Intraspecific Variation in Protists: Clues for Microevolution from Poteriospumella lacustris (Chrysophyceae)

Species delimitation in protists is still a challenge, attributable to the fact that protists are small, difficult to observe and many taxa are poor in morphological characters, whereas most current phylogenetic approaches only use few marker genes to measure genetic diversity. To address this problem, we assess genome-level divergence and microevolution in strains of the protist Poteriospumella lacustris, one of the first free-living, nonmodel organisms to study genome-wide intraspecific variation. Poteriospumella lacustris is a freshwater protist belonging to the Chrysophyceae with an assumed worldwide distribution. We examined three strains from different geographic regions (New Zealand, China, and Austria) by sequencing their genomes with the Illumina and PacBio platforms. The assembled genomes were small with 49-55 Mb but gene-rich with 16,000-19,000 genes, of which ∼8,000 genes could be assigned to functional categories. At least 68% of these genes were shared by all three species. Genetic variation occurred predominantly in genes presumably involved in ecological niche adaptation. Most surprisingly, we detected differences in genome ploidy between the strains (diploidy, triploidy, and tetraploidy). In analyzing intraspecific variation, several mechanisms of diversification were identified including SNPs, change of ploidy and genome size reduction.

Biodiversity of biological soil crusts from the Polar Regions revealed by metabarcoding

Biological soil crusts (BSCs) are amalgamations of autotrophic, heterotrophic and saprotrophic organisms. In the Polar Regions, these unique communities occupy essential ecological functions such as primary production, nitrogen fixation and ecosystem engineering. Here, we present the first molecular survey of BSCs from the Arctic and Antarctica focused on both eukaryotes and prokaryotes as well as passive and active biodiversity. Considering sequence abundance, Bryophyta is among the most abundant taxa in all analyzed BSCs suggesting that they were in a late successional stage. In terms of algal and cyanobacterial biodiversity, the genera Chloromonas, Coccomyxa, Elliptochloris and Nostoc were identified in all samples regardless of origin confirming their ubiquitous distribution. For the first time, we found the chrysophyte Spumella to be common in polar BSCs as it was present in all analyzed samples. Co-occurrence analysis revealed the presence of sulfur metabolizing microbes indicating that BSCs also play an important role for the sulfur cycle. In general, phototrophs were most abundant within the BSCs but there was also a diverse community of heterotrophs and saprotrophs. Our results show that BSCs are unique microecosystems in polar environments with an unexpectedly high biodiversity.

Killing the killer: predation between protists and predatory bacteria

Predation by microbes is one of the main drivers of bacterial mortality in the environment. In most ecosystems multiple micropredators compete at least partially for the same bacterial resource. Predatory interactions between these micropredators might lead to shifts within microbial communities. Integrating these interactions is therefore crucial for the understanding of ecosystem functioning. In this study, we investigated the predation between two groups of micropredators, i.e. phagotrophic protists and Bdellovibrio and like organisms (BALOs). BALOs are obligate predators of Gram-negative bacteria. We hypothesised that protists can prey upon BALOs despite the small size and high swimming speed of the latter, which makes them potentially hard to capture. Predation experiments including three protists, i.e. one filter feeder and two interception feeder, showed that BALOs are a relevant prey for these protists. The growth rate on BALOs differed for the respective protists. The filter feeding ciliate was growing equally well on the BALOs and on Escherichia coli, whereas the two flagellate species grew less well on the BALOs compared to E. coli. However, BALOs might not be a favourable food source in resource-rich environments as they are not enabling all protists to grow as much as on bacteria of bigger volume.

Comprehensive transcriptome analysis provides new insights into nutritional strategies and phylogenetic relationships of chrysophytes

Background

Chrysophytes are protist model species in ecology and ecophysiology and important grazers of bacteria-sized microorganisms and primary producers. However, they have not yet been investigated in detail at the molecular level, and no genomic and only little transcriptomic information is available. Chrysophytes exhibit different trophic modes: while phototrophic chrysophytes perform only photosynthesis, mixotrophs can gain carbon from bacterial food as well as from photosynthesis, and heterotrophs solely feed on bacteria-sized microorganisms. Recent phylogenies and megasystematics demonstrate an immense complexity of eukaryotic diversity with numerous transitions between phototrophic and heterotrophic organisms. The question we aim to answer is how the diverse nutritional strategies, accompanied or brought about by a reduction of the plasmid and size reduction in heterotrophic strains, affect physiology and molecular processes.

Results

We sequenced the mRNA of 18 chrysophyte strains on the Illumina HiSeq platform and analysed the transcriptomes to determine relations between the trophic mode (mixotrophic vs. heterotrophic) and gene expression. We observed an enrichment of genes for photosynthesis, porphyrin and chlorophyll metabolism for phototrophic and mixotrophic strains that can perform photosynthesis. Genes involved in nutrient absorption, environmental information processing and various transporters (e.g., monosaccharide, peptide, lipid transporters) were present or highly expressed only in heterotrophic strains that have to sense, digest and absorb bacterial food. We furthermore present a transcriptome-based alignment-free phylogeny construction approach using transcripts assembled from short reads to determine the evolutionary relationships between the strains and the possible influence of nutritional strategies on the reconstructed phylogeny. We discuss the resulting phylogenies in comparison to those from established approaches based on ribosomal RNA and orthologous genes. Finally, we make functionally annotated reference transcriptomes of each strain available to the community, significantly enhancing publicly available data on Chrysophyceae.

