Microbiotic Crusts Research Articles

Unearthing the Molecular Phylodiversity of Desert Soil Green Algae (Chlorophyta)

Deserts are not usually considered biodiversity hotspots, but desert microbiotic crust communities exhibit a rich diversity of both eukaryotic and prokaryotic life forms. Like many communities dominated by microscopic organisms, they defy characterization by traditional species-counting approaches to assessing biodiversity. Here we use exclusive molecular phylodiversity (E) to quantify the amount of evolutionary divergence unique to desert-dwelling green algae (Chlorophyta) in microbiotic crust communities. Given a phylogenetic tree with branch lengths expressed in units of expected substitutions per site, E is the total length of all tree segments representing exclusively desert lineages. Using MCMC to integrate over tree topologies and branch lengths provides 95% Bayesian credible intervals for phylodiversity measures. We found substantial exclusive molecular phylodiversity based on 18S rDNA data, showing that desert lineages are distantly related to their nearest aquatic relatives. Our results challenge conventional wisdom, which holds that there was a single origin of terrestrial green plants and that green algae are merely incidental visitors rather than indigenous components of desert communities. We identify examples of lineage diversification within deserts and at least 12 separate transitions from aquatic to terrestrial life apart from the most celebrated transition leading to the embryophyte land plants. [Bayesian phylogenetics; biodiversity; exclusive molecular phylodiversity; microbiotic crusts.].

Evolutionary characteristics of the artificially revegetated shrub ecosystem in the Tengger Desert, northern China

AbstractThe rain‐fed sand‐binding vegetation which stabilizes the migrating desert dunes in the Shapotou area at the southeastern edge of the Tengger Desert was initiated in 1956. The shrubs initially employed were predominantly Caragana korshinskii, Hedysarum scoparium, and Artemisia ordosica, and a desert shrub ecosystem with a dwarf shrub and microbiotic soil crust cover on the stabilized sand dunes has since developed. Since 1956 the success of this effort has not only ensured the smooth operation of the Baotou–Lanzhou railway in the sand dune area but has also played an important role in the restoration of the local eco‐environment; therefore, it is viewed as a successful model for desertification control and ecological restoration along the transport infrastructure in the arid desert region of China. Some of the effects of recovery from desertification and ecological restoration on soil properties are shown by the increase in the distribution of fine soil particles, organic matter, and nutrients. The physical surface structure of the stabilized sand dunes, and inorganic soil crusts formed by atmospheric dust, have also led to the gradual formation of microbiotic soil crusts. Sand dune stabilization is associated with: (1) decreased soil particle size, (2) increased total N, (3) increased thickness of microbiotic crusts, (4) increased thickness of subsoil, and (5) an increase in volumetric soil moisture in the near‐surface environment. After 17 years of dune stabilization, both the number of shrubs and community biomass decreased. The number of microbes, plant species and vegetation cover, all attained a maximum after the dunes had stabilized for 40 years. There is a significant positive correlation between the fractal dimension of soil particle size distribution (PSD) and the clay content of the shallow soil profile in the desert shrub ecosystem; the longer the period of dune stabilization, the greater the soil clay content in the shallow soil profiles (0–3 cm), and the greater the fractal dimension of soil PSD. This reflects the fact that during the revegetation processes, the soil structure is better developed, especially in the upper profile. Hence, the migrating sand dune becomes more stabilized. Therefore, the fractal model can be used to describe the texture and fertility of the soil, and, along with the degree of stability of the previously migrating sand dunes, can be used as an integrated quantitative index to evaluate the revegetation practice in the sand dune areas and their stabilization.

