Three reference genomes for freshwater diatom ecology and evolution

Diatoms are an important component of marine and freshwater ecosystems. Although the majority of described diatom species live in freshwater systems, genome sequencing efforts have focused primarily on marine species. Genomic resources for freshwater species have the potential to improve our understanding of diatom ecology and evolution, particularly in the context of major environmental shifts. We used long- and short-read sequencing platforms to assemble reference genomes for three freshwater diatom species, all in the order Thalalassiosirales, which are abundant in the plankton of oceans, lakes, reservoirs, and rivers worldwide. We targeted three species that cover the breadth of phylogenetic diversity in the cyclostephanoid clade of Thalassiosirales: Cyclostephanos tholiformis (JALLPB020000000), Discostella pseudostelligera (JALLBG020000000), and Praestephanos triporus (JALLAZ020000000). The reference genome for D. pseudostelligera was considerably smaller (39 Mb) than those of both P. triporus (73 Mb) and C. tholiformis (177 Mb). Long-read sequencing allowed for the assembly of scaffold-level genomes, including regions rich in repetitive DNA. Compared to short-read assemblies, long-read assemblies increased the contig N50 length as much as 37-fold and reduced the number of contigs by more than 88%. Transcriptome-guided annotation of the protein-coding genes identified between 10,000 and 12,000 genes. This work provides further demonstration of the value of long-read sequencing and provides novel genomic resources for understanding the ecology and evolution of freshwater diatoms.

High-quality genome assembly and annotation of Thalassiosira rotula (synonym of Thalassiosira gravida)

Diatoms are unicellular eukaryotic microorganisms thriving in most aquatic environments thanks to the expression of biosynthetic pathways for secondary metabolites involved in defence and adaptation to environmental changes. The sequencing of the transcriptome of the cosmopolitan diatom Thalassiosira rotula Meunier 1910 (synonym of Thalassiosira gravida Cleve 1896) and of the metagenome of its associated microbiome revealed the presence of biosynthetic pathways synthesising molecules and compounds useful for the algae survival and with potential biotechnological applications. Here we present the genome of a Neapolitan T. rotula strain, which is 672 Mbp in size due to a high proportion of repetitive elements (63.59%) and segmental duplications (14%), while the number of predicted genes resulted to be comparable to that of smaller diatom genomes. DNA methylation was predominantly located in transposable elements.

First regional reference database of northern Adriatic diatom transcriptomes

Marine microbial communities form the basis for the functioning of marine ecosystems and the conservation of biodiversity. With the application of metagenomics and metatranscriptomics in marine environmental studies, significant progress has been made in analysing the functioning of microbial communities as a whole. These molecular techniques are highly dependent on reliable, well-characterised, comprehensive and taxonomically diverse sequenced reference transcriptomes of microbial organisms. Here we present a set of 12 individual transcriptome assemblies derived from 6 representative diatom species from the northern Adriatic Sea grown under 2 environmentally relevant growth conditions (phosphate replete vs. phosphate deprived). After filtering the reads and assembly, an average number of 64,932 transcripts per assembly was obtained, of which an average of 8856 were assigned to functionally known proteins. Of all assigned transcripts, an average of 6483 proteins were taxonomically assigned to diatoms (Bacillariophyta). On average, a higher number of assigned proteins was detected in the transcriptome assemblies of diatoms grown under replete media condition. On average, 50% of the mapped proteins were shared between the two growth conditions. All recorded proteins in the dataset were classified into 24 COG categories, with approximately 25% belonging to the unknown function and the remaining 75% belonging to all other categories. The resulting diatom reference database for the northern Adriatic, focussing on the response to nutrient limitation as characteristic for the region and predicted for the future world oceans, provides a valuable resource for analysing environmental metatranscriptome and metagenome data. Each northern Adriatic transcriptome can also be used by itself as a reference database for the (meta)transcriptomes and gene expression studies of the associated species that will be generated in the future.