Conclusions

Our study is the first comprehensive transcriptomic characterisation of a diverse set of Chrysophyceaen strains. In addition, we showcase the possibility of inferring phylogenies from assembled transcriptomes using an alignment-free approach. The raw and functionally annotated data we provide will prove beneficial for further examination of the diversity within this taxon. Our molecular characterisation of different trophic modes presents a first such example.

Microbial eukaryote plankton communities of high-mountain lakes from three continents exhibit strong biogeographic patterns

Microbial eukaryotes hold a key role in aquatic ecosystem functioning. Yet, their diversity in freshwater lakes, particularly in high-mountain lakes, is relatively unknown compared with the marine environment. Low nutrient availability, low water temperature and high ultraviolet radiation make most high-mountain lakes extremely challenging habitats for life and require specific molecular and physiological adaptations. We therefore expected that these ecosystems support a plankton diversity that differs notably from other freshwater lakes. In addition, we hypothesized that the communities under study exhibit geographic structuring. Our rationale was that geographic dispersal of small-sized eukaryotes in high-mountain lakes over continental distances seems difficult. We analysed hypervariable V4 fragments of the SSU rRNA gene to compare the genetic microbial eukaryote diversity in high-mountain lakes located in the European Alps, the Chilean Altiplano and the Ethiopian Bale Mountains. Microbial eukaryotes were not globally distributed corroborating patterns found for bacteria, multicellular animals and plants. Instead, the plankton community composition emerged as a highly specific fingerprint of a geographic region even on higher taxonomic levels. The intraregional heterogeneity of the investigated lakes was mirrored in shifts in microbial eukaryote community structure, which, however, was much less pronounced compared with interregional beta-diversity. Statistical analyses revealed that on a regional scale, environmental factors are strong predictors for plankton community structures in high-mountain lakes. While on long-distance scales (>10 000 km), isolation by distance is the most plausible scenario, on intermediate scales (up to 6000 km), both contemporary environmental factors and historical contingencies interact to shift plankton community structures.

Small but Manifold - Hidden Diversity in "Spumella-like Flagellates"

Colourless, nonscaled chrysophytes comprise morphologically similar or even indistinguishable flagellates which are important bacterivors in water and soil crucial for ecosystem functioning. However, phylogenetic analyses indicate a multiple origin of such colourless, nonscaled flagellate lineages. These flagellates are often referred to as “Spumella‐like flagellates” in ecological and biogeographic studies. Although this denomination reflects an assumed polyphyly, it obscures the phylogenetic and taxonomic diversity of this important flagellate group and, thus, hinders progress in lineage‐ and taxon‐specific ecological surveys. The smallest representatives of colourless chrysophytes have been addressed in very few taxonomic studies although they are among the dominant flagellates in field communities. To overcome the blurred picture and set the field for further investigation in biogeography and ecology of the organisms in question, we studied a set of strains of specifically small, colourless, nonscaled chrysomonad flagellates by means of electron microscopy and molecular analyses. They were isolated by a filtration‐acclimatisation approach focusing on flagellates of around 5 μm. We present the phylogenetic position of eight different lineages on both the ordinal and the generic level. Accordingly, we describe the new genera Apoikiospumella, Chromulinospumella, Segregatospumella, Cornospumella and Acrispumella Boenigk et Grossmann n. g. and different species within them.

Assembling the challenging puzzle of algal biodiversity: species delimitation within the genus Asterochloris (Trebouxiophyceae, Chlorophyta)

The genus Asterochloris represents one of the most common, widespread, and diverse taxa of lichen photobionts. In this report, we describe and characterize six new species (A. echinata, A. friedlii, A. gaertneri, A. leprarii, A. lobophora, and A. woessiae) that were identified during our recent investigation of photobiont diversity. We found that the species differed genetically, morphologically, ecologically, and with respect to their mycobiont partners. Statistical analyses revealed significant morphological differentiation of all six newly described species, as well as their separation from previously described Asterochloris species. Chloroplast morphology represented the best morphological marker for species delineation. In fact, each species can be recognized by the dominance and unique assemblage of particular chloroplast types. Although genetically well recognized by rapidly evolving internal transcribed spacer rDNA and actin intron markers, all 13 investigated Asterochloris species shared identical small subunit rDNA sequences. We therefore demonstrated that morphologically and ecologically diverse species can frequently be grouped into a single taxonomic unit in whole-transcriptome sequencing studies, considerably affecting the resulting estimates of species diversity. Finally, we demonstrated the presence of isogamous sexual reproduction in Asterochloris, disputing the current symbiotic dogma of the loss of sexual reproduction in algal symbionts.

