Diversity of Bacteria Cultured from Arid Soils and Sedimentary Rocks under Conditions of Available Water Deficiency

Abstract

The diversity of bacteria cultured from the soil of the Negev desert (Israel, sample SN2) and the sedimentary rock of the Sahara Desert (Tunisia, sample Alg) has been studied. To assess the ability of bacteria to metabolize at different levels of moisture availability and to reveal bacterial diversity more fully, culturing was carried out on R2A medium with the addition of glycerol to establish a certain level of water activity (Aw) in range from 1.0 to 0.9 (with a step of 0.01 Aw). After incubation, unique morphotypes of cultured bacteria were isolated, described, identified by 16S rRNA sequencing, and tested for the ability to grow in the Aw gradient in pure cultures. After incubation and isolation, 355 strains were identified and tested. Culturable bacteria were found at Aw 0.95 and higher. With a decrease in Aw from 1 to 0.95, the number of cultured bacteria decreased from 105 and 107 CFU/g in samples SN2 and Alg, respectively, to 2 × 104 CFU/g in both studied samples. As a result of culturing, representatives of 34 genera of bacteria were isolated, mainly representatives of the phylum Actinobacteria; representatives of the genera Arthrobacter, Kocuria, and Pseudarthrobacter dominated. At this, 38 strains with low similarity of nucleotide sequences with databases and, probably, being representatives of previously undescribed species of the genera Agrococcus, Arthrobacter, Bacillus, Brachybacterium, Cellulomonas, Conyzicola, Kocuria, Microbacterium, Okibacterium, Rathayibacter, and Sphingomonas were revealed. Testing the strains for their ability to grow in pure culture in a gradient of Aw values revealed 18 strains of the genera Arthrobacter, Kocuria, Brachybacterium, Serratia, and Leucobacter capable of growing at Aw 0.91. The study confirms the data that desert soils and rocks are a depository of previously undescribed bacterial species and can also be a valuable source of biotechnologically promising strains.