Assessing level of development and successional stages in biological soil crusts with biological indicators

Deterministic processes influence bacterial more than fungal community assembly during the development of biological soil crusts in the desert ecosystem

Biological soil crusts (biocrusts) constitute a crucial biological component of the soil surface in arid and semi-arid ecosystems. Understanding the variations in soil microbial community assembly across biocrust successional stages is essential for a deeper comprehension of microbial biodiversity and desert ecosystem functioning. However, knowledge about the mechanisms of microbial community assembly and the factors influencing its development remains limited. In this study, we utilized amplicons sequencing to assess the compositions of bacterial and fungal communities in bare sand and three types of biocrusts (light cyanobacterial biocrusts, dark cyanobacterial biocrusts, and moss crusts). Subsequently, we analyzed the ecological processes shaping microbial community composition and structure, along with the influencing factors. Our results revealed a significant increase in bacterial diversity and no significant changes in fungal diversity during biocrust development. The relative abundances of the copiotrophic bacteria (e.g., Actinobacteria, Acidobacteria, and Bacteroidetes) showed significant increases, while oligotrophic bacteria (e.g., Proteobacteria and Firmicutes) decreased over time. Moreover, the relative abundances of Ascomycota, which exhibit strong resistance to adverse environmental conditions, significantly decreased, whereas Basidiomycota, known for their ability to degrade lignin, significantly increased throughout biocrust development. Additionally, stochastic processes (dispersal limitation and drift) predominantly drove the assemblies of both bacterial and fungal communities. However, the relative importance of deterministic processes (homogeneous selection) in bacterial assembly increased during biocrust development. Structural equation modeling indicated that bacterial community assembly was primarily related to soil water content, whereas fungal community assembly was primarily related to total organic carbon. These findings provide a scientific foundation for investigating the formation and development of biocrusts, and further insights into the conservation and sustainable management of biocrust resources under future climate change scenarios.

Biocrusts enhance soil organic carbon stability and regulate the fate of new-input carbon in semiarid desert ecosystems

Given their global prevalence, dryland (including hyperarid, arid, semiarid, and dry subhumid regions) ecosystems are critical for supporting soil organic carbon (SOC) stocks, with even small changes in such SOC pools affecting the global carbon (C) cycling. Biocrusts play an essential role in supporting C cycling in semiarid ecosystems. However, the influence of biocrusts and their successional stages on SOC and its fraction contents, as well as their role in regulating new input C into SOC fractions remain largely unknown. In this study, we collected continuous samples of bare soil (BS) and three successional stages of biocrust soils (cyanobacterial (CC), low-cover moss (LM), and high-cover moss (HM)) at 0-5 cm depth every month for one year in a semiarid desert ecosystem. We analyzed SOC changes among the samples and their fraction contents including: labile organic C (LOC) (composed of microbial biomass C (MBC), dissolved organic C (DOC), and easily oxidized organic C (EOC)) and recalcitrant organic C (ROC) fractions, soil nutrient content including: ammonium (NH4+-N), nitrate (NO3--N), and available phosphorus (AP), and soil temperature and moisture. We also conducted a 13C pulse-labelling experiment in the field to accurately quantify the effects of biocrust successional stage on exogenous C allocation to SOC fractions. Our results showed that the three successional stages of biocrust (CC-LM-HM) increased SOC and ROC contents by an average of 5.3 ± 3.6 g kg-1 and 4.0 ± 3.0 g kg-1, respectively; and the MBC, DOC, and EOC contents increased by an average of 41.7 ± 24.8 mg kg-1, 28.7 ± 12.6 mg kg-1, and 1.2 ± 0.6 g kg-1, respectively, compared to that of BS. These increases were attributed to an increase in photosynthetic pigment content, higher nutrient levels, and more suitable microclimates (e.g., higher moisture and more moderate temperature) during biocrust succession. More importantly, SOC stability was greatly improved with biocrust succession from cyanobacteria to moss, as evidenced by the reduction in soil EOC:SOC and EOC:ROC ratios by an average of 50 ± 34 % and 99 ± 67 %, respectively, while the ROC:SOC ratio increased by 33 ± 16 % with biocrust succession compared to those of BS. The biocrust SOC, DOC, and MBC 13C contents at different stages were on average 0.096 ± 0.034 mg kg-1, 0.010 ± 0.005 mg kg-1, and 0.014 ± 0.005 mg kg-1 higher than those of BS. Similarly, the allocation of new-input C among the DOC and MBC at different biocrust stages (19 ± 10 %) was significantly higher than that of BS (9 ± 6 %). New-input C into the biocrusts was fixed by microbes (43 ± 18 %) within ∼10 days and converted into other forms of C (85 ± 5 %) after 80 days. Our study provides a new perspective on how biocrusts support C cycling in semiarid desert ecosystems by mediating new C inputs into diverse fractional contents, and highlights the significance of biocrust successional stages in maintaining soil C stocks and stability in the dryland soil system.

