荒漠地表生物土壤结皮形成与演替特征概述

土壤表面结皮是世界范围内干旱沙漠地区土壤表面广泛存在的自然现象,包括物理结皮和生物土壤结皮两大类型。其中,生物土壤结皮作为干旱沙漠地区特殊环境的产物,是由细菌、真菌、蓝绿藻、地衣和苔藓植物与土壤形成的有机复合体。它的形成使土壤表面在物理、化学和生物学特性上均明显不同于松散沙土,具有较强的抗风蚀功能和重要的生态效应,成为干旱沙漠地区植被演替的重要基础。随着形成生物土壤结皮的物种更替,维持结皮结构的主要胶结方式亦随之发生变化,即由胞外多糖的粘结作用逐渐转变为蓝藻和荒漠藻的藻丝体、地衣菌丝体以及苔藓植物假根的缠绕和捆绑作用,物种更替是结皮微结构和胶结方式转化的生物基础。生物土壤结皮的形成通常可以分为以下几个阶段:生物土壤结皮的早期阶段(土壤酶和土壤微生物),藻结皮阶段、地衣结皮阶段和苔藓结皮阶段。即随着土壤微生物在沙土表面的生长,随后出现丝状蓝藻和荒漠藻类植物,形成以藻类植物为主体的荒漠藻结皮;当土壤表面得到一定固定后,便开始出现地衣和苔藓植物,形成以地衣和苔藓植物为优势的生物结皮类型。其中,前一阶段的完成又为后一阶段的开始提供良好的环境条件。当环境条件适宜时,生物土壤结皮也可以不经历其中某个阶段而直接发育到更高级的阶段。     

基于克隆文库法研究古尔班通古特沙漠蓝藻多样性

该研究采用克隆文库法研究古尔班通古特沙漠藻类生物结皮中蓝藻多样性及分布。在古尔班通古特沙漠不同区域采集10份藻类结皮代表性土样,分别构建古尔班通古特沙漠蓝藻16S rRNA和psbA基因克隆文库,并进行系统发育分析,对蓝藻多样性和丰度与环境因子进行关联分析,研究蓝藻分布特点及影响因子。结果显示:(1)16S rRNA基因系统发育树中包括具有明确分类地位的蓝藻有6科10属(占总克隆文库的94.85%)和一个未分类蓝藻属,其中颤藻属(Oscillatoria)和微鞘藻属(Microcoleus)分别占克隆文库的42.54%和37.16%,为古尔班通古特沙漠蓝藻优势属;psbA基因系统发育树中仅鉴定有蓝藻类群4科4属,但优势属与前者结果一致。(2)10个样点藻类结皮中所含蓝藻种类不尽相同,但每个采样点都出现颤藻属和微鞘藻属,证明这二者是古尔班通古特沙漠藻类结皮中的优势属;且样点Gur2和Gur17中蓝藻种类较多,Gur3、Gur5和Gur9中种类较少,但Gur2、Gur3、Gur5和Gur17相对地理位置较近,表明地理位置不是影响蓝藻分布的主要因素。(3)RDA(Redundancy analysis)分析结果显示,微生物量氮(MBN)和土壤有机碳(SOC)对蓝藻多样性影响程度最大,其次是硝态氮(NO~-_3-N)、微生物量碳(MBC),全磷(TP)和全钾(TK)对其影响程度最小。研究表明,古尔班通古特沙漠不同区域沙漠蓝藻多样性和土壤理化性质具有空间异质性,综合分析可得沙漠中南部藻类结皮土壤营养最为丰富,蓝藻多样性较高,而东部和西部土壤营养较为贫瘠,蓝藻丰富度和多样性较低。     

Phosphate-solubilizing peanut associated bacteria: screening for plant growth-promoting activities

Carbon and nitrogen are supplied by a variety of sources in the desert food web; both vascular and non-vascular plants and cyanobacteria supply carbon, and cyanobacteria and plant-associated rhizosphere bacteria are sources of biological nitrogen fixation. The objective of this study was to compare the relative influence of vascular plants and biological soil crusts on desert soil nematode and protozoan abundance and community composition. In the first experiment, biological soil crusts were removed by physical trampling. Treatments with crust removed had fewer nematodes and a greater relative ratio of bacterivores to microphytophages than treatments with intact crust. However, protozoa composition was similar with or without the presence of crusts. In a second experiment, nematode community composition was characterized along a spatial gradient away from stems of grasses or shrubs. Although nematodes generally occurred in increasing abundance nearer to plant stems, some genera (such as the enrichment-type Panagrolaimus) increased disproportionately more than others (such as the stress-tolerant Acromoldavicus). We propose that the impact of biological soil crusts and desert plants on soil microfauna, as reflected in the community composition of microbivorous nematodes, is a combination of carbon input, microclimate amelioration, and altered soil hydrology.     

