The relevance of terrestrial and aquatic history in microbial community assembly along a boreal terrestrial-aquatic continuum
Inland waters form complex hydrological networks acting as a bridge of abiotic and biotic matter between the terrestrial milieu and ultimately the oceans. While microbial taxa traverse through diverse habitats along the network, microbial communities are modified and re-assembled, forming complex patterns of ecological successions. Seldom, however, is the potential effect of upstream assembly on the downstream community composition considered within aquatic networks. Here, we reconstructed the microbial succession along a true land-freshwater-estuary continuum within La Romaine river watershed in Eastern Québec for three years. We captured hydrological seasonality by sampling in spring and summer, and differentiated the total from the reactive community by sequencing both 16S rRNA genes and transcripts. By examining how DNA-/RNA-based assemblages diverge and converge along the continuum, we were able to capture temporal shifts in the relative importance of assembly processes, with mass effects dominant in spring, while species selection became stronger in summer. Spatially, the point of strongest selection within the network differed between seasons, indicating towards a shifting window in selection hotspots that moves depending on the hydrological conditions. Furthermore, the unreactive fraction was composed of a diverse pool of origins along the whole aquatic network, while a large proportion of the reactive community could be traced back to soils and soilwaters. Notably, taxa within the reactive fraction were recruited along the entire rank-abundance curve. Overall, our findings highlight the importance of considering upstream history, hydrological seasonality and the reactive microbial fraction to fully understand microbial community assembly on a watershed scale.
Masumi Stadler and Paul A. del Giorgio
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