Differential Expression of Stress Adaptation Genes in a Diatom Ulnaria acus under Different Culture Conditions

Diatoms are a group of unicellular eukaryotes that are essential primary producers in aquatic ecosystems. The dynamic nature of their habitat necessitates a quick and specific response to various stresses. However, the molecular mechanisms of their physiological adaptations are still underexplored. In this work, we study the response of the cosmopolitan freshwater diatom Ulnaria acus (Bacillariophyceae, Fragilariophycidae, Licmophorales, Ulnariaceae, Ulnaria) in relation to a range of stress factors, namely silica deficiency, prolonged cultivation, and interaction with an algicidal bacterium. Fluorescent staining and light microscopy were used to determine the physiological state of cells under these stresses. To explore molecular reactions, we studied the genes involved in the stress response-type III metacaspase (MC), metacaspase-like proteases (MCP), death-specific protein (DSP), delta-1-pyrroline-5-carboxylate dehydrogenase (ALDH12), and glutathione synthetase (GSHS). We have described the structure of these genes, analyzed the predicted amino acid sequences, and measured their expression dynamics in vitro using qRT-PCR. We demonstrated that the expression of UaMC1, UaMC3, and UaDSP increased during the first five days of silicon starvation. On the seventh day, it was replaced with the expression of UaMC2, UaGSHS, and UaALDH. After 45 days of culture, cells stopped growing, and the expression of UaMC1, UaMC2, UaGSHS, and UaDSP increased. Exposure to an algicidal bacterial filtrate induced a higher expression of UaMC1 and UaGSHS. Thus, we can conclude that these proteins are involved in diatoms' adaptions to environmental changes. Further, these data show that the molecular adaptation mechanisms in diatoms depend on the nature and exposure duration of a stress factor.

Expansion of photoreception-related gene families may drive ecological adaptation of the dominant diatom species Skeletonema marinoi

Diatom species of the genus Skeletonema are dominant in global coastal waters with important roles in marine primary production and global biogeochemical cycling. Many Skeletonema species have been extensively studied also because they can cause harmful algae blooms (HABs) with negative impacts on marine ecosystems and aquaculture. In this study, the first chromosome-level assembly of the genome of Skeletonema marinoi was constructed. The genome size was 64.99 Mb with a contig N50 of 1.95 Mb. Up to 97.12 % of contigs were successfully anchored on 24 chromosomes. Analysis of the annotated genes revealed 28 large syntenic blocks with 2397 collinear gene pairs in the genome of S. marinoi, suggesting large-scale segmental duplication events in evolution. Substantial expansion of light-harvesting genes encoding fucoxanthin-chlorophyll a/c binding proteins, as well as expansion of photoreceptor gene families encoding aureochromes and cyptochromes (CRY) in S. marinoi were found, which may have shaped ecological adaptation of S. marinoi. In conclusion, the construction of the first high-quality Skeletonema genome assembly offers valuable clues on the ecological and evolutionary characteristics of this dominant coastal diatom species.

Analysis of Predicted Amino Acid Sequences of Diatom Microtubule Center Components

Diatoms synthesize species-specific exoskeletons inside cells under the control of the cytoskeleton and microtubule center. Previous studies have been conducted with the visualization of the microtubule center; however, its composition has not been studied and reliably established. In the present study, several components of MTOC in diatoms, GCP (gamma complex proteins), Aurora A, and centrins have been identified. Analysis of the predicted amino acid sequences of these proteins revealed structural features typical for diatoms. We analyzed the conserved amino acids and the motives necessary for the functioning of proteins. Phylogenetic analysis of GCP showed that all major groups of diatoms are distributed over phylogenetic trees according to their systematic position. This work is a theoretical study; however, it allows drawing some conclusions about the functioning of the studied components and possible ways to regulate them.

