Haloalkaliphilic diazotrophs in soda solonchak soils

Abstract

Nitrogen fixation (NF) potential was measured in more than 40 samples of soda solonchak soils with the pH of water extract between 9.5 and 11.0 collected in several locations of Central Asia and in Egypt, using the acetylene reduction method. NF was detected in most of the samples. Maximal rates were observed under microaerophilic-anaerobic conditions with glucose as a substrate. In most cases, the NF negatively correlated with salt content and alkalinity. Five enrichments at pH 10 under micro-oxic conditions with glucose resulted in stable haloalkaliphilic mixed cultures, with diazotrophic component(s) active up to 2.0-3.0 M total Na(+). The cultures were dominated by Gram-positive spore-forming bacteria. Molecular cloning of nifH genes demonstrated the presence of two phylogenetic lineages of diazotrophs in the enrichments affiliated with the low-GC Gram-positive bacteria (in rRNA groups 1 and 6 of bacilli and in Clostridiales). Isolation of pure cultures of haloalkaliphilic diazotrophs from micro-oxic enrichments yielded nine strains, comprising two phylogenetic lineages. Most of the isolates (eight) were affiliated with the aerotolerant fermentative haloalkaliphilic bacterium Amphibacillus tropicus and a single strain clustered with the obligately anaerobic haloalkaliphile Bacillus arseniciselenatis. Diazotrophy has never been recognized previously in these groups of Gram-positive bacteria. Overall, the results demonstrated the existence, in soda solonchak soils, of a novel group of free-living fermentative diazotrophic bacteria active at extremely haloalkaline conditions.

Cultivable Nitrogen Fixing Bacteria from Extremely Alkaline-Saline Soils

“…ammonification, nitrification and denitrification), nitrogen fixation is affected by environmental factors, such as temperature, pH, oxygen, heavy metals and mineral nutrients [40]- [42]. Our study documents nitrogen-fixing cultivable bacteria obtained by enrichment ·g −1 soil), similar to those found in other terrestrial environments, such as in soda solonchak soils and the rhizosphere of some plants [12] [14] [40]. In this work, the high EC, however, affected negatively the nitrogen fixation activity, as it was ten-times lower in Tex-High compared to the Tex-Low soil ( Table 1).…”

“…pH is also an important factor that affects nitrogen fixation [12] [43] reported that in soda solonchak soils maximum nitrogen fixation activity was obtained at pH 9.8, but at pH 7.3 and 10.5 it decreased 80%. Microorganisms involved in key processes of important biochemical cycles are probably well adapted to extreme conditions.…”

“…Activity of nitrogen fixating bacteria, however, is often inhibited by large concentrations of salts [10]- [13]. Nevertheless, Sorokin et al [12] demonstrated that diazotrophs might still be active in soda soils with moderate salinity and alkalinity. Isolation of pure cultures of haloalkaliphilic diazotrophs from several locations in Central Asia and Egypt through micro-oxic enrichments of soils yielded the aerotolerant fermentative haloalkaliphilic bacterium Amphibacillus tropicus and the obligately anaerobic haloalkaliphile Bacillus arseniciselenatis.…”

The Resurrection Plant Tripogon Spicatus (Poaceae) Harbors a Diversity of Plant Growth Promoting Bacteria in Northeastern Brazilian Caatinga

“…Halotolerance under diazotrophic conditions of Enterobacteriaceae and Firmicutes is quite variable and can range from total inhibition, through the addition of low concentrations of salt in the culture medium, to no effect (Bilal et al, 1990;Sorokin et al, 2008;Jha et al, 2012). High tolerance to salts in vitro can indicate promising bacteria for plant inoculation in saline field environments.…”

Natronobacillus azotifigens gen. nov., sp. nov., an anaerobic diazotrophic haloalkaliphile from soda-rich habitats

“…The presence of two groups of diazotrophic Gram-positive bacteria in haloalkaliphilic enrichment cultures was identified (Sorokin et al 2008a). In this paper, a novel branch of obligately fermentative diazotrophic Gram-positive bacteria is described.…”