Page 222 - Algae

Working with Light 205

steady-state electron turnover rate through the PSII reaction center (1/t PSII ):

1
P max ¼ n (5:34)
t PSII

This equation says that increasing the number of photosynthetic units (but not their size) P max ,
that is, the saturation level, increases, and that P max cannot be derived from measurements of light
absorption (Figure 5.15c). In other words, above the saturation point (E k ), the light-dependent reac-
tion are producing more ATP and NADPH than can be used by the light-independent reaction for
CO 2 fixation, that is, increasing irradiance no longer causes any increase in photosynthetic rate.
Above E k and under normal condition, availability of CO 2 is the limiting factor, because the
concentration of CO 2 in the atmosphere is very low (0.035% v/v).
Saturating irradiances show some correlation with habitat, but generally, they are low com-
22 21
pared with full sun. Intertidal species require 400–600 mmol m sec (ca. 10% of the full sun
22 21
irradiance), upper and midsublittoral species 150–250 mmol m sec and deep sublittoral
species less than 100 mmol m 22 sec 21 . Diatoms under ice saturate at 5 mmol m 22 sec 21 .
Further increase in irradiance beyond light saturation can lead to a reduction in photosynthetic
rate from the maximum saturation level. This reduction, which is dependent on both the irradiance
and the duration of the exposure, is often termed photoinhibition. Photoinhibition can be thought as
a modification of P max either by a reduction in the number of photosynthetic units or by an increase
in the maximum turnover rate [Equation (5.34)]; thus photoinhibition leads a reduction in the
photochemical efficiency of PSII, through a reduction in the population of functional (O 2 evolving)
reaction centers. Increasing irradiance levels increase the probability that more than one photon,
two for example, strike the same reaction centers at the same time. The added energies of two
blue photons, for example, could be very harmful as the resulting energy will correspond to a
UV photon and could damage the chromophores.

PHOTOACCLIMATION

Photoacclimation is a complex light-response that changes cellular activities on many time scales.
The aquatic environment presents a highly variable irradiance (E) field with changes occurring over
a wide range of time scales. For example, changes in E on short time scales can result from focusing
and defocusing of radiation by waves at the surface. Longer time scale changes can result from vari-
able cloud cover or turbulent motion that transports phytoplankton across the exponential E gradi-
ent of the surface mixed layer. In a well-mixed surface layer, phytoplankton experiences long
periods of low E interspersed by short periods of saturating or even supersaturating E. The
diurnal solar cycle causes changes in E on even longer time scales. To cope with the highly variable
radiation environment, phytoplankton has developed numerous strategies to optimize photo-
synthesis, while minimizing susceptibility to photodamage. Photosynthetic acclimation to E over
time scales of hours to days proceeds through changes in cellular pigmentation or structural charac-
teristics, for example, size and number of photosynthetic units. On shorter time scales, cells adjust
photon utilization efficiencies by changing the distribution of harvested energy between photo-
systems (state transitions) or by dissipating excess energy through non-photochemical processes,
for example, xanthophyll cycle or photoinhibition.
Therefore, photoacclimation involves change in macromolecular composition in photosynthe-
tical apparatus. It is relatively easy to observe acclimation in unicellular algae and seeweeds, where
chlorophylls per cell or per unit surface can increase five- to ten-fold as irradiance decreases. The
response is not a linear function of irradiance; rather at extremely low light levels, cells often
become a bit chlorotic, and on exposure to slightly higher (but still low) irradiance, chlorophyll
reaches a maximum. Increase in irradiance leads to a decrease in the cellular complement of chloro-
phylls until a minimum value is reached. The absolute irradiance levels that induce these effects