Desert Crusts Research Articles

Controlling photosynthetic energy conversion by small conformational changes.

Control phenomena in biology usually refer to changes in gene expression and protein translation and modification. In this paper, another mode of regulation is highlighted; we propose that photosynthetic organisms can harness the interplay between localization and delocalization of energy transfer by utilizing small conformational changes in the structure of light-harvesting complexes. We examine the mechanism of energy transfer in photosynthetic pigment-protein complexes, first through the scope of theoretical work and then by in vitro studies of these complexes. Next, the biological relevance to evolutionary fitness of this localization-delocalization switch is explored by in vivo experiments on desert crust and marine cyanobacteria, which are both exposed to rapidly changing environmental conditions. These examples demonstrate the flexibility and low energy cost of this mechanism, making it a competitive survival strategy.

Keep your friends close and your competitors closer: novel interspecies interaction in desert biological sand crusts

ABSTRACT The green alga Chlorella sp. and the filamentous cyanobacterium Leptolyngbya sp., regularly cope with various stressors including frequent hydration–desiccation cycles. When grown in axenic cultures, Chlorella sp. is unable to resurrect even after very slow desiccation unless desiccation takes place in the presence of Leptolyngbya sp. or a lysate thereof. When Chlorella sp. was provided with extracellular polysaccharides from Leptolyngbya sp., or its main sugar constituents, it was able to be resurrected after mild desiccation. These findings suggest that desiccation tolerance of Chlorella sp. in the field depends on the presence of Leptolyngbya sp. or components thereof. Chlorella sp. was originally isolated as a contaminant in a decaying stationary culture of Leptolyngbya sp. that was established by the addition of a cyanobacterial growth media to biological soil crusts. Co-incubation with a dialysis tube containing Leptolyngbya sp. severely inhibited growth of Chlorella sp. These data suggest that secondary metabolites excreted by Leptolyngbya sp., the nature of which is yet unknown, inhibit growth of Chlorella sp. Thus, though Leptolyngbya sp. enables the survival of Chlorella sp. in a desert habitat, it simultaneously inhibits growth of this potential competitor. The combination of positive and negative effects of Leptolyngbya sp. on fitness of Chlorella sp. creates a scenario in which the former tightly controls the abundance of the latter.

Are the phosphate oxygen isotopes of Saharan dust a robust tracer of atmospheric P source?

Abstract Aeolian dust is a significant source of phosphorus (P) in various ecosystems. The Saharan desert is the major dust source on earth and it contributes atmospheric P to terrestrial ecosystems which often depend on these inputs. Since estimations on the contribution of dust to the atmospheric P cycle vary between studies, accurate estimations on the aerosol-P inputs to ecosystems can be enhanced by an identification method for dust from the other atmospheric P sources. In the current study, we explored the use of the oxygen stable isotopes in phosphate, δ18OP, as a marker for desert dust P. To characterize the δ18OP in Saharan dust, we analyzed atmospheric particles sampled in Puerto Rico, which has good exposure in summer to African dust with little exposure to wildfires, local, or anthropogenic influence on atmospheric inputs. Additionally, to evaluate possible isotopic fractionations in desert dust sources, which could affect the soil δ18OP before transport, we analyzed the δ18OP in Israeli desert soils that served as an analog. Saharan dust storms sampled at Puerto Rico were found to have δ18OP values of 22.5 ± 0.3‰ in the resin fraction (which represents labile inorganic P), similar to previously reported values of Saharan dust sampled in Israel. The data from Israel desert soils demonstrated that biological activity in desert crusts can have a small effect (with a maximum difference of 3.3‰ between the topsoil and the subsoil) on the δ18OP of desert soils, mainly when developed soil crust is present, as a result of intracellular isotopic equilibration between oxygen in phosphate and soil water. However, biological crusts are not considered a major dust source. Additionally, Saharan dust sampled in Israel and in Puerto Rico had a δ18OP similar to apatite minerals derived from sedimentary phosphorites. Thus, we suggest the dust source had little or no biological activity. Furthermore, the δ18OP in Saharan dust was distinctive from other atmospheric P sources, such as pollen and wildfire ash, and as a result, the δ18OP can be used to distinguish between these atmospheric P main sources.