Dune Blowouts as Microbial Hotspots and the Changes of Overall Microbial Activity and Photosynthetic Biomass Along with Succession of Biological Soil Crusts

Biological soil crust (BSC) constitutes a consortium of cyanobacteria, algae, lichen, mosses, and heterotrophic microorganisms, forming a miniature ecosystem within the uppermost soil layer. The biomass of different organisms forming BSC and their activity changes along with succession. Previous studies focused primarily on BSC in hyper-arid/arid regions, whereas the ecophysiology of BSC in temperate climates is still not well recognized. In order to determine changes in overall microbial activity and photosynthetic biomass in BSC at different stages of the succession of inland sandy grasslands, we analyzed dehydrogenase activity and determined the content of photosynthetic pigments. We also compared these parameters between BSC developed on the dune ridges and aeolian blowouts in the initial stage of succession. Our study revealed a significant increase in both photosynthetic biomass and overall microbial activity in BSC as the succession of inland shifting sands progresses. We found that chl a concentration in BSC could be considered a useful quantitative indicator of both the presence of photoautotrophs and the degree of soil crust development in warm-summer humid continental climates. The photosynthetic biomass was closely related to increased microbial activity in BSC, which suggests that photoautotrophs constitute a major BSC component. Dune blowouts constitute environmental niches facilitating the development of BSC, compared to dune ridges. High biomass of microorganisms in the dune blowouts may be associated with a high amount of organic material and more favorable moisture conditions. We conclude that deflation fields are key places for keeping a mosaic of habitats in the area of shifting sands and can be a reservoir of microorganisms supporting further settlement of dune slopes by BSC.

Balanced biogeographic and local environmental effects determine the patterns of microbial diversity in biocrusts at multi-scales

Introduction

Biodiversity maintenance and its underlying mechanisms are central issues of ecology. However, predicting the composition turnovers of microbial communities at multiple spatial scales remains greatly challenging because they are obscured by the inconsistent impacts of climatic and local edaphic conditions on the assembly process.

Methods

Based on the Illumina MeSeq 16S/18S rRNA sequencing technology, we investigated soil bacterial and eukaryotic communities in biocrusts with different successional levels at a subcontinental scale of Northern China.

Results

Results showed that irrespective of spatial scale, bacterial α diversity increased but eukaryotic diversity decreased with the primary succession, whereas both β diversities decreased at the subcontinental scale compared with smaller scales, indicating that the biogeographic pattern of soil microorganisms was balanced by successional convergence and distance decay effect. We found that the convergence of bacterial and eukaryotic communities was attributed to the turnovers of generalist and specialist species, respectively. In this process, edaphic and climatic factors showed unique roles in the changes of diversity at local/subcontinental scales. Moreover, the taxonomic diversity tended to be more susceptible to climatic and edaphic conditions, while biotic factors (photosynthesis and pigments) were more important to phylogenetic diversity.

Conclusion

Taken together, our study provided comprehensive insights into understanding the pattern of microbial diversity at multiple spatial scales of drylands.

Impact of dehydration on the physiochemical properties of Nostoc calcicola BOT1 and its untargeted metabolic profiling through UHPLC-HRMS

The global population growth has led to a higher demand for food production, necessitating improvements in agricultural productivity. However, abiotic and biotic stresses pose significant challenges, reducing crop yields and impacting economic and social welfare. Drought, in particular, severely constrains agriculture, resulting in unproductive soil, reduced farmland, and jeopardized food security. Recently, the role of cyanobacteria from soil biocrusts in rehabilitating degraded land has gained attention due to their ability to enhance soil fertility and prevent erosion. The present study focused on Nostoc calcicola BOT1, an aquatic, diazotrophic cyanobacterial strain collected from an agricultural field at Banaras Hindu University, Varanasi, India. The aim was to investigate the effects of different dehydration treatments, specifically air drying (AD) and desiccator drying (DD) at various time intervals, on the physicochemical properties of N. calcicola BOT1. The impact of dehydration was assessed by analyzing the photosynthetic efficiency, pigments, biomolecules (carbohydrates, lipids, proteins, osmoprotectants), stress biomarkers, and non-enzymatic antioxidants. Furthermore, an analysis of the metabolic profiles of 96-hour DD and control mats was conducted using UHPLC-HRMS. Notably, there was a significant decrease in amino acid levels, while phenolic content, fatty acids, and lipids increased. These changes in metabolic activity during dehydration highlighted the presence of metabolite pools that contribute to the physiological and biochemical adjustments of N. calcicola BOT1, mitigating the impact of dehydration to some extent. Overall, present study demonstrated the accumulation of biochemical and non-enzymatic antioxidants in dehydrated mats, which could be utilized to stabilize unfavorable environmental conditions. Additionally, the strain N. calcicola BOT1 holds promise as a biofertilizer for semi-arid regions.

Protist Diversity Responses to Experimental N Deposition in Biological Crusts of a Semiarid Mediterranean Ecosystem

Biological soil crusts (BSC) are associations of different macro and microorganisms and aggregated soil particles located on the surface of soils in many different habitats. BSC harbour a diverse and complex community of ciliates and testate amoebae. These phagotrophic protists play an important role in C and N recycling in soil ecosystems but have not been frequently studied in BSC. In this context, the effects of three increasing N inputs on ciliates and testate amoebae in crusts from a semi-arid Mediterranean ecosystem were evaluated. A field experiment with artificial N-deposition was designed to mimic the effects caused by anthropogenic N depositions. The results have shown that the protist populations of these semi-arid Mediterranean environments have lower species richness than other soil environments. The increase in N produces a net loss of diversity in the populations studied and shifts in the community structure. It has also been shown that some ciliates and testate amoebae, due to their population responses to increased N concentrations, could potentially be used as bio-indicators of N contamination in these BSCs.

