What is the function of phycoerythrin in Prochlorocccus marinus ?

Abstract

Prochlorococcus, one of the dominant cyanobacteria in the world¿s oceans, comprises an unusual pigment composition. It uniquely possesses Divinyl-(DV) Chl a and b that are bound to PCBs, the major antenna proteins of Prochlorophytes. Within Prochlorococcus, ecotypes exist that are adapted to low and high light, respectively. Low light-adapted strains are capable to build a functional form of phycoerythrin (PE), whereas in high light-adapted strains only remnants of this gene cluster are left. Genes that code for the alpha and gamma PE subunit, as well as several genes that are involved in the synthesis of phycobilins are lacking. Here we present data showing energetical coupling of PE in low light-adapted strains using synchrotron radiation to perform time-resolved fluorescence measurements. Fluorescence decay lifetimes of 550 ps were measured, indicating that any light-harvesting energy transfer must be unusually slow. After decoupling, the decay lifetime slowed to 1 ns. This lifetime corresponds to those measured for decoupled phycobilins. Additionally, fluorescence yields of DV-Chl b and PE were determined thus enabling us to estimate the PE-contribution to the total light-harvesting capacity. To investigate whether the presence of a reduced beta PE form in high light-adapted ecotypes can be generalised, different laboratory cultures and field samples from the Red Sea were screened using PCR and degenerated primers. In all tested strains, as well as in the field samples a very particular form of beta PE was found suggesting its functional relevance in these strains.