CANCELLED - Lighting the dark: composition and interaction networks in dark biocrusts in the Mexican Chihuahuan desert

Background/Question/Methods

Biological soil crusts (biocrust) form diverse communities of cyanobacteria, lichens, and bryophytes on the soil surface; they are key components of many dryland ecosystems worldwide. Over the last few decades, “dark biocrust” has been referred to as contiguous, dark covers on dryland soils, constituted by dark-colored heterocystous cyanobacteria, with interspersed black soil lichens and micro fungi. In the heart of the Chihuahuan desert in Mexico, vast swathes of dark crust cover hundreds of hectares of open creosote desert scrub and halophytic grasslands in the Mapimi Biosphere Reserve. In this study we asked: how do contrasting microsites (beneath canopy, open interspace) associated with the most abundant shrub and grass species influence the diversity, composition, and ecological interaction networks of the bacterial and fungal communities of dark crust? We collected a total of 32 dark biocrust and soil samples beneath plant canopies and open interspaces of sites dominated by the shrubs Larrea tridentata and Prosopis glandulosa, and the grasses Pleuraphis mutica and Sporobolus airoides. We examined the bacterial and fungal communities of these samples using amplicon-based metabarcoding. Thereafter, we applied an interaction network analysis based on the Lotka-Volterra approach to infer positive and negative ecological interactions assessing the microbial community profile.

Results/Conclusions

Overall, we identified a total of 15 bacterial phyla, with Cyanobacteria being the most abundant phylum in all samples. Bacterial composition of the two microhabitats (under plant canopy and interspace) did not differ considering L. tridentata, P. glandulosa, and P. mutica. However, bacterial composition was different comparing the microhabitats linked to S. airoides sites (PERMANOVA, P=0.041), where there were 17 soil physical and chemical properties that significantly influenced the bacterial communities, mostly related to grass microhabitats. Neither plant species nor microsites influenced bacterial richness and diversity differentially. In dark crust fungal communities, Ascomycota was the overwhelmingly dominant phylum; only in the P. mutica site the fungal composition was different between the microhabitats (PERMANOVA, P =0.047). Nine soil variables were highly correlated with the fungal community composition in grass sites. Fungal diversity showed significant differences across sites (PERMANOVA, P = 0.001) but not at the microsite level. The cross-domain (bacterial-fungal) community network analysis identified six key taxa corresponding to the diverse shrub and grass microhabitats, mainly belonging to Cyanobacteria, with more than 200 interactions with other taxa. Despite the visual homogeneity of the dark crusts, different sites had unique key taxa and interaction networks, revealing an unaccounted-for diversity of dark crusts.