The Microbiomes in Lichen and Moss Biocrust Contribute Differently to Carbon and Nitrogen Cycles in Arid Ecosystems

Biological soil crusts (biocrusts) are distributed in arid and semiarid regions across the globe. Microorganisms are an essential component in biocrusts. They add and accelerate critical biochemical processes. However, little is known about the functional genes and metabolic processes of microbiomes in lichen and moss biocrust. This study used shotgun metagenomic sequencing to compare the microbiomes of lichen-dominated and moss-dominated biocrust and reveal the microbial genes and metabolic pathways involved in carbon and nitrogen cycling. The results showed that Actinobacteria, Bacteroidetes, and Acidobacteria were more abundant in moss biocrust than lichen biocrust, while Proteobacteria and Cyanobacteria were more abundant in lichen biocrust than moss biocrust. The relative abundance of carbohydrate-active enzymes and enzymes associated with carbon and nitrogen metabolism differed significantly between microbiomes of the two biocrust types. However, in the microbial communities of both biocrust types, respiration pathways dominated over carbon fixation pathways. The genes encoding carbon monoxide dehydrogenase were more abundant than those encoding ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCo) involved in carbon fixation. Similarly, metabolic N-pathway diversity was dominated by nitrogen reduction, followed by denitrification, with nitrogen fixation the lowest proportion. Gene diversity involved in N cycling differed between the microbiomes of the two biocrust types. Assimilatory nitrate reduction genes had higher relative abundance in lichen biocrust, whereas dissimilatory nitrate reduction genes had higher relative abundance in moss biocrust. As dissolved organic carbon and soil organic carbon are considered the main drivers of the community structure in the microbiome of biocrust, these results indicate that biocrust type has a pivotal role in microbial diversity and related biogeochemical cycling.

Physical Disturbance Reduces Cyanobacterial Relative Abundance and Substrate Metabolism Potential of Biological Soil Crusts on a Gold Mine Tailing of Central China

As the critical ecological engineers, biological soil crusts (biocrusts) are considered to play essential roles in improving substrate conditions during ecological rehabilitation processes. Physical disturbance, however, often leads to the degradation of biocrusts, and it remains unclear how the physical disturbance affects biocrust microorganisms and their related metabolism. In this study, the photosynthetic biomass (indicated by chlorophyll a), nutrients, enzyme activities, and bacterial communities of biocrusts were investigated in a gold mine tailing of Central China to evaluate the impact of physical disturbance on biocrusts during the rehabilitation process of gold mine tailings. The results show that physical disturbance significantly reduced the photosynthetic biomass, nutrient contents (organic carbon, ammonium nitrogen, nitrate nitrogen, and total phosphorus), and enzyme activities (β-glucosidase, sucrase, nitrogenase, neutral phosphatase, and urease) of biocrusts in the mine tailings. Furthermore, 16S rDNA sequencing showed that physical disturbance strongly changed the composition, structure, and interactions of the bacterial community, leading to a shift from a cyanobacteria dominated community to a heterotrophic bacteria (proteobacteria, actinobacteria, and acidobacteria) dominated community and a more complex bacterial network (higher complexity, nodes, and edges). Altogether, our results show that the biocrusts dominated by cyanobacteria could also develop in the tailings of humid region, and the dominants (e.g., Microcoleus) were the same as those from dryland biocrusts; nevertheless, physical disturbance significantly reduced cyanobacterial relative abundance in biocrusts. Based on our findings, we propose the future work on cyanobacterial inoculation (e.g., Microcoleus), which is expected to promote substrate metabolism and accumulation, ultimately accelerating the development of biocrusts and the subsequent ecological restoration of tailings.

Valorization of poultry litter using Acutodesmus obliquus and its integrated application for lipids and fertilizer production

Microalgae are recognized as potential candidates for resource recovery from wastewater and projected for biorefinery models. This study was undertaken to evaluate the potential of poultry litter and municipal wastewater as nutrient and water sources, for the cultivation of Acutodesmus obliquus for lipids production for biodiesel application. The efficacy of lipid extracted biomass (LEA) as fertilizer for mung bean crops was also assessed in microcosm. A. obliquus cultivation in acid pre-treated poultry litter extract (PPLE) showed maximum biomass production of 1.90 g L^-1, which was 74.67% and 12.61% higher than the raw poultry litter extract (RPPE) and BG11 respectively. Higher NO₃-N, NH₃-N, and PO₄-P removal of 79.51%, 81.82%, and 80.52% respectively were observed in PPLE as compared to RPLE treatment. The highest biomass (140.36 mg L^-1 d^-1), lipids (38.49 mg L^-1 d^-1), and carbohydrates (49.55 mg L^-1 d^-1) productivities were observed in the PPLE medium. The application of LEA as a fertilizer for mung bean crops showed improvement in plant growth and soil microbial activity. A maximum increase in organic carbon (59.5%) and dehydrogenase activity (130.8%) was observed in LEA amended soil which was significantly higher than chemical fertilizer (CF) control in 30 days. Whilst plant fresh weight and leaf chlorophyll in the LEA amended soil was comparable to whole algal biomass (WA) and CF control. The strategy developed could be a basis for sustainable biorefinery for the valorization of wastewater for the production of microalgae-derived biofuel and byproducts for agricultural application.

