Acclimation of the photosynthetic apparatus to changes of irradiance, temperature and nutrient availability, involving regulation of the chlorophyll a:carbon ratio (g), is a universal feature of all phytoplankton studied to date. We derive a dynamic regulatory model that predicts the dependencies of 8 and growth rate ( U ) on irradiance, daylength, temperature and nutrient availdbilitv. Thc model requires specification of 4 parameters to describe the light-dependencies of 8 and g under nutr~ent-saturating conditions at constant temperature. These are the maximum value of 8 (B,,), the in~tial slope of the chl a-specific photosynthesis-light response curve (aCh'), the mdximum carbon-spcclfic photosynt h e s~s rate (P:;) and the cost of biosynthesis (C). The influences of temperature and nutnent availability are accommodated through their effects on P: . The temperature dependence is described by the slope of an Arrhenius plot and the nutrient dependence IS described through the half saturation constant (K,) of the Monod equation. Fidelity of the model results to empirical studies suggests that microalgal cells adjust 0 in response to an imbalance between the rate of light absorption and the energy demands for photosynthesis and biosynthesis.

The photosynthesis-irradiance response (PE) curve, in which mass-specific photosynthetic rates are plotted versus irradiance, is commonly used to characterize photoacclimation. The interpretation of PE curves depends critically on the currency in which mass is expressed. Normalizing the light-limited rate to chl a yields the chl a -specific initial slope ( ␣ chl ). This is proportional to the light absorption coefficient (a chl ), the proportionality factor being the photon efficiency of photosynthesis ( m ). Thus, ␣ chl is the product of a chl and m . In microalgae ␣ chl typically shows little ( Ͻ 20%) phenotypic variability because declines of m under conditions of high-light stress are accompanied by increases of a chl . The variation of ␣ chl among species is dominated by changes in a chl due to differences in pigment complement and pigment packaging. In contrast to the microalgae, ␣ chl declines as irradiance increases in the cyanobacteria where phycobiliproteins dominate light absorption because of plasticity in the phycobiliprotein:chl a ratio. By definition, light-saturated photosynthesis (P m ) is limited by a factor other than the rate of light absorption. Normalizing P m to organic carbon concentration to obtain P m C allows a direct comparison with growth rates. Within species, P m C is independent of growth irradiance. Among species, P m C covaries with the resource-saturated growth rate. The chl a :C ratio is a key physiological variable because the appropriate currencies for normalizing light-limited and light-saturated photosynthetic rates are, respectively, chl a and carbon. Typically, chl a :C is reduced to about 40% of its maximum value at an irradiance that supports 50% of the species-specific maximum growth rate and light-harvesting accessory pigments show similar or greater declines. In the steady state, this down-regulation of pigment content prevents microalgae and cyanobacteria from maximizing photosynthetic rates throughout the light-limited region for growth. The reason for down-regulation of light harvesting, and therefore loss of potential photosynthetic gain at moderately limiting irradiances, is unknown. However, it is clear that maximizing the rate of photosynthetic carbon assimilation is not the only criterion governing photoacclimation.

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Fast repetition rate (FRR) fluorescence can provide highly resolved estimates of light absorption by photosystem II (PSII), a variable that is critical to bio-optical determinations of phytoplankton productivity. We compared estimates of chlorophyll a-specific light absorption by PSII, a chl PSII , using both biophysical (FRR) and optical (chlorophyll a-specific light absorption coefficient, a chl ) techniques on cultures of phytoplankton from diverse taxa. Biophysical determinations of a chl PSII were obtained from the product of the effective light absorption cross-section of PSII (σ PSII ), measured by FRR fluorescence, and the ratio of PSII reaction centers to chlorophyll a (n PSII ), measured by oxygen flash yields. Both parameters were highly variable between individual taxa. In mixtures of algae, the estimates of σ PSII were largely determined by the taxon that dominated fluorescence intensity. Independent optical estimates of a chl PSII were obtained by weighting the light absorption spectra of photosynthetic pigments by the fluorescence excitation spectra. The biophysical and optical estimates of a chl PSII were highly correlated (r 2 = 0.94) with a slope that was not significantly different from 1 and an intercept of 0. Estimates of productivity using biophysical or optical absorption measurements should therefore be comparable when the latter are adjusted to account for the proportion of light provided for photochemistry into both photosystem I and II. Finally, we show how simultaneous measurements of σ PSII and a chl PSII can be used to derive n PSII where flash-yield measurements are impractical, as is almost universally the case in field measurements.