When Is A Lineage A Species? A Case Study In Myxacorys gen. nov. (Synechococcales: Cyanobacteria) With The Description of Two New Species From The Americas

Soil cyanobacteria are crucial components of biological soil crusts and carry out many functions in dryland ecosystems. Despite this importance, their taxonomy and population genetics remain poorly known. We isolated 42 strains of simple filamentous cyanobacteria previously identified as Pseudophormidium hollerbachianum from 26 desert locations in the North and South America and characterized these strains using a total evidence approach, that is, using both morphological and molecular data to arrive at taxonomic decisions. Based on a phylogenetic analysis of 16S rRNA gene sequences, we propose and characterize Myxacorys gen. nov. with two new species Myxacorys chilensis, the generitype, and M. californica. We also found distinct 16S‐23S ITS sequence variability within species in our dataset. Especially interesting was the presence of two distinct lineages of M. californica obtained from locations in close spatial proximity (within a few meters to kilometers from each other) suggesting niche differentiation. The detection of such unrecognized lineage‐level variability in soil cyanobacteria has important implications for biocrust restoration practices and conservation efforts.     

古尔班通古特沙漠生物结皮微鞘藻分类学研究

实验研究了从古尔班通古特沙漠生物土壤结皮中分离纯化培养出的11株与微鞘藻(Microcoleus)形态接近的丝状蓝藻,通过形态特征、16S rRNA和ITS二级结构相结合的多相分析方法对其进行分类学研究。研究结果表明,实验藻株隶属于微鞘藻科(Microcoleaceae)的微鞘藻属(Microcoleus)和束脉藻属(Symplocastrum),其中包括2个中国新记录种:斯坦微鞘藻(Microcoleus steenstrupii)和细长束脉藻(Symplocastrum flechtnerii),另外还有具鞘微鞘藻(Microcoleus vaginatus)和类似斯坦微鞘藻的存疑物种。藻丝多少与排列方式、细胞大小与末端细胞形状,以及16S rRNA系统发育位置是确定微鞘藻(Microcoleus)与束脉藻(Symplocastrum)属于不同物种的关键依据, ITS二级结构是区分属内不同物种的重要参考。     

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.     

铜陵铜尾矿废弃地生物土壤结皮中的蓝藻多样性

生物土壤结皮是生态系统原生演替过程中的一个早期阶段,在铜陵铜尾矿废弃地自然生态恢复过程中生物土壤结皮在尾矿废弃地表面广泛分布.以生长在铜陵杨山冲和铜官山2处铜尾矿废弃地的生物土壤结皮为研究对象,运用常规培养方法和变性梯度凝胶电泳技术(PCR-DGGE)对不同群落生物土壤结皮中的蓝藻多样性及优势类群进行研究.结果表明2种研究方法所获得的蓝藻种类组成具有明显差异.显微观察结果表明常规培养试验中主要蓝藻类群为微囊藻属(Microcystis)、色球藻属(Chroococcus)、颤藻属(Oscillatoria)、念珠藻属(Nostoc)和浮鞘丝藻属(Planktolyngbya),其中优势种类主要为铜绿微囊藻(Microcystis aeruginosa)、断裂颤藻(Oscillatoria fracta)和细浮鞘丝藻(Planktolyngbya subtilis);提取样品中微生物总DNA,对蓝藻16SrRNA进行PCR-DGGE分析,回收DGGE图谱中24个条带进行测序分析,结果显示,所有序列与GenBank数据库中的近缘蓝藻的相似性系数均在93％以上,其中优势蓝藻类群主要隶属于微鞘藻属(Microcoleus)和细鞘丝藻属(Leptolyngbya),裸地(YL)处和木贼群落下尾矿表面(YM)的生物土壤结皮中优势蓝藻类群主要为微鞘藻属,而黄色真藓-藻类混合结皮(YT)和白茅群落( YB,TG)下的生物土壤结皮中的优势类群主要隶属于细鞘丝藻属.     