Spatial and temporal distribution of aerobic anoxygenic phototrophic bacteria: key functional groups in biological soil crusts

Several significant ecosystem services are performed by biological soil crusts (BSCs) in drylands, wherein photoautotrophic microorganisms are commonly critical contributors. However, aerobic anoxygenic phototrophic bacteria (AAnPB) are rarely reported in BSCs, despite being the second major branch of Earth's phototrophic microbes. Here, we collected different types of BSCs and their subsoils from temperate deserts, investigated distributions of AAnPB communities among BSCs using cultivation and high-throughput sequencing approaches, predicted keystone species by co-occurrence network analysis, and verified their effects on BSCs formation through microcosm experiments. The absolute abundances and diversity of AAnPB were higher in BSCs and were closely related with BSCs successional stages, as well as soil organic carbon contents. AAnPB communities in both BSCs and their subsoils were dominated by Proteobacteria and Alphaproteobacteria, specifically Acetobacteraceae, Rhodospirillaceae, Roseiflexaceae, Sphingomonadaceae and Caulobacteraceae families. Mean annual precipitation, pH and available nutrients were the primary factors that shaped AAnPB community structures. The predicted keystone species belonged to the families Acetobacteraceae, Rhodospirillaceae and Sphingomonadanceae. Microcosm experiments demonstrated that inoculation with strains from the three families greatly accelerated the formation and development of BSCs. These observations suggest that AAnPB are likely important functional groups in BSCs that significantly contribute to their formation and important ecosystem services.

Beneficial biofilms for land rehabilitation and fertilization

The acquisition of a biofilm lifestyle is common in nature for microorganisms. It increases their biotic and abiotic stress tolerance and their capability to provide ecosystem services. Although diminutive communities, soil beneficial biofilms are essential for nutrient cycling, soil stabilization and direct or indirect promotion of plant development. Some biofilms represent valid biotechnological tools to deal with problems related to soil degradation, which threat food quality and the maintenance of ecosystem functions. Three genres of biofilms: rhizobacterial biofilms, fungal-bacterial biofilms and biocrusts are reviewed, and their beneficial effects on the environment outlined. Their induction by microbial inoculation represents a potential eco-friendly and sustainable approach to restore lost ecosystem functions and counteract the effects of soil erosion. Yet, some existing knowledge and methodological gaps, that will be discussed here, still hamper the optimization of this technology, and its application at its full potential.

High Arctic biocrusts: characterization of the exopolysaccharidic matrix

Biocrusts can be found in a wide array of habitats, where they provide important ecosystem services. These microbial associations are particularly important in High Arctic environments, where biocrust colonize the newly exposed barren soil after glacier retreat and significantly contribute to soil stabilization and nutrient cycling. Starting from incipient, structurally simple biolayers, they develop in complexity, increasing from the glacier terminus. Starting from a simple community structure, mainly constituted by cyanobacteria, heterotrophic bacteria and fungi immersed in a self-secreted extracellular polymeric matrix (cyanobacterial crusts), they later may recruit mosses and lichens (moss crusts and lichen crusts, respectively). The extracellular polymeric matrix protects the biocrust community from abiotic constraints, notably drought and freezing stress, from external physical harming factors, and from predation. The physicochemical characteristics of the extracellular matrix are related to several of its properties, such as its soil-stabilizing effect and water retention. We analysed the chemical (monosaccharidic composition) and macromolecular (molecular weight distribution) properties of the extracellular polymeric matrix of biocrusts with different morphologies collected in northwestern Spitsbergen, Norway. The uronic acid content and molecular weight (MW) distribution of the extracellular polysaccharidic matrices (EPMs) appeared in accordance with the developmental stages of the biocrusts. The MW distribution also showed significant differences between the samples, possibly reflecting differences in microbial enzymatic activities leading to the degradation of high-MW polymers into smaller compounds. The MW distribution profiles presented some important differences, reflecting differences in environmental conditions and, probably, the seasonal variance in microbial community composition that is known to characterize the environment examined in the present study.

Habitat-specific environmental factors regulate spatial variability of soil bacterial communities in biocrusts across northern China's drylands

Biocrusts are common biotic components in dryland ecosystems worldwide, they contain diverse soil organisms and effectively enhance soil stability and perform a series of key ecological functions. However, the geographical pattern of microbial communities in biocrusts is rarely assessed, despite it is closely related to the spatial variation of ecosystem functions in drylands. We assessed soil bacterial communities in biocrusts across four ecosystems (Gobi, desert, desert steppe and grassland) in a precipitation gradient (16-566 mm yr^-1) in northern China. Bacterial OTU number and phylogenetic diversity did not linearly increase with decreasing aridity, they were significantly lower in Gobi and similar among desert, desert steppe and grassland. Soil bacterial community composition in Gobi and desert were different than those in desert steppe and grassland, and they were similar between Gobi and desert, this suggests the key role of habitat in structuring soil bacterial communities. The geographic pattern of soil bacterial communities was strongly influenced by both geographic distance and environmental factors. The first explanatory factor for the geographic variation of bacterial community dissimilarity differed among four ecosystems, being aridity in Gobi and desert, precipitation in desert steppe, and soil inorganic nitrogen in grassland. The geographic pattern of the bacterial functional group profile showed a similar pattern with community composition across four ecosystems, and the groups of containing mobile elements and gram negative bacteria were more abundant in drier habitats of Gobi and desert. Our results reveal the non-linear changes in diversity, composition and functional group of soil bacterial communities in biocrusts across the precipitation gradient from hyper-arid to semi-humid regions, and suggest that the geographic distance and habitat-specific environmental factors determine the distribution of soil bacterial communities in different ecosystems.