Growth, Viability, and Death of Planktonic and Biofilm Sphingomonas desiccabilis in Simulated Martian Brines.

Aqueous solutions on Mars are theorized to contain very different ion compositions than those on Earth. To determine the effect of such solutions on typical environmental micro-organisms, which could be released from robotic spacecraft or human exploration activity, we investigated the resistance of Sphingomonas desiccabilis to brines that simulate the composition of martian aqueous environments. S. desiccabilis is a desiccation-resistant, biofilm-forming microbe found in desert crusts. The viability of cells in both planktonic and biofilm forms was measured after exposure to simulated martian brines. Planktonic cells showed a loss of viability over the course of several hours in almost all of the seven brines tested. Biofilms conferred greater resistance to all the brines, including those with low water activity and pH, but even cells in biofilms showed a complete loss of viability in <6 h in the harsher brines and in <2 days in the less harsh brines. One brine, however, allowed the microbes to maintain viability over several days, despite having a water activity and pH lower and ionic strength higher than brines that reduced viability over the same timescales, suggesting important ion-specific effects. These data show that biofilm-forming cells have a greater capacity to resist martian aqueous extremes, but that evaporative or deliquescent brines are likely to be destructive to many organisms over relatively short timescales, with implications for the habitability of Mars and for micro-organisms dispersed by robotic or human explorers.

Changes in aggregation states of light-harvesting complexes as a mechanism for modulating energy transfer in desert crust cyanobacteria

In this paper we propose an energy dissipation mechanism that is completely reliant on changes in the aggregation state of the phycobilisome light-harvesting antenna components. All photosynthetic organisms regulate the efficiency of excitation energy transfer (EET) to fit light energy supply to biochemical demands. Not many do this to the extent required of desert crust cyanobacteria. Following predawn dew deposition, they harvest light energy with maximum efficiency until desiccating in the early morning hours. In the desiccated state, absorbed energy is completely quenched. Time and spectrally resolved fluorescence emission measurements of the desiccated desert crust Leptolyngbya ohadii strain identified (i) reduced EET between phycobilisome components, (ii) shorter fluorescence lifetimes, and (iii) red shift in the emission spectra, compared with the hydrated state. These changes coincide with a loss of the ordered phycobilisome structure, evident from small-angle neutron and X-ray scattering and cryo-transmission electron microscopy data. Based on these observations we propose a model where in the hydrated state the organized rod structure of the phycobilisome supports directional EET to reaction centers with minimal losses due to thermal dissipation. In the desiccated state this structure is lost, giving way to more random aggregates. The resulting EET path will exhibit increased coupling to the environment and enhanced quenching.

Cell damage caused by ultraviolet B radiation in the desert cyanobacterium Phormidium tenue and its recovery process

Phormidium tenue, a cyanobacterium that grows in the topsoil of biological soil crusts (BSCs), has the highest recovery rate among desert crust cyanobacteria after exposure to ultraviolet B (UV-B) radiation. However, the mechanism underlying its recovery process is unclear. To address this issue, we measured chlorophyll a fluorescence, generation of reactive oxygen species (ROS), lipid peroxidation, and repair of DNA breakage in P. tenue following exposure to UV-B. We found that UV-B radiation at all doses tested reduced photosynthesis and induced cell damage in P. tenue. However, P. tenue responded to UV-B radiation by rapidly reducing photosynthetic activity, which protects the cell by leaking less ROS. Antioxidant enzymes, DNA damage repair systems, and UV absorbing pigments were then induced to mitigate the damage caused by UV-B radiation. The addition of exogenous antioxidant chemicals ascorbate and N-acetylcysteine also mitigated the harmful effects caused by UV-B radiation and enhanced the recovery process. These chemicals could aid in the resistance of P. tenue to the exposure of intense UV-B radiation in desertified areas when inoculated onto the sand surface to form artificial algal crusts.

