Niche adaptation promoted the evolutionary diversification of tiny ocean predators.

Unicellular eukaryotic predators play a crucial role in the functioning of the ocean ecosystem by recycling nutrients and energy that are channeled to upper trophic levels. Traditionally, these evolutionarily diverse organisms have been combined into a single functional group (heterotrophic flagellates), overlooking their organismal differences. Here, we investigated four evolutionarily related species belonging to one cosmopolitan group of uncultured marine picoeukaryotic predators: marine stramenopiles (MAST)-4 (species A, B, C, and E). Co-occurrence and distribution analyses in the global surface ocean indicated contrasting patterns in MAST-4A and C, suggesting adaptation to different temperatures. We then investigated whether these spatial distribution patterns were mirrored by MAST-4 genomic content using single-cell genomics. Analyses of 69 single cells recovered 66 to 83% of the MAST-4A/B/C/E genomes, which displayed substantial interspecies divergence. MAST-4 genomes were similar in terms of broad gene functional categories, but they differed in enzymes of ecological relevance, such as glycoside hydrolases (GHs), which are part of the food degradation machinery in MAST-4. Interestingly, MAST-4 species featuring a similar GH composition (A and C) coexcluded each other in the surface global ocean, while species with a different set of GHs (B and C) appeared to be able to coexist, suggesting further niche diversification associated with prey digestion. We propose that differential niche adaptation to temperature and prey type has promoted adaptive evolutionary diversification in MAST-4. We show that minute ocean predators from the same phylogenetic group may have different biogeography and genomic content, which needs to be accounted for to better comprehend marine food webs.

Comparative genomics reveals new functional insights in uncultured MAST species.

Heterotrophic lineages of stramenopiles exhibit enormous diversity in morphology, lifestyle, and habitat. Among them, the marine stramenopiles (MASTs) represent numerous independent lineages that are only known from environmental sequences retrieved from marine samples. The core energy metabolism characterizing these unicellular eukaryotes is poorly understood. Here, we used single-cell genomics to retrieve, annotate, and compare the genomes of 15 MAST species, obtained by coassembling sequences from 140 individual cells sampled from the marine surface plankton. Functional annotations from their gene repertoires are compatible with all of them being phagocytotic. The unique presence of rhodopsin genes in MAST species, together with their widespread expression in oceanic waters, supports the idea that MASTs may be capable of using sunlight to thrive in the photic ocean. Additional subsets of genes used in phagocytosis, such as proton pumps for vacuole acidification and peptidases for prey digestion, did not reveal particular trends in MAST genomes as compared with nonphagocytotic stramenopiles, except a larger presence and diversity of V-PPase genes. Our analysis reflects the complexity of phagocytosis machinery in microbial eukaryotes, which contrasts with the well-defined set of genes for photosynthesis. These new genomic data provide the essential framework to study ecophysiology of uncultured species and to gain better understanding of the function of rhodopsins and related carotenoids in stramenopiles.

Mediocremonas mediterraneus, a New Member within the Developea.

The stramenopiles are a large and diverse group of eukaryotes that possess various lifestyles required to thrive in a broad array of environments. The stramenopiles branch with the alveolates, rhizarians, and telonemids, forming the supergroup TSAR. Here, we present a new genus and species of aquatic nanoflagellated stramenopile: Mediocremonas mediterraneus, a free-swimming heterotrophic predator. M. mediterraneus cell bodies measure between 2.0-4.0 µm in length and 1.2-3.7 µm in width, possessing two flagella and an oval body morphology. The growth and grazing rate of M. mediterraneus in batch cultures ranges from 0.68 to 1.83 d-1 and 1.99 to 5.38 bacteria/h, respectively. M. mediterraneus was found to be 93.9% phylogenetically similar with Developayella elegans and 94.7% with Develorapax marinus, two members within the class Developea. The phylogenetic position of the Developea and the ability of M. mediterraneus to remain in culture make it a good candidate for further genomic studies that could help us to better understand phagotrophy in marine systems as well as the transition from heterotrophy to phototrophy within the stramenopiles.

