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Don A. Cowan - One of the best experts on this subject based on the ideXlab platform.
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novel lichen dominated hypolithic communities in the Namib Desert
Microbial Ecology, 2021Co-Authors: Asuncion De Los Rios, Don A. Cowan, Gillian Maggskolling, Isaac Garridobenavent, Alicia Limon, Errol D Cason, Angel ValverdeAbstract:The ventral surfaces of translucent rocks from hot Desert pavements often harbor hypolithic microbial communities, which are mostly dominated by cyanobacteria. The Namib Desert fog belt supports extensive hypolithic colonization of quartz rocks, which are also colonized by lichens on their dorsal surfaces. Here, we aim to evaluate whether lichens colonize the ventral surface of the rocks (i.e., show hypolithic lifestyle) and compare the bacterial composition of these coastal hypolithic communities with those found inland. Fungal DNA barcoding and fungal and bacterial Illumina metabarcoding were combined with electron microscopy to characterize the composition and spatial structure of hypolithic communities from two (coastal and inland) areas in the Namib Desert. We report, for the first time, the structure and composition of lichen-dominated hypolithic communities found in the coastal zone of the Namib Desert with extensive epilithic lichen cover. Lichen modified areoles with inverted morphology of the genus Stellarangia (three lineages) and Buellia (two lineages) were the main components of these hypolithic communities. Some of these lineages were also found in epilithic habitats. These lichen-dominated hypolithic communities differed in structural organization and bacterial community composition from those found in inland areas. The hypolithic lichen colonization characterized here seems not to be an extension of epilithic or biological soil crust lichen growths but the result of specific sublithic microenvironmental conditions. Moisture derived from fog and dew could be the main driver of this unique colonization.
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differences in precipitation regime shape microbial community composition and functional potential in Namib Desert soils
Microbial Ecology, 2021Co-Authors: Yashini Naidoo, Angel Valverde, Rian Pierneef, Don A. CowanAbstract:Precipitation is one of the major constraints influencing the diversity, structure, and activity of soil microbial communities in Desert ecosystems. However, the effect of changes in precipitation on soil microbial communities in arid soil microbiomes remains unresolved. In this study, using 16S rRNA gene high-throughput sequencing and shotgun metagenome sequencing, we explored changes in taxonomic composition and functional potential across two zones in the Namib Desert with contrasting precipitation regime. We found that precipitation regime had no effect on taxonomic and functional alpha-diversity, but that microbial community composition and functional potential (beta-diversity) changed with increased precipitation. For instance, Acidobacteriota and ‘resistance to antibiotics and toxic compounds’ related genes were relatively more abundant in the high-rainfall zone. These changes were largely due to a small set of microbial taxa, some of which were present in low abundance (i.e. members of the rare biosphere). Overall, these results indicate that key climatic factors (i.e. precipitation) shape the taxonomic and functional attributes of the arid soil microbiome. This research provides insight into how changes in precipitation patterns associated with global climate change may impact microbial community structure and function in Desert soils.
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nutrient acquisition rather than stress response over diel cycles drives microbial transcription in a hyper arid Namib Desert soil
Frontiers in Microbiology, 2019Co-Authors: Carlos Leonsobrino, Jean-baptiste Ramond, Gillian Maggskolling, Don A. CowanAbstract:Hot Desert surface soils are characterised by extremely low water activities for large parts of any annual cycle. It is widely assumed that microbial processes in such soils are very limited. Here we present the first metatranscriptomic survey of microbial community function in a low water activity hyperarid Desert soil. Sequencing of total mRNA revealed a diverse and active community, dominated by Actinobacteria. Metatranscriptomic analysis of samples taken at different times over three days indicated that functional diel variations were limited at the whole community level, and mostly affected the eukaryotic subpopulation which was induced during the cooler night hours. High levels of transcription of chemoautotrophic carbon fixation genes contrasted with limited expression of photosynthetic genes, indicating that chemoautotrophy is an important alternative to photosynthesis for carbon cycling in desiccated Desert soils. Analysis of the transcriptional levels of key N-cycling genes provided strong evidence that soil nitrate was the dominant nitrogen input source. Transcriptional network analyses and taxon-resolved functional profiling suggested that nutrient acquisition processes, and not diurnal environmental variation, were the main drivers of community activity in hyperarid Namib Desert soil. While we also observed significant levels of expression of common stress response genes, these genes were not dominant hubs in the co-occurrence network.