Cyanobacterial composition and spatial distribution based on pyrosequencing data in the Gurbantunggut Desert, Northwestern China.

Cyanobacteria are the primary colonizers and form a dominant component of soil photosynthetic communities in biological soil crusts. They are crucial in improving soil environments, namely accumulating soil carbon and nitrogen. Many classical studies have examined cyanobacterial diversity in desert crusts, but relatively few comprehensive molecular surveys have been conducted. We used 454 pyrosequencing of 16S rRNA to investigate cyanobacterial composition and distribution on regional scales in the Gurbantunggut Desert. The relationship between cyanobacterial distribution and environmental factors was also explored. A total of 24,973 cyanobacteria partial 16S rRNA gene sequences were obtained, and 507OTUs were selected, as most OTUs had very few reads. Among these, 347 OTU sequences were of cyanobacteria origin, belonging to Oscillatoriales, Nostocales, Chroococcales, and uncultured cyanobacterium clone, respectively. Microcoleus vaginatus, Chroococcidiopsis spp. and M. steenstrupii were the dominant species in most areas of the Gurbantunggut Desert. Compared with other desert, the Gurbantunggut Desert differed in the prominence of Chroococcidiopsis spp. and lack of Pseudanabaenales. Species composition and abundance of cyanobacteria also showed distinct variations. Soil texture, precipitation, and nutrients and salt levels affected cyanobacterial distribution. Increased precipitation was helpful in improving cyanobacterial diversity. A higher content of coarse sand promoted the colonization and growth of Oscillatoriales and some phylotypes of Chroococcales. The fine-textured soil with higher nutrients and salts supported more varied populations of cyanobacteria, namely some heterocystous cyanobacteria. The results suggested that the Gurbantunggut Desert was rich in cyanobacteria and that precipitation was a primary regulating factor for cyanobacterial composition on a regional scale.

An easily reversible structural change underlies mechanisms enabling desert crust cyanobacteria to survive desiccation

Biological desert sand crusts are the foundation of desert ecosystems, stabilizing the sands and allowing colonization by higher order organisms. The first colonizers of the desert sands are cyanobacteria. Facing the harsh conditions of the desert, these organisms must withstand frequent desiccation–hydration cycles, combined with high light intensities. Here, we characterize structural and functional modifications to the photosynthetic apparatus that enable a cyanobacterium, Leptolyngbya sp., to thrive under these conditions.Using multiple in vivo spectroscopic and imaging techniques, we identified two complementary mechanisms for dissipating absorbed energy in the desiccated state. The first mechanism involves the reorganization of the phycobilisome antenna system, increasing excitonic coupling between antenna components. This provides better energy dissipation in the antenna rather than directed exciton transfer to the reaction center. The second mechanism is driven by constriction of the thylakoid lumen which limits diffusion of plastocyanin to P700. The accumulation of P700+ not only prevents light-induced charge separation but also efficiently quenches excitation energy.These protection mechanisms employ existing components of the photosynthetic apparatus, forming two distinct functional modes. Small changes in the structure of the thylakoid membranes are sufficient for quenching of all absorbed energy in the desiccated state, protecting the photosynthetic apparatus from photoinhibitory damage. These changes can be easily reversed upon rehydration, returning the system to its high photosynthetic quantum efficiency.

