Abstract
One of the major contemporary challenges in microbial ecology has been to discriminate the reactive core from the random, unreactive components of bacterial communities. In previous work we used the spatial abundance distributions of bacterioplankton across boreal lakes of Québec to group taxa into four distinct categories that reflect either hydrology-mediated dispersal along the aquatic network or environmental selection mechanisms within lakes. Here, we test whether this categorization derived from the spatial distribution of taxa is maintained over time, by analyzing the temporal dynamics of the operational taxonomic units (OTUs) within those spatially derived categories along an annual cycle in the oligotrophic lake Croche (Québec, Canada), and assessing the coherence in the patterns of abundance, occurrence, and environmental range of these OTUs over space and time. We report that the temporal dynamics of most taxa within a single lake are largely coherent with those derived from their spatial distribution over large spatial scales, suggesting that these properties must be intrinsic of particular taxa. We also identified a set of rare taxa cataloged as having a random occupancy based on their spatial distribution, but which showed clear seasonality and abundance peaks along the year, yet these comprised a very small fraction of the total rare OTUs. We conclude that the presence of most rare bacterioplankton taxa in boreal lakes is random, since both their temporal and spatial dynamics suggest links to passive downstream transport and persistence in freshwater networks, rather than environmental selection.
Highlights
It is well-established that most aquatic bacterial communities show a recurrent structure in which a relatively small number of abundant bacteria coexist with a vast number of extremely rare taxa that comprise the long tail of the so-called “rare biosphere” (Pedrós-Alió, 2006, 2012; Sogin et al, 2006)
In that previous study we had determined the spatial abundance distribution (SpAD) of individual bacterial operational taxonomic units (OTUs) across 198 widely different boreal lakes in Québec, and we used these to group taxa into categories of spatial abundance distributions (SpADs)
This partition of the structure of lake bacterial communities into four groups of taxa with different SpADs suggested that lake communities are composed of a core of taxa whose distribution is linked to active growth and in-lake environmental selection, and an enormous fraction of rare bacteria (94% total OTUs) whose presence seems random and linked to hydrology-mediated transport
Summary
It is well-established that most aquatic bacterial communities show a recurrent structure in which a relatively small number of abundant bacteria coexist with a vast number of extremely rare taxa that comprise the long tail of the so-called “rare biosphere” (Pedrós-Alió, 2006, 2012; Sogin et al, 2006). Since freshwater bacterioplankton communities are known to be highly influenced by the immigration of taxa from upstream terrestrial or aquatic ecosystems (Crump et al, 2012; Ruiz-González et al, 2015; Savio et al, 2015; Niño-García et al, 2016a), we further explored the dynamics of the resulting categories by retracing these OTUs to the fluvial networks associated to the lakes, to distinguish between taxa whose presence and abundance is driven by downstream transport versus those subjected to environmental selection within lakes Two of these SpAD categories (normal-like and bimodal) seemed to comprise the core of bacterioplankton communities, which included taxa whose patterns of spatial distribution were clearly linked to active growth within lakes, and to selection by local conditions (i.e., reactive taxa). Our results suggest that the ecological features associated to categories derived from the large scale spatial distribution patterns of taxa are overwhelmingly maintained over time, and that the core component of these bacterial communities is consistent over time and space, at least in this boreal region
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