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ancient landscapes of the Namib Desert harbor high levels of genetic variability and deeply divergent lineages for collembola
Ecology and Evolution, 2019Co-Authors: Gemma E Collins, Gillian Maggskolling, Ian D Hogg, Janine R Baxter, Don A. CowanAbstract:Aim To assess spatial patterns of genetic and species-level diversity for Namib Desert Collembola using mitochondrial DNA cytochrome c oxidase subunit I (COI) gene sequences. Location Namib Desert gravel plains. Taxon Collembola (springtails). Methods A total of 77 soil samples were collected along NE-SW (60 km) and E-W (160 km) transects from within a 4,000 km2 area of the Namib Desert gravel plains. We extracted 434 springtails from the 37 samples which contained Collembola and sequenced them at the COI gene locus. In the absence of specific taxonomic keys and previous genetic data for these taxa, we used Generalized Mixed Yule Coalescent (GMYC) analyses to provide putative species-level designations. Results We obtained 341 successful COI sequences, 175 of which were unique haplotypes. GMYC analyses identified 30 putative species, with up to 28% sequence divergence (uncorrected p-distance). The distribution of genetic variants was disjunct, with 97% of haplotypes and 70% of "GMYC species" found only at single sites. Main conclusions Dispersal events, although rare, may be facilitated by environmental events such as prevailing onshore winds or occasional flow of rainwater to the coast. We conclude that the high genetic diversity we observed is the result of ancient springtail lineages, patchy distribution of suitable habitats, and limited dispersal (gene flow) among habitable locations.
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Data_Sheet_1_Nutrient Acquisition, Rather Than Stress Response Over Diel Cycles, Drives Microbial Transcription in a Hyper-Arid Namib Desert Soil.doc
2019Co-Authors: Carlos León-sobrino, Jean-baptiste Ramond, Gillian Maggs-kölling, Don A. CowanAbstract:Hot Desert surface soils are characterized by extremely low water activities for large parts of any annual cycle. It is widely assumed that microbial processes in such soils are very limited. Here we present the first metatranscriptomic survey of microbial community function in a low water activity hyperarid Desert soil. Sequencing of total mRNA revealed a diverse and active community, dominated by Actinobacteria. Metatranscriptomic analysis of samples taken at different times over 3 days indicated that functional diel variations were limited at the whole community level, and mostly affected the eukaryotic subpopulation which was induced during the cooler night hours. High levels of transcription of chemoautotrophic carbon fixation genes contrasted with limited expression of photosynthetic genes, indicating that chemoautotrophy is an important alternative to photosynthesis for carbon cycling in desiccated Desert soils. Analysis of the transcriptional levels of key N-cycling genes provided strong evidence that soil nitrate was the dominant nitrogen input source. Transcriptional network analyses and taxon-resolved functional profiling suggested that nutrient acquisition processes, and not diurnal environmental variation, were the main drivers of community activity in hyperarid Namib Desert soil. While we also observed significant levels of expression of common stress response genes, these genes were not dominant hubs in the co-occurrence network.
Mary Seely - One of the best experts on this subject based on the ideXlab platform.
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Namib Desert primary productivity is driven by cryptic microbial community n fixation
Scientific Reports, 2018Co-Authors: Jean-baptiste Ramond, Mary Seely, Stephan Woodborne, Grant Hall, Don A. CowanAbstract:Carbon exchange in drylands is typically low, but during significant rainfall events (wet anomalies) drylands act as a C sink. During these anomalies the limitation on C uptake switches from water to nitrogen. In the Namib Desert of southern Africa, the N inventory in soil organic matter available for mineralisation is insufficient to support the observed increase in primary productivity. The C4 grasses that flourish after rainfall events are not capable of N fixation, and so there is no clear mechanism for adequate N fixation in dryland ecosystems to support rapid C uptake. Here we demonstrate that N fixation by photoautotrophic hypolithic communities forms the basis for the N budget for plant productivity events in the Namib Desert. Stable N isotope (δ15N) values of Namib Desert hypolithic biomass, and surface and subsurface soils were measured over 3 years across dune and gravel plain biotopes. Hypoliths showed significantly higher biomass and lower δ15N values than soil organic matter. The δ15N values of hypoliths approach the theoretical values for nitrogen fixation. Our results are strongly indicative that hypolithic communities are the foundation of productivity after rain events in the Namib Desert and are likely to play similar roles in other arid environments.
