Selection of salt tolerant purple nonsulfur bacteria producing 5-aminolevulinic acid (ALA) and reducing methane emissions from microbial rice straw degradation

Saline soil is one major problem that causes low productivity on rice, while paddy fields are also one source of methane (CH₄) emissions that can have a considerable impact on global warming. By using purple nonsulfur bacteria (PNSB) it is possible to overcome both these serious problems because PNSB produce 5-aminolevulinic acid (ALA), a compound that reduces salt stress for plants, and they are also good competitors for methanogens in the paddy fields. Therefore, the objective of this study was to select salt resistant PNSB with an ability to release ALA and reduce CH₄ emissions. The PNSB isolates used were obtained from saline paddy fields in southern Thailand and the best were selected on the basis of their good growth under conditions of aerobic dark and anaerobic light while being salt tolerant, released ALA and reduced CH₄ emissions under microaerobic light conditions. Among the 272 PNSB isolates, TN114 was the best ALA producer while PP803 was the best to reduce CH₄ emissions. Both selected PNSB strains grew well in glutamate acetate (GA) broth containing NaCl up to 6.00% but 0.25% NaCl was their optimal concentration for growth. Under salt stress with 0.25% NaCl, the strain TN114 released 25.63μM ALA in GA broth. In scaled-up experiments, when it was mixed with native soil flora in rice straw broth the strain TN114 released 13.44μM ALA and the strain PP803 reduced CH₄ emissions by 88.41%. These two strains were identified as Rhodopseudomonas palustris according to both traditional and 16S rRNA identifications. Overall the results demonstrated that they had a good potential as plant growth stimulating bacteria under salt stress, due to the release of ALA and also reduce CH₄ emissions.