001% at 400 nm. Available data indicate that
the F for CO 2 range even higher, perhaps as much as
15–20-fold. The yields for CO 2 production must thus
approach, if not exceed, those for O 2
À and H 2 O 2 .
This result is somewhat surprising, since it implies
that about one CO 2 is produced for each electron
transferred from the CDOM to O 2 , further implying
a high average redox state for CDOM. Although
CDOM (i.e. humic substances) is known to contain
significant numbers of carboxyl moieties that could
serve as the source of the CO 2 , the yield for CO 2
production, relative to O 2
À and H 2 O 2 , would be expected to fall rapidly as these groups were removed
photochemically; available evidence suggests that
this does not occur. An alternative explanation is that
other species, perhaps the CDOM itself, is acting as
an electron acceptor. Regardless of mechanism,
existing information indicates that CO 2 is the dominant product of CDOM photolysis (Figure 3).
A recent estimate suggests that the annual global
photoproduction of CO in the oceans could be as
high as 0.82 Â 10
15 g C. Assuming that CO 2 photoproduction is 15–20 times higher than that for CO,
values for CO 2 formation could reach from 12 to
16 Â 10
15 g C y
À1
. To place these numbers in perspective, the estimated annual input of terrestrial
dissolved organic carbon to the oceans (0.2 Â 10
15 g
C y
À1
) is only 1.3–1.7% of the calculated annual
CO 2 photoproduction, which is itself about 2–3% of
the oceanic dissolved organic carbon pool. These
calculated CO 2 (and CO) photoproduction rates
may be high due to a number of assumptions, including (1) the complete absorption of UV radiation
by the CDOM throughout the oceans, (2) constant
quantum yields (or action spectra) for production
independent of locale or light history, and (3) neglecting mass transfer limitations associated with
physical mixing. Nevertheless, these estimates clearly
highlight the potential impact of abiotic photochemistry on the oceanic carbon cycle. Moreover,
the products of this photochemistry are generated
in near-surface waters where exchange with the
atmosphere can take place readily.
Like the LMW organic compounds, bacteria can
oxidize CO to CO 2 ; this consumption takes place in
competition with the release of CO to the atmosphere. Due to its photochemical production, CO
exists at supersaturated concentrations in the surface
waters of most of the Earth’s oceans. Recent estimates indicate that global oceanic CO emissions
could range from 0.013 Â 10
15 g y
À1
–1.2 Â 10
15 g y
À1
(see above). The upper estimate is based on calculated photochemical fluxes (see below) and the assumption that all CO produced is emitted to the
atmosphere. The lower estimate was calculated using
air–sea gas exchange equations and extensive measurements of CO concentrations in the surface waters
PHOTOCHEMICAL PROCESSES 93
Consumption of LMW Organic
Compounds and Trace Gases
The photolysis of CDOM produces a suite of LMW
organic compounds and a number of trace gases. The
production of these species presumably occurs
through radical and fragmentation reactions arising
from the net oxidative flow of electrons from CDOM
to O 2 (see above), although the exact mechanism(s)
have yet to be established. Most LMW organic
compounds produced contain three or fewer carbon
atoms and include such species as acetaldehyde,
acetate, acetone, formaldehyde, formate, glyoxal,
glyoxalate, methylglyoxal, propanal, and pyruvate.
The F values for the production of individual compounds are low, B0.001–0.0001%, with wavelengths in the UV-B the most effective; efficiencies
decrease rapidly with increasing wavelength. Available evidence indicates that the F for O 2
À and H 2 O 2
production are about one to two orders of magnitude
larger than those for the LMW organic compounds,
so it appears that the sum of the production rates for
the known LMW organic compounds is small with
respect to the flux of photochemical equivalents from
CDOM to O 2 .
Most, if not all, of these products are rapidly taken
up and respired by bacteria to CO 2 . Numerous investigators have presented evidence supporting enhanced microbial activity in waters exposed to
sunlight, with bacterial activities increasing from
1.5- to almost 6-fold depending presumably on the
length and type of light exposure, and the concentration and source of the CDOM. Recently, biologically labile nitrogen-containing compounds such
as ammonia and amino acids have also been reported
to be produced photochemically from CDOM. Because CDOM is normally considered to be biologically refractive, this recent work highlights the
important role that abiotic photochemistry plays in
the degradation of CDOM, not only through direct
photoreactions, but also through the formation of
biologically available products that can be respired
to CO 2 or used as nutrients by biota. A recent estimate suggests that the utilization of biologically labile photoproducts could account for as much as
21% of the bacterial production in some near-surface
waters.
Carbon dioxide and carbon monoxide are major
products of the direct photolysis of CDOM (Figure 3). Quantum yields for CO production are about
an order of magnitude smaller than those for O 2
À and
H 2 O 2 production, ranging from B0.01% at 300 nm
to B0.001% at 400 nm. Available data indicate that
the F for CO 2 range even higher, perhaps as much as
15–20-fold. The yields for CO 2 production must thus
approach, if not exceed, those for O 2
À and H 2 O 2 .
This result is somewhat surprising, since it implies
that about one CO 2 is produced for each electron
transferred from the CDOM to O 2 , further implying
a high average redox state for CDOM. Although
CDOM (i.e. humic substances) is known to contain
significant numbers of carboxyl moieties that could
serve as the source of the CO 2 , the yield for CO 2
production, relative to O 2
À and H 2 O 2 , would be expected to fall rapidly as these groups were removed
photochemically; available evidence suggests that
this does not occur. An alternative explanation is that
other species, perhaps the CDOM itself, is acting as
an electron acceptor. Regardless of mechanism,
existing information indicates that CO 2 is the dominant product of CDOM photolysis (Figure 3).
A recent estimate suggests that the annual global
photoproduction of CO in the oceans could be as
high as 0.82 Â 10
15 g C. Assuming that CO 2 photoproduction is 15–20 times higher than that for CO,
values for CO 2 formation could reach from 12 to
16 Â 10
15 g C y
À1
. To place these numbers in perspective, the estimated annual input of terrestrial
dissolved organic carbon to the oceans (0.2 Â 10
15 g
C y
À1
) is only 1.3–1.7% of the calculated annual
CO 2 photoproduction, which is itself about 2–3% of
the oceanic dissolved organic carbon pool. These
calculated CO 2 (and CO) photoproduction rates
may be high due to a number of assumptions, including (1) the complete absorption of UV radiation
by the CDOM throughout the oceans, (2) constant
quantum yields (or action spectra) for production
independent of locale or light history, and (3) neglecting mass transfer limitations associated with
physical mixing. Nevertheless, these estimates clearly
highlight the potential impact of abiotic photochemistry on the oceanic carbon cycle. Moreover,
the products of this photochemistry are generated
in near-surface waters where exchange with the
atmosphere can take place readily.
Like the LMW organic compounds, bacteria can
oxidize CO to CO 2 ; this consumption takes place in
competition with the release of CO to the atmosphere. Due to its photochemical production, CO
exists at supersaturated concentrations in the surface
waters of most of the Earth’s oceans. Recent estimates indicate that global oceanic CO emissions
could range from 0.013 Â 10
15 g y
À1
–1.2 Â 10
15 g y
À1
(see above). The upper estimate is based on calculated photochemical fluxes (see below) and the assumption that all CO produced is emitted to the
atmosphere. The lower estimate was calculated using
air–sea gas exchange equations and extensive measurements of CO concentrations in the surface waters
PHOTOCHEMICAL PROCESSES 93