Multifunctionality of biocrusts is positively predicted by network topologies consistent with interspecies facilitation

The potential of biodiversity loss to impair the delivery of ecosystem services has motived ecologists to better understand the relationship between biodiversity and ecosystem functioning. Although increasing evidence underlines the collective contribution of different biodiversity components on the simultaneous performance of multiple functions (multifunctionality), we know little about the trade‐offs between individual diversity effects and the extent to which they determine multifunctionality differentially. Here, at a subcontinental scale of 62 dryland sites, we show in phototrophic microbiota of biological soil crusts (biocrusts) that, whereas richness alone is unable to guarantee the maxima of multifunctional performance, interspecies facilitation and compositional identity are particularly stronger but often neglected predictors. The inconsistent effects of different biodiversity components imply that soil multifunctionality can be lost despite certain species remaining present. Moreover, we reveal a significant empirical association between species functional importance and its topological feature in co‐occurrence networks, indicating a functional signal of species interaction. Nevertheless, abundant species tend to isolate and merely interact within small topological structures, but rare species were tightly connected in complicated network modules. Our findings suggest that abundant and rare species of soil phototrophs exhibit distinct functional relevance. These results give a comprehensive view of how soil constructive species drive multifunctionality in biocrusts and ultimately promote a deeper understanding of the consequences of biodiversity loss in real‐world ecosystems.

Water Regulation in Cyanobacterial Biocrusts from Drylands: Negative Impacts of Anthropogenic Disturbance

Arid and semi-arid ecosystems are characterized by patchy vegetation and variable resource availability. The interplant spaces of these ecosystems are very often covered by cyanobacteria-dominated biocrusts, which are the primary colonizers of terrestrial ecosystems and key in facilitating the succession of other biocrust organisms and plants. Cyanobacterial biocrusts regulate the horizontal and vertical fluxes of water, carbon and nutrients into and from the soil and play crucial hydrological, geomorphological and ecological roles in these ecosystems. In this paper, we analyze the influence of cyanobacterial biocrusts on water balance components (infiltration-runoff, evaporation, soil moisture and non-rainfall water inputs (NRWIs)) in representative semiarid ecosystems in southeastern Spain. The influence of cyanobacterial biocrusts, in two stages of their development, on runoff-infiltration was studied by rainfall simulation and in field plots under natural rainfall at different spatial scales. Results showed that cover, exopolysaccharide content, roughness, organic carbon, total nitrogen, available water holding capacity, aggregate stability, and other properties increased with the development of the cyanobacterial biocrust. Due to the effects on these soil properties, runoff generation was lower in well-developed than in incipient-cyanobacterial biocrusts under both simulated and natural rainfall and on different spatial scales. Runoff yield decreased at coarser spatial scales due to re-infiltration along the hillslope, thus decreasing hydrological connectivity. Soil moisture monitoring at 0.03 m depth revealed higher moisture content and slower soil water loss in plots covered by cyanobacterial biocrusts compared to bare soils. Non-rainfall water inputs were also higher under well-developed cyanobacterial biocrusts than in bare soils. Disturbance of cyanobacterial biocrusts seriously affected the water balance by increasing runoff, decreasing soil moisture and accelerating soil water loss, at the same time that led to a very significant increase in sediment yield. The recovery of biocrust cover after disturbance can be relatively fast, but its growth rate is strongly conditioned by microclimate. The results of this paper show the important influence of cyanobacterial biocrust in modulating the different processes supporting the capacity of these ecosystems to provide key services such as water regulation or erosion control, and also the important impacts of their anthropic disturbance.

Population dynamics of Echinops gmelinii Turcz. at different successional stages of biological soil crusts in a temperate desert in China

The effects of biological soil crusts (BSC) on vascular plant growth can be positive, neutral or negative, and little information is available on the impacts of different BSC successional stages on vascular plant population dynamics. We analysed seedling emergence, survival, plant growth and reproduction in response to different BSC successional stages (i.e. habitats: bare soil, cyanobacteria, lichen and moss crusts) in natural populations of Echinops gmelinii Turcz. in the Tengger Desert of northwest China. The winter annual E. gmelinii is a dominant pioneer herb after sand stabilisation. During the early stages of BSC succession, the studied populations of E. gmelinii were characterised by high density, plant growth and fecundity. As the BSC succession proceeded beyond moss crusts, the fecundity decreased sharply, which limited seedling recruitment. Differences in seedling survival among the successional stages were not evident, indicating that BSC have little effect on survival in arid desert regions. Moreover, E. gmelinii biomass allocation exhibited low plasticity, and only reproductive allocation was sensitive to the various habitats. Our results further suggest that the negative effects of BSC succession on population dynamics are primarily driven by increasing topsoil water-holding capacity and decreasing rain water infiltration into deeper soil. We conclude that BSC succession drives population dynamics of E. gmelinii, primarily via its effect on soil moisture. The primary cause for E. gmelinii population decline during the moss-dominated stage of BSC succession is decreased fecundity of individual plants, with declining seed mass possibly reducing the success of seedling establishment.

Small-Scale Spatial Heterogeneity of Photosynthetic Fluorescence Associated with Biological Soil Crust Succession in the Tengger Desert, China

