Excitation energy relaxation in a symbiotic cyanobacterium, Prochloron didemni, occurring in coral-reef ascidians, and in a free-living cyanobacterium, Prochlorothrix hollandica

The marine cyanobacterium Prochloron is a unique photosynthetic organism that lives in obligate symbiosis with colonial ascidians. We compared Prochloron harbored in four different host species and cultured Prochlorothrix by means of spectroscopic measurements, including time-resolved fluorescence, to investigate host-induced differences in light-harvesting strategies between the cyanobacteria. The light-harvesting efficiency of photosystems including antenna Pcb, PS II–PS I connection, and pigment status, especially that of PS I Red Chls, were different among the four samples. We also discuss relationships between these observed characteristics and the light conditions, to which Prochloron cells are exposed, influenced by distribution pattern in the host colonies, presence or absence of tunic spicules, and microenvironments within the ascidians' habitat.