The microstructure of microbiotic crust and its influence on wind erosion for a sandy soil surface in the Gurbantunggut Desert of Northwestern China

Microscopic examination of microbiotic soil crust shows that the profiles of soil with a dense cyanobacterial cover had two different layers: a surface thin layer composed of aeolian-born materials and an organic layer formed by filamentous cyanobacteria associated with sand particles. The results indicate that microbiotic cover is an important determinant of sand fixation in the Gurbantunggut Desert, northern part of Xinjiang, Northwestern China. Microscopic examination of microbiotic crusts in this study revealed an intricate network of filamentous cyanobacteria and exopolysaccharides, which binds and entraps sand grains and conglutinate fine particles with each other. Resistance to wind erosion paralleled the different disturbance levels on microbiotic soil crust. Sandy soil surface disturbances resulted in greatly decreased soil resistance to wind erosion. Maximum wind tunnel velocity in this test (25 m s − 1 ) did not lead to any wind erosion on the surface of undisturbed microbiotic soil crust, i.e. 100% covered by microbiotic soil crust. As for different disturbance levels, the highest threshold friction velocity was seen in the sand surface with 10% disturbance of microbiotic crust. The surface microbiotic soil crusts have great effects on wind erosion rates. Wind erosion rates for sandy soil with 0% crust cover was about 46, 21, 17 times the soil with 90% crust cover at wind velocities of 18, 22, 25 m s − 1 , respectively. This study confirms that the planners and managers of nature reserves in this area should understand the important ecological roles of microbiotic crust in desert ecosystems. The reduction of trampling on the soil will eventually result in the re-establishment of biological crusts and their associated organisms, and ultimately lead to lower levels of wind erosion. Additionally, strategies should be developed to manage livestock and oil exploration in order to avoid concentrated zones of impact.

Recovery patterns of microbiotic soil crusts 70 years after arsenic contamination

The Mercur Gold Mine tailings pond experienced a dam failure more than 70 years ago, creating a denuded area that was the subject of this study. Arsenic levels in the surface soils ranged from 45 to 3237 mg As kg −1. Vascular plants, mosses, algae, and lichens all showed a threshold effect to elevated arsenic. Over 2100 mg As kg −1, chlorophyll a was consistently depressed. Over 1700 mg As kg −1, grasses, annuals and mosses were absent, and over 1400 mg As kg −1, cyanobacteria were notably affected. We conclude that the microbiotic crusts will not recover so long as the soil remains contaminated.

Ecological and environmental explanation of microbiotic crusts on sand dune scales in the Gurbantonggut Desert, Xinjiang

Abstract Results obtained from the field investigation and the analysis in laboratory show that many species of microbiotic crusts of lichens, mosses and algae develop extensively in the Gurbantonggut Desert, Xinjiang. The formation, species and distribution are closely related to the environmental conditions at the different positions of sand dunes. The animalcule crusts develop mainly on the mobile or semi-mobile sand surface of dune tops, the alga crusts develop mainly at the upper to middle parts of dune slopes, the lichen crusts develop at middle and lower parts of dune slopes, and the moss crusts are mainly distributed at the lower part of dune slopes and the interdune low-lands. The species, thickness and developing degree of microbiotic crusts increase from the upper part to the middle and lower parts of dune slopes and the interdune lowlands, and an obvious contrast between the microbiotic crusts and the different species of plant communities forms. The development and differentiation of microbio...

CRYPTIC SPECIES OF SCENEDESMUS (CHLOROPHYTA) FROM DESERT SOIL COMMUNITIES OF WESTERN NORTH AMERICA1

Nine isolates of unicellular green algae were obtained from six geographically separate desert microbiotic crust communities in western North America. Microscopically, eight isolates strongly resembled unicellular forms of Scenedesmus obliquus (Turpin) Kützing. They are oval or crescent shaped, often flattened on one side, with knobby cell apices. SEM indicated a lack of wall ornamentation. Fine filaments connecting cells pole to pole were observed in some isolates, as previously documented in Scenedesmus (Dactylococcus) dissociatus and S. obliquus. The ninth isolate was spherical, without knobby apices or connections between cells, and was similar to unicellular forms that were originally classified as species of Chlorella (Scenedesmus vacuolatus and S. rubescens). None of the isolates formed coenobia in liquid culture. Phylogenetic analysis of the 18S rRNA gene placed all desert isolates in the genus Scenedesmus, separating them into two or three weakly resolved groups along with published sequences of other Scenedesmus isolates. Phylogenetic analyses of the internal transcribed spacer region revealed well‐supported lineages of desert algae that were unsupported with 18S data alone. The eight S. obliquus‐like desert strains formed two distinct clades that excluded the S. obliquus strains from geographically widespread nondesert habitats. The ninth strain was outside of the S. obliquus group, associated with S. raciborskii and S. pectinatus. These results demonstrate three lineages of Scenedesmus from desert soils and provide robust support for the presence of cryptic species in S. obliquus, a morphospecies that is said to have a cosmopolitan distribution. Three new species of Scenedesmus are described.