Revealing the Differences in Ulnaria acus and Fragilaria radians Distribution in Lake Baikal via Analysis of Existing Metabarcoding Data

Two diatom species, Ulnaria acus and Fragilaria radians, are morphologically very similar and often coexist, which makes it difficult to compare their abundances. However, they are easily separated by molecular data; thus, in this work, we attempted to estimate the differences in their spatial and temporal distribution from existing metabarcoding datasets. Reanalyzing published sequences with an ASV-based pipeline and ad hoc classification routine allowed us to estimate the relative abundances of the two species, increasing the precision compared to usual OTU-based analyses. Existing data permit qualitative comparisons between two species that cannot be differentiated by other methods, detecting the distinct seasonal peaks and spatial distributions of F. radians and U. acus.

Delimitation of Some Taxa of Ulnaria and Fragilaria (Bacillariophyceae) Based on Genetic, Morphological Data and Mating Compatibility

Fragilaria and Ulnaria are two closely related diatom genera for which the delimitation and circumscription of several species is unclear. We studied strains isolated from Lake Baikal and compared them with the species from freshwater reservoirs in Europe and Asia using phylogenetic and species delimitation methods, microscopy and interclonal crossing experiments. The results of the phylogenetic analyses of the fragments of rbcL and 18S rRNA genes revealed that baikalian F. radians clade was independent from the representatives of the genus from other localities. Among Ulnaria we found the following 18S rRNA phylogenetic tree groups at species level: U. acus, U. ulna and U. danica. Genetic distance between genera varied between 3.9–10.2% substitutions in rbcL gene and 3.2–11.5% in 18S rRNA. The boundary between intraspecies and interspecies polymorphism for studied species of Ulnaria and Fragilaria in these marker genes was around 0.8% substitutions. Morphometric characters of individual strains showed their variability and division into F. radians, U. acus and U. ulna together with U. danica. Strains of U. acus and U. danica from different localities of Europe and Asia were sexually compatible inside the species. Sexual reproduction has never been observed in monoclonal cultures, either between this species or with strains of the Fragilaria.

Molecular Insights into Lipoxygenases in Diatoms Based on Structure Prediction: a Pioneering Study on Lipoxygenases Found in Pseudo-nitzschia arenysensis and Fragilariopsis cylindrus

Diatoms produce a variety of oxylipins which are oxygenated polyunsaturated fatty acids and are involved in chemical defense and intercellular communication, among other roles. Although the chemistry of diatom oxylipins has long been studied, the enzymes involved in their production, in particular lipoxygenase (LOX), which catalyzes the initial reaction of the synthesis, have not been discovered in diatom genomes. Recently, diatom LOXs were found in two species, Pseudo-nitzschia arenysensis (PaLOX) and Fragilariopsis cylindrus (FcLOX); however, the enzymology of these LOXs is largely unknown. In this review article, we discuss the potential functions of the diatom LOXs based on previously reported structures of LOXs derived from various organisms other than diatoms. Since the structures of PaLOX and FcLOX have not yet been solved, we discussed their functions, such as regio- and stereospecificities, on the basis of their structures predicted using a computational tool based on deep learning technology. Both diatom LOXs were predicted to conserve common core domains with relatively wide substrate-binding pockets. The stereo-determinant residues in PaLOX and FcLOX suggest S specificity. We assume that the highly conserved common core domain can be a clue to reveal unidentified lox genes from the accumulated diatom genome information with the aid of high-throughput structure prediction tools and structure-based alignment tools in the near future.

What Was Old Is New Again: The Pennate Diatom Haslea ostrearia (Gaillon) Simonsen in the Multi-Omic Age

The marine pennate diatom Haslea ostrearia has long been known for its characteristic blue pigment marennine, which is responsible for the greening of invertebrate gills, a natural phenomenon of great importance for the oyster industry. For two centuries, this taxon was considered unique; however, the recent description of a new blue Haslea species revealed unsuspected biodiversity. Marennine-like pigments are natural blue dyes that display various biological activities—e.g., antibacterial, antioxidant and antiproliferative—with a great potential for applications in the food, feed, cosmetic and health industries. Regarding fundamental prospects, researchers use model organisms as standards to study cellular and physiological processes in other organisms, and there is a growing and crucial need for more, new and unconventional model organisms to better correspond to the diversity of the tree of life. The present work, thus, advocates for establishing H. ostrearia as a new model organism by presenting its pros and cons—i.e., the interesting aspects of this peculiar diatom (representative of benthic-epiphytic phytoplankton, with original behavior and chemodiversity, controlled sexual reproduction, fundamental and applied-oriented importance, reference genome, and transcriptome will soon be available); it will also present the difficulties encountered before this becomes a reality as it is for other diatom models (the genetics of the species in its infancy, the transformation feasibility to be explored, the routine methods needed to cryopreserve strains of interest).

