Global freshwater distribution of Telonemia protists

Abstract

Telonemia are one of the oldest identified marine protists that for most part of their history have been recognized as a distinct incertae sedis lineage. Today, their evolutionary proximity to the SAR supergroup (Stramenopiles, Alveolates, and Rhizaria) is firmly established. However, their ecological distribution and importance as a natural predatory flagellate, especially in freshwater food webs, still remain unclear. To unravel the distribution and diversity of the phylum Telonemia in freshwater habitats, we examined over a thousand freshwater metagenomes from all over the world. In addition, to directly quantify absolute abundances, we analyzed 407 samples from 97 lakes and reservoirs using Catalyzed Reporter Deposition-Fluorescence in situ Hybridization (CARD-FISH). We recovered Telonemia 18S rRNA gene sequences from hundreds of metagenomic samples from a wide variety of habitats, indicating a global distribution of this phylum. However, even after this extensive sampling, our phylogenetic analysis did not reveal any new major clades, suggesting current molecular surveys are near to capturing the full diversity within this group. We observed excellent concordance between CARD-FISH analyses and estimates of abundances from metagenomes. Both approaches suggest that Telonemia are largely absent from shallow lakes and prefer to inhabit the colder hypolimnion of lakes and reservoirs in the Northern Hemisphere, where they frequently bloom, reaching 10%-20% of the total heterotrophic flagellate population, making them important predatory flagellates in the freshwater food web.

Keywords: CARD-FISH; Telonemia; freshwater lakes; metagenomics; microbial food webs; predatory flagellate.

Figure 1

(A) Maximum likelihood phylogenetic tree of representative Telonemia 18S rRNA gene sequences clustered at 97% nucleotide identity (shown as a cladogram). The outgroup sequences are not shown but indicated by an arrow. Ultrafast bootstrap values are shown at each node. Two clades, Telo-1 and Telo-2 as defined in previous publications, are shown in different colors. Sequences not classified or <1% are shown as “other.” Isolate sequences are shown in bold. The origin of each sequence (clone library, metagenomic, or isolate) is indicated by colored squares. The length of each sequence, and the number of sequences in each sequence cluster are shown at the right as barcharts. Number of sequences in clusters with >10 sequences is shown (B) Telonemia abundances estimated with 18S rRNA gene sequences from metagenomes and (C) in a metagenomic time-series of Řimov reservoir.

Figure 2

(A) Geographic locations of lake samples used for CARD-FISH in this work. Samples are color-coded according to the maximum %Telonemia found at that site using CARD-FISH (see key top left). Sites where Telonemia was not detected at all are shown as empty circles. A complete list of all samples is provided in Supplementary Table S1. (B) CARD-FISH counts (%Telonemia) in CARD-FISH filters (only those more than 1% are shown here), sorted by decreasing temperature. Epilimnion and hypolimnion samples, along with lake trophic status are shown in different colors.

Figure 3

CARD-FISH images of Telonemia targeted by the probe Telo-1250. Green: CARD-FISH probe, blue: DAPI, red: autofluorescence (A) from Lake Cinciș (20 m) (B) from Řimov reservoir (0.5 m), Telonemia ingesting a Rhodomonas. A larger Cryptomonas and smaller Rhodomonas are also seen left and right, respectively, and (C) a dividing Telonemia cell from Breiter Luzin hypolimnion (50 m). All scale bars are 10 μm, panel (A) is in magnification 40X, and panels (B) and (C) are in 100X magnification. (D) Four annual time-series from Řimov reservoir, Lake Biwa, Kličava reservoir and Žlutice reservoir showing relative abundances of Telonemia (using CARD-FISH) in epilimnion and hypolimnion. Temperatures are shown as green (epilimnion) and gray-dotted (hypolimnion) lines. (E) Relative abundances of Telonemia in epilimnion and hypolimnion during a high-resolution sampling of a spring phytoplankton bloom in Řimov reservoir. Counts of the most abundant Cryptophyte (Rhodomonas) are shown as a blue background and temperatures as lines.