Prochlorophyta as a proposed new division of algae

Abstract

UNICELLULAR algae associated with ascidians from tropical Pacific shores have been reported by various biologists^1–5. They are bright green, generally spherical and about 10–20 µm in diameter, and they seem to have no clearly delimited nucleus or plastids. Such cells (identified as Synechocystis didemni), found associated with surfaces of Didemnum colonies on the Pacific coast of Mexico, have been shown by electron microscopy to be prokaryotic^6,7, which suggests that they are cyanophytes, that is, blue-green algae. Although all known blue-green algae (other than a few apochlorotic types) contain phycoerythrin, phycocyanin, or both, however, these ascidian symbionts are apple green and contain no detectable bilin pigments. Furthermore, like the eukaryotic algae in the divisions Chlorophyta and Euglenophyta, they contain two chlorophyll components, separable by chromatography and provisionally identifiable as chlorophylls a and b (ref. 8), whereas no cyanophytes are known to contain chlorophyll b. The assignment of S. didemni to any of the established algal divisions, therefore, presents a major taxonomic problem.