The dark side of the moon: first insights into the microbiome structure and function of one of the last glacier-fed streams in Africa

Abstract

The glaciers on Africa's 'Mountains of the Moon' (Rwenzori National Park, Uganda) are predicted to disappear within the next decades owing to climate change. Consequently, the glacier-fed streams (GFSs) that drain them will vanish, along with their resident microbial communities. Despite the relevance of microbial communities for performing ecosystem processes in equatorial GFSs, their ecology remains understudied. Here, we show that the benthic microbiome from the Mt. Stanley GFS is distinct at several levels from other GFSs. Specifically, several novel taxa were present, and usually common groups such as Chrysophytes and Polaromonas exhibited lower relative abundances compared to higher-latitude GFSs, while cyanobacteria and diatoms were more abundant. The rich primary producer community in this GFS likely results from the greater environmental stability of the Afrotropics, and accordingly, heterotrophic processes dominated in the bacterial community. Metagenomics revealed that almost all prokaryotes in the Mt. Stanley GFS are capable of organic carbon oxidation, while greater than 80% have the potential for fermentation and acetate oxidation. Our findings suggest a close coupling between photoautotrophs and other microbes in this GFS, and provide a glimpse into the future for high-latitude GFSs globally where primary production is projected to increase with ongoing glacier shrinkage.

Keywords: Africa; Uganda; biofilms; climate change; microbial ecology; tropical glacier.

Figure 1.

Photographs showing the Mt. Stanley Glacier and its stream channel to the bottom left (a), and the bottom of the Mt. Stanley glacier-fed stream with angular sediments, green algae and mosses (b).

Figure 2.

Taxonomic composition of the microbiome in the Mt. Stanley GFS. Relative abundances of bacterial (a) and eukaryotic groups (b) in epilithic and epipsammic biofilms. Each bar represents an average of three patches from one upstream and one downstream reach. (c) Patterns of epipsammic bacterial community composition across mountain ranges illustrated on a PCoA based on Bray–Curtis dissimilarity. Ellipses represent the 95% confidence intervals.

Figure 3.

Abundance of organisms (expressed in percentages) involved in different carbon, nitrogen and sulfur cycling in epilithic and epipsammic communities of the GFS. Each sub-pathway is indicated as a step with the corresponding percentage of genomes encoding the respective genes. The flow grams were created using a modified script from METABOLIC [50].

Figure 4.

Heatmap representing the number of different CAZymes genes per prokaryotic MAGs (purple colour gradient) in the 15 most abundant taxonomic classes. For all CAZymes categories, the sum of the coverage of each gene is indicated on the right (black colour gradient). The numbers in parenthesis correspond to the number of MAGs per classes. The CAZYmes categories are CE (carbohydrate esterases), GH (glycoside hydrolases), GT (glycosyltransferases) and CBM (carbohydrate-binding modules).

Figure 5.

Abundance of prokaryotic MAGs able to degrade polysaccharides coloured by taxa. The size of the circles is representative of the number of MAGs of a specific class possessing the genes known to be involved in the degradation of the selected compounds (i.e. cellulase, fructose, fucose, galacturonic acid, sucrose and xylan).