Glossary of Terms:
Photosynthesis
is a biochemical reaction in which carbon dioxide, water and light energy
ultimately produce oxygen and carbohydrates; it is a link between the
inorganic and organic worlds. The
rate of photosynthesis is generally proportional to the amount of
available light. Light quantity (intensity) and quality (spectral
composition) are important for plant growth.
The “Z” Scheme for
photosynthesis. Light
energy is harvested by Photosystem I (PS I) and Photosystem II (PS II).
Oxygen evolution occurs in PS II; an interruption of reactions in
PS II stops the electron flow to PS I, thus effectively stopping
photosynthesis.
The photosynthetic
electron transport system in all oxygenic organisms is composed of
Photosystems I and II. Both systems include special forms of Chlorophyll
A – Photosystem I includes a form of the Chlorophyll A pigment with a
specific absorbance of 700 nm (red light) that is called P-700.
Photosystem II contains the reaction center responsible for oxygen
evolution; it contains a special Chlorophyll A that absorbs light at 680
nm (red light) that is called P-680.
Photosystem I is dependent upon the proper functioning of
Photosystem II – if the photochemical reactions in Photosystem II are
inhibited, Photosystem I is inhibited as well.
Zooxanthellae contain
chlorophylls A (both P-680 and P-700 in addition to “regular”
Chlorophyll A that collects light in the violet, blue and red portions of
the visible light spectrum). Pigments that harvest light energy outside of 680 nm and 700
nm and make it available for photosynthesis are called Accessory
or Antennae Pigments.
Antennae pigments include Chlorophyll C2, peridinin and
beta-carotene.
Botanists and
phycologists use terms with which one should be familiar.
These include:
Photosynthetically
Active Radiation (PAR):
A measure of visible
light intensity (400-700 nm) obtained by using a quantum meter. PAR is simply a count of photons falling upon a surface
in a given time and is reported as “micro Mols per square meter per
second” (µMols·m2·sec). Quantum
meters report all wavelengths between 400 and 700 nanometers. However, they report only light intensity and do not
account for spectral quality. Generally,
maximum solar PAR values are 2,000 – 2,100 µMols·m2·sec. PAR is something of an outlaw in the scientific
community; it is not recognized as a standard unit, however most major
works in the field (notably Kirk (1983), among others) state compensation
and saturation points (see below) in PAR units.
(Since PAR is a relative new-comer to science, it has not been
recognized by CIE (Commission Internationale de L’Eclairage) or the
International System of Units (SI) – both had already adopted standards
for measuring light intensity.
Lack of recognition by either of these committees should not
undermine the importance of PAR measurements.
Incidentally, divide µMols·m2·sec (of sunlight) by 4.6 to
convert to watts per square meter per second (which is a SI-recognized
unit.) A quantum meter is
better suited for reporting light intensity than lux meters.
Lux meters are photometric in their response, that is, they
“see” light as the human eye does and have a maximum sensitivity to
green wavelengths. The human eye is not especially sensitive to those
wavelengths known to promote photosynthesis (violet, blue and red).
Generally, noontime lux measurements made on cloudless days in the
tropics range from 100,000 – 120,000 lux.
Maximum PAR
is the highest measurement
made under standardized conditions (for our cases, the lamps are
3.5" above the PAR sensor. This replicates the distance
from the lamp to aquarium water surface in many cases.
Blue PAR
is determined by using
the PAR sensor and subtracting glass cut-off filters. A yellow
filter removes blue wavelengths, red removes green, and blue removes
red. The amount of radiation subtracted is added together and
the "blue" PAR is divided by the sum of all 3 to arrive at
an approximation of broadband PAR in each case. (These are
exact, but since all lamps are tested under the same conditions, it
allows us to compare lamps.)
Compensation
Point is usually
defined as the minimum amount of light required for oxygen production to
meet the zooxanthellae/coral host respiratory requirements.
Corals have the ability to absorb oxygen from the surrounding water
(as they do in darkness); however, insufficient light energy may also
result in low production of photosynthetic lipids. During periods of
prolonged darkness (or inadequate light) zooxanthellae will then use their
energy reserves until they are depleted and a sort of starvation occurs,
usually resulting in irreversible damage or death.
Compensation points vary from specimen to specimen and often depend
upon their light history. Compensation
points in low light adapted corals may be just a few µMols·m2·sec or
much higher in high light adapted corals (350 µMols·m2·sec or ~17,500
lux; see Kirk, 1983). It
should be understood that light intensity should exceed the
zooxanthella’s compensation point.
Saturation
Point Photosynthetic
rates are proportional to light intensity only to a certain point.
The Saturation Point has been met when photosynthesis is at a
maximum, and increasing light will no longer increase the rate of
photosynthesis. Saturation
occurs when the photosynthesis electron transport systems are operating at
full capacity. Exceeding the saturation point is pointless, and from a
practical standpoint, results in needlessly high electric bills.
If light energy greatly exceeds the saturation point, Photoinhibition
may occur. Photoinhibition is generally defined as any occurrence
interrupting the normal electron flow in photosynthesis.
There are two types of photoinhibition – dynamic and chronic.
The first is chronic
photoinhibition that involves
irreversible damage to Photosystem II and were synthesis of new
“photosynthetic proteins” must occur before normal photochemistry may
resume (Brown et al, 1999). Dynamic
photoinhibition involves
reversible photochemical reactions that divert excess light energy away
from Photosystem II through thermal dissipation.
This “quenching” of photosynthesis involves reversible changes
in xanthophylls diadinoxanthin and diatoxanthin. Dynamic photoinhibition
protects the zooxanthellae (through absorption of violet through
yellow-green wavelengths of 400-550 nm) from high levels of
photosynthetically produced oxygen radicals, including hydrogen peroxide.
Not all strains of zooxanthellae have the ability to produce xanthophylls
and therefore may have little resistance to the effects of high light
intensity.
|
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| * These
researchers detected no measurable quantities of Diadinxanthin and
Diatoxanthin. |