Long‐Term Ecosystem Effects of Sand‐Binding Vegetation in the Tengger Desert, Northern China

Abstract The planting of sand‐binding vegetation in the Shapotou region at the southeastern edge of the Tengger Desert began in 1956. Over the past 46 years, it has not only insured the smooth operation of the Baotou–Lanzhou railway in the sand dune section but has also played an important role in the restoration of the local eco‐environment; therefore, it is viewed as a successful model for desertification control and ecological restoration along the transport line in the arid desert region of China. Long‐term monitoring and focused research show that within 4–5 years of establishment of sand‐binding vegetation, the physical surface structure of the sand dunes stabilized, and inorganic soil crusts formed by atmospheric dust gradually turned into microbiotic crusts. Among the organisms comprising these crusts are cryptogams such as desert algae and mosses. In the 46 years since establishing sand‐binding vegetation, some 24 algal species occurred in the crusts. However, only five moss species were identified, which was fewer than the species number in the crust of naturally fixed sand dunes. Other results of the planting were that near‐surface wind velocity in the 46‐year‐old vegetation area was reduced by 54.2% compared with that in the moving sand area; soil organic matter increased from 0.06% in moving sand dunes to 1.34% in the 46‐year‐old vegetation area; the main nutrients N, P, K, etc., in the desert ecosystem increased; soil physicochemical properties improved; and soil‐forming processes occurred in the dune surface layer. Overall, establishment of sand‐binding vegetation significantly impacted soil water cycles, creating favorable conditions for colonization by many herbaceous species. These herbaceous species, in turn, facilitated the colonization and persistence of birds, insects, soil animals, and desert animals. Forty‐six years later, some 28 bird species and 50 insect species were identified in the vegetated dune field. Thus, establishment of a relatively simple community of sand‐binding species led to the transformation of the relatively barren dune environment into a desert ecosystem with complex structure, composition, and function. This restoration effort shows the potential for short‐term manipulation of environmental variables (i.e., plant cover via artificial vegetation establishment) to begin the long‐term process of ecological restoration, particularly in arid climates, and demonstrates several techniques that can be used to scientifically monitor progress in large‐scale restoration projects.

Wind tunnel simulation of aeolian sandy soil erodibility under human disturbance

Inappropriate anthropogenic activities such as overcultivation and overgrazing of steppe and excessive collection of fuelwood are largely responsible for current desertification in China. However, quantitative information concerning the impacts of human disturbance on soil erosion remains sparse. This study investigated aeolian sandy soil erodibility under human disturbance by wind tunnel simulation. The fixed aeolian sandy soils were taken from an artificial vegetation protective system on mobile dunes at the southeastern edge of the Tengger Desert. The results indicate that: (1) human disturbance such as cultivation can accelerate the erodibility of the fixed aeolian sandy soil. The ratio between total soil loss from the undisturbed soil and the cultivated soil is about 0.004; (2) surface vegetation and microbiotic crust are the main factors responsible for the natural wind erodibility of the fixed sandy soil. Wind erosion rate increases with decreasing percent of the vegetation and crust cover; (3) the grain size distribution shows a higher percentage of particles in the range >1.0 mm and a lower percentage in the range <0.05 mm for the cultivated sandy soil than for the undisturbed fixed sandy soil.