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the impact of fog on soil moisture dynamics in the Namib Desert
Author, 2018Co-Authors: Lixin Wang, Kudzai Farai Kaseke, Roland Vogt, Mary SeelyAbstract:Abstract Soil moisture is a crucial component supporting vegetation dynamics in drylands. Despite increasing attention on fog in dryland ecosystems, the statistical characterization of fog distribution and how fog affects soil moisture dynamics have not been seen in literature. To this end, daily fog records over two years (Dec 1, 2014–Nov 1, 2016) from three sites within the Namib Desert were used to characterize fog distribution. Two sites were located within the Gobabeb Research and Training Center vicinity, the gravel plains and the sand dunes. The third site was located at the gravel plains, Kleinberg. A subset of the fog data during rainless period was used to investigate the effect of fog on soil moisture. A stochastic modeling framework was used to simulate the effect of fog on soil moisture dynamics. Our results showed that fog distribution can be characterized by a Poisson process with two parameters (arrival rate λ and average depth α (mm)). Fog and soil moisture observations from eighty (Aug 19, 2015–Nov 6, 2015) rainless days indicated a moderate positive relationship between soil moisture and fog in the Gobabeb gravel plains, a weaker relationship in the Gobabeb sand dunes while no relationship was observed at the Kleinberg site. The modeling results suggested that mean and major peaks of soil moisture dynamics can be captured by the fog modeling. Our field observations demonstrated the effects of fog on soil moisture dynamics during rainless periods at some locations, which has important implications on soil biogeochemical processes. The statistical characterization and modeling of fog distribution are of great value to predict fog distribution and investigate the effects of potential changes in fog distribution on soil moisture dynamics.
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fog spatial distributions over the central Namib Desert an isotope approach
Author, 2018Co-Authors: Kudzai Farai Kaseke, Mary Seely, Chao Tian, Lixin Wang, Roland Vogt, Theo Wassenaar, Roland MushiAbstract:Fog is a characteristic feature of the Namib Desert and is essential to life in this fog dependent system. It is often acknowledged that advective fog from the ocean is the dominant fog type over the Namib Desert fog-zone but recent evidence suggests that other fog types occur in this area. Knowledge of the existence and spatial distribution of different fog types will enhance the mechanistic understanding of fog formation and potential changes in this region, but such knowledge is limited in literature. In this study, we investigated fog spatial variations within the Namib Desert fog-zone by applying stable isotope (δ^(18)O and δ^2H) techniques to differentiate various fog types and identify their source waters. Isotope based results showed that at least three types of fog (advective, radiation and mixed) occurred in this region and what appears as a single fog event may include all three types. Results suggest that radiation fog was the dominant fog type during our study period. The results also suggest that advective fog (with Atlantic Ocean origins) either dissipated 30-50 km inland and the residual humidity combined with locally derived moisture to form mixed fog or advective fog incorporated local moisture along its trajectory inland resulting in mixed fog. Fog in the Namib Desert was consistently depleted in ^(18)O and ^2H compared to rainfall and this was attributed to sub-cloud evaporation of the rainfall as well as different sources of fog and rainfall. Sub-cloud evaporation led to enrichment of ^(18)O and ^2H in rainfall beyond that of the first stage condensate, fog. Advective fog is often considered the architect of the fog-zone in the Namib Desert, but our results demonstrated multiple dominant fog types during the study period, suggesting knowledge of both fog frequency and fog type is needed to better predict climate change impacts on the fog-zone.
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Namib Desert soil microbial community diversity assembly and function along a natural xeric gradient
Microbial Ecology, 2018Co-Authors: Vincent Scola, Jean-baptiste Ramond, Olivier Zablocki, Evelien M Adriaenssens, Aline Frossard, Riegardt M Johnson, Mary SeelyAbstract:The hyperarid Namib Desert is a coastal Desert in southwestern Africa and one of the oldest and driest Deserts on the planet. It is characterized by a west/east increasing precipitation gradient and by regular coastal fog events (extending up to 75 km inland) that can also provide soil moisture. In this study, we evaluated the role of this natural aridity and xeric gradient on edaphic microbial community structure and function in the Namib Desert. A total of 80 individual soil samples were collected at 10-km intervals along a 190-km transect from the fog-dominated western coastal region to the eastern Desert boundary. Seventeen physicochemical parameters were measured for each soil sample. Soil parameters reflected the three a priori defined climatic/xeric zones along the transect (“fog,” “low rain,” and “high rain”). Microbial community structures were characterized by terminal restriction fragment length polymorphism fingerprinting and shotgun metaviromics, and their functional capacities were determined by extracellular enzyme activity assays. Both microbial community structures and activities differed significantly between the three xeric zones. The deep sequencing of surface soil metavirome libraries also showed shifts in viral composition along the xeric transect. While bacterial community assembly was influenced by soil chemistry and stochasticity along the transect, variations in community “function” were apparently tuned by xeric stress.
