A mechanistic model is developed to present the photosynthetic response of
phytoplankton to irradiance at the physiological level. The model is operat
ed on photosynthetic units (PSU), and each PSU is assumed to have two state
s: reactive and activated. Light absorption that drives a reactive PSU into
the activated state results from the effective absorption of the PSU. Tran
sitions between the two states are asymmetrical in rate. A PSU in the react
ive state becomes activated much faster than it recovers from the activated
state to the reactive one. The turnover time for an activated PSU to trans
it into the reactive one is defined by the turnover time of the electron tr
ansport chain. The present model yields a photosynthesis-irradiance curve (
PE-curve) in a hyperbola, which is described by three physiological paramet
ers: effective cross-section (sigma (PSII)), turnover time of electron tran
sport chain (tau) and number of PSUs (N). The PE-curve has an initial slope
of sigma (PSII) x N, a half-saturated irradiance of 1/(sigma (PSII)), and
a maximal photosynthetic rate of Nlc at the saturated irradiance. The PE-cu
rve from the present model is comparable to the empirical function based on
the target theory described by the Poisson distribution. (C) 2001 Academic
Press.