Water film fauna of microbiotic crusts of a warm desert

Twenty-eight microbiotic crusts containing cyanobacteria, lichens and bryophytes, collected from desert soils of the Grand Canyon in northern Arizona, between Lees Ferry and Whitmore Wash in May 2000, were examined for water film fauna. Fifty-one species of ciliates, 28 of amoebae, 17 of testacea, four metazoan taxa, and a number of flagellate morphotypes were found. In the non-flagellated protozoan groups, r-selected bactivores, responding rapidly to wetting, quickly exploiting resources, and possessing efficient encysting abilities, predominated. The few nematodes were principally stylet-bearing tylenchids, feeding on fungal hyphae and moss rhizoids.

Development of immobilized cyanobacterial amendments for reclamation of microbiotic soil crusts

Cyanobacteria were immobilized on hemp cloth, which was subsequently cut into fine pieces for use as a soil amendment. The amendment is intended for speeding recovery of microbiotic soil crusts in semi-arid and arid lands where such crusts have been destroyed by anthropogenic activities. Microcoleus vaginatus, Schizothrix calcicola, and Nostoc were used to create amendments, but most of the experiments in this study utilized the Microcoleus amendment, as it is the most cosmopolitan and ecologically important cyanobacterial taxon in desert soil crusts. The amendment was found to retain its viability in storage for at least 18 months. M. vaginatus grew best in CT and Z8+ (plus vitamins) media with aeration and addition of KHCO 3 . A field test of Fort Irwin National Training Center, Mojave Desert, California, showed significant increases in phototrophic biomass 6 and 12 months after inoculation, but the stimulans was not evident at 18 months. Four other field tests did not show significant gains. We suspect the failure of the field tests is due to a loss of cyanobacterial viability after inoculation but before favorable growth conditions. Cyanobacteria were possibly killed by UV irradiation or photo-oxidation in the hot desert environment M. vaginatus is known to live below the surface in desert soils, and so likely is sensitive to full sunlight. Laboratory experiments in full sunlight verified this hypothesis.

98 Phylodiversity of green algae (chlorophyta) from desert microbiotic crusts

Deserts are not thought of as biodiversity hotspots, but desert microbiotic crust communities represent a largely unknown community type rich in diversity of eukaryotic and prokaryotic taxa. These ecologically important communities have received much attention because of their role in nutrient cycling and soil stabilization in deserts, but they defy characterization by the traditional approach to assessing biodiversity by counting species. While genetically diverse, taxa characteristic of desert crusts are difficult to identify to the species level due to convergent evolution toward simple morphologies, phenotypic plasticity, or poor knowledge about particular lineages. Focusing on the green algae, we show that while biodiversity is difficult to measure in these communities, phylodiversity provides a surrogate measure that more accurately portrays the diversity of organisms, and one that is standardized across the variety of life histories, reproductive strategies and morphological variability that creates problems with species‐counting measures. Bayesian phylogenetic inference uses MCMC simulation to generate phylogenies sampled in proportion to their Bayesian posterior probability. The length of a segment in any of these trees corresponds to the amount of change in the lineage, measured as the expected number of substitutions/nucleotide site. Comparisons of segment lengths corresponding to desert vs. other green algal lineages provides a means of addressing questions of relative genetic diversity, or phylodiversity, without complications arising from the difficulty of counting species. Our data illustrate the impact of desert green algae to overall knowledge of the green algal phylogenetic tree.

