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In arid and semiarid areas of the planet, biological soil crusts (BSCs) participate in functions critical to the ecosystem sustainability and are usually located in inter-vegetation spaces. These crusts are structurally and in terms of biomass dominated by mosses and lichens. Associated with them, a great variety of fungi, bacteria, and cyanobacteria have been found. Despite their ecological importance, major details of the functionalities of the microbial communities inhabiting BSCs and the underlying soil are still missing. Here, we explore the biomass, functionality, and protein-based phylogeny of the microbial communities associated with lichen-dominated BSCs under a semiarid climate. Two lichen-dominated BSCs (Fulgensia desertorum -dominated BSC, F-BSC; and Squamarina cartilaginea -dominated BSC, S-BSC) were sampled in a semiarid soil located in South-East Spain. Samples of the BSCs and the underlying soils were collected. The biomass and activity of the microbial community in both BSCs was higher than in the soil underlying them, as estimated by their PLFA content, basal respiration, and enzyme activities. The factor analysis of PLFAs and the putative phylogenetic analysis of the identified proteins indicated scarce differences in the composition of the microbial communities of the two biocrusts at the phylum level. In contrast, community-level physiological profiles (CLPPs) revealed that the microbial communities inhabiting BSCs metabolized substrates differently. The identification of ribulose-1,5 biphosphate carboxylase oxygenase (RuBisCO) peptides points to a specialization in carbon fixation which is carried out by some cyanobacterial species in F-BSC, such as Prochlorothrix hollandica and Microcoleus vaginatus, but not in S-BSC. The structure of the microbial community in the soil underlying each BSC was different, as estimated by factor analysis of PLFAs, but such differences did not correspond to changes in the functional structure, as indicated by the CLPPs. The multidisciplinary approach used in this study revealed remarkable differences between the microbial communities inhabiting F. desertorum and S. cartilaginea -dominated biological crusts, whereas the microbial communities in the soils underlying the two crusts behaved similarly. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs) cover >35% of the Earth’s land area and contribute to important ecological functions in arid and semiarid ecosystems, including erosion reduction, hydrological cycling, and nutrient cycling. Artificial rapid cultivation of BSCs can provide a novel alternative to traditional biological methods for controlling soil and water loss such as the planting of trees, shrubs, and grasses. At present, little is known regarding the cultivation of BSCs in the field due to lack of knowledge regarding the influencing factors that control BSCs growth. Thus, we determined the effects of various environmental factors (shade; watering; N, P, K, and Ca concentrations) on the growth of cyanobacteria-dominated BSCs from the Sonoran Desert in the southwestern United States. The soil surface changes and chlorophyll a concentrations were used as proxies of BSC growth and development. After 4 months, five factors were found to impact BSC growth with the following order of importance: NH4NO3 ≈ watering frequency>shading>CaCO3 ≈ KH2PO4. The soil water content was the primary positive factor affecting BSC growth, and BSCs that were watered every 5 days harbored greater biomass than those watered every 10 days. Groups that received NH4NO3 consistently exhibited poor growth, suggesting that fixed N amendment may suppress BSC growth. The effect of shading on the BSC biomass was inconsistent and depended on many factors including the soil water content and availability of nutrients. KH2PO4 and CaCO3 had nonsignificant effects on BSC growth. Collectively, our results indicate that the rapid restoration of BSCs can be controlled and realized by artificial “broadcasting” cultivation through the optimization of environmental factors. [ABSTRACT FROM AUTHOR]

Soil microbial communities in dryland ecosystems play important roles as root associates of the widely spaced plants and as the dominant members of biological soil crusts (biocrusts) colonizing the plant interspaces. We employed rRNA gene sequencing (bacterial 16S/fungal large subunit) and shotgun metagenomic sequencing to compare the microbial communities inhabiting the root zones of the dominant shrub, Larrea tridentata (creosote bush), and the interspace biocrusts in a Mojave desert shrubland within the Nevada Free Air CO2 Enrichment (FACE) experiment. Most of the numerically abundant bacteria and fungi were present in both the biocrusts and root zones, although the proportional abundance of those members differed significantly between habitats. Biocrust bacteria were predominantly Cyanobacteria while root zones harbored significantly more Actinobacteria and Proteobacteria. Pezizomycetes fungi dominated the biocrusts while Dothideomycetes were highest in root zones. Functional gene abundances in metagenome sequence datasets reflected the taxonomic differences noted in the 16S rRNA datasets. For example, functional categories related to photosynthesis, circadian clock proteins, and heterocyst-associated genes were enriched in the biocrusts, where populations of Cyanobacteria were larger. Genes related to potassium metabolism were also more abundant in the biocrusts, suggesting differences in nutrient cycling between biocrusts and root zones. Finally, ten years of elevated atmospheric CO2 did not result in large shifts in taxonomic composition of the bacterial or fungal communities or the functional gene inventories in the shotgun metagenomes. [ABSTRACT FROM AUTHOR]

