Page 223 - Algae Anatomy, Biochemistry, and Biotechnology

206 Algae: Anatomy, Biochemistry, and Biotechnology

are species specific and chlorotic response is not universal. The changes in pigmentation resulting
from photoacclimation have profound consequences for light absorption properties of the cells.
First, cells acclimated to high irradiance levels generally have high carotenoids concentration rela-
tive to chlorophyll a. Carotenoids such as b-carotene and zeaxanthin do not transfer excitation
energy to the reaction centers and consequently act to screen the cell from excess light. Some xan-
tophylls such as lutein, transfer excitation energy but with reduced efficiency, and therefore effec-
tively reduce the functional absorption cross-section of the associated photosystem. Because these
carotenoids absorb light without a concomitant increase in the functional cross-section of PSII,
organisms acclimated to high irradiance levels often have lower maximum quantum yield of photo-
synthetic O 2 evolution. Second, when cells acclimate to low irradiance levels, the subsequent
increase in pigmentation is associated with a decrease in functional optical absorption section nor-
malized to chlorophyll a. This effect is due primarly to the self-shading of the chromophores
between layers of thylakoids membranes, and is an inverse function of the number of membranes
in the chloroplast, that is, the more the membranes, the lower the optical cross-section. Thus, as
cells accumulate chlorophyll, each chlorophyll molecule becomes less effective in light absorption;
a doubling of cellular chlorophyll does not produce a doubling in the rate of light absorption. The
reduction in the chlorophyll-specific optical absorption cross-section can be visualized considering
the fate of photons incident on two stacks of thylakoid membranes: one is a thin stack from a cell
acclimated to high irradiance levels and the second is a thick stack from a cell acclimated to low
irradiance levels. The probability of a photon passing through a thick stack of membranes without
being absorbed is small compared with a thin stack of membranes. This so-called “package effect”
reduces the effectiveness of increased pigmentation in harvesting light and has important impli-
cations for the capital costs of light harvesting, that is, the investments in the physical structures
of the organism required for the metabolic processes. The diminution in the optical absorption
cross-section with increased chlorophyll is also a function of cell size: the larger the cell the
more important is this effect. At some point a cell is, for most practical purposes, optically black
and further increases in pigment levels confer no advantage in light absorption.
There are two basic photoacclimation responses in algae. In one, acclimation is accomplished
primarily by changes in the number of photosynthetic reaction centers, while the effective absorp-
tion cross-section of the reaction centers remains relatively constant. The second is characterized by
relatively large changes in the functional size of the antennae serving the reaction centers, while the
number of reaction centers remains relatively constant. Complementary changes in either of the
responses produce the same effect on the initial slope of the photosynthesis–irradiance curve.
As the functional size of the antennae serving PSII, and not the number of rection centers, deter-
mines the light-saturation parameter, organisms that vary the cross-section would tend to have
more control over this parameter, as long as the turnover time remains constant.

SUGGESTED READING
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quantum sensor measuring yield photon flux and photosynthetic photon flux, HortScience, 28,
1197–1200, 1993.
Behrenfeld, M. J., Prasil, O., Babin, M., and Bruyant, F., In search of a physiological basis for covariations in
light-limited and light-saturated photosynthesis, Journal of Phycology, 40, 4–25, 2004.
Blanchard, G. F., Guarini, J.-M., Dang, C., and Richard, P., Characterizing and quantifying photoinhibition in
intertidal microphytobenthos, Journal of Phycology, 40, 692–696, 2004.
Buhrer, H., Light within algal cultures; implications from light intensity within a lens, Aquatic Science, 62,
91–103, 2000.