Single Cell Genomics Reveals Viruses Consumed by Marine Protists.

The predominant model of the role of viruses in the marine trophic web is that of the "viral shunt," where viral infection funnels a substantial fraction of the microbial primary and secondary production back to the pool of dissolved organic matter. Here, we analyzed the composition of non-eukaryotic DNA associated with individual cells of small, planktonic protists in the Gulf of Maine (GoM) and the Mediterranean Sea. We found viral DNA associated with a substantial fraction cells from the GoM (51%) and the Mediterranean Sea (35%). While Mediterranean SAGs contained a larger proportion of cells containing bacterial sequences (49%), a smaller fraction of cells contained bacterial sequences in the GoM (19%). In GoM cells, nearly identical bacteriophage and ssDNA virus sequences where found across diverse lineages of protists, suggesting many of these viruses are non-infective. The fraction of cells containing viral DNA varied among protistan lineages and reached 100% in Picozoa and Choanozoa. These two groups also contained significantly higher numbers of viral sequences than other identified taxa. We consider mechanisms that may explain the presence of viral DNA in protistan cells and conclude that protistan predation on free viral particles contributed to the observed patterns. These findings confirm prior experiments with protistan isolates and indicate that the viral shunt is complemented by a viral link in the marine microbial food web. This link may constitute a sink of viral particles in the ocean and has implications for the flow of carbon through the microbial food web.

Single cell ecogenomics reveals mating types of individual cells and ssDNA viral infections in the smallest photosynthetic eukaryotes.

Planktonic photosynthetic organisms of the class Mamiellophyceae include the smallest eukaryotes (less than 2 µm), are globally distributed and form the basis of coastal marine ecosystems. Eight complete fully annotated 13-22 Mb genomes from three genera, Ostreococcus, Bathycoccus and Micromonas, are available from previously isolated clonal cultured strains and provide an ideal resource to explore the scope and challenges of analysing single cell amplified genomes (SAGs) isolated from a natural environment. We assembled data from 12 SAGs sampled during the Tara Oceans expedition to gain biological insights about their in situ ecology, which might be lost by isolation and strain culture. Although the assembled nuclear genomes were incomplete, they were large enough to infer the mating types of four Ostreococcus SAGs. The systematic occurrence of sequences from the mitochondria and chloroplast, representing less than 3% of the total cell's DNA, intimates that SAGs provide suitable substrates for detection of non-target sequences, such as those of virions. Analysis of the non-Mamiellophyceae assemblies, following filtering out cross-contaminations during the sequencing process, revealed two novel 1.6 and 1.8 kb circular DNA viruses, and the presence of specific Bacterial and Oomycete sequences suggests that these organisms might co-occur with the Mamiellales. This article is part of a discussion meeting issue 'Single cell ecology'.

Reconstruction of protein domain evolution using single-cell amplified genomes of uncultured choanoflagellates sheds light on the origin of animals.

Understanding the origins of animal multicellularity is a fundamental biological question. Recent genome data have unravelled the role that co-option of pre-existing genes played in the origin of animals. However, there were also some important genetic novelties at the onset of Metazoa. To have a clear understanding of the specific genetic innovations and how they appeared, we need the broadest taxon sampling possible, especially among early-branching animals and their unicellular relatives. Here, we take advantage of single-cell genomics to expand our understanding of the genomic diversity of choanoflagellates, the sister-group to animals. With these genomes, we have performed an updated and taxon-rich reconstruction of protein evolution from the Last Eukaryotic Common Ancestor (LECA) to animals. Our novel data re-defines the origin of some genes previously thought to be metazoan-specific, like the POU transcription factor, which we show appeared earlier in evolution. Moreover, our data indicate that the acquisition of new genes at the stem of Metazoa was mainly driven by duplications and protein domain rearrangement processes at the stem of Metazoa. Furthermore, our analysis allowed us to reveal protein domains that are essential to the maintenance of animal multicellularity. Our analyses also demonstrate the utility of single-cell genomics from uncultured taxa to address evolutionary questions. This article is part of a discussion meeting issue 'Single cell ecology'.