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metaviromes of extracellular soil viruses along a Namib Desert aridity gradient
Genome Announcements, 2017Co-Authors: Mary Seely, Jean-baptiste Ramond, Olivier Zablocki, Evelien M Adriaenssens, Aline Frossard, Don A. CowanAbstract:The Namib Desert in southwest Africa is hyperarid and composed of distinct microbial communities affected by a longitudinal aridity gradient. Here, we report four soil metaviromes from the Namib Desert, assessed using deep sequencing of metavirome libraries prepared from DNA extracted from gravel plain surface soils.
Jean-baptiste Ramond - One of the best experts on this subject based on the ideXlab platform.
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nutrient acquisition rather than stress response over diel cycles drives microbial transcription in a hyper arid Namib Desert soil
Frontiers in Microbiology, 2019Co-Authors: Carlos Leonsobrino, Jean-baptiste Ramond, Gillian Maggskolling, Don A. CowanAbstract:Hot Desert surface soils are characterised by extremely low water activities for large parts of any annual cycle. It is widely assumed that microbial processes in such soils are very limited. Here we present the first metatranscriptomic survey of microbial community function in a low water activity hyperarid Desert soil. Sequencing of total mRNA revealed a diverse and active community, dominated by Actinobacteria. Metatranscriptomic analysis of samples taken at different times over three days indicated that functional diel variations were limited at the whole community level, and mostly affected the eukaryotic subpopulation which was induced during the cooler night hours. High levels of transcription of chemoautotrophic carbon fixation genes contrasted with limited expression of photosynthetic genes, indicating that chemoautotrophy is an important alternative to photosynthesis for carbon cycling in desiccated Desert soils. Analysis of the transcriptional levels of key N-cycling genes provided strong evidence that soil nitrate was the dominant nitrogen input source. Transcriptional network analyses and taxon-resolved functional profiling suggested that nutrient acquisition processes, and not diurnal environmental variation, were the main drivers of community activity in hyperarid Namib Desert soil. While we also observed significant levels of expression of common stress response genes, these genes were not dominant hubs in the co-occurrence network.
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Data_Sheet_1_Nutrient Acquisition, Rather Than Stress Response Over Diel Cycles, Drives Microbial Transcription in a Hyper-Arid Namib Desert Soil.doc
2019Co-Authors: Carlos León-sobrino, Jean-baptiste Ramond, Gillian Maggs-kölling, Don A. CowanAbstract:Hot Desert surface soils are characterized by extremely low water activities for large parts of any annual cycle. It is widely assumed that microbial processes in such soils are very limited. Here we present the first metatranscriptomic survey of microbial community function in a low water activity hyperarid Desert soil. Sequencing of total mRNA revealed a diverse and active community, dominated by Actinobacteria. Metatranscriptomic analysis of samples taken at different times over 3 days indicated that functional diel variations were limited at the whole community level, and mostly affected the eukaryotic subpopulation which was induced during the cooler night hours. High levels of transcription of chemoautotrophic carbon fixation genes contrasted with limited expression of photosynthetic genes, indicating that chemoautotrophy is an important alternative to photosynthesis for carbon cycling in desiccated Desert soils. Analysis of the transcriptional levels of key N-cycling genes provided strong evidence that soil nitrate was the dominant nitrogen input source. Transcriptional network analyses and taxon-resolved functional profiling suggested that nutrient acquisition processes, and not diurnal environmental variation, were the main drivers of community activity in hyperarid Namib Desert soil. While we also observed significant levels of expression of common stress response genes, these genes were not dominant hubs in the co-occurrence network.