The role of symbiosis in the transition of some eukaryotes from aquatic to terrestrial environments

Symbiosis may have played a far greater role in biological evolution than was previously thought. The symbiosis that made the colonization of land by plants possible was as a consequence of the development of arbuscular mycorrhizae. However, the present review draws attention to the role of lichens in assisting in this transition and to the phenomenon of lichenization. The recent discovery of lichen fossils in marine phosphorites in China and molecular clock estimates indicate that lichenized fungi were already present in Precambrian seas and, like contemporary species, played a role as pioneers in occupying new habitats. There is evidence that the holistic properties of associations between fungi and cyanobacteria and/or green algae facilitated the transition onto land and the subsequent colonization of terrestrial habitats. A key role in this process was played by poikilohydry. The algal components of delichenized fungi, along with lichens and photosynthetic aquatic organisms all contributed to the increase in atmospheric oxygen. Lichens, fungi and cyanobacteria settling on land were undoubtedly important in the formation of soils and thereafter enhancing their fertility. It is suggested that vascular and other green plants were able grow on these primitive soils that were stabilized by the growth of lichens, algae and cyanobacteria in a similar way to those which play a role in desert crusts at the present time.

Chloroplast phylogenomic analysis resolves deep-level relationships within the green algal class Trebouxiophyceae

BackgroundThe green algae represent one of the most successful groups of photosynthetic eukaryotes, but compared to their land plant relatives, surprisingly little is known about their evolutionary history. This is in great part due to the difficulty of recognizing species diversity behind morphologically similar organisms. The Trebouxiophyceae is a species-rich class of the Chlorophyta that includes symbionts (e.g. lichenized algae) as well as free-living green algae. Members of this group display remarkable ecological variation, occurring in aquatic, terrestrial and aeroterrestrial environments. Because a reliable backbone phylogeny is essential to understand the evolutionary history of the Trebouxiophyceae, we sought to identify the relationships among the major trebouxiophycean lineages that have been previously recognized in nuclear-encoded 18S rRNA phylogenies. To this end, we used a chloroplast phylogenomic approach.ResultsWe determined the sequences of 29 chlorophyte chloroplast genomes and assembled amino acid and nucleotide data sets derived from 79 chloroplast genes of 61 chlorophytes, including 35 trebouxiophyceans. The amino acid- and nucleotide-based phylogenies inferred using maximum likelihood and Bayesian methods and various models of sequence evolution revealed essentially the same relationships for the trebouxiophyceans. Two major groups were identified: a strongly supported clade of 29 taxa (core trebouxiophyceans) that is sister to the Chlorophyceae + Ulvophyceae and a clade comprising the Chlorellales and Pedinophyceae that represents a basal divergence relative to the former group. The core trebouxiophyceans form a grade of strongly supported clades that include a novel lineage represented by the desert crust alga Pleurastrosarcina brevispinosa. The assemblage composed of the Oocystis and Geminella clades is the deepest divergence of the core trebouxiophyceans. Like most of the chlorellaleans, early-diverging core trebouxiophyceans are predominantly planktonic species, whereas core trebouxiophyceans occupying more derived lineages are mostly terrestrial or aeroterrestrial algae.ConclusionsOur phylogenomic study provides a solid foundation for addressing fundamental questions related to the biology and ecology of the Trebouxiophyceae. The inferred trees reveal that this class is not monophyletic; they offer new insights not only into the internal structure of the class but also into the lifestyle of its founding members and subsequent adaptations to changing environments.

Neurotoxic amino acids and their isomers in desert environments

Cyanobacteria are capable of producing a wide range of bioactive compounds including highly toxic molecules which affect a variety of molecular targets. These toxins are well known from aquatic environments, and their occurrence in terrestrial environments is now gaining attention. Of the toxins produced by cyanobacteria, β-N-methylamino-l-alanine (BMAA) is a neurotoxic amino acid linked to human neurodegenerative diseases including Amyotrophic Lateral Sclerosis (ALS), Alzheimer's and Parkinson's disease. Two isomers of BMAA are known to be produced by cyanobacteria in axenic culture and in cyanobacterial blooms. Desert crust material was assessed for the presence of BMAA isomers. In addition to BMAA, 2,4-diaminobutyric acid (DAB) and N-(2-aminoethyl)glycine (AEG), were found to be present in desert environments. Both BMAA and DAB are known to be toxic. A preliminary assessment of AEG toxicity was performed with Artemia salina. At high concentrations, AEG was shown to cause mortality of A. salina with paralysis observed at lower concentrations. These findings show that the co-occurrence of BMAA, DAB and AEG may lead to adverse human and animal health effects and future research should consider the interaction of these three isomers for their effect on human health in arid environments.