Evaluation of single-cell genomics to address evolutionary questions using three SAGs of the choanoflagellate Monosiga brevicollis.

Single-cell genomics (SCG) appeared as a powerful technique to get genomic information from uncultured organisms. However, SCG techniques suffer from biases at the whole genome amplification step that can lead to extremely variable numbers of genome recovery (5-100%). Thus, it is unclear how useful can SCG be to address evolutionary questions on uncultured microbial eukaryotes. To provide some insights into this, we here analysed 3 single-cell amplified genomes (SAGs) of the choanoflagellate Monosiga brevicollis, whose genome is known. Our results show that each SAG has a different, independent bias, yielding different levels of genome recovery for each cell (6-36%). Genes often appear fragmented and are split into more genes during annotation. Thus, analyses of gene gain and losses, gene architectures, synteny and other genomic features can not be addressed with a single SAG. However, the recovery of phylogenetically-informative protein domains can be up to 55%. This means SAG data can be used to perform accurate phylogenomic analyses. Finally, we also confirm that the co-assembly of several SAGs improves the general genomic recovery. Overall, our data show that, besides important current limitations, SAGs can still provide interesting and novel insights from poorly-known, uncultured organisms.

Viral to metazoan marine plankton nucleotide sequences from the Tara Oceans expedition.

A unique collection of oceanic samples was gathered by the Tara Oceans expeditions (2009-2013), targeting plankton organisms ranging from viruses to metazoans, and providing rich environmental context measurements. Thanks to recent advances in the field of genomics, extensive sequencing has been performed for a deep genomic analysis of this huge collection of samples. A strategy based on different approaches, such as metabarcoding, metagenomics, single-cell genomics and metatranscriptomics, has been chosen for analysis of size-fractionated plankton communities. Here, we provide detailed procedures applied for genomic data generation, from nucleic acids extraction to sequence production, and we describe registries of genomics datasets available at the European Nucleotide Archive (ENA, www.ebi.ac.uk/ena). The association of these metadata to the experimental procedures applied for their generation will help the scientific community to access these data and facilitate their analysis. This paper complements other efforts to provide a full description of experiments and open science resources generated from the Tara Oceans project, further extending their value for the study of the world's planktonic ecosystems.

Exploring Microdiversity in Novel Kordia sp. (Bacteroidetes) with Proteorhodopsin from the Tropical Indian Ocean via Single Amplified Genomes.

Marine Bacteroidetes constitute a very abundant bacterioplankton group in the oceans that plays a key role in recycling particulate organic matter and includes several photoheterotrophic members containing proteorhodopsin. Relatively few marine Bacteroidetes species have been described and, moreover, they correspond to cultured isolates, which in most cases do not represent the actual abundant or ecologically relevant microorganisms in the natural environment. In this study, we explored the microdiversity of 98 Single Amplified Genomes (SAGs) retrieved from the surface waters of the underexplored North Indian Ocean, whose most closely related isolate is Kordia algicida OT-1. Using Multi Locus Sequencing Analysis (MLSA) we found no microdiversity in the tested conserved phylogenetic markers (16S rRNA and 23S rRNA genes), the fast-evolving Internal Transcribed Spacer and the functional markers proteorhodopsin and the beta-subunit of RNA polymerase. Furthermore, we carried out a Fragment Recruitment Analysis (FRA) with marine metagenomes to learn about the distribution and dynamics of this microorganism in different locations, depths and size fractions. This analysis indicated that this taxon belongs to the rare biosphere, showing its highest abundance after upwelling-induced phytoplankton blooms and sinking to the deep ocean with large organic matter particles. This uncultured Kordia lineage likely represents a novel Kordia species (Kordia sp. CFSAG39SUR) that contains the proteorhodopsin gene and has a widespread spatial and vertical distribution. The combination of SAGs and MLSA makes a valuable approach to infer putative ecological roles of uncultured abundant microorganisms.