Effects of altered temperature and precipitation on desert protozoa associated with biological soil crusts

Biological soil crusts are diverse assemblages of bacteria, cyanobacteria, algae, fungi, lichens, and mosses that cover much of arid land soils. The objective of this study was to quantify protozoa associated with biological soil crusts and test the response of protozoa to increased temperature and precipitation as is predicted by some global climate models. Protozoa were more abundant when associated with cyanobacteria/lichen crusts than with cyanobacteria crusts alone. Amoebae, flagellates, and ciliates originating from the Colorado Plateau desert (cool desert, primarily winter precipitation) declined 50-, 10-, and 100-fold, respectively, when moved in field mesocosms to the Chihuahuan Desert (hot desert, primarily summer rain). However, this was not observed in protozoa collected from the Chihuahuan Desert and moved to the Sonoran desert (hot desert, also summer rain, but warmer than Chihuahuan Desert). Protozoa in culture began to encyst at 37 degrees C. Cysts survived the upper end of daily temperatures (37-55 degrees C), and could be stimulated to excyst if temperatures were reduced to 15 degrees C or lower. Results from this study suggest that cool desert protozoa are influenced negatively by increased summer precipitation during excessive summer temperatures, and that desert protozoa may be adapted to a specific desert's temperature and precipitation regime.     

Bacterial Communities in Soil Under Moss and Lichen-Moss Crusts

Biological soil crusts are symbiotic microbial communities formed by green algae, mosses, fungi, lichens, cyanobacteria and bacteria in different proportions. Crusts contribute to soil fertility and favour water retention and infiltration. However, little is known about the bacterial community structure in soil under the crusts. Soil was sampled under a moss crust (considered the MOSS group), lichen plus moss (considered the LICHEN group) and bare soil (considered the BARE group) and the microbial communities determined using nearly full 16S rRNA gene libraries. Bacteria belonging to seven different phyla were found and the Acidobacteria and Alphaproteobacteria were the dominant in each group. The crusts affected negatively the abundance of the Burkholderiales. The phylogenetic diversity and bacterial community membership were different in the LICHEN group compared to the BARE and MOSS groups, but not species richness and community structure. The beta diversity analysis also revealed a different bacterial community structure beneath the LICHEN and MOSS crusts, suggesting species-specific influence. This is a first insight into the effect of a biological soil crust on the bacterial community structure in an organic matter rich soil of a high altitude mountain forest.     

Cyanobacteria inhabiting biological soil crusts of a polar desert: Sør Rondane Mountains,Antarctica

Molecular and morphological methods were applied to study cyanobacterial community composition in biological soil crusts (BSCs) from four areas (two nunataks and two ridges) in the Sør Rondane Mountains, Antarctica. The sampling sites serve as control areas for open top chambers (OTCs) that were put in place in 2010 at the time of sample collection and will be compared with BSC samples taken from the OTCs in the future. Cyanobacterial cell biovolume was estimated using epifluorescence microscopy, which revealed the dominance of filamentous cyanobacteria in all studied sites except the Utsteinen ridge, where unicellular cyanobacteria were the most abundant. Cyanobacterial diversity was studied by a combination of molecular fingerprinting methods based on the 16S rRNA gene (denaturing gradient gel electrophoresis (DGGE) and 454 pyrosequencing) using cyanobacteria-specific primers. The number of DGGE sequences obtained per site was variable and, therefore, a high-throughput method was subsequently employed to improve the diversity coverage. Consistent with previous surveys in Antarctica, both methods showed that filamentous cyanobacteria, such as Leptolyngbya sp., Phormidium sp. and Microcoleus sp., were dominant in the studied sites.In addition, the studied localities differed in substrate type, climatic conditions and soil parameters, which probably resulted in differences in cyanobacterial community composition. Furthermore, the BSC growing on gneiss pebbles had lower cyanobacterial abundances than BSCs associated with granitic substrates.     

