Namib Research Articles

Nature-inspired surface for simultaneously harvesting water and triboelectric energy from ambient humidity using polymer brush coatings

Atmospheric water harvesting provides a promising, sustainable solution for alleviating the ever-growing water and energy crisis. Here, inspired by the Namib desert beetle, a wettability patterned surface was designed to simultaneously harvest water and triboelectric energy from ambient moisture. Indium tin oxide (ITO) conductive glass was coated by environmentally friendly perfluoropolyether (PFPE) polymer brushes to obtain a hydrophobic surface, decorated by hydrophilic patterns. The PFPE brushes enabled the droplet to slide down and served as the tribonegative material. The water and triboelectric energy harvesting performance of the hydrophilic-hydrophobic “patterned water and energy harvesting” (P-WEH) system was then investigated. In this regard, a water collection rate of 703 mg cm-2 h-1 was achieved when the P-WEH was placed within an artificial fog. The influence of pattern size and tilt angle, and their relationship with water droplet volume and speed on the triboelectric output, were measured. The effect of the water harvesting area on the output performance of the P-WEH was investigated. The P-WEH with an area of 100 cm2 and a tilt angle of 60° exhibited a high output current of 8.15 µA and a maximum output power of 3.35 µW. Finally, the P-WEH was integrated into a four-season greenhouse to demonstrate its application in reducing external water-energy consumption. This study presents insights into the design of simultaneous water and energy harvesting systems and may contribute to building a sustainable society.

Stellarangia namibensis (Teloschistaceae) out of the Namib Desert

During lichenological fieldwork along the coast of the Atlantic Sahara, Morocco, we collected some remarkably long lobed “Caloplaca” specimens, which turned out to be Stellarangia namibensis. This is the first record of this species in the northern hemisphere, and represents a significant extension of the known range of a lichen previously thought to be endemic to the Namib Desert.

A ca. 39,000-year record of vegetation and climate change from the margin of the Namib Sand Sea

AbstractThis paper presents the first continuous multi-proxy record of climate and vegetation change from the central Namib Desert extending over much of the last ca. 39,000 years. Derived from rock hyrax middens, evidence from stable carbon and nitrogen isotopes, pollen, and microcharcoal reveals significant differences between glacial-age and Holocene climates and vegetation types. Although still arid to semi-arid, conditions during Marine Oxygen Isotope Stages (MIS) 2–3 were significantly more humid than in the Late Holocene. Considerable associated vegetation change is apparent, with cooler temperatures and higher/more-regular rainfall promoting the westward expansion of relatively mesic shrubby karroid vegetation during MIS 2–3. With the last glacial–interglacial transition, increasing temperatures and less/less-regular rainfall resulted in marked vegetation changes and the establishment of current xeric grasslands. The inter-plant spacing of the karroid vegetation promoted by wetter conditions does not carry fire effectively, and the microcharcoal record indicates that more extensive fires may develop only with the development of grassier vegetation under drier conditions. As with other terrestrial records from the Namib Desert and environs, no Cape flora elements were found to support previously hypothesised expansion of the Fynbos Biome during the last glacial period.

Evolution at the arid extreme: the influence of climate on sand termite colonies and fairy circles of the Namib Desert.

In the hyperarid Namib Desert, the sand termite Psammotermes allocerus Silvestri, 1908 (Isoptera: Rhinotermitidae) establishes colonies that create conspicuous, barren patches known as 'fairy circles' on permeable, sandy soils. The central bare areas of fairy circles serve the key function of storing moisture received from sparse rainfall. The sandy soil texture allows rapid infiltration and percolation of precipitation, while localized herbivory by the termites creates the bare patch, thereby reducing the rapid loss of soil moisture by the uptake and transpiration of water by plants. The resulting storage of rain water even during prolonged periods of drought enables perennial life in hyperarid desert environments and forms a globally unique example of ecosystem engineering by social insects. During the past decade, most publications primarily debated the origin of fairy circles. Here, we contribute to the special issue with a focus on the functional and evolutionary dimension of the structure of the Psammotermes colony with two differing nest types and two spatially separated key resources, as a successful adaptation to extreme desert environment. The paper is primarily a review and a synthesis of previous work, with the inclusion of new, relevant findings. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

