All strains are obligate photoautrophs and are around 0.6–1.7 µm in diameter. This cluster is, however, further divided into a population that either contains (cluster 5.1) or does not contain (cluster 5.2) phycoerythrin. The reference strains are WH8103 for the phycoerythrin-containing strains and WH5701 for those strains that lack this pigment.
All strains of this group are obligate photoautotrophs, measuring 0.6–1.7 µm in diameter. This cluster is further divided into two groups: one containing phycoerythrin (cluster 5.1) and the other lacking it (cluster 5.2). The reference strains for these groups are WH8103 (phycoerythrin-containing) and WH5701 (lacking phycoerythrin).