Photoacclimation of Prochlorococcus sp. (prochlorophyta) strains isolated from the North Atlantic and the Mediterranean Sea

Abstract

Two Atlantic (SARC and NATL1) strains and one Mediterranean (MED) strain of Prochlorococcus sp., a recently discovered marine, free-living prochlorophyte, were grown over a range of "white" irradiances (Ig) and under low blue light to examine their photoacclimation capacity. All three strains contained divinyl (DV) chlorophylls (Chl) a and b, both distinguishable from "normal" Chls by their red-shifted blue absorption maximum, a Chl c-like pigment at low concentration, zeaxanthin, and α-carotene. The presence of two phaeophytin 6 peaks in acidified extracts from both Atlantic strains grown at high Ig suggests that these strains also had a normal ChI 6-like pigment. In these strains, the total Chl b to DV-Chl a molar ratio decreased from about 1 at 7.5 μmol quanta m-2 s-1 to 0.4 to 0.5 at 133 μmol quanta m-2 s-1. In contrast, the MED strain always had a low DV-Chl b to DV-Chl a molar ratio, ranging between 0.13 at low Ig and 0.08 at high Ig. The discrepancies between the Atlantic and MED strains could result from differences either in the number of light-harvesting complexes (LHC) II per photosystem II or in the Chl b-binding capacity of the apoproteins constituting LHC II. Photosynthesis was saturated at approximately 5 fg C(fg Chl)-1 h-1 or 6 fg C cell-1 h-1, and growth was saturated at approximately 0.45 d-1 for both MED and SARG strains at 18°C, but saturating irradiances differed between strains. Atlantic strains exhibited increased light-saturated rates and quantum yield for carbon fixation under blue light.