Physiological Responses of a Diazotrophic Cyanobacterium to Acidification of Paddy Floodwater: N(2) Fixation, Photosynthesis, and Oxidative–Antioxidative Characteristics

Long-term of excessive fertilization using nitrogen (N) chemical fertilizer caused the acidification of paddy soils. Presently, the impacts of soil acidification on physiological characteristics of diazotrophic cyanobacteria remain unknown. In order to elucidate this issue, the effects of paddy floodwater acidification on activities of respiration, photosynthetic oxygen evolution, and N(2) fixation of a paddy diazotrophic cyanobacterium Aliinostoc sp. YYLX235 were investigated in this study. In addition, the origination and quenching of intracellular reactive oxygen species (ROS) were analyzed. The acidification of paddy floodwater decreased intracellular pH and interfered in energy flux from light-harvesting chlorophyll antenna to the reaction center of photosystem II (PS II). Activities of respiration, photosynthetic oxygen evolution, and N(2) fixation were decreased by the acidification of paddy floodwater. Accompanied with an increase in ROS, the level of antioxidative system increased. Superoxide dismutase (SOD) and catalase (CAT) were the main enzymatic ROS scavengers in the cells of YYLX235; reduced glutathione (GSH) was the main non-enzymatic antioxidant. Antioxidants and oxidants in the cells of YYLX235 lost balance when the pH of paddy floodwater fell to 5.0 and 4.0, and lipid oxidative damage happened. The results presented in this study suggest that the acidification of paddy soil severely interfered in the photosynthesis of diazotrophic cyanobacteria and induced the production of ROS, which in turn resulted in oxidative damage on diazotrophic cyanobacteria and a decrease in cell vitality.