Use of foliar 15N and 13C abundance to evaluate effects of microbiotic crust on nitrogen and water utilization in Pinus massoniana in deteriorated pine stands of south China

We compared the foliar δ15N and δ13C values of Pinus massoniana growing on soils with and without microbiotic crust to examine the influence of the microbiotic crust on N and water use in plants in deteriorated watersheds in southern China. At our study site, litterfall and undergrowth had been intensively removed for fuel and soil N concentration was extremely low. Microbiotic crust covered the lower slope within the watersheds and pine trees were taller here than on the middle and upper slopes, although the crust reduced the amount of rainfall that could penetrate the soil. The foliar δ15N values were greater (closer to zero) in pine trees growing on soil covered with microbiotic crust on the lower slope than on the middle and upper slopes, which lacked the microbiotic crust. These data suggest that P. massoniana may depend on N fixed by the microbiotic crust on the lower slope, and on N carried by precipitation on the middle and upper slopes. The microbiotic crust did not influence foliar δ13C, an index for water use efficiency, in P. massoniana. The fact that P. massoniana biomass was greater on the lower slope, which is less permeable to rainfall, suggests that P. massoniana growth may be limited by the amount of available N rather than by water. The microbiotic crust may improve plant productivity by increasing N availability, despite its negative effect on water availability.

Microbiotic crust control of runoff generation on sand dunes in the Negev Desert

The general belief that dune sands in arid regions are not likely to generate runoff and that runoff necessitates wet conditions is examined. Runoff generation was measured within an arid dune field in the western Negev Desert, Israel, during 1990–1994 on 16 plots (1.2–6.6 m2), constructed along a continuum of crust‐covered crust. The relationships between chlorophyll a and carbohydrates of the microbiotic crust and runoff yield were studied. Whereas chlorophyll a (15–45 mg m−2) and carbohydrates (4.9–17.6 g m−2) of the cyanobacterial crusts showed positive linear relationships with runoff coefficients, a significant reduction in runoff yield was obtained once the moss‐dominated crust (with 53–54.5 mg m−2 chlorophyll a and 28.5–28.9 g m−2 of carbohydrates), which inhabited the wettest habitat within the dune field, was also included in the analysis. Contrary to previous assumptions that an increase in runoff generation occurs on arid and semiarid sand dunes in wet climates, wet climates may lead to the establishment of moss‐dominated crusts that in turn decrease runoff yield.

Cyanobacterial inoculation of heated soils: effect on microorganisms of C and N cycles and on chemical composition in soil surface

Physiological groups of soil microorganisms, total C and N and available nutrients were investigated in four heated (350 °C, 1 h) soils (one Ortic Podsol over sandstone and three Humic Cambisol over granite, schist or limestone) inoculated (1.5 μg chlorophyll a g −1 soil or 3.0 μg chlorophyll a g −1 soil) with four cyanobacterial strains of the genus Oscillatoria, Nostoc or Scytonema and a mixture of them. Cyanobacterial inoculation promoted the formation of microbiotic crusts which contained a relatively high number of NH 4 +-producers (7.4×10 9 g −1 crust), starch-mineralizing microbes (1.7×10 8 g −1 crust), cellulose-mineralizing microbes (1.4×10 6 g −1 crust) and NO 2 − and NO 3 − producers (6.9×10 4 and 7.3×10 3 g −1 crust, respectively). These crusts showed a wide range of C and N contents with an average of 293 g C kg −1 crust and 50 g N kg −1 crust, respectively. In general, Ca was the most abundant available nutrient (804 mg kg −1 crust), followed by Mg (269 mg kg −1 crust), K (173 mg kg −1 crust), Na (164 mg kg −1 crust) and P (129 mg kg −1 crust). There were close positive correlations among all the biotic and abiotic components of the crusts. Biofertilization with cyanobacteria induced great microbial proliferation as well as high increases in organic matter and nutrients in the surface of the heated soils. In general, cellulolytics were increased by four logarithmic units, amylolytics and ammonifiers by three logarithmic units and nitrifiers by more than two logarithmic units. C and N contents rose an average of 275 g C kg −1 soil and 50 g N kg −1 soil while the C:N ratio decreased up to 7 units. Among the available nutrients the highest increase was for Ca (315 mg kg −1 soil) followed by Mg (189 mg kg −1 soil), K (111 mg kg −1 soil), Na (109 mg kg −1 soil) and P (89 mg kg −1 soil). Fluctuations of the microbial groups as well as those of organic matter and nutrients were positively correlated. The efficacy of inoculation depended on both the type of soil and the class of inoculum. The best treatment was the mixture of the four strains and, whatever the inoculum used, the soil over lime showed the most developed crust followed by the soils over schist, granite and sandstone. In the medium term there were not significant differences between the two inocula amounts tested. These results showed that inoculation of burned soils with alien N 2-fixing cyanobacteria may be a biotechnological means of promoting microbiotic crust formation, enhancing C and N cycling microorganisms and increasing organic matter and nutrient contents in heated soils.

