Light responses in the green sulfur bacterium Prosthecochloris aestuarii: changes in prosthecae length, ultrastructure, and antenna pigment composition

The morphology (mainly prosthecae length), ultrastructure. and antenna pigm ent composition of the green sulfur bacterium Prosthecochloris aestuarii ch anged when grown under different light intensities. At light intensities of 0.5 and 5 mu mol quanta m(-2) s(-1), the cells had a star-like morphology. Prosthecae, the characteristic appendages of the genus Prosthecochloris. w ere 232 nm and 194 nm long, respectively. In contrast, when grown at 100 mu mol quanta m(-2) s(-1), these appendages were shorter (98 nm) and the cell s appeared more rod-shaped. Transmission electron microscopy revealed a sig nificant decrease in the cell perimeter to area ratio and in the number of chlorosomes per linear gm of membrane as light intensity increased. In addi tion to these morphological and ultrastructural responses, Prosthecochloris aestuarii exhibited changes in its pigment composition as a function of li ght regime. Lower specific pigment content and synthesis rates were found i n cultures grown at light intensities above 5 pmol quanta m(-2) s(-1). A bl ue shift in the bacteriochlorophyll (BChl) c Q(y) absorption maximum of up to 17.5 nm was observed under saturating light conditions (100 mu mol quant a m(-2) s(-1)). This displacement was accompanied by changes in the composi tion of BChl c homologs and by a very low carotenoid content. The morpholog ical, ultrastructural and functional changes exhibited by Prosthecochloris aestuarii revealed the strong light-response capacity of this bacterium to both high and low photon-flux densities.