Multi-bioinspired Janus-shedding fabric: One-way sweat transport and rapid sweat removal for improved personal moisture management

In recent years, Janus fabrics that can effectively facilitate personal moisture management have received great attention. However, they are still limited in sweat shedding issue of their superhydrophilic outer surface. Here, inspired by the lotus leaf and Namib Desert beetles, we propose a Janus-shedding fabric with superhydrophilic/hydrophobic patterns and asymmetric wetting property. Through femtosecond-laser microfabrication, we successfully construct superhydrophilic/hydrophobic patterns on the outside surface of Janus fabrics. The method prevents the usage of toxic chemical reagent, thus is completely environmentally-friendly. The as-prepared Janus-shedding fabrics possess one-way liquid transport property. More importantly, the fabrics can achieve rapid liquid/sweat removal on their hydrophilic outer surfaces. The liquid shedding mechanism can be explained that the hydrophilic/hydrophobic patterns on the outer fabric surface can create less surface tension of water which can only fix small volume of water. Moreover, they can maintain human body temperature 3.2 ℃ lower than covered with pristine cotton fabrics. This work provides new insights for the design and fabrication of advanced smart fabrics with both sweat transport and removal property for satisfying the growing demands of personal comfort.

3D-printing-assisted fabrication of hierarchically structured biomimetic surfaces with dual-wettability for water harvesting

Freshwater acquisition methods under various environments are required because water scarcity has intensified worldwide. Furthermore, as water is an essential resource for humans, a freshwater acquisition method that can be utilized even under harsh conditions, such as waterless and polluted water environments, is highly required. In this study, a three-dimensional (3D) printing-assisted hierarchically structured surface with dual-wettability (i.e., surface with both hydrophobic and hydrophilic region) for fog harvesting was developed by mimicking the biological features (i.e., cactus spines and elytra of Namib Desert beetles) that have effective characteristics for fog harvesting. The cactus-shaped surface exhibited self-transportation ability of water droplet, derived from the Laplace pressure gradient. Additionally, microgrooved patterns of the cactus spines were implemented using the staircase effect of 3D printing. Moreover, a partial metal deposition method using wax-based masking was introduced to realize the dual wettability of the elytra of the Namib Desert beetle. Consequently, the proposed surface exhibited the best performance (average weight of 7.85 g for 10 min) for fog harvesting, which was enhanced by the synergetic effect between the Laplace pressure gradient and surface energy gradient. These results support a novel freshwater production system that can be utilized even in harsh conditions, such as waterless and polluted water environments.

Revisiting the Acheulean at Namib IV in the Namib Desert, Namibia

ABSTRACT Namib IV (S23° 44.829’, E14° 19.720’) is frequently cited, as it is one of few Earlier Stone Age sites in the Sand Sea of the Namib Desert. The site was first investigated in 1978 by Myra Shackley, who described 582 artifacts on the surface of a pan as representing an Acheulean butchery site. Descriptions of the artifacts, their number, and area were inconsistently reported. Recently rediscovered, the site of Namib IV is a rare example of a tool-rich and fossil fauna-bearing pan system in the Namib Sand Sea. This project aims to investigate when, how, and under what environmental conditions hominins utilized these landscapes. This article presents the first archaeological research conducted at the site in over 40 years. Typological and technological data was collected from surface-exposed artifacts and large cutting tools (LCTs) and compared to Shackley’s assemblage. Data demonstrate that her collection is representative of the Namib IV site and raise many new questions about the original research and the site.

