Physiological Responses of a Diazotrophic Cyanobacterium to Acidification of Paddy Floodwater: N2 Fixation, Photosynthesis, and Oxidative–Antioxidative Characteristics

Abstract

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 N2 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 N2 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.

Biotoxicity of NH4+ on Nostoc Sphaeroides (diazotrophic cyanobacteria) in paddy floodwater

“…ROS are byproducts of aerobic metabolism (Sies et al, 2022;Jadhav et al, 2011). Under normal conditions, the production and scavenging of intracellular ROS are in dynamic balance (Yan et al, 2022;Yu et al, 2020). The results presented this study showed that the ROS and the MDA content in GXM increased significantly when the NH + 4 concentration was 67.30 mg L −1 .…”

“…The paddy soil was collected from the base of Cultivation Garden of Hunan Agricultural University (28 • 18'25" N, 113 • 07'34" E). Simulation of paddy floodwater were prepared according to the methods described by Yan et al (2022). In brief, deionized water (dH 2 O) was mixed with soil in a ratio of 10:9 (W:W), with a flooding depth of 1cm, and incubated at 25 • C under darkness for 15 d. Paddy floodwater was obtained by centrifugation at 12,000 ×g for 10 min.…”

“…In this study, the growth characteristics of GXM was evaluated by the variation of Chl a concentration in conical flasks. Chl a in cells of GXM was extracted using 95% ethanol and determined spectrophotometrically at 665 nm, 649 nm (Yan et al, 2022). Chl a concentration was calculated using the following equations.…”