Potential of Phototrophic Purple Nonsulfur Bacteria to Fix Nitrogen in Rice Fields

Biological nitrogen fixation catalyzed by Mo-nitrogenase of symbiotic diazotrophs has attracted interest because its potential to supply plant-available nitrogen offers an alternative way of using chemical fertilizers for sustainable agriculture. Phototrophic purple nonsulfur bacteria (PNSB) diazotrophically grow under light anaerobic conditions and can be isolated from photic and microaerobic zones of rice fields. Therefore, PNSB as asymbiotic diazotrophs contribute to nitrogen fixation in rice fields. An attempt to measure nitrogen in the oxidized surface layer of paddy soil estimates that approximately 6–8 kg N/ha/year might be accumulated by phototrophic microorganisms. Species of PNSB possess one of or both alternative nitrogenases, V-nitrogenase and Fe-nitrogenase, which are found in asymbiotic diazotrophs, in addition to Mo-nitrogenase. The regulatory networks control nitrogenase activity in response to ammonium, molecular oxygen, and light irradiation. Laboratory and field studies have revealed effectiveness of PNSB inoculation to rice cultures on increases of nitrogen gain, plant growth, and/or grain yield. In this review, properties of the nitrogenase isozymes and regulation of nitrogenase activities in PNSB are described, and research challenges and potential of PNSB inoculation to rice cultures are discussed from a viewpoint of their applications as nitrogen biofertilizer.

Genetic diversity and geographic distribution of the Bemisia tabaci species complex in Bangladesh

Bemisia tabaci (Gennadius) is a species complex consisting of at least 40 cryptic species. Although the genetic diversity of B. tabaci has been studied in various regions, little is known about distribution in Bangladesh, which is covered by the Bengal delta, the largest delta on Earth. We conducted an extensive survey throughout the country and determined the nucleotide sequence of mitochondrial cytochrome c oxidase subunit 1 (COI) from 110 individuals. We then examined phylogenetic relationships. The results identified four cryptic species that expressed distinct interspecific variation but low intraspecific variation. Asia I was the most abundant, both Asia II 1 and Asia II 5 were moderately abundant, and Asia II 10 was found only in the central region. COI sequences of each cryptic species were distinctive and differentiated into many haplotypes. Our study provides important information to better understand the genetic diversity and geographic distribution of cryptic species in Bangladesh and nearby countries.