Sand termite herbivory causes Namibia´ s fairy circles – A response to

In parts of Angola, Namibia and South Africa the sparse vegetation at the margin of the Namib Desert is often dotted with roughly circular bare patches. The origin of these “fairy circles” (FC) is subject of an ongoing debate. In a recent article in PPEES, Getzin et al. (2022) provided assessments of grasses and termites combined with soil moisture measurements, in and near to fairy circles in several areas in Namibia. In their interpretation they state that termite herbivory is not causing this grass death as the plants had undamaged roots. Instead they propose that the matrix grasses severely depleted the water in FCs. Here, we use a comprehensive, detailed body of measurements and assessments collated during the last 14 years to propose an alternative interpretation. We structure our interpretation with four statements, each of them based on shown evidence: (1) Long-term soil moisture measurements confirm that the soil beneath the dry topsoil of the bare patch of fairy circles contains an equal or, especially during the biologically active season, higher amount of moisture than the surrounding matrix, at any given time. The grasses of the fairy circles bare patch die during the moist phase of the first weeks after a rain, before even the soil beneath the matrix vegetation gets depleted by transpiration. (2) Within the sandy soils of fairy circle landscapes, there is no sufficiently strong “uptake –diffusion feedback” that could cause a horizontal movement of soil moisture over several meters within a few days. (3) The grasses of the fairy circles bare patch first die at the centre of the bare patch and later towards the margin. (4) The grass in the bare patch of fairy circles dies because of damage to roots due to herbivory by sand termites.

Net diversification rates of the woody plant genus Petalidium (Acanthaceae) are highest in the ancient and arid Namib Desert

At present, tropical arid biomes house less woody plant species diversity than tropical moist biomes, which could be due to lower rates of evolutionary diversification in the recent or distant past. Here, we study the evolutionary diversification of Petalidium (Acanthaceae), a genus of 36 species of woody shrubs found in the Namib Desert of southwest Africa, and surrounding areas. We generated a new, nearly fully sampled and temporally calibrated phylogeny for Petalidium using RADseq SNP data and secondary calibrations. We then investigated variation in net diversification rate across the phylogeny, the ancestral climatic niche of lineages and the link between the two. We find that arid climatic conditions are linked with increased rates of net species diversification in the genus. Despite its great age, the Namib Desert clearly hosts young plant radiations. This apparent contradiction can be explained by a scenario of high evolutionary turnover, in this case potentially caused by alternating hyper-arid and relatively mesic phases. Hyper-arid phases could result in high plant mortality and extinction of species, leading to ecological opportunity and diversification during mesic phases. Taken together, our results contribute to a growing body of literature that shows evidence for elevated rates of plant diversification in the Quaternary in arid biomes across the globe.

First record of the North African Launaea arborescens in southern Africa

The first record of the non-native, naturalised Launaea arborescens (Batt.) Murb. in the Namib Desert raised questions of its origin and whether or not it could pose a threat to the indigenous vegetation. The North African plant was introduced in a forestry nursery in the Kuiseb Delta and some individuals were also planted outside the nursery in the early 1970s. They have maintained a likely viable population for nearly 50 years, but have so far not been observed elsewhere and thus appear not to be spreading.

Contributions of hypolithic communities to surface soil organic carbon across a hyperarid-to-arid climate gradient

Carbon (C) in soils accounts for a substantial and dynamic portion of the global C cycle, with concentrations of soil organic carbon (SOC) typically closely linked to vascular plant productivity. However, C fixed by non-vascular photosynthetic organisms may account for a sizeable proportion of SOC in locations where vascular plants are not abundant. In the hyperarid Namib Desert, vascular plant growth is very limited and largely ephemeral. Extreme abiotic conditions even limit establishment of cyanobacterial soil crusts, with photoautotrophic cyanobacteria largely restricted to the undersides of translucent quartz clasts that buffer environmental extremes. The importance of these ‘hypoliths’ in enhancing SOC pools, and how this may vary with climate and clast physical characteristics, remains unknown. We worked across a rainfall and fog gradient in the central Namib to assess quartz clast size and distribution, factors affecting the probability of hypolithic colonization, and the landscape-level influence of hypoliths on surface SOC pools. Clast colonization increased with clast thickness and with increasing mean annual rainfall. SOC and chlorophyll a (a proxy for cyanobacterial biomass) concentrations were greater under colonized quartz clasts than under non-quartz clasts or in bare soil. Landscape-level SOC, estimated by combining SOC concentration with the distribution and colonization of quartz clasts, was greatest at the site with the highest rainfall and lowest at a mid-gradient site with moderate rainfall and fog. Climate change scenarios that promote quartz colonization have the potential to double SOC pools at the mid-gradient site, although SOC changes would be more muted at other sites. Low vascular plant and biocrust cover in the Namib Desert allows hypolithic communities to play an outsized role in SOC pools; a loss of all hypolithic cyanobacteria would lead to 10–20% declines in surface SOC pools. Future climate change has the potential to shift surface SOC if it alters cyanobacterial colonization of quartz.

