This study was conducted to evaluate selected biomolecular characteristics of rice root-associated diazotrophs isolated from the Tanjong Karang rice irrigation project area of Malaysia. Soil and rice plant samples were collected from seven soil series belonging to order Inceptisol (USDA soil taxonomy). A total of 38 diazotrophs were isolated using a nitrogen-free medium. The biochemical properties of the isolated bacteria, such as nitrogenase activity, indoleacetic acid (IAA) production and sugar utilization, were measured. According to a cluster analysis of Jaccard's similarity coefficients, the genetic similarities among the isolated diazotrophs ranged from 10% to 100%. A dendogram constructed using the unweighted pair-group method with arithmetic mean (UPGMA) showed that the isolated diazotrophs clustered into 12 groups. The genomic DNA rep-PCR data were subjected to a principal component analysis, and the first four principal components (PC) accounted for 52.46% of the total variation among the 38 diazotrophs. The 10 diazotrophs that tested highly positive in the acetylene reduction assay (ARA) were identified as Bacillus spp. (9 diazotrophs) and Burkholderia sp. (Sb16) using the partial 16S rRNA gene sequence analysis. In the analysis of the biochemical characteristics, three principal components were accounted for approximately 85% of the total variation among the identified diazotrophs. The examination of root colonization using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) proved that two of the isolated diazotrophs (Sb16 and Sb26) were able to colonize the surface and interior of rice roots and fixed 22%-24% of the total tissue nitrogen from the atmosphere. In general, the tropical soils (Inceptisols) of the Tanjong Karang rice irrigation project area in Malaysia harbor a diverse group of diazotrophs that exhibit a large variation of biomolecular characteristics.
This work investigates the effect of heterocyst toward biohydrogen production by A. variabilis. The heterocyst frequency was artificially promoted by adding an amino acid analog, in this case DL-7-azatryptophan into the growth medium. The frequency of heterocyst differentiation was found to be proportional to the concentration of azatryptophan (0-25 µM) in the medium. Conversely, the growth and nitrogenase activity were gradually suppressed. In addition, there was also a distinct shortening of the cells filaments and detachment of heterocyst from the vegetative cells. Analysis on the hydrogen production performance revealed that both the frequency and distribution of heterocyst in the filaments affected the rate of hydrogen production. The highest hydrogen production rate and yield (41 µmol H2 mg chl a(-1) h(-1) and 97 mL H2 mg chl a(-1), respectively) were achieved by cells previously grown in 15 µM of azatryptophan with 14.5 % of heterocyst frequency. The existence of more isolated heterocyst has been shown to cause a relative loss in nitrogenase activity thus lowering the hydrogen production rate.
A study was carried out to determine the effects of paraquat, pretilachlor and 2, 4-D on growth and nitrogen fixing activity of Stenotrophomonas maltophilia (Sb16) and pH of Jensen's N-free medium. The growth of Sb16 and pH of medium were significantly reduced with full (X) and double (2X) doses of tested herbicides, but nitrogen fixing activity was decreased by 2X doses. The nitrogenase activity had the highest value in samples treated with 1/2X of 2, 4-D on fifth incubation day, but 2X of 2, 4-D had the most adverse effect. An inhibition in the growth and nitrogenase activity was recovered on the last days of incubation.
The production of nitrogenase enzyme and auxins by free living diazotrophs has the potential to influence the growth of host plants. In this study, diazotrophs were grown in the presence of various concentrations of nitogen (N) to determine the optimal concentration of N for microbial growth stimulation, promotion of gaseous N (N2) fixation, and phytohormone production. Therefore, we investigate whether different levels of N supplied to Herbaspirillum seropedicae (Z78) have significant effects on nitrogenase activity and auxin production. The highest nitrogenase activity and the lowest auxin production of H. seropedicae (Z78) were both recorded at 0 gL(-1) of NH4Cl. Higher levels of external N caused a significant decrease in the nitrogenase activity and an increased production of auxins. In a subsequent test, two different inoculum sizes of Z78 (10(6) and 10(12) cfu/ml) were used to study the effect of different percentages of acetylene on nitrogenase activity of the inoculum via the acetylene reduction assay (ARA). The results showed that the optimal amount of acetylene required for nitrogenase enzyme activity was 5% for the 10(6) cfu/ml inoculum, whereas the higher inoculum size (10(12) cfu/ml) required at least 10% of acetylene for optimal nitrogenase activity. These findings provide a clearer understanding of the effects of N levels on diazotrophic nitrogenase activity and auxin production, which are important factors influencing plant growth.