Divinyl chlorophyll a-specific absorption coefficients and absorption efficiency factors for Prochlorococcus marinus: kinetics of photoacclimation

The photosynthetic prokaryote Prochlorococcus appears to have a high capaci ty to modify its physiological and optical properties in response to change s in available irradiance. In order to study the time scale of variations i n light absorption induced by photoacclimation, 2 strains (MED, clone CCMP 1378, and SARG, unialgal strain) were grown in batch cultures at high (56.7 mu mol quanta m(-2) s(-1)) or low (8.4 mu mol quanta m(-2) s(-1)) irradian ce. Then the temporal changes over 3 to 4 d in spectral absorption coeffici ents, pigment composition, cell number density and size distribution were f ollowed for cultures transferred from high to low irradiance, and vice vers a. Both strains experienced significant changes in their divinyl-chlorophyl l 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, whi le 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 fr om 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 photoacclimatio n was not achieved after 3 d. The efficiency factors for absorption, <(Q(a) )over bar>(lambda), and the a*(lambda) coefficients, were reconstructed fro m theory at the different photoacclimation stages from the cell characteris tics, i.e. intracellular concentrations of the various pigments and cell si ze distribution. This permitted the determination of the parameters which a re mainly responsible for the observed changes. The <(Q(a))over bar>(lambda ) values (and therefore the package effect) are enhanced at low irradiance by 2 effects resulting from photoacclimation: the increase of the intracell ular concentration of divinyl-chlorophyll a, and (for the SARG strain only) the increase of the intracellular concentration of chlorophyll b. In addit ion, the presence of zeaxanthin, in stable amounts within the cells whateve r the irradiance, enhances the <(Q(a))over bar>(lambda) values for all ligh t conditions, and thus 'moderates' its variations with irradiance. Contrary to what is commonly admitted because of its tiny size, the absorption effi ciency 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 t he lower part of the euphotic zone. This reduction directly affects the amo unt of absorbed energy usable for photosynthesis.