On the effects of hybrid surface wettability on the structure of cavitating flow using implicit large eddy simulation

BackgroundSupercavitation flow is characterized by the presence of a supercavity, which can be unstable and cause unwanted motion or even loss of control of an object. To address this issue, flow control techniques are often employed. These techniques aim to manipulate the flow of liquid around the object and improve stability and control. Some of these methods include using control surfaces, injecting fluids, or more advanced techniques such as active flow control or smart materials. MethodThe aim of this study is to use computational methods to investigate the behaviour of supercavitation flow over a Clark-Y hydrofoil with different contact angle patterns (hybrid wettability) inspired by the Namib desert beetle. The hybrid wettability consists of six distinct patterns with a 160-degree contact angle for superhydrophobic surfaces and a five-degree contact angle for superhydrophilic surfaces. To predict the dynamic and unsteady behaviours of the supercavitation flow with a cavity number of 0.4, the implicit large eddy simulation (ILES) technique and Kunz mass transfer model are used. The compressive volume of fluid technique is used in conjunction with dynamic adaptive mesh refinement to track the interface between the vapour and liquid phases. The simulations are conducted using the interPhaseChangeFoam two-phase flow solver within the OpenFOAM framework. This study analyses the time-averaged and instantaneous fluid dynamic properties of the supercavitation flow of the hydrofoil with a hybrid surface, including pressure, velocity, vorticity fields, wake flow, recirculation zone, and liquid volume fraction. In addition, it examines the instantaneous cavity leading edge and flow separation, the vortical flow structure, vorticity stretching and dilatation, spanwise flow details, the creation of a low-pressure zone behind the hybrid hydrofoil, streamwise velocity fluctuations, and the cavity dynamics over an entire cycle. FindingsThe results show that the contact angle has a significant effect on the structure of the supercavitation flow. Specifically, the use of a superhydrophobic surface on the pressure side and trailing edge of a hydrofoil, along with a superhydrophilic surface on the leading edge, can reduce the thickness of the wake zone, the length of the cloud cavity, and flow instability. Additionally, this approach can delay the onset of cavitation.

Soil fungal diversity and assembly along a xeric stress gradient in the central Namib Desert

The Namib Desert of south-western Africa is one of the oldest deserts in the world and possesses unique geographical, biological and climatic features. While research through the last decade has generated a comprehensive survey of the prokaryotic communities in Namib Desert soils, little is yet known about the diversity and function of edaphic fungal communities, and even less of their responses to aridity. In this study, we have characterized soil fungal community diversity across the longitudinal xeric gradient across the Namib desert (for convenience, divided into the western fog zone, the central low-rainfall zone and the eastern high-rainfall zone), using internal transcribed sequence (ITS) metabarcoding. Ascomycota, Basidiomycota and Chytridiomycota consistently dominated the Namib Desert edaphic fungal communities and a core mycobiome composed of only 15 taxa, dominated by members of the class Dothideomycetes (Ascomycota), was identified. However, fungal community structures were significantly different in the fog, low-rainfall and high-rainfall zones. Furthermore, Namib Desert gravel plain fungal community assembly was driven by both deterministic and stochastic processes; the latter dominating in the all three xeric zones. We also present data that suggest that the inland limit of fog penetration represents an ecological barrier to fungal dispersal across the Namib Desert.