Desert crust microorganisms, their environment, and human health

This article reviews current knowledge on cyanobacteria, the dominant primary producers, and other microorganisms in arid desert environments. These microorganisms have developed an array of adaptations to hot, arid climates with intense UV radiation, extreme diurnal temperature fluctuations, and high soil salinity. Crust microorganisms positively contribute to their harsh ecosystems, by preventing evapotranspiration, fixing nitrogen, and blocking solar radiation. In doing so, desert crust prevents soil erosion and facilitates the establishment of plant species. However, like aquatic cyanobacteria, desert cyanobacteria have the potential to produce toxins linked to human and animal illness. Furthermore, the impact of terrestrial cyanobacterial toxins on human health in desert regions is poorly understood. A largely ignored, but potentially important human exposure route for cyanotoxins in desert environments is through the inhalation of desert crusts during dust storms and anthropogenic activity. Future work in this field should include the characterization of toxins produced in desert regions as well as the presence of toxins in clinical and environmental materials.

The Diversity and Abundance of Bacteria and Oxygenic Phototrophs in Saline Biological Desert Crusts in Xinjiang, Northwest China

Although microorganisms, particularly oxygenic phototrophs, are known as the major players in the biogeochemical cycles of elements in desert soil ecosystems and have received extensive attention, still little is known about the effects of salinity on the composition and abundances of microbial community in desert soils. In this study, the diversity and abundance of bacteria and oxygenic phototrophs in biological desert crusts from Xinjiang province, which were under different salinity conditions, were investigated by using clone library and quantitative PCR (qPCR). The 16S rRNA gene phylogenetic analysis showed that cyanobacteria, mainly Microcoleus vagnitus of the order Oscillatoriales, were predominant in the low saline crusts, while other phototrophs, such as diatom, were the main microorganism group responsible for the oxygenic photosynthesis in the high saline crusts. Furthermore, the higher salt content in crusts may stimulate the growth of other bacteria, including Deinococcus-Thermus, Bacteroidetes, and some subdivisions of Proteobacteria (β-, γ-, and δ-Proteobacteria). The cpcBA-IGS gene analysis revealed the existence of novel M. vagnitus strains in this area. The qPCR results showed that the abundance of oxygenic phototrophs was significantly higher under lower saline condition than that in the higher saline crusts, suggesting that the higher salinity in desert crusts could suppress the numbers of total bacteria and phototrophic bacteria but did highly improve the diversity of salt-tolerant bacteria.

Diversity of free-living and lichenized fungal communities in biological soil crusts of the Sultanate of Oman and their role in improving soil properties

Biological soil crusts of arid and semiarid regions are one of the least explored habitats with respect to the diversity of their fungal communities and the Arabian deserts, in particular, remains mycologically poorly investigated. Here, we investigate the diversity of free-living and lichen-forming fungal communities associated with crusts at two locations in Oman, using intensive cultivation and pyrosequencing, and their role in improving soil stability and hydrology. A total of 226 fungal isolates were recovered and phylogenetic analysis placed 98% of the isolates within the Ascomycota phylum, most of which belonged to Dothideomycetes class and Pleosporales order. The isolates were phylogenetically affiliated to 101 different species within 44 different genera with >55% of the total isolates belonging to Alternaria, Aspergillus, Cladosporium, Cochliobolus, Fusarium, Myrothecium, Phoma and Ulocladium. Using pyrosequencing, a total of 26,998 sequence reads were obtained with Ascomycota, Basidiomycota and Chytridiomycota encompassing >96% of the total sequences. In cyanobacterial crusts, between 67.2 and 70.6% of the total fungal sequences belonged to the classes Dothideomycetes and Eurotiomycetes with the dominance of yeast-like fungi of the genera Sarcinomyces and Aureobasidium. On the other hand, the sequences obtained from the lichen crusts mainly belonged to the classes Lichinomycetes, Lecanoromycetes and Eurotiomycetes. Among the identified lichens were Placidium lacinulatum, Psora decipiens, Peccania fontqueriana, Stromatella bermudana, Verrucaria chiloensis, Pecania arizonica, Lempholemma polyanthes and Lichinella cribellifera. Although detected fungi confirmed earlier trends in fungal diversity in other deserts, quite a number of isolates and sequences representing novel taxa were recovered. The presence of lichen in crusts improved their resistance to erosion and increased their water holding capacity. We conclude that desert crusts of Oman harbor a large diversity of fungal communities that varies with crust type and desert ecoregions, and the presence of lichens in these crusts improves soil characteristics.