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Nutrient Acquisition, Rather Than Stress Response Over Diel Cycles, Drives Microbial Transcription in a Hyper-Arid Namib Desert Soil
Frontiers Media S.A., 2019Co-Authors: Carlos León-sobrino, Jean-baptiste Ramond, Gillian Maggs-kölling, Don A. CowanAbstract:Hot Desert surface soils are characterized by extremely low water activities for large parts of any annual cycle. It is widely assumed that microbial processes in such soils are very limited. Here we present the first metatranscriptomic survey of microbial community function in a low water activity hyperarid Desert soil. Sequencing of total mRNA revealed a diverse and active community, dominated by Actinobacteria. Metatranscriptomic analysis of samples taken at different times over 3 days indicated that functional diel variations were limited at the whole community level, and mostly affected the eukaryotic subpopulation which was induced during the cooler night hours. High levels of transcription of chemoautotrophic carbon fixation genes contrasted with limited expression of photosynthetic genes, indicating that chemoautotrophy is an important alternative to photosynthesis for carbon cycling in desiccated Desert soils. Analysis of the transcriptional levels of key N-cycling genes provided strong evidence that soil nitrate was the dominant nitrogen input source. Transcriptional network analyses and taxon-resolved functional profiling suggested that nutrient acquisition processes, and not diurnal environmental variation, were the main drivers of community activity in hyperarid Namib Desert soil. While we also observed significant levels of expression of common stress response genes, these genes were not dominant hubs in the co-occurrence network
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Nutrient acquisition, rather than stress response over diel cycles, drives microbial transcription in a dessicated Namib Desert soil
bioRxiv, 2018Co-Authors: Carlos León-sobrino, Jean-baptiste Ramond, Gillian Maggs-kölling, Don A. CowanAbstract:Hot Desert surface soils are characterised by extremely low water activities for large parts of any annual cycle. It is widely assumed that microbial processes in such soils are very limited. Here we present the first metatranscriptomic survey of microbial community function in a low water activity hyperarid Desert soil. Sequencing of total mRNA revealed a diverse and active community, dominated by Actinobacteria. Metatranscriptomic analysis of samples taken at different times over three days indicated that most functions did not fluctuate on a diel basis, except for a eukaryotic subpopulation which was induced during the cooler night hours. High levels of transcription of chemoautotrophic carbon fixation genes contrasted with limited expression of photosynthetic genes, indicating that chemoautotrophy is an important alternative to photosynthesis for carbon cycling in desiccated Desert soils. Analysis of the transcriptional levels of key N-cycling genes provided strong evidence that soil nitrate was the dominant nitrogen input source. Transcriptional network analyses and taxon-resolved functional profiling suggested that nutrient acquisition processes, and not diurnal environmental variation, were the main drivers of community activity in hyperarid Namib Desert soil. While we also observed significant levels of expression of common stress response genes, these genes were not dominant hubs in the co-occurrence network.
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Namib Desert primary productivity is driven by cryptic microbial community n fixation
Scientific Reports, 2018Co-Authors: Jean-baptiste Ramond, Mary Seely, Stephan Woodborne, Grant Hall, Don A. CowanAbstract:Carbon exchange in drylands is typically low, but during significant rainfall events (wet anomalies) drylands act as a C sink. During these anomalies the limitation on C uptake switches from water to nitrogen. In the Namib Desert of southern Africa, the N inventory in soil organic matter available for mineralisation is insufficient to support the observed increase in primary productivity. The C4 grasses that flourish after rainfall events are not capable of N fixation, and so there is no clear mechanism for adequate N fixation in dryland ecosystems to support rapid C uptake. Here we demonstrate that N fixation by photoautotrophic hypolithic communities forms the basis for the N budget for plant productivity events in the Namib Desert. Stable N isotope (δ15N) values of Namib Desert hypolithic biomass, and surface and subsurface soils were measured over 3 years across dune and gravel plain biotopes. Hypoliths showed significantly higher biomass and lower δ15N values than soil organic matter. The δ15N values of hypoliths approach the theoretical values for nitrogen fixation. Our results are strongly indicative that hypolithic communities are the foundation of productivity after rain events in the Namib Desert and are likely to play similar roles in other arid environments.
Angel Valverde - One of the best experts on this subject based on the ideXlab platform.
