Characterization of Elliptochloris marina (Trebouxiophyceae, Chlorophyta) the Green Algal Symbiont of Two Intertidal Sea Anemones in the Genus Anthopleura (Cnidaria)

Abstract

In the eastern Pacific Ocean, symbioses between microscopic single-celled algae and the sea anemones Anthopleura xanthogrammica Brandt and Anthopleura elegantissima Brandt are an important component of the intertidal ecosystem. Both anemone species form symbioses with photosynthetic eukaryotes from two phyla: green algae (Chlorophyta), and dinoflagellates in the genus Symbiodinium. The genetic diversity and geographic distribution of the Symbiodinium symbionts in the Anthopleura spp. have been characterized; identity, genetic variation and distribution of the green algal symbiont have received limited examination. This study uses molecular data to expand what is known about the identity, diversity, distribution and phylogenetic relationships of the green algal symbiont of A. xanthogrammica and A. elegantissima. Sequence data from the chloroplast-encoded rbcL gene and nuclear-encoded SSU rDNA are used to examine the phylogenetic placement of the green symbiont. These data, along with morphological examinations, allowed this alga to be formally assigned to a new species, Elliptochloris marina (Trebouxiophyceae, Chlorophyta). The chloroplast-encoded rbcL gene and the nuclear-encoded Internal Transcribed Spacer (ITS) regions are used to examine the genetic diversity and distribution of the green algal symbionts of Anthopleura. Analyses of chloroplast-encoded rbcL data are consistent with a single species of green alga in A. xanthogrammica through out its geographic range, and with in the two host anemones in Washington state. Analyses of the more variable ITS data supported a single well-mixed population of Elliptochloris marina hosted throughout the known range, with no population structuring among biogeographic regions, between host species or among seasons. A unique chloroplast gene order was described for Elliptochloris marina and four different gene orders recovered among species in the closely related genera Elliptochloris, Hemichloris and Coccomyxa. This is the first study to demonstrated chloroplast gene order variation within congeneric green algae.