光合成細菌の弱光への生理的適応とその生態的意義

抄録

Physiological adaptability of photosynthetic sulfur bacterium to low light environment and its ecological meaning were examined by using sulfur bacterium, Chromatium strain D. When Chromatium was cultured under 1 klux and 20 klux of continuous light, the high light culture showed a growth rate twice as high as that of the low light culture. The amount of bacteriochlorophyll in cells was also remarkably changed. The BChl content per unit cellular carbon or nitrogen was 2-3 times higher in the low light culture than in the high light culture. The carotenoid content per unit cellular nitrogen also showed the higher values in the higher light cultures. The light absorption of intact cells was influenced by the light conditions. The low light culture indicated a higher photosynthetic rate at low light intensities compared with the high light culture, and the photosynthetic rate based on cellular carbon was noticeably high for the low light culture. This may be due to the high content of BChl in a cell. By using the mathematical model, the depth-profiles of daily photosynthesis of both the low and high light cultures were estimated. The low light culture showed higher photosynthetic rates compared with the high light culture in the deeper layers where the light intensity is less than 10% of the surface illumination. The intensive growth of photosynthetic sulfur bacteria at the depth of dim light near the upper boundary of reductive zone may be supported by their photosynthetic adaptability to low light environment coupling with the occurrence of hydrogen donors such as H₂S.