Study on Diversity and Binding-sand Effect of Moss on Biotic Crusts of Fixed Dunes

The study area, Shapotou district (37°27' N, 104°57' E) of Ningxia Hui Autonomous Region is situated in the southeastern fringe of the Tengger Desert of China. A study of species identification, coverage and biomass in the soil microbiotic crusts of fixed sand dunes was conducted in Shapotou district, from which we found two families, seven genera and sixteen species of mosses. Didymodon constrictus (Mitt. ) Saito and D. tectorwn ( C. Muell. ) Saito were reduced to synonymy of D. vinealis ( Brid.) Zand. by Zander and Red-fearn. Crossidium chloronotos (Brid. ) Limpr. , Crossidium aberrans Holz. Bartr. Aloina obliquifolia ( C. Muell. ) Broth. and Tortilla atrovirens ( Sm. ) lindb. are new to Shapotou district. Bryum argenteum is an eu-rytopic species that survives on all kinds of habitat, and is the dominant component that forms moss synusia. The community of Didymodon vinealis, D. rigidulus Hedw. var. ditrichoides and Syntrichia bidentata are found on hillocks and are randomly distributed in crusts. Diversity of species is the highest in this area in the natural fixed dunes, with sixteen species, followed by five species of mosses (31.3%) in the fixed dunes of 1956, four species of mosses (25.0%) in the fixed dunes of 1964, and only two species of mosses (12.5%) in the fixed dunes of 1981. The total coverage of mosses and algae increased with the fixed-sand time, hi different ages, the coverage of mosses and algae exist in an equal distance gradient, while in the fixed dunes they exist along the slope with the coverage of mosses decreasing as that of algae increased. A determination of biomass and binding-sand quantity of the dominant species-Bryum argenteum showed that its biomass (944.03 kg·hm-2) and binding-sand (3.925 × 104 kg·hm-2) quantity increased when the fixed sand time prolonged in moss crusts of the fixed dunes of 1950s. While the rate of binding-sand of Bryum argenteum decreased, its saturated water absorption had a positive correlation with biomass of 7.06 ×103 kg·hm-2 . Therefore the results indicate that the moss crusts had a strong ability to absorb and reserve water, which is of ecological significance in arid desert region.

The role of dew as a moisture source for sand microbiotic crusts in the Negev Desert, Israel

Dew and fog were measured periodically during the years 1992–1995 at the Hallamish dune field, western Negev Desert (average annual rain precipitation ca. 95 mm). A major portion of the dunes are covered by cyanobacterial and moss-dominated microbiotic crusts. Chlorophyll- a measurements and microscopic examination were carried out in order to detect the possible changes in crust biomass and phenological changes of the mosses following the occurrence of dew and fog. Dew and/or fog were present in over 50% of the late summer and fall mornings with an average yield of 0·1 mm on cloth attached to glass plates. This corresponds to ca. 0·03 mm for the upper 0·5 cm of the crust, and is less than the 0·1 mm threshold which implies liquid water and which was also found necessary for net photosynthesis of the cyanobacterial crust. This threshold was met only during heavy dew or fog events, during which 10–40% of the crust surface area was moistened. It was estimated that dew and fog could account for upto 3·2–9·4% of the total annual time of crust wetness and consequently activity due to liquid water. Nevertheless, microscopic examination of the dominant moss species at Nizzana, Bryum dunense, disclosed phenological changes, mainly resulting in growth of vegetative reproduction organs (bulbils), and sexual organs, gametangia (antheridia and archegonia). B.dunense may thus be adapted to the utilization of dew and fog precipitation for the production of nearly mature sexual organs. The moss may thus be capable of already utilizing the first winter rains for fertilization and sporophyte formation.

