Specific Microbial Communities Associate with the Rhizosphere of Welwitschia mirabilis, a Living Fossil

Abstract

Welwitschia mirabilis is an ancient and rare plant distributed along the western coast of Namibia and Angola. Several aspects of Welwitschia 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 W. mirabilis 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 Nitratireductor, Steroidobacter, Pseudonocardia and three Phylobacteriaceae) that together with Rhizophagus, an arbuscular mycorrhizal fungus, and other putative plant growth-promoting microbes may interact synergistically to promote Welwitschia growth.

Fig 1. Relative proportions of the (a) bacteria and (b) fungi associated with the Welwitschia rhizosphere and bulk soil.

Error bars indicate mean ± SE.

Fig 2. Heatmap displaying the most abundant genera for rhizosphere and bulk soil samples.

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).

Fig 3. nMDS ordination plot (UniFrac dissimilarity matrix).

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.

Fig 4. Venn diagram showing the number of shared phylotypes of (a) bacteria and (b) fungi between the rhizosphere and bulk soil communities.