Resilience of Freshwater Communities of Small Microbial Eukaryotes Undergoing Severe Drought Events

Abstract

Small and shallow aquatic ecosystems such as ponds and streams constitute a significant proportion of continental surface waters, especially in temperate zones. In comparison with bigger lakes and rivers, they harbor higher biodiversity but they also exhibit reduced buffering capacity face to environmental shifts, such that climate global change can affect them in a more drastic way. For instance, many temperate areas are predicted to undergo droughts with increasing frequency in the near future, which may lead to the temporal desiccation of streams and ponds. In this work, we monitored temporal dynamics of planktonic communities of microbial eukaryotes (cell size range: 0.2-5 μm) in one brook and one pond that experienced recurrent droughts from 1 to 5 consecutive months during a temporal survey carried out monthly for 2 years based on high-throughput 18S rDNA metabarcoding. During drought-induced desiccation events, protist communities present in the remaining dry sediment, though highly diverse, differed radically from their planktonic counterparts. However, after water refill, the aquatic protist assemblages recovered their original structure within a month. This rapid recovery indicates that these eukaryotic communities are resilient to droughts, most likely via the entrance in dormancy. This property is essential for the long-term survival and functional stability of small freshwater ecosystems.

Keywords: 18S rRNA; dormancy; plankton; protist; resilience; temporal dynamics.

FIGURE 1

Phylogenetic composition of microbial eukaryotic communities over a 2-years monthly temporal survey in the brook (Ru) Sainte Anne and the shallow lake La Claye. Light-colored bars correspond to plankton samples collected over the 2 years (Simon et al., 2015b). Un-masked, bright color bars show the relative proportion of eukaryotic lineages identified in dry sediment during desiccation events as well as in water samples collected in neighboring months right before and after droughts. The horizontal bar below the histograms indicates whether the corresponding histogram bars correspond to water (green) or sediment (brown) eukaryotic communities.

FIGURE 2

Average distribution of microbial eukaryotic taxa in water samples and dry sediment during droughts in Ru Sainte Anne and La Claye.

FIGURE 3

Non-Metric MultiDimentional Scaling (NMDS) plots corresponding to eukaryote communities from water and dry sediment samples collected along the 2-years survey in Ru Sainte Anne and La Claye (A), La Claye only (B), and Ru Sainte Anne only (C). In (B,C) ellipses group eukaryote communities of the same periods defined by drought events.

FIGURE 4

Differences in richness, diversity and evenness between all water and sediment communities in Ru Sainte Anne and La Claye. The thick line represents the median of the distribution; the lower and upper limits of the boxes correspond to the first and third quartile respectively. Whiskers extend to the minimal and maximal values. Notches are drawn to indicate whether medians from distinct distributions can be considered as different.

FIGURE 5

Distribution of the relative abundance of putative phototrophs, free living heterotrophs and parasites in water and sediment samples from La Claye. (A) All samples from La Claye; (B) four sediment samples from summer and autumn 2011 and four water samples from the same period in 2012. Thick lines indicate median values. The top and below limits of boxes indicate the third and first quartile respectively. Whiskers extend to the minimal and maximal values. Notches are drawn to indicate whether medians from distinct distributions can be considered as different. Putative phototrophs, free-living heterotrophs and parasites were defined as previously (Simon et al., 2015b).