Ultrastructural and Biogeochemical Features of Microbiotic Soil Crusts and their Implications in a Semi-Arid Habitat

This investigation was designed to explore the relationships between lichen symbionts (phycobiont and mycobiont) and the substrate on which they grow by examining the chemical and ultrastructural features of the lichen-soil interface. These lichens form an integral part of microbiotic soil crusts. Fragments of three different lichen biotypes growing over gypsum crystals and marls were fixed and embedded in resin. The lichen-substratum interface was then examined by scanning electron microscopy with backscattered electron imaging. In situ observation, microanalytical (EDS), and FT-Raman plus infrared spectroscopy of the lichen-substratum interface indicated that different ultrastructural features of the mycobiont were related to biogeochemical processes and Ca 2+ distribution in the soil crust. Phycobionts were observed to make direct contact with the substratum and to be surrounded by a nondifferentiated thallus structure. These observations suggest that they can grow outside the thallus in the early stages of lichen development in the semi-arid conditions of their habitat. The particular ultrastructural features of the lichen thallus and of the lichen-substratum interface appear to have marked effects on runoff phenomena and ponding generation of the surface.

Cyanobacterial Microbiotic Crusts in Eroded Soils of a Tropical Dry Forest in the Baja California Peninsula, Mexico

This study deals with the cyanobacterial composition, and the nitrogen fixation of four members, of cryptobiotic crusts collected from eroded soils in a transitional area between arid and tropical climatic environments. Identification was based on microscopic analyses. Morphotypes were identified directly from reactivated natural crusts and from cultured strains. The identified morphotypes were Scytonema cf. ocellatum, Scytonema sp., Microcoleus cf. paludosus, M. cf. sociatus, Calothrix cf. elenkinii, C. cf. marchica, Nostoc cf. microscopicum, and Phormidium sp. Results show that the cyanobacterial composition of the microbiotic crusts studied is different from those in warmer and cooler deserts, particularly the absence of Microcoleus cf. vaginatus and Nostoc cf. commune in our samples. Such differences could be caused by the transitional character of the area. The results of the acetylene reduction assay show that the capacity of nitrogen fixation of some morphotypes is limited to the heterocyst-forming morphotypes.

Green algae (Chlorophyta) of desert microbiotic crusts: diversity of North American taxa

AbstractGreen algae are present in desert soils as components of microbiotic communities that also include cyanobacteria and other prokaryotes, lichens, non­lichenized fungi, invertebrates, and other photosynthetic eukaryotes such as diatoms, eustigmatophytes, and xanthophytes. The green algae that occur in crusts are morphologically simple unicells, packets of cells, or weak filaments, yet represent a diverse assemblage of taxa spanning the classes Chlorophyceae, Trebouxiophyceae, and Charophyceae. As part of an ongoing study of the biodiversity of microbiotic crust communities in the western United States and Northern Mexico, a large number of green algae were isolated and characterized morphologically and genetically. Phylogenetic analyses using ribosomal RNA gene sequence data have greatly aided our understanding of the diversity and evolution of desert green algae. Our results indicate that desert green algae evolved from aquatic green algae at least five independent times. In addition, the desert green algae are derived from freshwater, not marine, green ancestors. Some lineages of green algae have a high proportion of desert taxa, while other lineages thus far have no known desert representatives. Many of the isolates are likely to be new taxa. These taxa represent independent lineages of green plants that have evolved to inhabit desert environments. Because they are distinct from but phylogenetically related to embryophyte taxa, these other "land plants" can offer important biochemical and physiological comparisons to desert­dwelling embryophytes.

Green Algae (Chlorophyta) of Desert Microbiotic Crusts: Diversity of North American Taxa

Green Algae (Chlorophyta) of Desert Microbiotic Crusts: Diversity of North American Taxa