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Although climate change is expected to increase the extent of drylands worldwide, the effect of drought on the soil microbiome is still insufficiently understood as for dominant but little characterized phyla like the Acidobacteria. In the present study the active acidobacterial communities of Namibian soils differing in type, physicochemical parameters, and land use were characterized by high-throughput sequencing. Water content, pH, major ions and nutrients were distinct for sandy soils, woodlands or dry agriculture on loamy sands. Soils were repeatedly sampled over a 2-year time period and covered consecutively a strong rainy, a dry, a normal rainy and a weak rainy season. The increasing drought had differential effects on different soils. Linear modeling of the soil water content across all sampling locations and sampling dates revealed that the accumulated precipitation of the preceding season had only a weak, but statistically significant effect, whereas woodland and irrigation exerted a strong positive effect on water content. The decrease in soil water content was accompanied by a pronounced decrease in the fraction of active Acidobacteria (7.9–0.7%) while overall bacterial community size/cell counts remained constant. Notably, the strongest decline in the relative fraction of Acidobacteria was observed after the first cycle of rainy and dry season, rather than after the weakest rainy season at the end of the observation period. Over the 2-year period, also the β-diversity of soil Acidobacteria changed. During the first year this change in composition was related to soil type (loamy sand) and land use (woodland) as explanatory variables. A total of 188 different acidobacterial sequence variants affiliated with the “Acidobacteriia,” Blastocatellia, and Vicinamibacteria changed significantly in abundance, suggesting either drought sensitivity or formation of dormant cell forms. Comparative physiological testing of 15 Namibian isolates revealed species-specific and differential responses in viability during long-term continuous desiccation or drying-rewetting cycles. These different responses were not determined by phylogenetic affiliation and provide a first explanation for the effect of drought on soil Acidobacteria. In conclusion, the response of acidobacterial communities to water availability is non-linear, most likely caused by the different physiological adaptations of the different taxa present.

Acidobacteria occur in a large variety of ecosystems worldwide and are particularly abundant and highly diverse in soils. In spite of their diversity, only few species have been characterized to date which makes

Members of the phylum Acidobacteria are abundant in a wide variety of soil environments. Despite this, previous cultivation attempts have frequently failed to retrieve representative phylotypes of Acidobacteria, which have, therefore, been discovered by culture-independent methods (13175 acidobacterial sequences in the SILVA database version 123;NR99) and only 47 species have been described so far. Strain Ac_5_C6(T) represents the first isolate of the globally widespread and abundant subdivision 6 Acidobacteria and is described in the present study. Cells of strain Ac_5_C6(T) were Gram-stain-negative, immotile rods that divided by binary fission. They formed yellow, extremely cohesive colonies and stable aggregates even in rapidly shaken liquid cultures. Ac_5_C6(T) was tolerant of a wide range of temperatures (12-40 degrees C) and pH values (4.7-9.0). It grew chemoorganoheterotrophically on a broad range of substrates including different sugars, organic acids, nucleic acids and complex proteinaceous compounds. The major fatty acids of Ac_5_C6(T) were iso-C-17:1 omega 9c, C-18:1 omega 7c and iso-C-15:0. Summed feature 3 (C-16:1 omega 7c/C-16:1 omega 6c), iso-C-17:0 and C-16:0 were also detected. Phosphatidylcholine, phosphatidylglycerol, diphosphatidylglycerol, phosphatidylethanolamine and an unidentified glycolipid were identified as polar lipids. The major quinone was MK-8. The DNA G+C content of Ac_5_C6(T) was 65.9 mol%. With 16S rRNA gene sequence similarities of 83-84 %, the closest described relatives were Acidicapsa borealis KA1(T), Acidobacterium capsulatum 161(T), Granulicella pectinovorans TPB6011(T), Occallatibacter riparius 277(T) and Paludibaculum fermentans P105(T). According to the morphological, physiological and molecular characteristics, the novel genus Vicinamibacter gen. nov., and the novel species, Vicinamibacter silvestris sp. nov. (type strain Ac_5_C6(T) = DSM 29464(T) = LMG 29035(T)) are proposed.