PHOTOCHEMICAL PROCESSES
N. V. Blough, University of Maryland, College Park,
MD, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Life on Earth is critically dependent on the spectral
quality and quantity of radiation received from the
sun. The absorption of visible light (wavelengths
from 400 to 700 nm) by pigments within terrestrial
and marine plants initiates a series of reactions that
ultimately transforms the light energy to chemical
energy, which is stored as reduced forms of carbon.
This complex photochemical process, known as
photosynthesis, not only provides all of the chemical
energy required for life on Earth’s surface, but also
acts to decrease the level of a major greenhouse gas,
CO 2 , in the atmosphere. By contrast, the absorption
of ultraviolet light in the UV-B (wavelengths from
280 to 320 nm) and UV-A (wavelengths from 320 to
400 nm) by plants (as well as other organisms) can
produce seriously deleterious effects (e.g. photoinhibition), leading to a decrease in the efficiency of
photosynthesis and direct DNA damage (UV-B), as
well as impairing or destroying other important
physiological processes. The level of UV-B radiation
received at the Earth’s surface depends on the concentration of ozone (O 3 ) in the stratosphere where it
is formed photochemically. The destruction of O 3 in
polar regions, leading to increased levels of surface
UV-B radiation in these locales, has been enhanced by
the release of man-made chlorofluorocarbons (CFCs),
but may also be influenced in part by the natural
production of halogenated compounds by biota.
These biotic photoprocesses have long been recognized as critical components of marine ecosystems
and air–sea gas exchange, and have been studied extensively. However, only within the last decade or so
has the impact of abiotic photoreactions on the
chemistry and biology of marine waters and their
possible coupling with atmospheric processes been
fully appreciated. Light is absorbed in the oceans not
only by phytoplankton and water, but also by colored
dissolved organic matter (CDOM), particulate detrital
matter (PDM), and other numerous trace lightabsorbing species. Light absorption by these constituents, primarily the CDOM, can have a number
of important chemical and biological consequences
including: (1) reduction of potentially harmful UV-B
and UV-A radiation within the water column;
(2) photo-oxidative degradation of organic matter
through the photochemical production of reactive
oxygen species (ROS) such as superoxide (O 2
À ),
hydrogen peroxide (H 2 O 2 ), the hydroxyl radical (OH)
and peroxy radicals (RO 2 ); (3) changes in metal ion
speciation through reactions with the ROS or through
direct photochemistry, resulting in the altered biological availability of some metals; (4) photochemical
production of a number of trace gases of importance
in the atmosphere such as CO 2 , CO, and carbonyl
sulfide (COS), and the destruction of others such as
dimethyl sulfide (DMS); (5) the photochemical production of biologically available low molecular
weight (LMW) organic compounds and the release of
available forms of nitrogen, thus potentially fueling
the growth of microorganisms from a biologically
resistant source material (the CDOM). These processes provide the focus of this article.
Optical Properties of the Abiotic
Constituents of Sea Waters
CDOM is a chemically complex material produced
by the decay of plants and algae. This material,
commonly referred to as gelbstoff, yellow substance,
gilvin or humic substances, can be transported from
land to the oceans by rivers or be formed directly in
marine waters by as yet poorly understood processes.
CDOM is the principal light-absorbing component
of the dissolved organic matter (DOM) pool in sea
waters, far exceeding the contributions of discrete
dissolved organic or inorganic light-absorbing compounds. CDOM absorption spectra are broad and
unstructured, and typically increase with decreasing
wavelength in an approximately exponential fashion
(Figure 1). Spectra have thus been parameterized
using the expression [1].
a l
ð Þ ¼ a l 0
ð Þ Á e
ÀS lÀl 0
ð
Þ
½1
aðlÞ and aðl 0 Þ are the absorption coefficients at
wavelength l and reference wavelength l 0 , respectively, and S defines how rapidly the absorption increases with decreasing wavelength. Absorption
coefficients are calculated from relation [2], where A
is the absorbance measured across pathlength, r.
a l
ð Þ ¼
2:303 Á A l
ð Þ
r
½2
89
N. V. Blough, University of Maryland, College Park,
MD, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Life on Earth is critically dependent on the spectral
quality and quantity of radiation received from the
sun. The absorption of visible light (wavelengths
from 400 to 700 nm) by pigments within terrestrial
and marine plants initiates a series of reactions that
ultimately transforms the light energy to chemical
energy, which is stored as reduced forms of carbon.