Phylogeny and structural peculiarities of the EB proteins of diatoms

The end-binding proteins are a family of microtubule-associated proteins; this family belongs to plus-end-tracking proteins (+TIPs) that regulate microtubule growth and stabilisation. Although the genes encoding EB proteins are found in all eukaryotic genomes, most studies of them have centred on one or another taxonomic group, without a broad comparative analysis. Here, we present a first phylogenetic analysis and a comparative analysis of domain structures of diatom EB proteins in comparison with other phyla of Chromista, red and green algae, as well as model organisms A. thaliana and H. sapiens. Phylogenetically, diatom EB proteins are separated into six clades, generally corresponding to the phylogeny of their respective organisms. The domain structure of this family is highly variable, but the CH and EBH domains responsible for binding tubulin and other MAPs are mostly conserved. Homologous modelling of the F. cylindrus EB protein shows that conserved motifs of the CH domain are positioned on the protein surface, which is necessary for their functioning. We hypothesise that high variance of the diatom C-terminal domain is caused by previously unknown interactions with a CAP-GLY motif of dynactin subunit p150. Our findings contribute to wider possibilities for further investigations of the cytoskeleton in diatoms.

Phylogenomic fingerprinting of tempo and functions of horizontal gene transfer within ochrophytes

Horizontal gene transfer (HGT) is an important source of novelty in eukaryotic genomes. This is particularly true for the ochrophytes, a diverse and important group of algae. Previous studies have shown that ochrophytes possess a mosaic of genes derived from bacteria and eukaryotic algae, acquired through chloroplast endosymbiosis and from HGTs, although understanding of the time points and mechanisms underpinning these transfers has been restricted by the depth of taxonomic sampling possible. We harness an expanded set of ochrophyte sequence libraries, alongside automated and manual phylogenetic annotation, in silico modeling, and experimental techniques, to assess the frequency and functions of HGT across this lineage. Through manual annotation of thousands of single-gene trees, we identify continuous bacterial HGT as the predominant source of recently arrived genes in the model diatom Phaeodactylum tricornutum Using a large-scale automated dataset, a multigene ochrophyte reference tree, and mathematical reconciliation of gene trees, we note a probable elevation of bacterial HGTs at foundational points in diatom evolution, following their divergence from other ochrophytes. Finally, we demonstrate that throughout ochrophyte evolutionary history, bacterial HGTs have been enriched in genes encoding secreted proteins. Our study provides insights into the sources and frequency of HGTs, and functional contributions that HGT has made to algal evolution.

Re-examination of two diatom reference genomes using long-read sequencing

Background

The marine diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum are valuable model organisms for exploring the evolution, diversity and ecology of this important algal group. Their reference genomes, published in 2004 and 2008, respectively, were the product of traditional Sanger sequencing. In the case of T. pseudonana, optical restriction site mapping was employed to further clarify and contextualize chromosome-level scaffolds. While both genomes are considered highly accurate and reasonably contiguous, they still contain many unresolved regions and unordered/unlinked scaffolds.

Results

We have used Oxford Nanopore Technologies long-read sequencing to update and validate the quality and contiguity of the T. pseudonana and P. tricornutum genomes. Fine-scale assessment of our long-read derived genome assemblies allowed us to resolve previously uncertain genomic regions, further characterize complex structural variation, and re-evaluate the repetitive DNA content of both genomes. We also identified 1862 previously undescribed genes in T. pseudonana. In P. tricornutum, we used transposable element detection software to identify 33 novel copia-type LTR-RT insertions, indicating ongoing activity and rapid expansion of this superfamily as the organism continues to be maintained in culture. Finally, Bionano optical mapping of P. tricornutum chromosomes was combined with long-read sequence data to explore the potential of long-read sequencing and optical mapping for resolving haplotypes.