In dryland regions, biological soil crusts (BSCs) have numerous important ecosystem functions. Crust species and functions are, however, highly spatially heterogeneous and remain poorly understood at a range of scales. In this study, chlorophyll fluorescence imaging was used to quantify millimeter-scale patterns in the distribution and activity of photosynthetic organisms in BSCs of different successional stages (including cyanobacterial, lichen, moss three main successional stages and three intermixed transitional stages) from the Tengger Desert, China. Chlorophyll fluorescence images derived from the Imaging PAM (Pulse Amplitude Modulation) showed that with the succession from cyanobacterial to lichen and to moss crusts, crust photosynthetic efficiency (including the maximum and effective photosynthetic efficiency, respectively) and fluorescence coverage increased significantly (P < 0.05), and that increasing photosynthetically active radiation (PAR) reduced the effective photosynthetic efficiency (Yield). The distribution of photosynthetic organisms in crusts determined Fv/Fm (ratio of variable fluorescence to maximum fluorescence) frequency pattern, although the photosynthetic heterogeneity (SHI index) was not significantly different (P > 0.05) between cyanobacterial and moss crusts, and showed a unimodal pattern of Fv/Fm values. In contrast, photosynthetic heterogeneity was significantly higher in lichen, cyanobacteria-moss and lichen-moss crusts (P < 0.05), with a bimodal pattern of Fv/Fm values. Point pattern analysis showed that the distribution pattern of chlorophyll fluorescence varied at different spatial scales and also among the different crust types. These new results provide a detailed (millimeter-scale) insight into crust photosynthetic mechanisms and spatial distribution patterns associated with their community types. Collectively, this information provides an improved theoretical basis for crust maintenance and management in dryland regions.

Soil Type and Cyanobacteria Species Influence the Macromolecular and Chemical Characteristics of the Polysaccharidic Matrix in Induced Biocrusts

Inoculation of soils with cyanobacteria is proposed as a sustainable biotechnological technique for restoration of degraded areas in drylands due to the important role that cyanobacteria and their exopolysaccharides (EPS) play in the environment. So far, few studies have analyzed the macromolecular and chemical characteristics of the polysaccharidic matrix in induced cyanobacterial biocrusts and the scarce existing studies have mainly focused on sandy soil textures. However, the characteristics of the cyanobacterial polysaccharidic matrix may greatly depend on soil type. The objective of this study was to examine the macromolecular distribution and monosaccharidic composition of the polysaccharidic matrix induced by inoculation of two cyanobacterial species common in arid environments, Phormidium ambiguum (non N-fixing) and Scytonema javanicum (N-fixing) in different soil types. S. javanicum promoted a higher release in the soil of the more soluble and less condensed EPS fraction (i.e., the loosely bound EPS fraction, LB-EPS), while P. ambiguum showed a higher release of the less soluble and more condensed EPS fraction (i.e., the tightly bound EPS fraction, TB-EPS). LB-EPSs were mainly composed of low MW molecules (< 50 kDa), while TB-EPSs were mainly composed of high MW molecules (1100-2000 kDa). The two EPS fractions showed a complex monosaccharidic composition (from 11 to 12 different types of monosaccharides), with glucose as the most abundant monosaccharide, in particular in the poorer soils characterized by lower organic C contents. In more C-rich soils, high abundances of galactose, mannose, and xylose were also found. Low abundance of uronic acids and hydrophobic monosaccharides, such as fucose and rhamnose, was found in the EPS extracted from the inoculated soils. Our results point to the influence of soil type on the macromolecular distribution and monosaccharide composition of the polysaccharidic matrix in induced biocrusts, which is likely to affect biocrust development and their role in soil structure and nutrient cycling in restored dryland soils.

Optimizing the Production of Nursery-Based Biological Soil Crusts for Restoration of Arid Land Soils

Biocrust communities provide important ecosystem services for arid land soils, such as soil surface stabilization promoting erosion resistance and contributing to overall soil fertility. Anthropogenic degradation to biocrust communities (through livestock grazing, agriculture, urban sprawl, and trampling) is common and significant, resulting in a loss of those ecosystem services. Losses impact both the health of the native ecosystem and the public health of local populations due to enhanced dust emissions. Because of this, approaches for biocrust restoration are being developed worldwide. Here, we present optimization of a nursery-based approach to scaling up the production of biocrust inoculum for field restoration with respect to temporal dynamics and reuse of biological materials. Unexpectedly, we also report on complex population dynamics, significant spatial variability, and lower than expected yields that we ascribe to the demonstrable presence of cyanobacterial pathogens, the spread of which may be enhanced by some of the nursery production standard practices.

Biological soil crusts (biocrusts) are topsoil communities formed by cyanobacteria or other microbial primary producers and are typical of arid and semiarid environments. Biocrusts promote a range of ecosystem services, such as erosion resistance and soil fertility, but their degradation by often anthropogenic disturbance brings about the loss of these services. This has prompted interest in developing restoration techniques. One approach is to source biocrust remnants from the area of interest for scale-up cultivation in a microbial “nursery” that produces large quantities of high-quality inoculum for field deployment. However, growth dynamics and the ability to reuse the produced inoculum for continued production have not been assessed. To optimize production, we followed nursery growth dynamics of biocrusts from cold (Great Basin) and hot (Chihuahuan) deserts. Peak phototrophic biomass was attained between 3 and 7 weeks in cold desert biocrusts and at 12 weeks in those from hot deserts. We also reused the resultant biocrust inoculum to seed successive incubations, tracking both phototroph biomass and cyanobacterial community structure using 16S rRNA gene amplicon sequencing. Hot desert biocrusts showed little to no viability upon reinoculation, while cold desert biocrusts continued to grow, but at the expense of progressive shifts in species composition. This leads us to discourage the reuse of nursery-grown inoculum. Surprisingly, growth was highly variable among replicates, and overall yields were low, a fact that we attribute to the demonstrable presence of virulent and stochastically distributed but hitherto unknown cyanobacterial pathogens. We provide recommendations to avoid pathogen incidence in the process.

