Ahuva Vonshak

Two causes for runoff initiation on microbiotic crusts: Hydrophobicity and pore clogging

Hydrophobicity and pore clogging are suggested as two mechanisms responsible for generating runoff over microbiotic crusts overlying dune sand. Although natural microbiotic crusts in the Hallamish dune field (Negev Desert, Israel) did not show any hydrophobicity, that was not the case with natural crusts subjected to long periods of continuous wetness in the lab. Monoalgal crusts, grown in the lab, also showed high hydrophobicity when dry. The hydrophobicity vanished, however, once the surface was wetted. Runoff on monoalgal lab-grown crusts was obtained when (i) the dry crust exhibited hydrophobic properties and (ii) the wetted crust no longer showed water repellence. Although runoff generation caused by hydrophobicity is expected, it is suggested that runoff initiation when the crusts do not exhibit any hydrophobicity stems from the high water absorption and swelling of the exopolysaccharide cyanobacterial sheaths causing pore clogging. The multi-layered structure of the crust and filament migration to the surface may enhance pore clogging. The experiments and their interpretation are supported by reinterpretation of published data.

Anaerobic metabolism of Nitrosomonas europaea

Nitrosomonas europaea is capable of maintaining an anaerobic metabolism, using pyruvate as an electron donor and nitrite as an electron acceptor; utilization of nitrite depends upon supply of both pyruvate and ammonia. The role of ammonia in this reaction was not determined. N europaea also assimilates CO2 anaerobically into cell material in the presence of nitrite (0.5–1.0 mM), pyruvate and ammonia. This reaction was partially inhibited by nitrite which apparently competed with CO2 for reducing power. Anaerobic “nitrite respiration” is sensitive to ionophores, FCCP being the most effective.

Assessment of pathogenic bacteria in treated graywater and irrigated soils

Reuse of graywater (GW) for irrigation is recognized as a sustainable solution for water conservation. One major impediment for reuse of GW is the possible presence of pathogenic microorganisms. The presence and abundance of six pathogens and indicators were investigated in three GW recirculating vertical flow constructed wetland treatment systems and their respective irrigated yard soils. The treated GW and soils were monitored once every two months for six months using real-time quantitative PCR. As a control, samples from four soils irrigated with fresh water (FW) were similarly analyzed for pathogens and indicators. Comparable types of pathogens and fecal indicator bacteria, including Escherichia coli, Klebsiella pneumoniae, Salmonella enterica, Pseudomonas aeruginosa, Enterococcus faecalis, and Shigella spp., were found in the treated GW, their corresponding irrigated soils and the FW-irrigated soils. Moreover, the abundance of these bacteria in the GW- and FW-irrigated soils was of the same order of magnitude, suggesting that the source of the pathogens cannot be established. Our results suggest that GW irrigation has no effect on the diversity and abundance of the tested pathogens and indicators in yard soils.

Factors inhibiting nitrification of ammonia in deep wastewater reservoirs

Abstract Nitrification of ammonia in wastewater reservoirs is very slow, in spite of the ubiquitous presence of nitrifying bacteria. In an attempt to identify the reasons, the effect of several factors on the rate of nitrification and viability of N. europaea was studied. Light, NH 2 OH and NH 2 NH 2 were found to be inhibitory to exponentially growing cells of Nitrosomonas europaea , while ammonia provided limited protection. Stationary phase cells were unaffected by light. Water samples from two wastewater reservoirs had a variable effect upon viability and nitrification of laboratory cultures of N. europaea upon illumination, while long-term incubation of N. europaea cells placed in semi-permeable containers at different depths in the reservoirs had generally no effect on their viability.

Perennials but not slope aspect affect the diversity of soil bacterial communities in the northern Negev Desert, Israel

Underneath the canopy of perennials in arid regions, moderate soil temperature and evaporation, as well as plant litter create islands of higher fertility in the low-productivity landscape, known as ‘resource islands’. The sparse distribution of these resource islands is mirrored by soil microbial communities, which mediate a large number of biogeochemical transformations underneath the plants. We explored the link between the bacterial community composition and two prevalent desert shrubs, Zygophyllum dumosum and Artemisia herba-alba, on northern- and southern-facing slopes in the northern highlands of the Negev Desert (Israel), at the end of a drought winter mild rainy season. We sequenced the bacterial community and analysed the physicochemical properties of the soil under the shrub canopies and from barren soil in replicate slopes. The soil bacterial diversity was independent of slope aspect, but differed according to shrub presence or type. Links between soil bacterial community composition and their associated desert shrubs were found, enabling us to link bacterial diversity with shrub type or barren soils. Our results suggest that plants and their associated bacterial communities are connected to survival and persistence under the harsh desert conditions.