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novel lichen dominated hypolithic communities in the Namib Desert
Microbial Ecology, 2021Co-Authors: Asuncion De Los Rios, Don A. Cowan, Gillian Maggskolling, Isaac Garridobenavent, Alicia Limon, Errol D Cason, Angel ValverdeAbstract:The ventral surfaces of translucent rocks from hot Desert pavements often harbor hypolithic microbial communities, which are mostly dominated by cyanobacteria. The Namib Desert fog belt supports extensive hypolithic colonization of quartz rocks, which are also colonized by lichens on their dorsal surfaces. Here, we aim to evaluate whether lichens colonize the ventral surface of the rocks (i.e., show hypolithic lifestyle) and compare the bacterial composition of these coastal hypolithic communities with those found inland. Fungal DNA barcoding and fungal and bacterial Illumina metabarcoding were combined with electron microscopy to characterize the composition and spatial structure of hypolithic communities from two (coastal and inland) areas in the Namib Desert. We report, for the first time, the structure and composition of lichen-dominated hypolithic communities found in the coastal zone of the Namib Desert with extensive epilithic lichen cover. Lichen modified areoles with inverted morphology of the genus Stellarangia (three lineages) and Buellia (two lineages) were the main components of these hypolithic communities. Some of these lineages were also found in epilithic habitats. These lichen-dominated hypolithic communities differed in structural organization and bacterial community composition from those found in inland areas. The hypolithic lichen colonization characterized here seems not to be an extension of epilithic or biological soil crust lichen growths but the result of specific sublithic microenvironmental conditions. Moisture derived from fog and dew could be the main driver of this unique colonization.
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differences in precipitation regime shape microbial community composition and functional potential in Namib Desert soils
Microbial Ecology, 2021Co-Authors: Yashini Naidoo, Angel Valverde, Rian Pierneef, Don A. CowanAbstract:Precipitation is one of the major constraints influencing the diversity, structure, and activity of soil microbial communities in Desert ecosystems. However, the effect of changes in precipitation on soil microbial communities in arid soil microbiomes remains unresolved. In this study, using 16S rRNA gene high-throughput sequencing and shotgun metagenome sequencing, we explored changes in taxonomic composition and functional potential across two zones in the Namib Desert with contrasting precipitation regime. We found that precipitation regime had no effect on taxonomic and functional alpha-diversity, but that microbial community composition and functional potential (beta-diversity) changed with increased precipitation. For instance, Acidobacteriota and ‘resistance to antibiotics and toxic compounds’ related genes were relatively more abundant in the high-rainfall zone. These changes were largely due to a small set of microbial taxa, some of which were present in low abundance (i.e. members of the rare biosphere). Overall, these results indicate that key climatic factors (i.e. precipitation) shape the taxonomic and functional attributes of the arid soil microbiome. This research provides insight into how changes in precipitation patterns associated with global climate change may impact microbial community structure and function in Desert soils.
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hypolithic and soil microbial community assembly along an aridity gradient in the Namib Desert
Extremophiles, 2013Co-Authors: Mary Seely, Marla I. Tuffin, Angel Valverde, Francesca Stomeo, Stephen B Pointing, Christopher P Mckay, Kimberley A Warrenrhodes, Don A. CowanAbstract:The Namib Desert is considered the oldest Desert in the world and hyperarid for the last 5 million years. However, the environmental buffering provided by quartz and other translucent rocks supports extensive hypolithic microbial communities. In this study, open soil and hypolithic microbial communities have been investigated along an East–West transect characterized by an inverse fog-rainfall gradient. Multivariate analysis showed that structurally different microbial communities occur in soil and in hypolithic zones. Using variation partitioning, we found that hypolithic communities exhibited a fog-related distribution as indicated by the significant East–West clustering. Sodium content was also an important environmental factor affecting the composition of both soil and hypolithic microbial communities. Finally, although null models for patterns in microbial communities were not supported by experimental data, the amount of unexplained variation (68–97 %) suggests that stochastic processes also play a role in the assembly of such communities in the Namib Desert.
Heather Viles - One of the best experts on this subject based on the ideXlab platform.
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weathering in the central Namib Desert Namibia controls processes and implications
Journal of Arid Environments, 2013Co-Authors: Heather Viles, Andrew GoudieAbstract:Abstract The hyper-arid Namib Desert is an ancient Desert of great lithological diversity. Weathering plays a key role in landscape evolution in the Namib which, in spite of its aridity, has a number of sources of moisture – rainfall, fogs, dews and groundwater seepage – which enhance weathering. Among the weathering processes that have been the subject of recent study in the central Namib are salt, thermal and lichen weathering which, in often complex associations of processes, contribute to the array of small scale weathering features found on marble and granite outcrops here. At the landscape scale weathering in the central Namib is also highly interrelated with erosion and the development of geochemical sediments (calcrete, gypcrete, tufas etc), whilst weathering ‘hotspots’ are thought to be important sources of fine sediment production. Over the long term weathering has played important roles in landscape evolution here which, despite localized weathering hotspots, has been very slow for much of the Cenozoic.