生物结皮粗糙特征——以古尔班通古特沙漠为例

摘要：空气动力粗糙度可以反映地表气流与下垫面的相互作用。古尔班通古特沙漠是我国最大的固定、半固定沙漠,其间广泛分布的生物结皮在稳定地表和改善环境方面意义重大。对未经扰动的4种类型生物结皮进行表面微形态观察,并通过风洞实验确定动力粗糙度Z0和摩阻风速u*,结果表明：（1）不同生物结皮类型,其组成和表面微形态等都具有明显差异。藻结皮以表面致密光滑为显著特征,由藻类分泌物和藻丝体粘结细粒物质所形成;地衣结皮表面藻类和真菌形成的叶状体匍匐沙面生长,呈现三维生长方式,形成有明显凹凸的壳状覆被;苔藓结皮以苔藓植物体密集丛生为特点,地上部分出现了茎叶分化,有一定的柔韧性。（2）就动力粗糙度的大小而言,是按地衣结皮>藻类－地衣结皮>苔藓结皮>藻结皮的顺序排列的,Z0平均值依次为（6.5890.850）mm、（4.1790.239）mm、（2.5420.357）mm和（0.3930.220）mm,与定床裸沙面的（0.0420.019）mm相比,生物结皮Z0值提高了10—150倍。随着风速的增大Z0值有所减小,其中以地衣结皮的减小趋势较为明显。（3）由风速廓线对比发现,四类生物结皮对气流阻滞作用的差异主要局限于4 cm以下的高度范围,风速越大这种差异也越大。各类生物结皮摩阻风速u*随风速增大而增大,其中藻结皮的增大速率明显低于其它三类结皮,说明藻结皮随风速增大的阻滞效应较其它三类结皮要差。（4）在净风条件下,地衣结皮具有最好的防风效果,其次为藻类－地衣结皮和苔藓结皮,藻结皮最差。当生物结皮破损后,床面结构和气流性质将发生变化,对空气动力学粗糙度和摩阻风速产生的影响将有待于进行更深入的研究。     

Hydrological response of biological soil crusts to global warming: A ten‐year simulative study

Biological soil crusts across the desert regions play a key role in regional ecological security and ecological health. They are vital biotic components of desert ecosystems that maintain soil stability, fix carbon and nitrogen, influence the establishment of vascular plants, and serve as habitats for a large number of arthropods and microorganisms, as well as influencing soil hydrological processes. Changes in temperature and precipitation are expected to influence the functioning of desert ecosystems by altering biotic components such as the species composition of biological soil crusts. However, it remains unclear how these important components will respond to the prolonged warming and reduced precipitation that is predicted to occur with climate change. To evaluate how the hydrological properties of these biological soil crusts respond to these alterations, we used open‐top chambers over a 10‐year period to simulate warming and reduced precipitation. Infiltration, dew entrapment, and evaporation were measured as surrogates of the hydrological functioning of biological soil crusts. It was found that the ongoing warming coupled with reduced precipitation will more strongly affect moss in crustal communities than lichens and cyanobacteria, which will lead to a direct alteration of the hydrological performance of biological soil crusts. Reductions in moss abundance, surface cover, and biomass resulted in a change in structure and function of crustal communities, decreased dew entrapment, and increased infiltration and evaporation of biological soil crusts in desert ecosystems, which further impacted on the desert soil water balance.     

不同生物土壤结皮微生物组跨膜转运蛋白基因多样性及差异

【背景】跨膜转运蛋白在微生物转运各种物质的过程中具有重要作用。【目的】通过比较原核微生物组磷酸转移酶(phosphotransferasesystem,PTS)系统和腺苷三磷酸结合盒(ATP-binding cassette,ABC)转运蛋白编码基因在两种不同生物土壤结皮中(藻结皮与藓结皮)的差异,以期揭示随着生物土壤结皮的发育演替,微生物组跨膜转运物质的生物学过程中的潜在变化趋势。【方法】对腾格里沙漠东南缘的藻结皮和藓结皮12个样品进行宏基因组测序,参照KEGG数据库PTS系统,与ABC转运蛋白代谢通路进行比较并筛选相关基因,分析其差异显著性。【结果】藻结皮和藓结皮PTS系统和ABC转运蛋白编码基因的多样性一致。在生物土壤结皮中共检测到16种PTS系统的转运蛋白的编码基因,具有显著性差异的有5种;检测到106种ABC转运蛋白的编码基因,具有显著性差异的有46种,并对这46种转运蛋白结合的底物以及变化趋势进行了详细的描述。【结论】生物土壤结皮发育演替过程中,微生物组从环境中摄取能够增加渗透势物质的潜力总体呈现降低趋势,转运氨基酸、细胞膜和细胞壁组分的潜力总体呈现增加趋势,对于矿物离子、...     

