Abstract
Bacterial and fungal communities in biofilms are important components in driving biogeochemical processes in stream ecosystems. Previous studies have well documented the patterns of bacterial alpha diversity in stream biofilms in glacier-fed streams, where, however, beta diversity of the microbial communities has received much less attention especially considering both bacterial and fungal communities. A focus on beta diversity can provide insights into the mechanisms driving community changes associated to large environmental fluctuations and disturbances, such as in glacier-fed streams. Moreover, modularity of co-occurrence networks can reveal more ecological and evolutionary properties of microbial communities beyond taxonomic groups. Here, integrating beta diversity and co-occurrence approach, we explored the network topology and modularity of the bacterial and fungal communities with consideration of environmental variation in glacier-fed streams in Central Asia. Combining results from hydrological modeling and normalized difference of vegetation index, this study highlighted that hydrological variables and vegetation status are major variables determining the environmental heterogeneity of glacier-fed streams. Bacterial communities formed a more complex and connected network, while the fungal communities formed a more clustered network. Moreover, the strong interrelations among the taxonomic dissimilarities of bacterial community (BC) and modules suggest they had common processes in driving diversity and taxonomic compositions across the heterogeneous environment. In contrast, fungal community (FC) and modules generally showed distinct driving processes to each other. Moreover, bacterial and fungal communities also had different driving processes. Furthermore, the variation of BC and modules were strongly correlated with hydrological properties and vegetation status but not with nutrients, while FC and modules (except one module) were not associated with environmental variation. Our results suggest that bacterial and fungal communities had distinct mechanisms in structuring microbial networks, and environmental variation had strong influences on bacterial communities but not on fungal communities. The fungal communities have unique assembly mechanisms and physiological properties which might lead to their insensitive responses to environmental variations compared to bacterial communities. Overall, beyond alpha diversity in previous studies, these results add our knowledge that bacterial and fungal communities have contrasting assembly mechanisms and respond differently to environmental variation in glacier-fed streams.
Highlights
Glaciers cover approximately 10% of the land surface on the Earth (Milner et al, 2017) and are important components of the hydrological cycle providing vital water resources (Barnett, Adam & Lettenmaier, 2005; Gardner et al, 2013; Zemp et al, 2015)
dissolved organic carbon (DOC) was negatively correlated with glacier source water (GS), total nitrogen (TN), and NO3−, while positively correlated with normalized difference vegetation index (NDVI) and pH (Fig. 2A)
Integrating beta diversity, our study provides the first co-occurrence network analyses of bacterial and fungal communities in glacier-fed streams
Summary
Glaciers cover approximately 10% of the land surface on the Earth (Milner et al, 2017) and are important components of the hydrological cycle providing vital water resources (Barnett, Adam & Lettenmaier, 2005; Gardner et al, 2013; Zemp et al, 2015). Previous studies have revealed that factors associated with glacier shrinkage have significant influences on the composition, diversity, and functional potential of bacterial communities in stream biofilms (Wilhelm et al, 2013, 2014; Ren, Gao & Elser, 2017; Ren et al, 2017). With the decrease in elevation, glacier coverage, and glacier source contribution to streamflow, as well as increase in distance to glacier terminus, bacterial communities showed increased alpha diversity as well as distinct taxonomic and functional compositions (Wilhelm et al, 2013, 2014; Ren, Gao & Elser, 2017; Ren et al, 2017). Comparing to alpha diversity, beta diversity is an underexplored facet of biodiversity (Mori, Isbell & Seidl, 2018), which accumulates from compositional variations among local assemblages and provides insights into the
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