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Environmental and Stress Physiology

CO₂ Fixation Rate and RuBisCO Content Increase in the Halotolerant Cyanobacterium, Aphanothece halophytica, Grown in High Salinities ¹

Research Institute for Biochemical Regulation, School of Agriculture, Nagoya University, Chikusa, Nagoya 464-01, Japan, Department of Anatomy, Yamanashi Medical School, Yamanashi 409-38, Japan

The growth of the halotolerant cyanobacterium Aphanothece halophytica, previously adapted to 0.5 molar NaCl, was optimal when NaCl concentration in culture medium was in the range 0.5 to 1.0 molar. The growth was delayed at either too low or too high salinities with lag time of ca. 0.5 day in 0.25 molar NaCl and ca. 2 days in 2 molar NaCl under the experimental conditions. However, the growth rates at the logarithmic phase were similar in the culture media containing NaCl in the range 0.25 to 2.0 molar. The capacity of photosynthetic CO₂ fixation increased 3.7-fold in the cells at the logarithmic phase as NaCl concentration in the culture medium increased from 0.25 to 2.0 molar. The protein level of ribulose 1,5-bisphosphate carboxylase/oxygenase was also found to increase with increasing salinity using both an immunoblotting method and protein A-gold immunoelectron microscopy. These results indicate that high photosynthetic capacity and high ribulose 1,5-bisphosphate carboxylase/oxygenase content may entail an important role in betaine synthesis and adaptation of the A. halophytica cells to high NaCl level.

¹ This research has been financially supported by the research grants from the Ministry of Education, Science and Culture (60560089, 61304004) and the Ishida foundation to T. T.

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