Phylogenetic placement of zoochlorellae (Chlorophyta), algal symbiont of the temperate sea anemone Anthopleura elegantissima

At northern latitudes the sea anemones Anthopleura elegantissima and its congener A. xanthogrammica contain unidentified green chlorophytes (zoochlorellae) in addition to dinophytes belonging to the genus Symbiodinium. This dual algal symbiosis, involving members of distinct algal phyla in one host, has been extensively studied from the perspective of the ecological and energetic consequences of hosting one symbiotic type over the other. However, the identity of the green algal symbiont has remained elusive. We determined the phylogenetic position of the marine zoochlorellae inhabiting A. elegantissima by comparing sequence data from two cellular compartments, the nuclear 18S ribosomal RNA gene region and the plastid-encoded rbcL gene. The results support the inclusion of these zoochlorellae in a clade of green algae that form symbioses with animal (Anthopleura elegantissima), fungal (the lichen genus Nephroma), and seed plant (Ginkgo) partners. This clade is distinct from the Chlorella symbionts of Hydra. The phylogenetic diversity of algal hosts observed in this clade indicates a predisposition for this group of algae to participate in symbioses. An integrative approach to the study of these algae, both within the host and in culture, should yield important clues about how algae become symbionts in other organisms.