Accessing the genomic information of unculturable oceanic picoeukaryotes by combining multiple single cells.

Pico-sized eukaryotes play key roles in the functioning of marine ecosystems, but we still have a limited knowledge on their ecology and evolution. The MAST-4 lineage is of particular interest, since it is widespread in surface oceans, presents ecotypic differentiation and has defied culturing efforts so far. Single cell genomics (SCG) are promising tools to retrieve genomic information from these uncultured organisms. However, SCG are based on whole genome amplification, which normally introduces amplification biases that limit the amount of genomic data retrieved from a single cell. Here, we increase the recovery of genomic information from two MAST-4 lineages by co-assembling short reads from multiple Single Amplified Genomes (SAGs) belonging to evolutionary closely related cells. We found that complementary genomic information is retrieved from different SAGs, generating co-assembly that features >74% of genome recovery, against about 20% when assembled individually. Even though this approach is not aimed at generating high-quality draft genomes, it allows accessing to the genomic information of microbes that would otherwise remain unreachable. Since most of the picoeukaryotes still remain uncultured, our work serves as a proof-of-concept that can be applied to other taxa in order to extract genomic data and address new ecological and evolutionary questions.

Survey of the green picoalga Bathycoccus genomes in the global ocean.

Bathycoccus is a cosmopolitan green micro-alga belonging to the Mamiellophyceae, a class of picophytoplankton that contains important contributors to oceanic primary production. A single species of Bathycoccus has been described while the existence of two ecotypes has been proposed based on metagenomic data. A genome is available for one strain corresponding to the described phenotype. We report a second genome assembly obtained by a single cell genomics approach corresponding to the second ecotype. The two Bathycoccus genomes are divergent enough to be unambiguously distinguishable in whole DNA metagenomic data although they possess identical sequence of the 18S rRNA gene including in the V9 region. Analysis of 122 global ocean whole DNA metagenome samples from the Tara-Oceans expedition reveals that populations of Bathycoccus that were previously identified by 18S rRNA V9 metabarcodes are only composed of these two genomes. Bathycoccus is relatively abundant and widely distributed in nutrient rich waters. The two genomes rarely co-occur and occupy distinct oceanic niches in particular with respect to depth. Metatranscriptomic data provide evidence for gain or loss of highly expressed genes in some samples, suggesting that the gene repertoire is modulated by environmental conditions.

Ocean plankton. Eukaryotic plankton diversity in the sunlit ocean.

Marine plankton support global biological and geochemical processes. Surveys of their biodiversity have hitherto been geographically restricted and have not accounted for the full range of plankton size. We assessed eukaryotic diversity from 334 size-fractionated photic-zone plankton communities collected across tropical and temperate oceans during the circumglobal Tara Oceans expedition. We analyzed 18S ribosomal DNA sequences across the intermediate plankton-size spectrum from the smallest unicellular eukaryotes (protists, >0.8 micrometers) to small animals of a few millimeters. Eukaryotic ribosomal diversity saturated at ~150,000 operational taxonomic units, about one-third of which could not be assigned to known eukaryotic groups. Diversity emerged at all taxonomic levels, both within the groups comprising the ~11,200 cataloged morphospecies of eukaryotic plankton and among twice as many other deep-branching lineages of unappreciated importance in plankton ecology studies. Most eukaryotic plankton biodiversity belonged to heterotrophic protistan groups, particularly those known to be parasites or symbiotic hosts.

The others: our biased perspective of eukaryotic genomes.

Understanding the origin and evolution of the eukaryotic cell and the full diversity of eukaryotes is relevant to many biological disciplines. However, our current understanding of eukaryotic genomes is extremely biased, leading to a skewed view of eukaryotic biology. We argue that a phylogeny-driven initiative to cover the full eukaryotic diversity is needed to overcome this bias. We encourage the community: (i) to sequence a representative of the neglected groups available at public culture collections, (ii) to increase our culturing efforts, and (iii) to embrace single cell genomics to access organisms refractory to propagation in culture. We hope that the community will welcome this proposal, explore the approaches suggested, and join efforts to sequence the full diversity of eukaryotes.