Passive restoration of vegetation and biological soil crusts following 80 years of exclusion from grazing across the Great Basin

Restoration targets for biological soil crusts are largely unknown. We surveyed seven 80‐year‐old grazing exclosures across northern Nevada for biocrusts to quantify reference conditions at relatively undisturbed sites. Exclosures were associated with the following plant communities: Wyoming big sagebrush, black sagebrush, and areas co‐dominated by winterfat and Wyoming big sagebrush. Cover of biocrusts and shrubs were generally higher than other plant groups at these sites, regardless of being inside or outside of the exclosures, suggesting these groups make up most of the native flora across the region. Important in forming soil structure, cyanobacteria of the order Oscillatoriales were less abundant and diverse in black sagebrush communities. Grazing had a negative effect on the abundance of Oscillatoriales but not the number of algal taxa, including cyanobacteria. Abundance of light algal crusts were not influenced by plant community or grazing. Dark algal crusts were generally less abundant on grazed sites. Influences of plant community and grazing were most apparent when accounting for reproductive rates of lichens and mosses based on establishment mechanisms. Abundance of shrubs, perennial grasses, Oscillatoriales, fast reproducing biocrusts and the number of algal and cyanobacterial taxa, varied by plant community, suggesting that restoration should be plant community specific. We demonstrate the affinity of rapidly reproducing biocrusts for winterfat‐Wyoming big sagebrush co‐dominated plant communities, regardless of grazing pressure. Across sites, the effects of grazing were most evident on the abundance of Oscillatoriales and slowly reproducing biocrusts following 80 years of cessation from grazing.     

Widespread desert affiliation of trebouxiophycean algae (Trebouxiophyceae,Chlorophyta) including discovery of three new desert genera

Unlike most other green algae, trebouxiophyceans are predominantly aerophytic and contain many symbiotic representatives. In recent years, a number of new terrestrial trebouxiophycean taxa were described from soils, tree bark, and lichens. The present phylogenetic study reveals three new lineages of free‐living trebouxiophyceans found in North American desert soil crusts and proposes new generic names to accommodate them: Desertella, Eremochloris, and Xerochlorella. This survey of desert isolates also led to discovery of representatives of seven existing genera of trebouxiophyceans. Two of these genera have never been reported to contain desert representatives and one was known previously only from aquatic habitats. Furthermore, we expand the known geographic range of the recently described genus Chloropyrula, heretofore only known from the Ural Mountains. We demonstrate that the diversity of trebouxiophyceans is still underestimated and poorly understood, and that most major trebouxiophycean lineages contain desert‐dwelling taxa.     

Chlorophytes of biological soil crusts in Gurbantunggut Desert, Xinjiang Autonomous Region, China

In this paper, chlorophytes collected from 253 biological soil crust samples in Gurbantunggut Desert in Xinjiang Autonomous Region, China were studied by field investigation and microscopical observation in lab. The flora composition, ecological distribution of chlorophytes in the desert and dynamic changes of species composition of chlorophytes in different developing stages of biological soil crusts are preliminarily analyzed. Results showed that there were 26 species belonging to 14 genera and 10 families, in which unicellular chlorophytes were dominant. There existed some differences in distribution of varied sand dune positions. The taxa of chlorophytes in leeward of sand dunes are most abundant, but the taxa in windward, interdune and the top of sand dunes reduced gradually. Chlorophytes were mainly distributed within the crust and the taxa of chlorophytes decrease obviously under the crust. In the devel-oping stages of the biological soil crust, species diversity of chlorophytes changed a little, but species composition pre-sented some differences. Chlorococcum humicola, Chlorella vulgaris, Chlamydomonas ovalis and Chlamydomonas sp. nearly existed in all developing stages of biological crusts. In several former stages of the biological soil crust there were spherical chlorophytes and filamentous ones. When moss crust formed, filamentous chlorophytes disappeared, such as Microspora and Ulothrix.     