Polyphasic characterization of cyanobacteria isolated from desert crusts, and the description of Desertifilum tharense gen. et sp. nov. (Oscillatoriales)

Dadheech P.K., Abed R.M.M., Mahmoud H., Krishna Mohan M. and Krienitz L. 2012. Polyphasic characterization of cyanobacteria isolated from desert crusts, and the description of Desertifilum tharense gen. et sp. nov. (Oscillatoriales) Phycologia 51: 260–270. DOI: 10.2216/09-51.1Four new cyanobacterial strains isolated from biological desert crusts in Thar Desert, India were characterized using a polyphasic approach. The strains were designated to two mophotypes, but all strains exhibited identical 16S ribosomal RNA (rRNA) gene sequences. On the basis of 16S rRNA phylogenetic reconstruction, the strains belonged to the Oscillatoriales order and formed a coherent cluster in the phylogenetic tree, with more than 5% sequence divergence to the closest relative belonging to a species of Microcoleus Desmazières ex Gomont. Our strains were different from the genus Microcoleus in phenotypic characters such as organization of thallus, trichome width, cell shape, gas vesicle, thylakoid arrangement and habitat. Although the strains shared some morphological similarities with members of Phormidiaceae, they differed in thylakoid pattern and 16S rRNA gene sequences. The type strain (PD2001/TDC17T) grew up to 45°C with optimum growth between 30 and 35°C. The sharp decrease in growth of that strain at 1% salinity indicates its sensitivity to salts. Phylogenetic, morphological, ultrastuctural and physiological analyses demonstrated that the investigated strains represented a novel cyanobacterial genus, for which the name Desertifilum tharense gen. et sp. nov. is proposed, with PD2001/TDC17T (= CCAP 1420/4T = BISR/CYANO/61T) as the type strain.

Cyanotoxins in desert environments may present a risk to human health

There have been few studies concerning cyanotoxins in desert environments, compared with the multitude of studies of cyanotoxins in aquatic environments. However, cyanobacteria are important primary producers in desert environments, where after seasonal rains they can grow rapidly both stabilising and fertilising arid habitats. Samples of cyanobacteria from wadis – dry, ephemeral river beds – and sabkha – supertidal salt flats – in Qatar were analysed for the presence of microcystins, nodularin, anatoxin-a, cylindrospermopsin and anatoxin-a(S). Microcystins were detected by HPLC-PDA and ELISA at concentrations between 1.5 and 53.7ngg−1 dry wt of crust. PCR products for the mycD gene for microcystin biosynthesis were detected after amplification of DNA from desert crust samples at two out of three sample sites. The presence of anatoxin-a(S) was also indicated by acetylcholine esterase inhibition assay. As a function of area of desert crust, microcystin concentrations were between 3 and 56μgm−2. Based on the concentration of microcystins detected in crust, with reference to the published inhalation NOAEL and LOAEL values via nasal spray inhalation of purified microcystin-LR in aqueous solution, and the amount of dust potentially inhaled by a person from these dried crusts, the dose of microcystins could exceed a calculated TDI value of 1–2ngkg−1day−1 for an average adult. The presence of microcystins, and potentially of anatoxin-a(S), in desert crusts has important implications for human health. Further studies are required to monitor desert dust storms for the presence of cyanotoxins. An understanding of the risks of inhaling particles containing cyanotoxins is also warranted.

