Fluvial network organization imprints on microbial co-occurrence networks

Abstract

Recent studies highlight linkages among the architecture of ecological networks, their persistence facing environmental disturbance, and the related patterns of biodiversity. A hitherto unresolved question is whether the structure of the landscape inhabited by organisms leaves an imprint on their ecological networks. We analyzed, based on pyrosequencing profiling of the biofilm communities in 114 streams, how features inherent to fluvial networks affect the co-occurrence networks that the microorganisms form in these biofilms. Our findings suggest that hydrology and metacommunity dynamics, both changing predictably across fluvial networks, affect the fragmentation of the microbial co-occurrence networks throughout the fluvial network. The loss of taxa from co-occurrence networks demonstrates that the removal of gatekeepers disproportionately contributed to network fragmentation, which has potential implications for the functions biofilms fulfill in stream ecosystems. Our findings are critical because of increased anthropogenic pressures deteriorating stream ecosystem integrity and biodiversity.stream networks | hydrological regime S treams and rivers sculpt the continental surface, forming fluvial networks (1), within which the biodiversity ranks among the highest on Earth (2). The dendritic nature of fluvial networks has been shown to affect the spatial and temporal patterns of microbial, invertebrate, and fish biodiversity (3-8). Ecological theory and observations posit that the local environment governs the dynamics and diversity of ecological communities in headwaters, the smallest and most abundant streams in fluvial networks. In contrast, dispersal ensures that communities further downstream are shaped not only by their immediate environment but also by upstream processes (3-9). Thus, the dynamics of the metacommunity, which comprises all interconnected communities in a landscape (10), are inextricably linked to the organization and hydrology of the fluvial network (5-8). This perception is essential to understand, predict, and manage streams and rivers and their resistance and resilience to human alterations across scales (that is, from patches to the catchment) (11).Ecological interactions are often usefully represented as networks (12). For example, analyses of food webs and mutualistic (e.g., pollination) networks have demonstrated that network organization can be linked to network persistence, to disturbance (12-16), or to species coexistence and richness (17). Microbial communities are so diverse and poorly studied that mapping out the interactions on the basis of biological knowledge is currently impossible for all but the simplest of habitats. Therefore, cooccurrence networks are increasingly used to infer microbial interactions (18,19) in soils (20), oceans (21), lakes (22), and even in global genomic surveys (23).A key question is whether the organization of microbial cooccurrence networks and their response to disturbance reflect physical characteristics inherent in fluvial networks such as geo...

The ecology and biogeochemistry of stream biofilms

“…This notion is supported by the observation that, under elevated flow velocity, Flavobacteriia adhere better to surfaces than Pseudomonas aeruginosa, which is a notable biofilm former 30 . Furthermore, analyses of co-occurrence networks identified Sphingobacteriia as a key taxonomic group in stream biofilms 31 , which agrees with the involvement of these bacteria in the formation and colonization of suspended aggregates in the ocean and lakes 29 . Other bacteria that are commonly found in stream biofilms, but at lower relative abundance, include Gammaproteobacteria, Deltaproteobacteria, Actinobacteria, Firmicutes, Gemmatimonadetes, Verrucomicrobia, Planctomycetes and DeinococcusThermus.…”

“…Analyses of co-occurrence networks generated from 16S rRNA sequencing data showed that community networks tend to fragment into more abundant, but smaller, clusters that may be sensitive to the hydrological regime and dispersal dynamics 31 . The same study also uncovered the role of typical biofilm formers, such as Sphingobacteriia, in the organization of biofilm community networks.…”

Altitudinal patterns of diversity and functional traits of metabolically active microorganisms in stream biofilms

“…In fact, dispersal limitation is commonly believed to be higher in the headwater reaches (Besemer et al, 2013), which would induce stronger constraints on the colonisation dynamics of microbial specialists. Furthermore, the combination of elevated physical disturbance (e.g., hydrology) on microbial communities (Widder et al, 2014) upstream with the observation that postdisturbance recovery of specialists is less successful than for generalists (Clavel et al, 2011) may further contribute to the spatial patterns of functional traits we observed in this study. Finally, benthic algae which provide the bulk DOC source in streams above the treeline exude compounds, often monomeric in nature, that do not require specialised metabolic pathways and that are therefore readily available to a wide range of microbial heterotrophs (Kaplan and Bott, 1989).…”

Tributary connectivity, confluence aggradation and network biodiversity

“…Confluences are important ecological elements for a number of other reasons too and there is broad recognition amongst ecologists that understanding confluence, link and networks effects is essential for improving understanding of the spatial and temporal distribution of river organisms and the collective health of river ecosystems (Davey and Lapointe, 2007;Poole 2002;Benda et al, 2004b;Thorp et al, 2006;Grant et al, 2007;Widder et al, 2014;Thorp, 2014;Erös and Grant, 2015;Lanthier et al 2015;Hauer 2015). The simple relations identified here, between the frequency and likelihood of tributary-driven aggradation and basin shape and size, support Benda's Network Dynamics Hypothesis (Benda et al, 2004b) In turn, to the extent that physical heterogeneity matters for biodiversity and ecosystem health, the cumulative ecological benefit of tributary-driven aggradation should be greater in more compact basins; a hypothesis that remains to be tested.…”