Conclusion

Despite its potential to yield highly contiguous scaffolds, long-read sequencing is not a panacea. Even for relatively small nuclear genomes such as those investigated herein, repetitive DNA sequences cause problems for current genome assembly algorithms. Determining whether a long-read derived genomic assembly is ‘better’ than one produced using traditional sequence data is not straightforward. Our revised reference genomes for P. tricornutum and T. pseudonana nevertheless provide additional insight into the structure and evolution of both genomes, thereby providing a more robust foundation for future diatom research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07666-3.

Mini-Review: Potential of Diatom-Derived Silica for Biomedical Applications

Diatoms are unicellular eukaryotic microalgae widely distributed in aquatic environments, possessing a porous silica cell wall known as frustule. Diatom frustules are considered as a sustainable source for several industrial applications because of their high biocompatibility and the easiness of surface functionalisation, which make frustules suitable for regenerative medicine and as drug carriers. Frustules are made of hydrated silica, and can be extracted and purified both from living and fossil diatoms using acid treatments or high temperatures. Biosilica frustules have proved to be suitable for biomedical applications, but, unfortunately, they are not officially recognised as safe by governmental food and medical agencies yet. In the present review, we highlight the frustule formation process, the most common purification techniques, as well as advantages and bottlenecks related to the employment of diatom-derived silica for medical purposes, suggesting possible solutions for a large-scale biosilica production.

Comprehensive and Functional Analysis of Horizontal Gene Transfer Events in Diatoms

Diatoms are a diverse group of mainly photosynthetic algae, responsible for 20% of worldwide oxygen production, which can rapidly respond to favorable conditions and often outcompete other phytoplankton. We investigated the contribution of horizontal gene transfer (HGT) to its ecological success. A large-scale phylogeny-based prokaryotic HGT detection procedure across nine sequenced diatoms showed that 3-5% of their proteome has a horizontal origin and a large influx occurred at the ancestor of diatoms. More than 90% of HGT genes are expressed, and species-specific HGT genes in Phaeodactylum tricornutum undergo strong purifying selection. Genes derived from HGT are implicated in several processes including environmental sensing and expand the metabolic toolbox. Cobalamin (vitamin B12) is an essential cofactor for roughly half of the diatoms and is only produced by bacteria. Five consecutive genes involved in the final synthesis of the cobalamin biosynthetic pathway, which could function as scavenging and repair genes, were detected as HGT. The full suite of these genes was detected in the cold-adapted diatom Fragilariopsis cylindrus. This might give diatoms originating from the Southern Ocean, a region typically depleted in cobalamin, a competitive advantage. Overall, we show that HGT is a prevalent mechanism that is actively used in diatoms to expand its adaptive capabilities.

Iron metabolism strategies in diatoms

Diatoms are one of the most successful group of photosynthetic eukaryotes in the contemporary ocean. They are ubiquitously distributed and are the most abundant primary producers in polar waters. Equally remarkable is their ability to tolerate iron deprivation and respond to periodic iron fertilization. Despite their relatively large cell sizes, diatoms tolerate iron limitation and frequently dominate iron-stimulated phytoplankton blooms, both natural and artificial. Here, we review the main iron use strategies of diatoms, including their ability to assimilate and store a range of iron sources, and the adaptations of their photosynthetic machinery and architecture to iron deprivation. Our synthesis relies on published literature and is complemented by a search of 82 diatom transcriptomes, including information collected from seven representatives of the most abundant diatom genera in the world's oceans.

