Photosymbiotic associations in planktonic foraminifera and radiolaria

Foraminifera, radiolaria and acantharia are relatively large (> 1 mm in most cases) unicellular eukaryotes that occur in pelagic oceanic communities. Commonly referred to as planktonic sarcodines, these organisms often harbor algal symbionts. The symbionts have been described as dinoflagellates, chrysophytes and prasinophytes based upon their morphology either in the host Or as free-living organisms in culture. To investigate the molecular taxonomic affiliations of the algae, and to determine the sequence variability between symbionts from individual hosts, we examined the small subunit ribosomal DNA sequences from symbionts isolated from planktonic foraminifera and radiolaria, The symbionts that we analyzed included dinoflagellates, prasinophytes and prymnesiophytes. We have, through our studies of planktonic sarcodine symbioses, and through comparison with other symbiotic associations (corals and lichens), observed that taxonomically distinct lineages of symbiotic algae are not uncommon. How do such different algae share the function of symbiosis, while other, more related algae, do not? We 9 propose that there are commonalties that exist between symbiotic algae that confer symbiotic 'competence', and the way to begin the search for these is to utilize the different algal symbiont lineages.