Effects of Light and Oxygen on Chlorophyll d Biosynthesis in a Marine Cyanobacterium Acaryochloris marina

A marine cyanobacterium Acaryochloris marina synthesizes chlorophyll (Chl) d as a major Chl. Chl d has a formyl group at its C3 position instead of a vinyl group in Chl a. This modification allows Chl d to absorb far-red light addition to visible light, yet the enzyme catalyzing the formation of the C3-formyl group has not been identified. In this study, we focused on light and oxygen, the most important external factors in Chl biosynthesis, to investigate their effects on Chl d biosynthesis in A. marina. The amount of Chl d in heterotrophic dark-grown cells was comparable to that in light-grown cells, indicating that A. marina has a light-independent pathway for Chl d biosynthesis. Under anoxic conditions, the amount of Chl d increased with growth in light conditions; however, no growth was observed in dark conditions, indicating that A. marina synthesizes Chl d normally even under such “micro-oxic” conditions caused by endogenous oxygen production. Although the oxygen requirement for Chl d biosynthesis could not be confirmed, interestingly, accumulation of pheophorbide d was observed in anoxic and dark conditions, suggesting that Chl d degradation is induced by anaerobicity and darkness.