The morphology of the marine cyanobacterium DC-2 and two light-harvesting complexes from it have been characterized. DC-2 has an outer cell wall sheath not previously observed, the purified phycoerythrin shows many unusual properties that distinguish it from all phycoerythrins characterized to date, and isolated phycobilisomes have a single absorption band at 640 nm in the phycocyanin-allophycocyanin region of the spectrum. On the basis of these observations we suggest that DC-2, rather than being a member of the Synechococcus group, should be placed in its own taxonomic group. In addition, the particular properties of the isolated phycoerythrin suggest that it may be representative of an early stage in the evolution of the phycoerythrins. These observations are of special interest in light of the contribution DC-2 and related cyanobacteria may make to global primary productivity. Several species of blue-green-and red-pigmented marine cyanobacteria that not only are abundant in the world's oceans but also may be responsible for a significant fraction ofprimary productivity have been discovered recently (1, 2). The red-pigmented cells have the virtue that they are readily distinguished from most other phytoplankton species by their principal in vivo fluorescence emission in the orange (560-580 nm), which undoubtedly arises from the presence ofthe red pigment-protein, phycoerythrin (PE). In some cyanobacteria and most red algae, PE serves as the major light-harvesting pigment for photosynthesis and is found in association with the other major phycobilipigments, phycocyanin and allophycocyanin. In vivo these phycobilipigments are aggregated into macromolecular arrays that form discrete organelles called phycobilisomes and are attached to the chlorophyll a-containing photosynthetic lamellae (3).Such an organization allows for excitation energy transfer from the shortest-wavelength-absorbing pigment, PE, to phycocyanin and allophycocyanin and ultimately to the chlorophyll a residing in the photochemical reaction centers ofphotosystems I and II (3-5). Therefore, the phycobilipigments serve two important functions in photosynthesis: (i) they increase photon capture under light-limited conditions and (ii) they increase the spectral range of light energy available for photosynthesis by absorbing light where chlorophyll a absorbs weakly and where there is the greatest transmission of light in the water column.Recent studies have demonstrated that the phycobiliproteins have highly conserved NH2-terminal sequences (6)(7)(8) and that phycobilisome structure is also fairly conservative (3). The dual requirements for efficient energy transfer from the phycobilipigments to chlorophyll a and for assembly of a phycobilisome seem to impose strong constraints on the nature ofthe phycobilipigments and their in vivo macromolecular organization.After examining the in vivo spectral properties of several of the recently discovered species ofcyanobacteria, it came to our attention that one of the PE-containing types termed DC-2 ...