Sulphur isotopes in the central Namib Desert ecosystem

<div><p>The Namib Desert is hyper-arid in terms of rainfall, but its ecology is influenced by frequent fog events. Fog utilisation by Namib biota has been well studied, but its role in nutrient deposition and cycling, particularly with respect to soil processes, still has open questions. Given its potential for distinguishing between various ecosystem components and fluxes, sulphur isotopic composition (δ³⁴S) is evaluated here as a passive tracer of aerosol deposition and plant water sources in the Namib. Measurements of δ³⁴S in Namib fog, groundwater, soils, plants and aerosols are presented and are consistent with the previously described system of sulphur cycling: primary marine sulphur accumulates as gypsum in the gravel plains and is redistributed by wind. Kuiseb River sediments had a wide range of δ³⁴S values, with several samples that were quite depleted relative to soils, plants, groundwater and gypsum of the gravel plains. This depleted signal appears more commonly in the fine (0.5, 1.0 <i>µ</i>m) rather than in the coarse (1.5, 7.6 <i>µ</i>m) aerosol size fractions. Fog and aerosol δ³⁴S values are consistent with local dust as a major sulphur source, limiting the utility of δ³⁴S as a unique tracer of fog deposition. It can still provide useful information in certain situations. For example, the 16.5‰ δ³⁴S value for the brackish groundwater at Hope Mine is distinct from the 10.2‰ value in <i>Welwitschia mirabilis</i> stem material at that site. This type of comparison could be one useful line of evidence in evaluating plant water sources.</p></div

Specific Microbial Communities Associate with the Rhizosphere of <i>Welwitschia mirabilis</i>, a Living Fossil

<div><p><i>Welwitschia mirabilis</i> is an ancient and rare plant distributed along the western coast of Namibia and Angola. Several aspects of <i>Welwitschia</i> biology and ecology have been investigated, but very little is known about the microbial communities associated with this plant. This study reports on the bacterial and fungal communities inhabiting the rhizosphere of <i>W</i>. <i>mirabilis</i> and the surrounding bulk soil. Rhizosphere communities were dominated by sequences of Alphaproteobacteria and Euromycetes, while Actinobacteria, Alphaproteobacteria, and fungi of the class Dothideomycetes jointly dominated bulk soil communities. Although microbial communities within the rhizosphere and soil samples were highly variable, very few “species” (OTUs defined at a 97% identity cut-off) were shared between these two environments. There was a small ‘core’ rhizosphere bacterial community (formed by <i>Nitratireductor</i>, <i>Steroidobacter</i>, <i>Pseudonocardia</i> and three <i>Phylobacteriaceae</i>) that together with <i>Rhizophagus</i>, an arbuscular mycorrhizal fungus, and other putative plant growth-promoting microbes may interact synergistically to promote <i>Welwitschia</i> growth.</p></div

nMDS ordination plot (UniFrac dissimilarity matrix).

<p>Each point represents the bacterial or fungal community of an individual sample. Rhizosphere communities are indicated by red diamonds, while bulk soil communities are denoted by blue diamonds.</p

Venn diagram showing the number of shared phylotypes of (a) bacteria and (b) fungi between the rhizosphere and bulk soil communities.

<p>Venn diagram showing the number of shared phylotypes of (a) bacteria and (b) fungi between the rhizosphere and bulk soil communities.</p

Relative proportions of the (a) bacteria and (b) fungi associated with the <i>Welwitschia</i> rhizosphere and bulk soil.

<p>Error bars indicate mean ± SE.</p

Heatmap displaying the most abundant genera for rhizosphere and bulk soil samples.

<p>Samples are clustered based on the percent relative abundance of the forty dominant genera (twenty bacteria and twenty fungal genera) shown as rows in this figure. Taxonomy for each genus is presented in the order: phylum, class, order, family, genus. Sample nomenclature indicates the sample type (S = bulk soil; R = rhizosphere), replicate (S = 1 to 5, R = 1 to 3) and pseudoreplicate (a, b).</p