Protective role of trehalose in the Namib desert ant, Ocymyrmex robustior.

Over recent decades, increasing attention has been paid to how low-molecular-weight molecules affect thermal tolerance in animals. Although the disaccharide sugar trehalose is known to serve as a thermal protectant in unicellular organisms, nothing is known about its potential role in insects. In this study, we investigated the effect of trehalose on heat tolerance in the Namib desert ant, Ocymyrmex robustior, one of the most thermotolerant animals found in terrestrial ecosystems. First, we tested whether a trehalose-supplemented diet increased worker survival following exposure to heat stress. Second, we assessed the degree of protein damage by comparing protein aggregation levels for trehalose-supplemented workers and control workers. Third, we compared the expression levels of three genes involved in trehalose metabolism. We found that trehalose supplementation significantly enhanced worker heat tolerance, increased metabolic levels of trehalose and reduced protein aggregation under conditions of heat stress. Expression levels of the three genes varied in a manner that was consistent with the maintenance of trehalose in the hemolymph and tissues under conditions of heat stress. Altogether, these results suggest that increased trehalose concentration may help protect Namib desert ant individuals against heat stress. More generally, they highlight the role played by sugar metabolites in boosting tolerance in extremophiles.

Fog Detection and Estimation Using CALIPSO Lidar Observations

AbstractMost fog detection from space cannot differentiate fog and low stratus clouds, and cannot estimate fog deposition. This study assessed the feasibility of using spaceborne lidar observations from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) in fog detection and estimation. We tested the method in the central Namib Desert, Namibia, where frequent fog events occur and fog observations are available. Results showed the CALIPSO backscatter signal at 532 nm can differentiate low clouds and fog due to its high‐resolution vertical profiles. Backscatter signals during fog events were significantly higher than those during non‐fog periods. The R2 between backscatter signals and fog observations was 0.85. Moreover, the backscatter signal was also sensitive to relative humidity variation (R2 = 0.66). These results indicate that the CALIPSO data are feasible to estimate fog occurrence and deposition, providing a new perspective for space‐based fog studies.

Radiative cooling layer boosting hydrophilic-hydrophobic patterned surface for efficient water harvesting

Water harvesting from fog is of considerable interest as a nominated solution to freshwater shortage in arid and underdeveloped regions. The unique hydrophilic and hydrophobic patterns on the dorsal surface of the Namib Desert Beetle have been shown to be an evolving and effective strategy for harvesting water from air. Illuminated by the Namib desert beetle, different elaborate patterns based on aluminum plates were established and their water harvesting efficiency was investigated from a comparative perspective. The results showed that the vein-like patterned surfaces had the highest (786.15 mg∙cm−2 h−1) water collection efficiency. However, the heat released during the condensation process affected the water collection rate. In order to reduce the re-evaporation rate of water droplets, a radiant cooling layer was designed on the other side of the water collection plate to release the condensation heat. The radiation cooling layer employed MgHPO4ꞏ0.78 H2O as the functional particle and P(VDF-HFP) as the adhesive. Benefitted from radiative cooling layer, the water collection efficiency could be increased by 198.51 mg∙cm−2 h−1. The water collection material with double-sided function structure has low preparation costs and green raw materials. This work provides an effective way to develop key materials with freshwater harvesting capabilities.

Bioinspired slippery asymmetric bumps of candle soot coating for condensation and directional transport of water