Neoproterozoic SHRIMP U–Pb zircon ages of silica-rich Dokhan Volcanics in the North Eastern Desert, Egypt

Chronology of Neoproterozoic volcanosedimentary successions remains controversial for many regions of the Arabian–Nubian Shield, including the Dokhan Volcanics of NE Egypt. New U–Pb zircon SHRIMP ages have been obtained for 10 silica-rich ignimbrites and two subvolcanic dacitic bodies, mapped as Dokhan Volcanics, from the North Eastern Desert of Egypt. Crystallization ages range between 592 ± 5 and 630 ± 6 Ma (Early Ediacaran). Apparently, the late consolidation of the Arabian–Nubian Shield was accompanied by the evolution of isolated volcanic centres and basin systems which developed during a period of approx. 40 Ma, independently in space and time and probably under changing tectonic regimes. The obtained age data together with other previously published reliable ages for Dokhan Volcanics suggest two main pulses of volcanic activity: 630–623 Ma and 618–592 Ma. Five samples contain inherited zircons, with ages of 669, 715–746, 847 and 1530 Ma, supporting models that North Eastern Desert crust is mainly juvenile Neoproterozoic crust.

Diversidad filogenética de especies de Microcoleus de costras biológicas de suelo

Microcoleus vaginatus has been proclaimed to be a cosmopolitan species. However, morphological studies performed on natural populations of cyanobacterial crusts in a region with a particular geological history at the southern part of the Baja California Peninsula, Mexico, called Sierra de la Laguna (SL), do not support the last assertion. We compared the community structure of cyanobacteria, with emphasis in the genus Microcoleus, in biological desert crusts from 10 different localities along the Baja California Peninsula. We analyzed natural cyanobacterial populations and matched them with type cultures from public collections: Microcoleus sociatus (Sammlung von Algenkulturen Germany- SAG 26.92), M. paludosus (SAG 1445. 1a), M. vaginatus (Pasteur Culture Collection-PCC 9802), and M. chthonoplastes (PCC 7420). Microscopy and cluster analysis of band patterns of 16S rRNA sequences from denaturing gradient gel electrophoresis (DGGE) were applied. The microscopic analysis of natural samples resulted in the undetection of morphotypes of M. vaginatus in 6 of the 10 localities, 4 or them from the SL. Comparison of band profiles from denaturing gradient gel electrophoresis showed signifi cant differences in the natural cyanobacterial community structure and the absence of the band that matches with that of M. vaginatus (PCC 9802) in all of the SL localities.

Bacterial diversity, pigments and nitrogen fixation of biological desert crusts from the Sultanate of Oman

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.

UV‐B‐induced Oxidative Damage and Protective Role of Exopolysaccharides in Desert Cyanobacterium Microcoleus vaginatus

UV-B-induced oxidative damage and the protective effect of exopolysaccharides (EPS) in Microcoleus vaginatus, a cyanobacterium isolated from desert crust, were investigated. After being irradiated with UV-B radiation, photosynthetic activity (Fv/Fm), cellular total carbohydrates, EPS and sucrose production of irradiated cells decreased, while reducing sugars, reactive oxygen species (ROS) generation, malondialdehyde (MDA) production and DNA strand breaks increased significantly. However, when pretreated with 100 mg/L exogenous EPS, EPS production in the culture medium of UV-B stressed cells decreased significantly; Fv/Fm, cellular total carbohydrates, reducing sugars and sucrose synthase (SS) activity of irradiated cells increased significantly, while ROS generation, MDA production and DNA strand breaks of irradiated cells decreased significantly. The results suggested that EPS exhibited a significant protective effect on DNA strand breaks and lipid peroxidation by effectively eliminating ROS induced by UV-B radiation in M. vaginatus.