Unicellular cryptophyte algae employ antenna proteins with phycobilin 
chromophores in their photosynthetic machinery. The mechanism of light 
harvesting in these organisms is significantly different than the energy 
funneling processes in phycobilisomes utilized by cyanobacteria and red algae. 
One of the most striking features of cryptophytes is the location of the 
water-soluble phycobiliproteins, which are contained within the intrathylakoid 
spaces and are not on the stromal side of the lamellae as in the red algae and 
cyanobacteria. Studies of mobility of phycobiliproteins at the lumenal side of 
the thylakoid membranes and how their diffusional behavior may influence the 
energy funneling steps in light harvesting are reported. Confocal microscopy and 
fluorescence recovery after photobleaching (FRAP) are used to measure the 
diffusion coefficient of phycoerythrin 545 (PE545), the primary light harvesting 
protein of Rhodomonas CS24, in vivo. It is concluded that the diffusion of PE545 
in the lumen is inhibited, suggesting possible membrane association or 
aggregation as a potential source of mobility hindrance.