Exploring the uncultured microeukaryote majority in the oceans: reevaluation of ribogroups within stramenopiles.

Molecular surveys in planktonic marine systems have unveiled a large novel diversity of small protists. A large part of this diversity belongs to basal heterotrophic stramenopiles and is distributed in a set of polyphyletic ribogroups (described from rDNA sequences) collectively named as MAST (MArine STramenopiles). In the few groups investigated, MAST cells are globally distributed and abundant bacterial grazers, therefore having a putatively large impact on marine ecosystem functioning. The main aim of this study is to reevaluate the MAST ribogroups described so far and to determine whether additional groups can be found. For this purpose, we used traditional and state-of-the-art molecular tools, combining 18S rDNA sequences from publicly available clone libraries, single amplified genomes (SAGs) of planktonic protists, and a pyrosequencing survey from coastal waters and sediments. Our analysis indicated a final set of 18 MAST groups plus 5 new ribogroups within Ochrophyta (named as MOCH). The MAST ribogroups were then analyzed in more detail. Seven were typical of anoxic systems and one of oxic sediments. The rest were clearly members of oxic marine picoplankton. We characterized the genetic diversity within each MAST group and defined subclades for the more diverse (46 subclades in 8 groups). The analyses of sequences within subclades revealed further ecological specializations. Our data provide a renovated framework for phylogenetic classification of the numerous MAST ribogroups and support the notion of a tight link between phylogeny and ecological distribution. These diverse and largely uncultured protists are widespread and ecologically relevant members of marine microbial assemblages.

Taming the smallest predators of the oceans.

Protists (unicellular eukaryotes) arguably account for most eukaryotic diversity and are central players of the biosphere. Known protist diversity and biology is largely based on cultured strains. Yet, environmental molecular surveys have unveiled entirely novel lineages that, as their prokaryotic counterparts, are essentially uncultured. Culture bias is an important drawback for any microbe-related science and is particularly severe for heterotrophic protists, which depend on organic food sources for growth. Here, we show how ecologically significant bacterivorous protists have been brought into culture by mimicking in situ conditions. Single cells sorted by serial dilution or flow cytometry were inoculated into seawater amended with natural bacterial assemblage at nearly in situ abundances. Strains belonging to lineages only known so far from environmental sequencing were isolated. Among them, Minorisa minuta gen. nov. sp. nov. forms a novel branch within Rhizaria, holding a key evolutionary position, and with an average size of 1.4 µm represents one of the smallest bacterial grazers known to date. It has a worldwide planktonic distribution and can account for 5% of heterotrophic protists communities in coastal waters. Physiological features of this strain can partly explain its success in the environment. Culturing ecologically relevant but elusive protists provide invaluable material for ecophysiology, genomics, ecosystem modeling and evolutionary issues.

High-throughput single-cell sequencing identifies photoheterotrophs and chemoautotrophs in freshwater bacterioplankton.

Recent discoveries suggest that photoheterotrophs (rhodopsin-containing bacteria (RBs) and aerobic anoxygenic phototrophs (AAPs)) and chemoautotrophs may be significant for marine and freshwater ecosystem productivity. However, their abundance and taxonomic identities remain largely unknown. We used a combination of single-cell and metagenomic DNA sequencing to study the predominant photoheterotrophs and chemoautotrophs inhabiting the euphotic zone of temperate, physicochemically diverse freshwater lakes. Multi-locus sequencing of 712 single amplified genomes, generated by fluorescence-activated cell sorting and whole genome multiple displacement amplification, showed that most of the cosmopolitan freshwater clusters contain photoheterotrophs. These comprised at least 10-23% of bacterioplankton, and RBs were the dominant fraction. Our data demonstrate that Actinobacteria, including clusters acI, Luna and acSTL, are the predominant freshwater RBs. We significantly broaden the known taxonomic range of freshwater RBs, to include Alpha-, Beta-, Gamma- and Deltaproteobacteria, Verrucomicrobia and Sphingobacteria. By sequencing single cells, we found evidence for inter-phyla horizontal gene transfer and recombination of rhodopsin genes and identified specific taxonomic groups involved in these evolutionary processes. Our data suggest that members of the ubiquitous betaproteobacteria Polynucleobacter spp. are the dominant AAPs in temperate freshwater lakes. Furthermore, the RuBisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) gene was found in several single cells of Betaproteobacteria, Bacteroidetes and Gammaproteobacteria, suggesting that chemoautotrophs may be more prevalent among aerobic bacterioplankton than previously thought. This study demonstrates the power of single-cell DNA sequencing addressing previously unresolved questions about the metabolic potential and evolutionary histories of uncultured microorganisms, which dominate most natural environments.