IMPORTANCE Biocrust communities provide important ecosystem services for arid land soils, such as soil surface stabilization promoting erosion resistance and contributing to overall soil fertility. Anthropogenic degradation to biocrust communities (through livestock grazing, agriculture, urban sprawl, and trampling) is common and significant, resulting in a loss of those ecosystem services. Losses impact both the health of the native ecosystem and the public health of local populations due to enhanced dust emissions. Because of this, approaches for biocrust restoration are being developed worldwide. Here, we present optimization of a nursery-based approach to scaling up the production of biocrust inoculum for field restoration with respect to temporal dynamics and reuse of biological materials. Unexpectedly, we also report on complex population dynamics, significant spatial variability, and lower than expected yields that we ascribe to the demonstrable presence of cyanobacterial pathogens, the spread of which may be enhanced by some of the nursery production standard practices.

Comparative physiological responses of Microcoleus vaginatus and Bryum argenteum to enhanced UV-B radiation under field conditions

UV-B radiation is an important environmental factor affecting the composition and function of biological soil crusts (BSCs). The aim of this study was to compare the effects of enhanced UV-B radiation on BSCs from Tengger Desert, north-western China, which are dominated by the cyanobacterium Microcoleus vaginatus Gom. and moss Bryum argenteum Hedw. The BSCs were exposed to four UV-B supplemental treatments, including 2.75 (control), 3.08, 3.25, and 3.41Wm-2, for 40 days under field condition. In both the studied organisms, UV-B radiation significantly affected the physiological properties (total flavonoids, soluble proteins, soluble sugars, and proline contents). While marginally enhanced UV-B radiation for a short period favoured the growth of M. vaginatus and B. argenteum, excessively high and prolonged UV-B radiation suppressed the physiological properties of the two organisms. Moreover, response index revealed that UV-B radiation had more detrimental effects on B. argenteum, suggesting that B. argenteum is more sensitive to UV-B radiation than M. vaginatus. The findings of this study could help to predict and evaluate the possible changes in the structure and function of desert ecosystems, based on the variation in physiological responses of M. vaginatus and B. argenteum to enhanced UV-B radiation.

Effects of Biological Soil Crusts on Enzyme Activities and Microbial Community in Soils of an Arid Ecosystem

Arid ecosystems constitute 41% of land's surface and play an important role in global carbon cycle. In particular, biological soil crusts (BSC) are known to be a hotspot of carbon fixation as well as mineralization in arid ecosystems. However, little information is available on carbon decomposition and microbes in BSC and key controlling variables for microbial activities in arid ecosystems. The current study, carried out in South Mediterranean arid ecosystem, aimed to evaluate the effects of intact and removed cyanobacteria/lichen crusts on soil properties, soil enzyme activities, and microbial abundances (bacteria and fungi). We compared five different treatments (bare soil, soil with intact cyanobacteria, soil with cyanobacteria removed, soil with intact lichens, and soil with lichens removed) in four different soil layers (0-5, 5-10, 10-15, and 15-20 cm). Regardless of soil treatments, activities of hydrolases and water content increased with increasing soil depth. The presence of lichens increased significantly hydrolase activities, which appeared to be associated with greater organic matter, nitrogen, and water contents. However, phenol oxidase was mainly controlled by pH and oxygen availability. Neither fungal nor bacterial abundance exhibited a significant correlation with enzyme activities suggesting that soil enzyme activities are mainly controlled by edaphic and environmental conditions rather than source microbes. Interestingly, the presence of lichens reduced the abundance of bacteria of which mechanism is still to be investigated.

A new biofilm based microalgal cultivation approach on shifting sand surface for desert cyanobacterium Microcoleus vaginatus

Biofilm based microalgal cultivation has recently received great attention because of its low water requirement and harvesting cost. However, the contradiction between microalgal attachment and harvesting still hinders the development of this technology. Therefore, in this study the most readily available and inexpensive shifting sand was used as attached substrate for microalgal (Microcoleus vaginatus) biofilm cultivation under different water conditions. After the inoculation, a stable and easily peeled microalgal biofilm formed through filamentous binding and exopolysaccharide cementing. In general, microalgal biomass, photosynthetic activity and exopolysaccharide accumulation were all significantly affected by the cultivation time, water content and their interaction (P<0.001). According to the maximal photosynthetic activity and microalgal productivity, cultivation time of microalgal biofilm on sand surface should be controlled around 15-25days, with water content at 10%. Based on the biofilm cultivation system, microalgal biomass yield reached up to 11gm^-2 eventually on the sand surface.

Analysis of environmental factors determining development and succession in biological soil crusts

Biological soil crusts play important ecological functions in arid and semi-arid regions, while different crust successional patterns appeared in different regions. Therefore in this study, the environmental conditions between Shapotou (with cyanobacterial, lichen and moss crusts) and Dalate Banner (with only cyanobacterial and moss crusts) regions of China were compared to investigate why lichen crusts only appeared in Shapotou; at the same time, artificial moss inoculation was conducted to find out the environmental factors promoting crust succession to moss stage. The results showed lichen crusts always developed from cyanobacterial crusts, which provide not only the stable soil surface, but also the biomass basis for lichen formation; furthermore, addition of crust physicochemical characteristics (primarily silt content) play a facilitating effect on lichen emergence (R(2)=0.53). The inoculation experiment demonstrated early crust soil surface and enough water holding content (>4%) provided the essential guarantee for moss germination. Our results show that there is heterogeneity in crust succession in different regions, which may be mainly affected by the ambient soil microenvironments. It is concluded that a positive feedback mechanism is expected between crust succession and ambient soil microenvironments; while a negative feedback mechanism forms between crust succession and free living cyanobacteria and algae.

