Marked seasonality and high spatial variability of protist communities in shallow freshwater systems

Abstract

Small eukaryotes have key roles in aquatic ecosystems, influencing their local environment, global biogeochemical cycles and climate. Their impact depends on community structure, which varies along time. However, very few studies take into account temporal variation. This is especially true for small, shallow freshwater systems, which remain largely understudied despite their wide variety, global surface and intense microbial activity. We have monthly followed changes in the community structure of small microbial eukaryotes (0.2-5 μm cell diameter) for 2 years in four ponds and one brook located in North-Western France based on massive 18S rDNA amplicon 454 pyrosequencing. We detected a total of 3742 stringently defined operational taxonomic units (OTUs) encompassing all recognized eukaryotic supergroups and lineages of uncertain affiliation. Although geographically close, protist communities in the five ecosystems were contrasting, with very few shared OTUs, suggesting that environmental selection mainly drives community structure. The temporal dynamics of different high-rank taxa appeared complex and rapid at monthly scales. Despite this, a clear and reproducible seasonality was observed. As expected, low-abundance OTUs dominated the community. Although some of them appeared sporadically or remained at low frequencies during the survey, others occasionally reached relatively high abundances, sometimes recurrently. This shows that at least a fraction of low-abundance eukaryotes constitutes a seed bank. The annual proportion of primary producers, free-living heterotrophs and parasites appeared remarkably constant among the different ecosystems, suggesting underlying trends of ecosystem carrying capacity for these functional groups.

Novel Diversity of Deeply Branching Holomycota and Unicellular Holozoans Revealed by Metabarcoding in Middle Paraná River, Argentina

“…This is in agreement with previous freshwater studies (Taib et al, 2013;Simon et al, 2014Simon et al, , 2015. The main reason of this large number might not be the abundance of the organisms themselves but the greater amount of rRNA gene copies in their genomes (Medinger et al, 2010;TorresMachorro et al, 2010;Pawlowski et al, 2011).…”

“…They were initially thought to be exclusively marine, although some clades have also been detected in a wide range of ecosystems, including fresh waters (Massana et al, 2004Simon et al, 2014Simon et al, , 2015. In our Paraná dataset, we detected several MAST groups and thus, confirmed its wider spatial distribution.…”

“…The abundance of this group is, however, not as large as in other surveys (Šlapeta et al, 2005;Taib et al, 2013;Simon et al, 2014). This pattern might be explained by the fluctuations of its abundance over time, variations in flow rate of the river or the presence of oxygenated waters (Šlapeta et al, 2005;Devercelli, 2006;Simon et al, 2015).…”

“…This is remarkable given the extreme oligotrophic condition of some of these lakes. There has also been research on ponds, brooks and other seasonal currents (Šlapeta et al, 2005;Simon et al, 2014Simon et al, , 2015. Some of these studies found novel eukaryotic clades, and showed the presence of some putatively marine-exclusive lineages in those freshwater environments.…”

“…However, a few new species have been recently described in fresh waters; these include the genera Syssomonas and Pigoraptor (Hehenberger et al, 2017) within Holozoa and Parvularia as an early-branching Holomycota (López-Escardó et al, 2018). Therefore, there is need for exploring the diversity of unicellular opisthokonts, particularly from freshwater environments where a wide range of ecological niches exist (Logares et al, 2009a;Simon et al, 2015). This heterogeneous nature can be a potential reservoir of a diverse array of new eukaryotic lineages (Šlapeta et al, 2005;Downing, 2010).…”