A cross-ecosystem approach to understand microbial assembly along an aquatic continuum

Abstract:

Inland waters form complex hydrological networks acting as a bridge of abiotic and biotic matter between the terrestrial milieu and ultimately the oceans. During transport through streams, rivers, and lakes, pools such as organic matter and microorganisms, are processed, modified and re-assembled, determining complex patterns of biogeochemical and ecological successions. The microbial succession that occurs along this aquatic continuum is strongly influenced by the upstream history of these communities (i.e. legacy effects), which include taxa that are recruited from the surrounding terrestrial ecosystems. The influence of upstream history on downstream communities will depend on the degree of connectivity within the network, including factors that influence the hydrologic regime such as spring snow-melt. Legacy effects are further modulated by both selective loss of taxa through death or dormancy, and local selection and growth of taxa in specific habitats. These processes have seldom, if ever, been examined along a true land-estuary aquatic continuum that is characterized by the presence of a diversity of lentic and lotic habitats. Here we reconstruct the microbial succession within the Romaine River watershed in Eastern Québec (14 500 km²), where we followed a terrestrial-aquatic continuum by sampling soil, soilwater, headwater streams, rivers and reservoirs up to the estuary in the Gulf of Saint-Lawrence for three years, covering three hydrologically different periods. In order to distinguish the total from the reactive fraction of microbial communities we sequenced the 16S rRNA amplicon for both DNA and RNA. Differences in microbial assemblages within the network was mainly driven by habitat type and seasons, however, dissimilarity between DNA and RNA assemblages was highest in spring but only in soilwater, hyporheic water and streams. DNA and RNA converged with increasing residence time along the continuum, indicating growth of locally selected taxa, and conversely, that the influx of inactive bacteria is largest in systems with the lowest water retention time. These results indicate that major assembly processes differ depending on the ecosystem’s connectivity and residence time and priority effects grow smaller as influences of the terrestrial surroundings decrease.