Estimating Protistan Diversity Using High-Throughput Sequencing

Sequencing hypervariable regions from the 18S rRNA gene is commonly employed to characterize protistan biodiversity, yet there are concerns that short reads do not provide the same taxonomic resolution as full-length sequences. A total of 7,432 full-length sequences were used to perform an in silico analysis of how sequences of various lengths and target regions impact downstream ecological interpretations. Sequences that were longer than 400 nucleotides and included the V4 hypervariable region generated results similar to those derived from full-length 18S rRNA gene sequences. Present high-throughput sequencing capabilities are approaching protistan diversity estimation comparable to whole gene sequences.

A morphogenetic survey on ciliate plankton from a mountain lake pinpoints the necessity of lineage-specific barcode markers in microbial ecology

Analyses of high-throughput environmental sequencing data have become the 'gold-standard' to address fundamental questions of microbial diversity, ecology and biogeography. Findings that emerged from sequencing are, e.g. the discovery of the extensive 'rare microbial biosphere' and its potential function as a seed-bank. Even though applied since several years, results from high-throughput environmental sequencing have hardly been validated. We assessed how well pyrosequenced amplicons [the hypervariable eukaryotic V4 region of the small subunit ribosomal RNA (SSU rRNA) gene] reflected morphotype ciliate plankton. Moreover, we assessed if amplicon sequencing had the potential to detect the annual ciliate plankton stock. In both cases, we identified significant quantitative and qualitative differences. Our study makes evident that taxon abundance distributions inferred from amplicon data are highly biased and do not mirror actual morphotype abundances at all. Potential reasons included cell losses after fixation, cryptic morphotypes, resting stages, insufficient sequence data availability of morphologically described species and the unsatisfying resolution of the V4 SSU rRNA fragment for accurate taxonomic assignments. The latter two underline the necessity of barcoding initiatives for eukaryotic microbes to better and fully exploit environmental amplicon data sets, which then will also allow studying the potential of seed-bank taxa as a buffer for environmental changes.

Ultrastructure and 18S rDNA phylogeny of Apoikia lindahlii comb. nov. (Chrysophyceae) and its epibiontic protists, Filos agilis gen. et sp. nov. (Bicosoecida) and Nanos amicus gen. et sp. nov. (Bicosoecida)

Three heterotrophic stramenopiles--Apoikia lindahlii comb. nov. (Chrysophyceae), Filos agilis gen. et sp. nov. (Bicosoecida), and Nanos amicus gen. et sp. nov. (Bicosoecida)--were isolated from acidic peat bogs. The biflagellate A. lindahlii forms loose irregular colonies from which swimming cells may detach, and produces extensive mucilaginous material containing bacterial cells. Phylogenetic analyses of small subunit rDNA sequences demonstrated that A. lindahlii branches within the Chrysophyceae. While A. lindahlii is an obligate heterotroph, ultrastructural observations revealed a leukoplast in the perinuclear region. The pico-sized uniflagellates F. agilis and N. amicus were isolated from separate lakes and within the mucilage of A. lindahlii, suggesting their close associations in natural habitats. In SSU rDNA phylogenies, F. agilis and N. amicus were closely related to the bicosoecids Adriamonas, Siluania, Paramonas, and Nerada. While Filos, Nanos, and Siluania are similar in light microscopic features, their SSU rDNA gene sequences differed significantly (>8% differences) and were not monophyletic. Both F. agilis and N. amicus have a cytostome/cytopharynx particle ingestion apparatus. Bacterial cells and material similar to the mucilage of A. lindahlii occurred within the food vacuole of F. agilis and N. amicus. The nature of association between A. lindahlii and its epibiontic bicosoecids is discussed.

Spatio-temporal variations in protistan communities along an O/HS gradient in the anoxic Framvaren Fjord (Norway)

Despite its relevance for ecology and biodiversity, the stability of spatial microeukaryote diversity patterns in time has received only little attention using gene-based strategies, and there is little knowledge about the relation of spatial vs. temporal variation. We addressed this subject by investigating seasonal fluctuations in protistan communities in three ecologically distinct marine habitats. We analyzed 3360 eukaryote small subunit rRNA gene sequences collected along an O(2)/H(2)S gradient in a Norwegian fjord in order to reveal shifts in protistan community composition and structure in three different seasons. In all nine clone libraries, ciliates and stramenopiles accounted for the largest proportion. Yet, as expected, at the phylotype level, the protistan communities from distinct habitats differed significantly, with the number of shared phylotypes between two habitats being as low as 18%. This confirmed previous notions that environmental factors along the stratification gradient shape biodiversity patterns. Surprisingly, the intrahabitat community composition and structure varied at a comparable order of magnitude over time, with only 18-28% phylotypes shared within the same habitat. Our study demonstrates that the consideration of local fluctuations in microeukaryote diversity over time offers additional information for diversity surveys and can significantly contribute to the revelation of spatial protistan community patterns.