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rapid salt weathering in the coastal Namib Desert implications for landscape development
Geomorphology, 2007Co-Authors: Heather Viles, Andrew GoudieAbstract:Data on the rate and style of breakdown on naturally occurring rock types are important to improving geomorphological understanding of landscape development in arid environments. Two years exposure of rock blocks cut from an English limestone (Bath Stone) as well as locally-relevant Namibian rocks (Karibib marble and Damara granite) within four distinctive microenvironments in and around a coastal salt pan environment reveals high rates of breakdown in the Bath Stone blocks, observable breakdown in the marble blocks, and weight gains and strength increases in the granite blocks. Breakdown is associated with high conductivities and large amounts of halite and gypsum within the surface sediments. Frequency of wetting and drying appears to be a major control on the resultant breakdown rates. These findings can be compared with recent data from further inland in the Namib Desert which suggest very slow rates of breakdown of similar blocks of Damara granite and Karibib marble, even where salts are readily available. High rates of rock breakdown within coastal salt pans appear to contribute to dust production and the effective flattening of relief along the coastal strip.
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microclimate and weathering in the central Namib Desert Namibia
Geomorphology, 2005Co-Authors: Heather VilesAbstract:Data on rock surface microclimates at four sites (Kleinberg, Vogelfederberg, Gobabeb, and Ganab) along a transect across the central Namib Desert over a 3-year period have been collected, alongside some shorter term data sets. Rock surface temperature (RST), air temperature, wind, relative humidity, and surface wetness data are presented here. Weathering of exposure blocks of local marble and granite has also been monitored over the 3-year period, with change observed visually and with scanning electron microscopy. Complex temporal patterns of rock temperature fluctuations are observed over the 3 years, although the four sites show very similar overall trends, suggesting spatial homogeneity in the rock surface temperature regime. Relative humidity and surface wetness data show clear differences between the four sites, related to the frequency of fogs. Granite blocks show no visible changes after 2 years of weathering, whereas marble blocks (especially those at Ganab) show SEM evidence of structural weakening after 1 year.
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terricolous lichens in the northern Namib Desert of Namibia distribution and community composition
Lichenologist, 2005Co-Authors: Jennifer S Lalley, Heather VilesAbstract:Terricolous lichens are the dominant vegetation in expansive areas of the Namib Desert, where fog is the main source of moisture and other vegetation is scarce. They play several important roles in soil crust stabilization and in the primary production of the Namib Desert ecosystem, yet little is known about the diversity and distribution of lichens in the northern unexplored regions of the Namib. To our knowledge, this study is the first survey of terricolous lichens to be carried out in the northern Namib Desert. Seven soil crust habitat types were identified in the study area, and a total of twenty-eight soil crust lichen species was recorded. The survey uncovered lichen species that may be unique to the northern Namib, including vagrant species, and common species were frequently found in a vagrant form. The crustose group was the most widely represented. Overall community compositions differed from those found in other regions of the Namib, and distribution patterns suggest a link to gravel clast size and physical soil crust type.
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the role of playas in pedogenic gypsum crust formation in the central Namib Desert a theoretical model
Earth Surface Processes and Landforms, 2001Co-Authors: Frank D Eckardt, Nicholas Drake, Kevin White, Andrew Goudie, Heather VilesAbstract:The formation of Namibia's extensive pedogenic gypsum crusts (CaSO4·2H2O) is interpreted in a new light. It is suggested that gypsum primarily precipitates at isolated points of evaporitic concentration, such as inland playas, and that deflation of evaporitic-rich gypsum dust from these playas contributes to the formation of pedogenic gypsum duricrusts on the coastal gravel plains of the Namib Desert surrounding these playas. This study establishes the nature, extent and distribution of playas in the Central Namib Desert and provides evidence for playa gypsum deflation and gravel plain deposition. Remote sensing shows the distribution of playas, captures ongoing deflation and provides evidence of gypsum deflation. It is proposed that, following primary marine aerosol deposition, both inland playas and coastal sabkhas generate gypsum which through the process of playa deflation and gravel plain redeposition contributes to the extensive pedogenic crusts found in the Namib Desert region. Copyright © 2001 John Wiley & Sons, Ltd.
