Presence of phycoerythrin in two strains of Prochlorococcus (Cyanobacteria) isolated from the subtropical north Pacific Ocean

Prochlorococcus is a ubiquitous marine oxyphotobacterium characterized by t he presence of DV-chl a and b. In addition, the type strain Prochlorococcus marinus Chisholm et al, CCMP 1375 (or SS120), an isolate from the Sargasso Sea, contains low levels of an unusual phycoerythrin. Until now, it has be en unclear if phycoerythrin occurs randomly within this systematic group an d if the molecular characteristics of this phycoerythrin are restricted to this single strain. Here, we show that two additional Prochlorococcus strai ns from the Pacific Ocean also contain similar low levels of phycoerythrin. DNA sequence and phylogenetic analyses demonstrated that this phycoerythri n is very similar to the phycoerythrin of P. marinus SS120 and differs from the classic cyanobacterial phycoerythrins. In contrast, a third isolate fr om the Arabian Sea lacks phycoerythrin, Based on the DV-chl b:a ratio and 1 6S rRNA sequence data, we classify the two Pacific phycoerythrin-containing isolates as low-light-adapted strains and the Arabian Sea isolate as a hig h-light-adapted strain. Thus, we provide further evidence to link. the phys iology of an individual genotype and the presence or absence of functional phycoerythrin genes within the genus Prochlorococcus.