The Seminavis robusta genome provides insights into the evolutionary adaptations of benthic diatoms

Benthic diatoms are the main primary producers in shallow freshwater and coastal environments, fulfilling important ecological functions such as nutrient cycling and sediment stabilization. However, little is known about their evolutionary adaptations to these highly structured but heterogeneous environments. Here, we report a reference genome for the marine biofilm-forming diatom Seminavis robusta, showing that gene family expansions are responsible for a quarter of all 36,254 protein-coding genes. Tandem duplications play a key role in extending the repertoire of specific gene functions, including light and oxygen sensing, which are probably central for its adaptation to benthic habitats. Genes differentially expressed during interactions with bacteria are strongly conserved in other benthic diatoms while many species-specific genes are strongly upregulated during sexual reproduction. Combined with re-sequencing data from 48 strains, our results offer insights into the genetic diversity and gene functions in benthic diatoms.

Genome-enabled phylogenetic and functional reconstruction of an araphid pennate diatom Plagiostriata sp. CCMP470, previously assigned as a radial centric diatom, and its bacterial commensal

Diatoms are an ecologically fundamental and highly diverse group of algae, dominating marine primary production in both open-water and coastal communities. The diatoms include both centric species, which may have radial or polar symmetry, and the pennates, which include raphid and araphid species and arose within the centric lineage. Here, we use combined microscopic and molecular information to reclassify a diatom strain CCMP470, previously annotated as a radial centric species related to Leptocylindrus danicus, as an araphid pennate species in the staurosiroid lineage, within the genus Plagiostriata. CCMP470 shares key ultrastructural features with Plagiostriata taxa, such as the presence of a sternum with parallel striae, and the presence of a highly reduced labiate process on its valve; and this evolutionary position is robustly supported by multigene phylogenetic analysis. We additionally present a draft genome of CCMP470, which is the first genome available for a staurosiroid lineage. 270 Pfams (19%) found in the CCMP470 genome are not known in other diatom genomes, which otherwise does not hold big novelties compared to genomes of non-staurosiroid diatoms. Notably, our DNA library contains the genome of a bacterium within the Rhodobacterales, an alpha-proteobacterial lineage known frequently to associate with algae. We demonstrate the presence of commensal alpha-proteobacterial sequences in other published algal genome and transcriptome datasets, which may indicate widespread and persistent co-occurrence.

De novo transcriptome assembly and analysis of the freshwater araphid diatom Fragilaria radians, Lake Baikal

Diatoms are a group of eukaryotic microalgae populating almost all aquatic and wet environments. Their abundance and species diversity make these organisms significant contributors to biogeochemical cycles and important components of aquatic ecosystems. Although significant progress has been made in studies of Diatoms (Bacillariophyta) over the last two decades, since the spread of "omics" technologies, our current knowledge of the molecular processes and gene regulatory networks that facilitate environmental adaptation remain incomplete. Here, we present a transcriptome analysis of Fragilaria radians isolated from Lake Baikal. The resulting assembly contains 27,446 transcripts encoding 21,996 putative proteins. The transcriptome assembly and annotation were coupled with quantitative experiments to search for differentially expressed transcripts between (i) exponential growth phase and dark-acclimated cell cultures, and (ii) those changing expression level during the early response to light treatment in dark-acclimated cells. The availability of F. radians genome and transcriptome data provides the basis for future targeted studies of this species. Furthermore, our results extend taxonomic and environmental sampling of Bacillariophyta, opening new opportunities for comparative omics-driven surveys.

Extensive chloroplast genome rearrangement amongst three closely related Halamphora spp. (Bacillariophyceae), and evidence for rapid evolution as compared to land plants

Diatoms are the most diverse lineage of algae, but the diversity of their chloroplast genomes, particularly within a genus, has not been well documented. Herein, we present three chloroplast genomes from the genus Halamphora (H. americana, H. calidilacuna, and H. coffeaeformis), the first pennate diatom genus to be represented by more than one species. Halamphora chloroplast genomes ranged in size from ~120 to 150 kb, representing a 24% size difference within the genus. Differences in genome size were due to changes in the length of the inverted repeat region, length of intergenic regions, and the variable presence of ORFs that appear to encode as-yet-undescribed proteins. All three species shared a set of 161 core features but differed in the presence of two genes, serC and tyrC of foreign and unknown origin, respectively. A comparison of these data to three previously published chloroplast genomes in the non-pennate genus Cyclotella (Thalassiosirales) revealed that Halamphora has undergone extensive chloroplast genome rearrangement compared to other genera, as well as containing variation within the genus. Finally, a comparison of Halamphora chloroplast genomes to those of land plants indicates diatom chloroplast genomes within this genus may be evolving at least ~4–7 times faster than those of land plants. Studies such as these provide deeper insights into diatom chloroplast evolution and important genetic resources for future analyses.