Unveiling in situ interactions between marine protists and bacteria through single cell sequencing.

Heterotrophic protists are a highly diverse and biogeochemically significant component of marine ecosystems, yet little is known about their species-specific prey preferences and symbiotic interactions in situ. Here we demonstrate how these previously unresolved questions can be addressed by sequencing the eukaryote and bacterial SSU rRNA genes from individual, uncultured protist cells collected from their natural marine environment and sorted by flow cytometry. We detected Pelagibacter ubique in association with a MAST-4 protist, an actinobacterium in association with a chrysophyte and three bacteroidetes in association with diverse protist groups. The presence of identical phylotypes among the putative prey and the free bacterioplankton in the same sample provides evidence for predator-prey interactions. Our results also suggest a discovery of novel symbionts, distantly related to Rickettsiales and the candidate divisions ZB3 and TG2, associated with Cercozoa and Chrysophyta cells. This study demonstrates the power of single cell sequencing to untangle ecological interactions between uncultured protists and prokaryotes.

A holistic approach to marine eco-systems biology.

The structure, robustness, and dynamics of ocean plankton ecosystems remain poorly understood due to sampling, analysis, and computational limitations. The Tara Oceans consortium organizes expeditions to help fill this gap at the global level.

Potential for chemolithoautotrophy among ubiquitous bacteria lineages in the dark ocean.

Recent studies suggest that unidentified prokaryotes fix inorganic carbon at globally significant rates in the immense dark ocean. Using single-cell sorting and whole-genome amplification of prokaryotes from two subtropical gyres, we obtained genomic DNA from 738 cells representing most cosmopolitan lineages. Multiple cells of Deltaproteobacteria cluster SAR324, Gammaproteobacteria clusters ARCTIC96BD-19 and Agg47, and some Oceanospirillales from the lower mesopelagic contained ribulose-1,5-bisphosphate carboxylase-oxygenase and sulfur oxidation genes. These results corroborated community DNA and RNA profiling from diverse geographic regions. The SAR324 genomes also suggested C(1) metabolism and a particle-associated life-style. Microautoradiography and fluorescence in situ hybridization confirmed bicarbonate uptake and particle association of SAR324 cells. Our study suggests potential chemolithoautotrophy in several uncultured Proteobacteria lineages that are ubiquitous in the dark oxygenated ocean and provides new perspective on carbon cycling in the ocean's largest habitat.

Targeted sorting of single virus-infected cells of the coccolithophore Emiliania huxleyi.

Discriminating infected from healthy cells is the first step to understanding the mechanisms and ecological implications of viral infection. We have developed a method for detecting, sorting, and performing molecular analysis of individual, infected cells of the important microalga Emiliania huxleyi, based on known physiological responses to viral infection. Of three fluorescent dyes tested, FM 1-43 (for detecting membrane blebbing) gave the most unequivocal and earliest separation of cells. Furthermore, we were able to amplify the genomes of single infected cells using Multiple Displacement Amplification. This novel method to reliably discriminate infected from healthy cells in cultures will allow researchers to answer numerous questions regarding the mechanisms and implications of viral infection of E. huxleyi. The method may be transferable to other virus-host systems.