For centuries, biodiversity has spellbound biologists focusing mainly on macroorganism's diversity and almost neglecting the geographic mediated dynamics of microbial communities. We surveyed the diversity of soil bacteria and archaea along a steep precipitation gradient ranging from the Negev Desert in the south of Israel (o100 mm annual rain) to the Mediterranean forests in the north (4900 mm annual rain). Soil samples were retrieved from triplicate plots at five long-term ecological research stations, collected from two types of patches: plant interspaces and underneath the predominant perennial at each site. The molecular fingerprint of each soil sample was taken using terminal restriction length polymorphism of the 16S rRNA gene to evaluate the bacterial and archaeal community composition and diversity within and across sites. The difference in community compositions was not statistically significant within sites (P ¼ 0.33 and 0.77 for bacteria and archaea, respectively), but it differed profoundly by ecosystem type. These differences could largely be explained by the precipitation gradient combined with the vegetation cover: the archaeal and bacterial operational taxonomic units were unique to each climatic region, that is, arid, semiarid and Mediterranean (P ¼ 0.0001, for both domains), as well as patch type (P ¼ 0.009 and 0.02 for bacteria and archaea, respectively). Our results suggest that unlike macroorganisms that are more diverse in the Mediterranean ecosystems compared with the desert sites, archaeal and bacterial diversities are not constrained by precipitation. However, the community composition is unique to the climate and vegetation cover that delineates each ecosystem.

The exploration of spatial patterns of abundance and diversity patterns along precipitation gradients has focused for centuries on plants and animals; microbial profiles along such gradients are largely unknown. We studied the effects of soil pH, nutrient concentration, salinity, and water content on bacterial abundance and diversity in soils collected from Mediterranean, semi-arid, and arid sites receiving approximately 400, 300, and 100 mm annual precipitation, respectively. Bacterial diversity was evaluated by terminal restriction fragment length polymorphism and clone library analyses and the patterns obtained varied with the climatic regions. Over 75% of the sequenced clones were unique to their environment, while ∼2% were shared by all sites, yet, the Mediterranean and semi-arid sites had more common clones (∼9%) than either had with the arid site (4.7% and 6%, respectively). The microbial abundance, estimated by phospholipid fatty acids and real-time quantitative PCR assays, was significantly lower in the arid region. Our results indicate that although soil bacterial abundance decreases with precipitation, bacterial diversity is independent of precipitation gradient. Furthermore, community composition was found to be unique to each ecosystem.

Abstract. Nitrate concentrations in groundwater have increased in many areas of the world. This causes serious concerns because of the link found between nitrate and the blue-baby syndrome, and of the possible formation of carcinogenic compounds in the digestive tract. Biological denitrification, bacteria-mediated reduction of nitrate to nitrogen gas, is a method used in the treatment of nitrate contaminated groundwater. The denitrifying microorganisms require carbon and energy substrates which may be organic or inorganic compounds. Treatment can take place in the aquifer (in situ treatment) or in above ground reactors. Numerous biological denitrification processes have been reported; this paper reviews some of this work and studies in progress in the author's laboratory. The choice of a biological denitrification system has to be considered on an individual basis. Although preventive measures are curbing the problem in some developed countries, nitrate pollution is still on the rise in many other countries. Innovative, low-cost biological denitrification processes are specially needed in developing countries.

Bacteria and nutrients were determined in upper soil samples collected underneath and between canopies of the dominant perennial in each of three sites along a steep precipitation gradient ranging from the Negev desert in the south of Israel to a Mediterranean forest in the north. Bacterial abundance, monitored by phospholipid fatty acid analysis, was significantly higher under the shrub canopy (compared to barren soils) in the arid and semi-arid sites but not in the Mediterranean soils. Bacterial community composition, determined using terminal restriction fragment length polymorphism and clone libraries, differed according to the sample's origin. Closer examination revealed that in the arid and semi-arid sites, α-Proteobacteria are more abundant under the shrub canopy, while barren soils are characterized by a higher abundance of Actinobacteria. The bacterial communities in the Mediterranean soils were similar in both patch types. These results correspond to the hypothesis of "resource islands", suggesting that shrub canopies provide a resource haven in low-resource landscapes. Yet, a survey of the physicochemical parameters of inter- and under-shrub soils could not attribute the changes in bacterial diversity to soil moisture, organic matter, or essential macronutrients. We suggest that in the nutrient-poor soils of the arid and semi-arid sites, bacteria occupying the soil under the shrub canopy may have longer growth periods under favorable conditions, resulting in their increased biomass and altered community composition.

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