Biological desert crusts are relatively common in the arid deserts of the Sultanate of Oman; however, little is known about their microbial community composition and role in soil fertilization. We compared three crusts from geographically different locations for their soil texture, bacterial community structure, pigment composition and nitrogenase activity. The crusts were growing on alkaline (pH 7.6-8.7) loamy sand and silty loam soils. Microscopically, Microcoleus vaginatus was the most abundant cyanobacterium, but Nostoc and Scytonema types dominated in cultures. The 16S rRNA gene sequences showed close similarities in the crusts' bacterial composition, with 77-81% of the total clones belonging to cyanobacteria and the rest distributed among Alpha- and Deltaproteobacteria, Bacteriodetes, Gemmatimonas and Planctomycetes. Thirty-seven percent of the cyanobacterial clones were affiliated with heterocystous types such as Nostoc, Scytonema, Brasilonema and Petalonema. Chlorophyll a concentrations suggest a similar abundance of phototrophs in all crusts. High levels of the UVA sunscreen scytonemin were detected in the exposed crusts. The three crusts exhibited comparable acetylene reduction rates in the light and in the dark, with a maximum rate of 58.5+/-2.6 micromol C(2)H(2) reduced m(-2) h(-1). We conclude that the crusts, regardless of their geographical location, were rich in heterocystous cyanobacteria that can fix nitrogen and could possibly improve soil stability and productivity.

In an intertidal flat of the German Wadden Sea, a large sedimentary pool of intracellular nitrate was discovered that by far exceeded the pool of nitrate that was freely dissolved in the porewater. Intracellular nitrate was even present deep in anoxic sediment layers where it might be used for anaerobic respiration processes. The origin and some of the ecological controls of this intracellular nitrate pool were investigated in a laboratory experiment. Sediment microcosms were set up with and without the abundant polychaete Hediste diversicolor that is known to stimulate nitrate production by microbial nitrification in the sediment. Additional treatments were amended with ammonium to mimic ammonium excretion by the worms or with allylthiourea (ATU) to inhibit nitrification by sediment bacteria. H. diversicolor and ammonium increased, while ATU decreased the intracellular nitrate pool in the sediment. Microsensor profiles of porewater nitrate showed that bacterial nitrification was enhanced by worms and ammonium addition. Thus, nitrification formed an important nitrate supply for the intracellular nitrate pool in the sediment. The vertical distribution of intracellular nitrate matched that of the photopigments chlorophyll a and fucoxanthin, strongly suggesting that diatoms were the main nitrate-storing organisms. Intracellular nitrate formation is thus stimulated by the interaction of phylogenetically distant groups of organisms: worms enhance nitrification by feeding on particulate organic matter, excreting ammonium and oxygenating the sediment; bacteria oxidise ammonium to nitrate in oxic sediment layers; and diatoms store nitrate intracellularly. KEY WORDS: Intertidal sediment · Wadden Sea · Nitrogen cycle · Intracellular nitrate · Microphytobenthos · Macrofauna · Nereis diversicolor · Trophic interaction Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 445: [181][182][183][184][185][186][187][188][189][190][191][192] 2012 and probably also intracellular nitrate for nitrogen assimilation (Lomas & Glibert 2000, Sundbäck & Miles 2000.The ability to store nitrate intracellularly may be particularly advantageous in intertidal flats, which are very dynamic ecosystems. Benthic organisms must cope with frequent changes in the availability of e.g. light, oxygen and nutrients due to tidal and diurnal rhythms. Another source of perturbation in intertidal flats is the presence of macrofauna that rework large amounts of sediment and the microorganisms therein (e.g. Bouchet et al. 2009). Some polychaetes construct deep-reaching burrows and enhance solute exchange between sediment and the water column due to their ventilation activity (Kristensen 2001). Many species of intertidal macrofauna feed on sediment microorganisms and thereby decrease microbial populations or keep them in the exponential growth phase (Herman et al. 2000, Blanchard et al. 2001). Under such transient conditions, the nitrate storage capacity awards sediment microorganisms with the steady availa...

The cyanobacterial layer (1-3 mm) of microbial mats of Wadi Muqshin, Sultanate of Oman, in hypersaline continental pools near the Empty Quarter desert were analyzed for cyanobacterial diversity, pigments composition and antibacterial and quorum sensing inhibition activities at the microbial community level. The salinity of the ponds reached up to 75 ‰, the temperature was around 49 °C and the salt composition was similar to seawater. Cyanobacteria in pools containing coccoid and filamentous forms similar to those normally found in salt ponds and in intertidal flats were found. Among the observed taxa were Microcoleus chthonoplastes, Spirulina subsalsa, Johannesbaptistia pellucida, Chroococcidiopsis sp., Aphanocapsa sp., Chroococcus sp., Gloeocapsa sp., Schizothrix sp. and Leptolyngbya sp. Using high performance liquid chromatography (HPLC), the pigments chlorophyll a, scytonemin, chlorophyllid a, β-carotene and fucoxanthin were detected, with the highest concentrations obtained for the ultraviolet (UV) sunscreen pigment scytonemin. The chemical extracts of the microorganisms in the mats cyanobacterial layer exhibited antibacterial activity against 3 out of 9 tested pathogenic bacterial strains and showed quorum sensing inhibition activity against the reporters Agrobacterium tumefaciens NTL4 (pZLR4) and Salmonella enterica S235 but not Chromobacterium violaceum CV017. We conclude that the studied inland hypersaline microbial mats harbor similar diversity to their counterparts in intertidal flats and evaporation ponds and could produce chemical compounds that are of biotechnological significance.

Flood-induced disturbance causes major shifts in the diversity and function of microbial communities in desert streams. We compared bacterial community structure (using automated rRNA intergenic spacer analysis, ARISA), pigment composition and nitrogen fixation rates in benthic microbial mats from stone surfaces collected from 6 sites along a desert stream in the Sultanate of Oman before and 2 wk after an intense flood. Flooding caused partial flushing out of old mats, and new mats re-established on the surfaces of rocks. Visually, the filamentous green algae Spirogyra spp. increased in abundance in the water column at most sites after the flood, and direct microscopy revealed that the newly developed mats were dominated by the heterocystous Calothrix spp. Multivariate analyses of ARISA profiles revealed that, in each mat, the structure of the bacterial community was different after the flood (analysis of similarities [ANOSIM], R = 0.49, p < 0.001), and pairwise comparison of the presence/absence of operational taxonomic units (OTUs) showed that only between 18 and 26% shared OTUs. The OTU richness in the newly developed microbial communities decreased in 6 mats after the flood, but remained similar or slightly increased in the other 6 mats. Out of the 9 detected pigments, chlorophyll a, scytonemin, fucoxanthin, diadinoxanthin and β-carotene showed site-specific changes with flooding. The average acetylene reduction rates increased after the flood at all sites; however, this increase was statistically significant at only 2 sites due to the high variance among samples. In conclusion, flooding resulted in the replacement of over 74% of the microbial communities within mats, while nitrogen fixation rates remained stable or increased.KEY WORDS: Microbial mats · Flooding · ARISA · Acetylene reduction rate · Pigments · Nitrogen fixation Resale or republication not permitted without written consent of the publisher

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