ABSTRACT: The photosynthetic prokaryote Prochlorococcus appears to have a high capacity to modify its physiological and optical properties in response to changes in available irradiance. In order to study the time scale of variations in light absorption induced by photoacclimation, 2 strains (MED, clone CCMP 1378, and SARG, unialgal strain) were grown in batch cultures at high (56.7 µmol quanta m^-2 s^-1) or low (8.4 µmol quanta m^-2 s^-1) irradiance. Then the temporal changes over 3 to 4 d in spectral absorption coefficients, pigment composition, cell number density and size distribution were followed for cultures transferred from high to low irradiance, and vice versa. Both strains experienced significant changes in their divinyl-chlorophyll a-specific absorption coefficients (a*) in response to the transfers. For the MED strain, photoacclimation appeared to be achieved within about 40 h (covering 2 cell generations) for the low to high irradiance transfer, while 3 to 4 d (corresponding to 1 doubling of the population) were necessary for the high to low irradiance transfer. For the SARG strain transferred from high to low irradiance, in spite of a rapid change in absorption during the first 25 h (i.e. within the same cell generation), full photoacclimation was not achieved after 3 d. The efficiency factors for absorption, <\2>Q_a--(l), and the a*(l) coefficients, were reconstructed from theory at the different photoacclimation stages from the cell characteristics, i.e. intracellular concentrations of the various pigments and cell size distribution. This permitted the determination of the parameters which are mainly responsible for the observed changes. The <\2>Q_a--(l) values (and therefore the package effect) are enhanced at low irradiance by 2 effects resulting from photoacclimation: the increase of the intracellular concentration of divinyl-chlorophyll a, and (for the SARG strain only) the increase of the intracellular concentration of chlorophyll b. In addition, the presence of zeaxanthin, in stable amounts within the cells whatever the irradiance, enhances the Q_a--(l) values for all light conditions, and thus 'moderates' its variations with irradiance. Contrary to what is commonly admitted because of its tiny size, the absorption efficiency of Prochlorococcus (per pigment unit) is not always maximal, but can be reduced by 20 to 25% at low irradiances, such as those prevailing in the lower part of the euphotic zone. This reduction directly affects the amount of absorbed energy usable for photosynthesis.

KEY WORDS: Prochlorococcus · Prochlorophyta · Optical properties · Absorption · Photoacclimation

Published in MEPS Vol. 188 (1999) on November 3
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999