Selection of acid sulfate purple nonsulfur bacteria for mitigating methane emissions from paddy soils in the Mekong Delta, Vietnam.

In lowland rice cultivation, climate change is increasingly intensified by rising greenhouse gas emissions, especially methane (CH4). This study aimed to: (i) isolate and select acid sulfate-tolerant purple nonsulfur bacteria (PNSB) capable of nutrient provision; and (ii) screen and identify PNSB strains that reduce CH4 emissions. A total of 60 soil and 60 water samples were collected from four acid sulfate rice-growing areas in the Mekong Delta, Vietnam. From these, 118 PNSB strains were isolated, and 100 were selected based on their acid-sulfate tolerance, nitrogen fixation, phosphate solubilization, and indole-3-acetic acid (IAA) production. Among them, strains WHA-1A, WHA-9B, and SPL-4A exhibited the greatest CH4 emission reduction under both microaerobic light (17.1-61.3%) and aerobic dark (48.2-75.2%) conditions. These strains also demonstrated nitrogen fixation (21.5-22.9 and 19.4-31.4 mg L- 1), solubilization of three insoluble phosphate forms-Fe-P, Al-P, and Ca-P for both conditions (21.9-29.6 and 11.9-14.8 mg L- 1; 8.12-15.1 and 19.0-30.2 mg L- 1; 12.5-27.0 and 18.5-31.4 mg L- 1, respectively), and IAA production (0.34-1.91 and 2.24-6.28 mg L- 1). Based on 16S rRNA gene sequencing, two strains WHA-1A and SPL-4A were identified as Rhodobacter sphaeroides, and WHA-9B was identified as Blastochloris sulfoviridis. These strains show promise for mitigating CH4 emissions and improving rice growth in acid sulfate soils.