Abstract: The ecology and functional role of biological soil crusts (BSCs) in arid and semi-arid zones have been extremely well studied. However, little is known about the biochemical properties related to the number and activity of the microbiota that form the crusts, even though information about these properties is very important for understanding many of the processes that affect the formations. In this study, several properties related to the activity and number of microorganisms (biomass-C, basal respiration, dehydrogenase activity and nitrogen mineralization potential) were determined at different depths (crusts, 0–0.5 cm; middle, 0.5–3 cm and deep, 3–5 cm layers) in two types of crusts (predominated by cyanobacteria and by lichens) in the Tabernas desert (Almeria, SE Spain). The absolute values of the above-mentioned properties and the values expressed relative to the total organic carbon (TOC) content were both much higher in the crust layers than in the surface horizons of soils under Mediterranean or Atlantic climates. A large part of the TOC in the BSCs was contained in the microbiota and another large part was readily metabolized during incubation of the crusts for 10 days at 25 °C. The net nitrogen mineralization rate was also high, and ammonification predominated in the crust layers, whereas nitrification predominated in the middle and deep layers. In all types of BSCs, the microbiota colonized the deep layers, although with greater intensity in the lichen-dominated BSCs than in the cyanobacterial BSCs. The results also indicate that hydrolytic enzymes are not stabilized on soil colloids and their activity depends only on the active microbiota. [Copyright &y& Elsevier]

Abstract: Biological soil crusts (BSCs) are highly specialized topsoil microbial communities commonly found in arid and semiarid environments, permeated by a polymeric matrix of polysaccharides. BSCs can in principle influence edaphic properties such as texture, pore formation and water retention, which in turn determine water distribution and biological activity in dry lands. This paper investigates the influence of biotic and abiotic factors on BSC hydraulic conductivity, a parameter gauging the ease with which water can move through the pore spaces. Texture, phototroph abundance, microbial composition, and extracellular carbohydrate content were considered as potentially relevant parameters in a correlational study of BSC samples that spanned 1.5 orders of magnitude in hydraulic conductivity. A newly developed, non-destructive extraction method enabled us to directly quantify the specific role of extracellular polysaccharides on soil permeability on a variety of samples. Hydraulic conductivity showed a strongest correlation with texture (positive with sand content, negative with silt and clay). A weaker negative correlation with carbohydrate content, especially with polysaccharides having a molecular weight < 100 kDa, was also detected. In multiple regression analyses texture (silt content) was sufficient to explain most of the variation in hydraulic conductivity However, experimental removal of polymeric carbohydrates, resulted invariably in a substantial decrease in hydraulic conductivity for any given sample (between 1.7 and 3.3 fold). Our results suggest that while soil texture determines overall hydraulic conductivity in BSCs, the presence of exopolysaccharides can significantly enhance it, likely by conferring a spongy structure to a BSC thus increasing the number of waterways within it. [Copyright &y& Elsevier]

Abstract: Semiarid ecosystems, also known as drylands, contain small amounts of carbon (C). To date only few studies have evaluated soil C dynamics in these ecosystems. Cyanobacterial soil crusts are considered a major source of organic C in semiarid ecosystems through photosynthesis, increasing soil organic C (SOC) pool as carbohydrates. This study considers cyanobacterial soil crusts as a source of C in the southwest Kalahari. Cyanobacterial soil crusts are well adapted to drylands because extracellular polysaccharide (EPS) secretions provide stabilization of soil and resistance to desiccation. The carbohydrate and chlorophyll a content were evaluated in the upper soil profile on Kalahari Sands and pan soils. Topsoil carbohydrate concentration decreased exponentially with depth. The carbohydrate content produced in cyanobacterial soil crusts in the southwest Kalahari represents up to 75% of the total SOC and is thus an essential component for the fertility of Kalahari Sand soils. [Copyright &y& Elsevier]

Abstract: Archaea are common and abundant members of biological soil crust communities across large-scale biogeographic provinces of arid North America. Regardless of microbial community development, archaeal populations averaged 2 × 107 16S rRNA gene copies per gram of soil, representing around 5% of the prokaryotic (total calculated bacterial and archaeal) numbers assessed by quantitative-PCR. In contrast, archaeal diversity, determined by denaturing gradient gel electrophoresis fingerprinting and clone libraries of 16S rRNA genes, was very restricted. Only six different phylotypes (all Crenarchaea) were detected, three of which were very dominant. Some phylotypes were widespread, while others were typical of Southern desert areas. [Copyright &y& Elsevier]

Abstract: It has been suggested that the dominance of N forms should shift from dissolved organic nitrogen (DON) to nitrate along a gradient of increasing N availability. We aimed to apply this model at a local scale within a semi-arid ecosystem showing a high spatial heterogeneity in the distribution of vegetation and soil resources. By doing this, we seek a better understanding of the N cycling in spatially heterogeneous ecosystems. We took soil samples from the three major sources of spatial heterogeneity: the grass Stipa tenacíssima, the N-fixing shrub Retama sphaerocarpa, and open areas. We also sampled the biological soil crust (BSC) located in the latter areas as another source of spatial heterogeneity. BSC microsites were classified by four levels of soil coverage, ranging from high coverage (66%) to bare soil. The proportion of nitrate, ammonium and DON was determined in all microsites. DON was the dominant N form for open areas, while nitrate was dominant under the canopy of Retama; these microsites contained the lowest and highest N availability, respectively. Under BSC, DON was the dominant N form. We found high temporal variability in the dominance of N forms for all microsites. Our results suggest that the biome-derived model of explaining N form dominance across N availability gradients may be extended to local gradients. [Copyright &y& Elsevier]