Natural species use their surface structures for water harvesting. It is the dire need of the hour to learn from nature, mimic its functions, and solve the water scarcity problem through dew condensation. However, the present microscale and nanoscale approaches still suffer from complicated fabrication methods and durability failure in harsh conditions. Here, we introduce a bioinspired slippery surface of candle soot particles which uses its bumps to enhance the growth of condensed water droplets. The Namib desert beetles-inspired convex topography, cacti-inspired asymmetric slope, and Nepenthes pitcher plant-inspired slippery surface of soot particles synergistically combines the large droplet nucleation density, fast droplet coalescence, and easy removal of droplet from the surface. The hydrophilic SLIPS of soot particles harvests water at a rate of 326.4 ± 9 mg/cm2 h-1 which is 70 % high as compared to hydrophobic SLIPS of soot particles. The fluorinated SLIPS reveals 32 % high water collection rate as compared to the methyl-terminated SLIPS of candle soot particles and the results are corroborated by calculating the intermolecular hydrogen bonding energies through DFT. The programmed motion of water droplet on a slippery surface of soot particles is also observed by applying laser light, heat, and magnetic field even against gravity. The slippery surface shows long-term stability due to nano-porosity, permeation of lubricant into soot particles and lubricant’s crosslinking ability with the binder. The developed surface is biocompatible, self-cleaning, and suitable for water harvesting particularly in cold and arid regions.

Plant speciation in the Namib Desert: potential origin of a widespread derivative species from a narrow endemic

ABSTRACT Background Parapatric (or ‘budding’) speciation is increasingly recognised as an important phenomenon in plant evolution but its role in extreme (e.g. desert) environments is poorly documented. Aims To test this speciation model in a hypothesised sister pair, the Southwest – North African disjunct Senecio flavus and its putative progenitor, the Namibian Desert endemic S. englerianus. Methods Phylogenetic inferences were combined with niche divergence tests, morphometrics, and experimental-genetic approaches. We also evaluated the potential role of an African Dry Corridor (ADC) in promoting the hypothesised northward expansion of S. flavus (from Namibia), using palaeodistribution models. Results Belonging to an isolated (potential ‘relict’) clade, the two morphologically distinct species showed pronounced niche divergence in Namibia and signs of digenic-epistatic hybrid incompatibility (based on F2 pollen fertility). The presence of ‘connate-fluked’ pappus hairs in S. flavus, likely increasing dispersal ability, is controlled by a single-gene locus. Conclusions Our results provide evidence for a possible (and rare) example of ‘budding’ speciation in which a wider-ranged derivative (S. flavus) originated at the periphery of a smaller-ranged progenitor (S. englerianus) in the Namib Desert region. The Southwest – North African disjunction of S. flavus could have been established by dispersal across intermediate ADC areas during periods of (Late) Pleistocene aridification.

Bare areas caused by Hodotermes mossambicus in the northern Namib Desert

The harvester termite Hodotermes mossambicus is not regarded as causing fairy circles. On the other hand, the harvester termites forage huge amounts of grass biomass in the Namib Desert and beyond. This article describes quite different patterns caused by Hodotermes mossambicus in the Namib Desert. In addition to the well-known homogeneous foraging pattern this article newly describes hexagonal patterns and rare circular patterns from the Kunene region in northwestern Namibia.

Evidence for Psammotermes allocerus termite nests as refugium for plant pathogenic microbes: a contribution to generation and maintenance of fairy circles in the Namib Desert

Fairy circles are circular, barren structures in dry grasslands and the mechanisms generating and maintaining them are currently under intense discussion. Here, we analysed bacterial and fungal communities in Namib Desert fairy circle soils, in the tapetum lining of termite nests, the gut of the prevailing sand termite Psammotermes allocerus, and also in the neighbouring grass plants. The total and active microbial communities were analysed through amplicon sequencing of both DNA and RNA extracts, respectively, species-specific PCRs, and through cultivation of bacteria and fungi from the various samples. In the sandy soils, the active bacterial communities were substantially different from the total communities, which suggested the fairy circle soils to be highly selective environments in comparison to other drylands or even temperate soils. This appears to be even more true for fungi as amplification of fungal genes from RNA extracts was not possible. However, we observed a large diversity among cultured fungal species in comparison to cultured bacterial species, specifically in tapetum samples. Our results support the hypothesis that Psammotermes allocerus acts as a vector for plant pathogenic fungi and that their nests below the bare patches of fairy circles provide a refugium for the fungal plant pathogens.