A family of silicon transporter structural genes in a pennate diatom Synedra ulna subsp. danica (Kütz.) Skabitsch

Silicon transporters (SIT) are the proteins, which capture silicic acid in the aquatic environment and direct it across the plasmalemma to the cytoplasm of diatoms. Diatoms utilize silicic acid to build species-specific ornamented exoskeletons and make a significant contribution to the global silica cycle, estimated at 240 ±40 Tmol a year. Recently SaSIT genes of the freshwater araphid pennate diatom Synedra acus subsp. radians are found to be present in the genome as a cluster of two structural genes (SaSIT-TD and SaSIT-TRI) encoding several concatenated copies of a SIT protein each. These structural genes could potentially be transformed into "mature" SIT proteins by means of posttranslational proteolytic cleavage. In the present study, we discovered three similar structural SuSIT genes in the genome of a closely related freshwater diatom Synedra ulna subsp. danica. Structural gene SuSIT1 is identical to structural gene SuSIT2, and the two are connected by a non-coding nucleotide DNA sequence. All the putative "mature" SITs contain conserved amino acid motifs, which are believed to be important in silicon transport. The data obtained suggest that the predicted "mature" SIT proteins may be the minimal units necessary for the transport of silicon is S. ulna subsp. danica. The comparative analysis of all available multi-SITs has allowed us to detect two conservative motifs YQXDXVYL and DXDID, located between the "mature" proteins. Aspartic acid-rich DXDID motif can, in our opinion, serve as a proteolysis site during the multi-SIT cleavage. The narrow distribution of the distances between CMLD and DXDID motifs can serve as additional evidence to the conservation of their function.

Intracellular metabolic pathway distribution in diatoms and tools for genome-enabled experimental diatom research

Diatoms are important primary producers in the oceans and can also dominate other aquatic habitats. One reason for the success of this phylogenetically relatively young group of unicellular organisms could be the impressive redundancy and diversity of metabolic isoenzymes in diatoms. This redundancy is a result of the evolutionary origin of diatom plastids by a eukaryote-eukaryote endosymbiosis, a process that implies temporary redundancy of functionally complete eukaryotic genomes. During the establishment of the plastids, this redundancy was partially reduced via gene losses, and was partially retained via gene transfer to the nucleus of the respective host cell. These gene transfers required re-assignment of intracellular targeting signals, a process that simultaneously altered the intracellular distribution of metabolic enzymes compared with the ancestral cells. Genome annotation, the correct assignment of the gene products and the prediction of putative function, strongly depends on the correct prediction of the intracellular targeting of a gene product. Here again diatoms are very peculiar, because the targeting systems for organelle import are partially different to those in land plants. In this review, we describe methods of predicting intracellular enzyme locations, highlight findings of metabolic peculiarities in diatoms and present genome-enabled approaches to study their metabolism.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Diatom Frustule Morphogenesis and Function: a Multidisciplinary Survey

Diatoms represent the major component of phytoplankton and are responsible for about 20-25% of global primary production. Hundreds of millions of years of evolution led to tens of thousands of species differing in dimensions and morphologies. In particular, diatom porous silica cell walls, the frustules, are characterized by an extraordinary, species-specific diversity. It is of great interest, among the marine biologists and geneticists community, to shed light on the origin and evolutionary advantage of this variability of dimensions, geometries and pore distributions. In the present article the main reported data related to frustule morphogenesis and functionalities with contributions from fundamental biology, genetics, mathematics, geometry and physics are reviewed.