Spatial Dominance and Inorganic Carbon Assimilation by Conspicuous Autotrophic Biofilms in a Physical and Chemical Gradient of a Cold Sulfurous Spring: The Role of Differential Ecological Strategies

The community composition and ecophysiological features of microbial autotrophic biofilms were studied in Fuente Podrida, a cold sulfur spring located in East Spain. We demonstrated how different ecophysiological strategies, such as resistance and/or utilization of sulfide and oxygen, light adaptation, or resistance to high water flow, allow each of the microorganisms described to efficiently colonize several areas within the environmental gradient. In the zone of the spring constantly influenced by sulfide-rich waters, biofilms were formed by purple bacteria, cyanobacteria, and filamentous colorless sulfur bacteria. Purple bacteria showed higher photosynthetic efficiency per pigment unit than cyanobacteria, although they were dominant only in anoxic areas. Two filamentous cyanobacteria, strain UVFP1 and strain UVFP2, were also abundant in the sulfide-rich area. Whereas the cyanobacterial strain UVFP2 shows a strategy based on the resistance to sulfide of oxygenic photosynthesis, strain UVFP1, additionally, has the capacity for sulfide-driven anoxygenic photosynthesis. Molecular phylogenetic analyses cluster the benthic strain UVFP1 with genus Planktothrix, but with no particular species, whereas UVFP2 does not closely cluster with any known cyanobacterial species. The colorless sulfur bacterium Thiothrix sp. extended throughout the zone in which both sulfide and oxygen were present, exhibiting its capacity for chemolithoautotrophic dark carbon fixation. Downstream from the source, where springwater mixes with well-oxygenated stream water and sulfide disappears, autotrophic biofilms were dominated by diatoms showing higher photosynthetic rates than cyanobacteria and, by a lesser extent, by a sulfide-sensitive cyanobacterium (strain UVFP3) well adapted to low light availability, although in the areas of higher water velocity far from the river shore, the dominance shifted to crust-forming cyanobacteria. Both types of microorganisms were highly sensitive to sulfide impeding them from occupying sulfide-rich areas of the spring. Sulfide, oxygen, light availability, and water velocity appear as the main factors structuring the autotrophic community of Fuente Podrida spring. An erratum to this article is availbale at .     

Biological Soil Crusts in a Xeric Florida Shrubland: Composition,Abundance, and Spatial Heterogeneity of Crusts with Different Disturbance Histories

Biological soil crusts consisting of algae, cyanobacteria, lichens, fungi, bacteria, and mosses are common in habitats where water and nutrients are limited and vascular plant cover is discontinuous. Crusts alter soil factors including water availability, nutrient content, and erosion susceptibility, and thus are likely to both directly and indirectly affect plants. To establish this link, we must first understand the crust landscape. We described the composition, abundance, and distribution of microalgae in crusts from a periodically burned, xeric Florida shrubland, with the goal of understanding the underlying variability they create for vascular plants, as well as the scale of that variability. This is the first comprehensive study of crusts in the southeastern United States, where the climate is mesic but sandy soils create xeric conditions. We found that crusts were both temporally and spatially heterogeneous in depth and species composition. For example, cyanobacteria and algae increased in abundance 10-15 years after fire and away from dominant shrubs. Chlorophyll a levels recovered rapidly from small-scale disturbance relative to intact crusts, but these disturbances added to crust patchiness. Plants less than 1 m apart can experience different crust environments that may alter plant fitness, plant interactions, and plant community composition.     

Lithic cyanobacterial communities in the polyextreme Sahara Desert: implications for the search for the limits of life

The hyperarid Sahara Desert presents extreme and persistent dry conditions with a limited number of hours during which the moisture availability, temperature and light allow phototrophic growth. Some cyanobacteria can live in these hostile conditions by seeking refuge under (hypolithic) or inside (endolithic) rocks, by colonizing porous spaces (cryptoendoliths) or fissures in stones (chasmoendoliths). Chroococcidiopsis spp. have been reported as the dominant or even the only phototrophs in these hot desert lithic communities. However, the results of this study reveal the high diversity of and variability in cyanobacteria among the sampled habitats in the Sahara Desert. The chasmoendolithic samples presented high coccoid cyanobacteria abundances, although the dominant cyanobacteria were distinct among different locations. A high predominance of a newly described cyanobacterium, Pseudoacaryochloris sahariense, was found in hard, compact, and more opaque stones with cryptoendolithic colonization. On the other hand, the hypolithic samples were dominated by filamentous, non-heterocystous cyanobacteria. Thermophysiological bioassays confirmed desiccation and extreme temperature tolerance as drivers in the cyanobacterial community composition of these lithic niches. The results of the present study provide key factors for understanding life strategies under polyextreme environmental conditions. The isolated strains, especially the newly described cyanobacterium P. sahariense, might represent suitable microorganisms in astrobiology studies aimed at investigating the limits of life.