Tracing the Origin of Planktonic Protists in an Ancient Lake

Abstract

Ancient lakes are among the most interesting models for evolution studies, because their biodiversity is the result of a complex combination of migration and speciation. Here, we investigate the origin of single celled planktonic eukaryotes from the oldest lake in the world – Lake Baikal. By using 18S rDNA metabarcoding we recovered 1,427 Operational Taxonomic Units (OTUs) belonging to protists populating surface waters (1-50 meters) and belonging to pico- and nano-plankton size fractions. The recovered communities resembled other lacustrine freshwater assemblages found elsewhere, especially the unclassified protists. However, our results suggest that a fraction of Baikal protists originated evolutionary recently from marine/brackish ancestors. Moreover, our results suggest that rapid radiation may have occurred among some protist taxa, partially mirroring what was already shown for multicellular organisms in Lake Baikal. We found potential species flocks in Stramenopiles, Alveolata, Opisthokonta, Archaeplastida, Rhizaria and Hacrobia. Putative flocks predominated in Chrysophytes, which are highly diverse in Lake Baikal. Some of the species, especially those from these flocks, may be endemic from Lake Baikal, because their 18S rDNA differed > 10% from known DNA. Overall, our study points to novel diversity of planktonic protists in Lake Baikal, some of which may have emerged in situ after evolutionary diversification.

Environmental drivers of plankton protist communities along latitudinal and vertical gradients in the oldest and deepest freshwater lake

“…Overall, despite methodological differences, our identified plankton protist communities were consistent with previous studies in surface waters or in a water column previously sampled in the Southern basin, with ciliates, dinoflagellates and ochrophytes being highly represented (Annenkova et al, 2020) (Yi et al, 2017).…”

“…To study the diversity and relative abundance of microbial eukaryotes in Lake Baikal plankton, we concentrated cells in the 0.2-30 µm diameter fraction by successive filtration steps. This fraction thus integrated pico-(0.2-2 µm), nano-(2-20 µm) and small microplankton (20-30 µm), covering a wider protistan spectrum than some previous comparative studies (Annenkova et al, 2020). We purified DNA and massively sequenced (MiSeq Illumina, 2x300 bp) multiplexed 18S rRNA gene V4-region amplicons.…”

“…Although marine-freshwater transitions are thought to be rare (Mukherjee et al, 2019) and salinity, a major driver of microbial community composition (Lozupone and Knight, 2007), high-throughput environmental studies are revealing an increasing number of typically marine eukaryotic lineages in freshwater systems. Among those are members of the parasitic perkinsids (Brate et al, 2010), haptophytes (Simon et al, 2013), Bolidophyceae (Richards and Bass, 2005;Annenkova et al, 2020) and several Marine Stramenopiles (MAST) clades (Massana et al, 2004;Massana et al, 2006), such as MAST-2, MAST-12, MAST-3 and possibly MAST-6 (Simon et al, 2015a). Recently, diplonemids, a cosmopolitan group of oceanic excavates particularly abundant and diverse in the deep ocean (Lara et al, 2009;de Vargas et al, 2015) were identified in deep freshwater lakes (Yi et al, 2017;Mukherjee et al, 2019).…”

“…Yi et al analyzed protist diversity by 454 sequencing of 18S rRNA gene V9region amplicons along the Southern basin water column (52-1450 m) (Yi et al, 2017). More recently, Annenkova et al determined the community structure of small protists (0.45-8 µm cell-size fraction) from surface waters (1-15-50 m) across the lake via 18S rRNA gene V4-region metabarcoding and suggested that some clades within known protist groups might be endemic (Annenkova et al, 2020).…”

The windblown: possible explanations for dinophyte DNA in forest soils

“…All known dinophytes are from marine or freshwater environments (Adl et al 2019). As they constitute a considerable fraction of the plankton and play an important role in the global aquatic ecosystem, HTS studies have detected dinophytes from waters sampled from the polar regions through to the tropics (de Vargas et al 2015;Le Bescot et al 2016;Elferink et al 2017;Decelle et al 2018;Lentendu et al 2018;Annenkova et al 2020;Giner et al 2020;Gottschling et al 2020). HTS studies have also detected DNA of dinophytes in terrestrial environments (Bates et al 2013;Geisen et al 2015;Mahé et al 2017;Venter et al 2017;Voss et al 2019), although they are not expected to be there.…”