Chimeric origins of ochrophytes and haptophytes revealed through an ancient plastid proteome

Plastids are supported by a wide range of proteins encoded within the nucleus and imported from the cytoplasm. These plastid-targeted proteins may originate from the endosymbiont, the host, or other sources entirely. Here, we identify and characterise 770 plastid-targeted proteins that are conserved across the ochrophytes, a major group of algae including diatoms, pelagophytes and kelps, that possess plastids derived from red algae. We show that the ancestral ochrophyte plastid proteome was an evolutionary chimera, with 25% of its phylogenetically tractable nucleus-encoded proteins deriving from green algae. We additionally show that functional mixing of host and plastid proteomes, such as through dual-targeting, is an ancestral feature of plastid evolution. Finally, we detect a clear phylogenetic signal from one ochrophyte subgroup, the lineage containing pelagophytes and dictyochophytes, in plastid-targeted proteins from another major algal lineage, the haptophytes. This may represent a possible serial endosymbiosis event deep in eukaryotic evolutionary history.

DOI:http://dx.doi.org/10.7554/eLife.23717.001

The cells of most plants and algae contain compartments called chloroplasts that enable them to capture energy from sunlight in a process known as photosynthesis. Chloroplasts are the remnants of photosynthetic bacteria that used to live freely in the environment until they were consumed by a larger cell. “Complex” chloroplasts can form if a cell that already has a chloroplast is swallowed by another cell.

The most abundant algae in the oceans are known as diatoms. These algae belong to a group called the stramenopiles, which also includes giant seaweeds such as kelp. The stramenopiles have a complex chloroplast that they acquired from a red alga (a relative of the seaweed used in sushi). However, some of the proteins in their chloroplasts are from other sources, such as the green algal relatives of plants, and it was not clear how these chloroplast proteins have contributed to the evolution of this group.

Many of the proteins that chloroplasts need to work properly are produced by the host cell and are then transported into the chloroplasts. Dorrell et al. studied the genetic material of many stramenopile species and identified 770 chloroplast-targeted proteins that are predicted to underpin the origins of this group. Experiments in a diatom called Phaeodactylum confirmed these predictions and show that many of these chloroplast-targeted proteins have been recruited from green algae, bacteria, and other compartments within the host cell to support the chloroplast.

Further experiments suggest that another major group of algae called the haptophytes once had a stramenopile chloroplast. The current haptophyte chloroplast does not come from the stramenopiles so the haptophytes appear to have replaced their chloroplasts at least once in their evolutionary history.

The findings show that algal chloroplasts are mosaics, supported by proteins from many different species. This helps us understand why certain species succeed in the wild and how they may respond to environmental changes in the oceans. In the future, these findings may help researchers to engineer new species of algae and plants for food and fuel production.

DOI:http://dx.doi.org/10.7554/eLife.23717.002

Recent progress in diatom genomics and epigenomics

Diatoms are one of the most diverse and successful groups of phytoplankton at the base of the food chain, sustaining life in the ocean and performing vital biogeochemical functions. The last fifteen years have witnessed the comprehensive analysis of several diatom genomes, revealing that they bear traces of their endosymbiotic origins from algal and heterotrophic ancestors, as well as significant gene transfer from bacteria. Their chimeric genomes are further regulated by a range of chromatin-based processes that are characteristic of both plant and animal genomes. We discuss the conservation of gene regulatory mechanisms in diatoms and propose that epigenetic processes may have a significant role in mediating responses to a highly dynamic and unpredictable environment in these organisms.

Unique configuration of genes of silicon transporter in the freshwater pennate diatom Synedra acus subsp. radians

The existence of the cluster of duplicated sit silicon transporter genes in the chromosome of the diatom Synedra acus subsp. radians was shown for the first time. Earlier, the localization of sit genes in the same chromosome and cluster formation caused by gene duplication was shown only for the marine raphid pennate diatom P. tricornutum. Only non-clustered sit genes were found in the genomes of other diatoms. It is reasonable to assume that sit tandem (sit-td) and sit triplet (sit-tri) genes of S. acus subsp. radians occurred as a result of gene duplication followed by divergence of gene copies.