A significant proportion of the Earth's surface is desert or in the process of desertification. The extreme environmental conditions that characterize these areas result in a surface that is essentially barren, with a limited range of higher plants and animals. Microbial communities are probably the dominant drivers of these systems, mediating key ecosystem processes. In this review, we examine the microbial communities of hot desert terrestrial biotopes (including soils, cryptic and refuge niches and plant-root-associated microbes) and the processes that govern their assembly. We also assess the possible effects of global climate change on hot desert microbial communities and the resulting feedback mechanisms. We conclude by discussing current gaps in our understanding of the microbiology of hot deserts and suggest fruitful avenues for future research.

The taxonomic status of the species Rhizobium leguminosarum, Rhizobium trifolii and Rhizobium phaseoli was analysed in this study on the basis of their molecular and phenotypic characteristics. According to the results, the type strain R. phaseoli ATCC 14482T does not belong to any of the already described species of the genus Rhizobium and it should therefore be considered as a different species. In contrast, the strains of R. trifolii examined belonged to R. leguminosarum and thus R. trifolii is a later synonym of R. leguminosarum. The results of the analysis of 16S–23S intergenic spacer region and rrs, recA and atpD gene sequences as well as those of DNA–DNA hybridization experiments and phenotypic characterizations showed that the type strains R. leguminosarum USDA 2370T and R. leguminosarum DSM 30132 do not belong to the same species. Taking into account that strain USDA 2370T corresponds to the original strain of this species, 3Hoq18T, this strain should be considered as the true type strain of R. leguminosarum whereas strain DSM 30132 should be reclassified as a different species, for which the name Rhizobium pisi sp. nov. is proposed (type strain, DSM 30132T=NCIMB 11478T).

Agronomy for Sustainable Development, An International Journal in Agriculture and Environment

Bacterial strain PETP02^Twas isolated from nodules ofTrifolium pratensegrowing in a Spanish soil. Phylogenetic analysis of the 16S rRNA gene sequence showed that this strain represents a member of the genusPhyllobacterium. However, divergence found with the 16S rRNA gene sequence of the single recognized species of this genus,Phyllobacterium myrsinacearum, indicated that strain PETP02^Tbelongs to a different species. The results of DNA–DNA hybridization, phenotypic tests and fatty acid analyses confirmed that this strain represents a novel species of the genusPhyllobacterium, for which the namePhyllobacterium trifoliisp. nov. is proposed. The type strain is PETP02^T(=LMG 22712^T=CECT 7015^T). This strain was strictly aerobic and used several carbohydrates as carbon source. It was not able to reduce nitrate. Aesculin hydrolysis was negative. It did not produce urease, arginine dihydrolase, gelatinase orβ-galactosidase. The DNA G+C content was 56·4 mol%. ThenodDgene of this strain showed a sequence closely related to those of strains able to nodulateLupinus. Infectivity tests showed that this strain is able to produce nodules in bothTrifolium repensandLupinus albus.

The species Phaseolus vulgaris is a promiscuous legume nodulated by several species of the family Rhizobiaceae. During a study of rhizobia nodulating this legume in Portugal, we isolated several strains that nodulate P. vulgaris effectively and also Macroptilium atropurpureum and Leucaena leucocephala, but they form ineffective nodules in Medicago sativa. According to phylogenetic analysis of the 16S rRNA gene sequence, the strains from this study belong to the genus Rhizobium, with Rhizobium rhizogenes and Rhizobium tropici as the closest related species, with 99.9 and 99.2 % similarity, respectively, between the type strains of these species and strain P1-7^T. The nodD and nifH genes carried by strain P1-7^T are phylogenetically related to those of other species nodulating Phaseolus. This strain does not carry virulence genes present in the type strain of R. rhizogenes, ATCC 11325^T. Analysis of the recA and atpD genes confirms this phylogenetic arrangement, showing low similarity with respect to those of R. rhizogenes ATCC 11325^T (91.9 and 94.1 % similarity, respectively) and R. tropici IIB CIAT 899^T (90.6 % and 91.8 % similarity, respectively). The intergenic spacer (ITS) of the strains from this study is phylogenetically divergent from those of R. rhizogenes ATCC 11235^T and R. tropici CIAT 899^T, with 85.9 and 82.8 % similarity, respectively, with respect to strain P1-7^T. The tRNA profile and two-primer random amplified polymorphic DNA pattern of strain P1-7^T are also different from those of R. rhizogenes ATCC 11235^T and R. tropici CIAT 899^T. The strains isolated in this study can be also differentiated from R. rhizogenes and R. tropici by several phenotypic characteristics. The results of DNA–DNA hybridization showed means of 28 and 25 % similarity between strain P1-7^T and R. rhizogenes ATCC 11235^T and R. tropici CIAT 899^T, respectively. All these data showed that the strains isolated in this study belong to a novel species of the genus Rhizobium, for which we propose the name Rhizobium lusitanum sp. nov.; the type strain is P1-7^T (=LMG 22705^T=CECT 7016^T).