Chlorophyll d was first reported as a minor, green and magnesium-containing pigment in various species of red macroalgae in 1943, but its in vivo occurrence remained unclear as it was suspected to be an artefact produced by the pigment extraction process. We isolated a previously undescribed oxygenic photosynthetic prokaryote from a suspension of algae squeezed out of the colonial ascidian Lissoclinum patella collected from the marine coast of the Palau islands. The cells are unicellular, spheroidal or ellipsoidal (1.5-2.0 μm in diameter, 2.0-3.0 μm in length) prokaryotes with about 10 to 15 layers of peripherally stacked thylakoid-like membranes (ultrastructural features similar to Prochlorophyta). The cell suspensions are green with an absorption maximum in the red region at 714-718 nm (distinct from other oxygenic photosynthetic organisms). Pigment analysis by HPLC, NMR and fast atom bombardment mass spectrometry confirmed that chlorophyll d is the predominant pigment (accounting for about 80% of the total lipid-soluble pigment) with only a small amount of chlorophyll a (less than 0.08% of the cell dry weight). Our results confirm the natural occurrence of chlorophyll d in this newly isolated oxygenic photosynthetic prokaryote. Moreover, the high content of chlorophyll d and low chlorophyll a/chlorophyll d ratio suggest a unique, as yet unknown light-harvesting system using chlorophyll d. We propose the name Acaryochloris marina Miyashita et Chihara gen. et sp. nov. for the new organism.