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.

Photoacclimation was examined in the marine diatom Skeletonema costatum, which was subjected to reciprocal shifts between irradiances of 50 (low-light) and 1,200 (high-light) mol photons m Ϫ2 s Ϫ1

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Thermal acclimation following changes from 6 mC to 15 mC and from 15 mC to 25 mC was investigated in the diatom Chaetoceros calcitrans. Cell division rate increased with temperature. Cellular carbon showed a slight decrease with an increase in temperature from 6 mC to 15 mC but showed no further change with an increase to 25 mC. Cells grown at 6 mC had low levels of light-harvesting components and a high carotenoid to chlorophyll a (Chla) ratio due to an increase in abundance of photoprotective pigments. The abundance of light-harvesting pigments increased with temperature and this was mirrored by a decrease in carotenoid : Chla ratio. All pigments showed an immediate change in abundance following each temperature shift. However, changes in photoprotective pigments reached a plateau after 12 h, in contrast to the lightharvesting pigments that took 4 days to become acclimated. Light-saturated rates of photosynthesis were at a minimum at 6 mC and increased twofold with temperature between 6 mC and 25 mC. Light-limited photosynthesis showed little change with temperature. It is suggested that, at low temperatures, the rate of carbon fixation is impaired due to a decrease in the activity of photosynthetic enzymes. This imparts an increase in excitation pressure due to an imbalance between light absorption and utilization. Cells respond to this by decreasing their light-harvesting capacity and increasing their ability to dissipate the excess energy. This fairly rapid regulation may be ecologically advantageous in areas where there are transient temperature changes.

Thermal acclimation following changes from 6 mC to 15 mC and from 15 mC to 25 mC was investigated in the diatom Chaetoceros calcitrans. Cell division rate increased with temperature. Cellular carbon showed a slight decrease with an increase in temperature from 6 mC to 15 mC but showed no further change with an increase to 25 mC. Cells grown at 6 mC had low levels of light-harvesting components and a high carotenoid to chlorophyll a (Chla) ratio due to an increase in abundance of photoprotective pigments. The abundance of light-harvesting pigments increased with temperature and this was mirrored by a decrease in carotenoid : Chla ratio. All pigments showed an immediate change in abundance following each temperature shift. However, changes in photoprotective pigments reached a plateau after 12 h, in contrast to the lightharvesting pigments that took 4 days to become acclimated. Light-saturated rates of photosynthesis were at a minimum at 6 mC and increased twofold with temperature between 6 mC and 25 mC. Light-limited photosynthesis showed little change with temperature. It is suggested that, at low temperatures, the rate of carbon fixation is impaired due to a decrease in the activity of photosynthetic enzymes. This imparts an increase in excitation pressure due to an imbalance between light absorption and utilization. Cells respond to this by decreasing their light-harvesting capacity and increasing their ability to dissipate the excess energy. This fairly rapid regulation may be ecologically advantageous in areas where there are transient temperature changes.