Effects of light and temperature on open cultivation of desert cyanobacterium Microcoleus vaginatus

Microalgae cultivation has recently been recognized as an important issue to deal with the increasingly prominent resource and environmental problems. In this study, desert cyanobacterium Microcoleus vaginatus was open cultivated in 4 different cultivation conditions in Qubqi Desert, and it was found Chlorella sp., Scenedesmus sp. and Navicula sp. were the main contaminating microalgal species during the cultivation. High light intensity alone was responsible for the green algae contamination, but the accompanied high temperature was beneficial to cyanobacterial growth, and the maximum biomass productivity acquired was 41.3mgL(-1)d(-1). Low temperature was more suitable for contaminating diatoms' growth, although all the microalgae (including the target and contaminating) are still demand for a degree of light intensity, at least average daily light intensity >5μEm(-2)s(-1). As a whole, cultivation time, conditions and their interaction had a significant impact on microalgal photosynthetic activity (Fv/Fm), biomass and exopolysaccharides content (P<0.001).

Spatial distribution of microbial communities associated with dune landform in the Gurbantunggut Desert, China

The microbial community compositions and potential ammonia oxidation in the topsoil at different positions of sand dune (stoss slope, crest, lee slope, and interdune) from the Gurbantunggut Desert, the largest semi-fixed desert in China, were investigated using several molecular methods. Actinobacteria and Proteobacteria (especially Alphaproteobacteria) were commonly the dominant taxa across all soil samples. Bacterial communities were similar in soils collected from the stoss slopes and interdunes (HC-BSCs, biological soil crusts with a high abundance of cyanobacteria), containing more abundant cyanobacterial populations (16.9-24.5%) than those (0.2-0.7% of Cyanobacteria) in the crests and lee slopes (LC-BSCs, biological soil crusts with a low abundance of cyanobacteria). The Cyanobacteria were mainly composed of Microcoleus spp., and quantitative PCR analysis revealed that 16S rRNA gene copy numbers of Cyanobacteria (especially genus Microcoleus) were at least two orders of magnitude higher in HC-BSCs than in LC-BSCs. Heterotrophic Geodermatophilus spp. frequently occurred in HC-BSCs (2.5-8.0%), whereas genera Arthrobacter, Bacillus, and Segetibacter were significantly abundant in LC-BSC communities. By comparison, the desert archaeal communities were less complex, and were dominated by Nitrososphaera spp. The amoA gene abundance of ammonia-oxidizing archaea (AOA) was higher than that of ammonia-oxidizing bacteria (AOB) in all soil samples, particularly in the interdunal soils (10(6)-10(8) archaeal amoA gene copies per gram dry soil), indicating that AOA possibly dominate the ammonia oxidation at the interdunes.

Community succession of bacteria and eukaryotes in dune ecosystems of Gurbantünggüt Desert, Northwest China

Pyrosequencing and quantitative polymerase chain reaction of small subunit rRNA genes were used to provide a comprehensive examination of bacterial, cyanobacterial, and eukaryotic communities in the biological soil crusts (BSCs) of Gurbantünggüt Desert sand dunes (China). Three succession stages were recognized based on the analyses of eukaryotic communities: a late succession stage of BSCs in a swale with eukaryotes mainly related to the Bryophyta clade, an initial succession stage in a slope with barely any eukaryotic phototrophic microorganisms detected, and an intermediate succession type detected from both the swale and slope BSCs dominated by the phylum Chlorophyta. Moreover, the cyanobacterial community dominated all of the BSCs (48.2-69.5% of the total bacteria) and differed among the three succession stages: sequences related to Microcoleus steenstrupii and the genus Scytonema were abundant in the later succession stage, whereas both the initial and intermediate stages were dominated by Microcoleus vaginatus. Compared with swales, BSCs from slopes are exposed to a harsher environment, e.g., higher irradiance and lower water availability, and thus may be restricted from developing to a higher succession stage. Other disturbances such as wind and grazing may explain the different succession stages observed in swales or slopes. However, no clear differences were detected from non-phototrophic bacterial communities of the three succession stages, and sequences related to Alphaproteobacteria and Actinobacteria were most abundant in all the BSCs. The closest matches for the most frequent non-phototrophic bacterial genera were mainly derived from harsh environments, indicating the robustness of these genera.

Artificially accelerating the reversal of desertification: cyanobacterial inoculation facilitates the succession of vegetation communities

Desertification has been recognized as a global environmental problem, and one region experiencing ongoing desertification is the eastern edge of Qubqi Desert (Inner Mongolia). To investigate the facilitating effects of cyanobacterial inoculation technology on the desertification control along this steppe-desert transition region, artificial cyanobacterial crusts were constructed with two filamentous cyanobacteria 3 and 8 years ago combined with Salix planting. The results showed that no crusts formed after 3 years of fixation only with Salix planting, whereas after cyanobacterial inoculation, the crusts formed quickly and gradually succeed to moss crusts. During that course, topsoil environments were gradually improved, providing the necessary material basis for the regeneration of vascular plants. In this investigation, total 27 species of vascular plants had regenerated in the experimental region, mainly belonging to Asteraceae, Poaceae, Chenopodiaceae and Leguminosae. Using space time substitution, the dominant species along with the application of cyanobacterial inoculation technology succeeded from Agriophyllum squarrosum ultimately to Leymus chinensis. In addition, it was found that the shady side of the dunes is more conducive to crust development and succession of vegetation communities. Conclusively, our results indicate artificial cyanobacterial inoculation technology is an effective and desirable path for desertification control.