This complex photochemical process, known as
photosynthesis, not only provides all of the chemical
energy required for life on Earth’s surface, but also
acts to decrease the level of a major greenhouse gas,
CO 2 , in the atmosphere. By contrast, the absorption
of ultraviolet light in the UV-B (wavelengths from
280 to 320 nm) and UV-A (wavelengths from 320 to
400 nm) by plants (as well as other organisms) can
produce seriously deleterious effects (e.g. photoinhibition), leading to a decrease in the efficiency of
photosynthesis and direct DNA damage (UV-B), as
well as impairing or destroying other important
physiological processes. The level of UV-B radiation
received at the Earth’s surface depends on the concentration of ozone (O 3 ) in the stratosphere where it
is formed photochemically. The destruction of O 3 in
polar regions, leading to increased levels of surface
UV-B radiation in these locales, has been enhanced by
the release of man-made chlorofluorocarbons (CFCs),
but may also be influenced in part by the natural
production of halogenated compounds by biota.
These biotic photoprocesses have long been recognized as critical components of marine ecosystems
and air–sea gas exchange, and have been studied extensively. However, only within the last decade or so
has the impact of abiotic photoreactions on the
chemistry and biology of marine waters and their
possible coupling with atmospheric processes been
fully appreciated. Light is absorbed in the oceans not
only by phytoplankton and water, but also by colored
dissolved organic matter (CDOM), particulate detrital
matter (PDM), and other numerous trace lightabsorbing species. Light absorption by these constituents, primarily the CDOM, can have a number
of important chemical and biological consequences
including: (1) reduction of potentially harmful UV-B
and UV-A radiation within the water column;
(2) photo-oxidative degradation of organic matter
through the photochemical production of reactive
oxygen species (ROS) such as superoxide (O 2
À ),
hydrogen peroxide (H 2 O 2 ), the hydroxyl radical (OH)
and peroxy radicals (RO 2 ); (3) changes in metal ion
speciation through reactions with the ROS or through
direct photochemistry, resulting in the altered biological availability of some metals; (4) photochemical
production of a number of trace gases of importance
in the atmosphere such as CO 2 , CO, and carbonyl
sulfide (COS), and the destruction of others such as
dimethyl sulfide (DMS); (5) the photochemical production of biologically available low molecular
weight (LMW) organic compounds and the release of
available forms of nitrogen, thus potentially fueling
the growth of microorganisms from a biologically
resistant source material (the CDOM). These processes provide the focus of this article.
Optical Properties of the Abiotic
Constituents of Sea Waters
CDOM is a chemically complex material produced
by the decay of plants and algae. This material,
commonly referred to as gelbstoff, yellow substance,
gilvin or humic substances, can be transported from
land to the oceans by rivers or be formed directly in
marine waters by as yet poorly understood processes.
CDOM is the principal light-absorbing component
of the dissolved organic matter (DOM) pool in sea
waters, far exceeding the contributions of discrete
dissolved organic or inorganic light-absorbing compounds. CDOM absorption spectra are broad and
unstructured, and typically increase with decreasing
wavelength in an approximately exponential fashion
(Figure 1). Spectra have thus been parameterized
using the expression [1].
a l
ð Þ ¼ a l 0
ð Þ Á e
ÀS lÀl 0
ð
Þ
½1
aðlÞ and aðl 0 Þ are the absorption coefficients at
wavelength l and reference wavelength l 0 , respectively, and S defines how rapidly the absorption increases with decreasing wavelength. Absorption
coefficients are calculated from relation [2], where A
is the absorbance measured across pathlength, r.
a l
ð Þ ¼
2:303 Á A l
ð Þ
r
½2
89