Pigment Composition of a Novel Oxygenic Photosynthetic Prokaryote Containing Chlorophyll d as the Major Chlorophyll

The principal pigment found in the majority of oxygenic photosynthetic organisms is known to be chlorophyll a. However, we isolated a new oxygenic photosynthetic prokaryote that contained chlorophyll d as a predominant pigment with chlorophyll a being a minor pigment. Chlorophyll d had previously been noted but its natural occurrence and function remained unclear. Cells of the new prokaryote had an absorption maximum at red region of 714–718 nm due to chlorophyll d absorption, but no characteristic absorption peak of chlorophyll a around 680 nm was observed. Chlorophyll d of the new organism was identified spectrophotometrically in several solvents and its chemical structure was confirmed by NMR and FABMS analysis. The cell also contained a chlorophyll c-like pigment, zeaxanthin and a-carotene but not chlorophyll b and β-carotene. The content of chlorophyll d accounted for more than 2% of the cell dry weight, while the content of chlorophyll a was less than 0.1%. The chlorophyll a/d ratio remained between 0.03 and 0.09 under different culture conditions. The light absorption characteristics and the high content of chlorophyll d along with the small content of chlorophyll a indicated the existence of a new light utilization mechanism involving chlorophyll d.