Increasing Aridity Impacts the Network Cohesion of the Arid Soil Microbiome.

The extreme precipitation gradient present in the Atacama Desert, Chile provides an opportunity to study potential impacts of aridity on the soil microbiome. Hyperarid regions of the desert support significantly less diverse microbial communities than more humid arid soils. In addition, a broad gradient in phylogenetic diversity is observed that correlates strongly with average soil relative humidity (AvgSoilRH) within the range of 17% to 100%. Significant changes in microbial community structure were also associated with the presence or absence of vegetation. What impact do these aridity-driven changes in microbial community structure have on the integrity of the soil microbiome? In this study, we use network analysis to evaluate effects of decreasing AvgSoilRH on co-occurrence patterns between members of the soil microbiome. The results reveal that OTUs primarily associated with arid sites characterized by higher AvgSoilRH are involved in more densely connected networks than those associated with the lower-AvgSoilRH hyperarid sites. In fact, an analysis of arid and hyperarid subnetworks revealed a break down in community cohesion. Arid communities retained an integrated topology, whereas the hyperarid subnetwork revealed a significant loss in graph cohesion that we termed the island effect. The average path length and betweenness centralization network metrics were lower for arid communities than hyperarid communities. A network simulation confirmed that the observed decrease in community connectivity was not simply an artifact of reduced community richness. The significance of a decrease in community connectivity with increasing aridity is unknown, however it could have implications for the resilience of the respective microbial communities and their potential contributions to ecosystem function.