Insights into the role of DNA methylation in diatoms by genome-wide profiling in Phaeodactylum tricornutum

Comparative RNA-Seq of Ten Phaeodactylum tricornutum Accessions: Unravelling Criteria for Robust Strain Selection from a Bioproduction Point of View

The production of biologics in mammalian cells is hindered by some limitations including high production costs, prompting the exploration of other alternative expression systems that are cheaper and sustainable like microalgae. Successful productions of biologics such as monoclonal antibodies have already been demonstrated in the diatom Phaeodactylum tricornutum; however, limited production yields still remain compared to mammalian cells. Therefore, efforts are needed to make this microalga more competitive as a cell biofactory. Among the seventeen reported accessions of P. tricornutum, ten have been mainly studied so far. Among them, some have already been used to produce high-value-added molecules such as biologics. The use of "omics" is increasingly being described as useful for the improvement of both upstream and downstream steps in bioprocesses using mammalian cells. Therefore, in this context, we performed an RNA-Seq analysis of the ten most used P. tricornutum accessions (Pt1 to Pt10) and deciphered the differential gene expression in pathways that could affect bioproduction of biologics in P. tricornutum. Our results highlighted the benefits of certain accessions such as Pt9 or Pt4 for the production of biologics. Indeed, these accessions seem to be more advantageous. Moreover, these results contribute to a better understanding of the molecular and cellular biology of P. tricornutum.

Genome-wide assessment of genetic diversity and transcript variations in 17 accessions of the model diatom Phaeodactylum tricornutum

Diatoms, a prominent group of phytoplankton, have a significant impact on both the oceanic food chain and carbon sequestration, thereby playing a crucial role in regulating the climate. These highly diverse organisms show a wide geographic distribution across various latitudes. In addition to their ecological significance, diatoms represent a vital source of bioactive compounds that are widely used in biotechnology applications. In the present study, we investigated the genetic and transcriptomic diversity of 17 accessions of the model diatom Phaeodactylum tricornutum including those sampled a century ago as well as more recently collected accessions. The analysis of the data reveals a higher genetic diversity and the emergence of novel clades, indicating an increasing diversity within the P. tricornutum population structure, compared to the previous study and a persistent long-term balancing selection of genes in old and newly sampled accessions. However, the study did not establish a clear link between the year of sampling and genetic diversity, thereby, rejecting the hypothesis of loss of heterozygoty in cultured strains. Transcript analysis identified novel transcript including noncoding RNA and other categories of small RNA such as PiwiRNAs. Additionally, transcripts analysis using differential expression as well as Weighted Gene Correlation Network Analysis has provided evidence that the suppression or downregulation of genes cannot be solely attributed to loss-of-function mutations. This implies that other contributing factors, such as epigenetic modifications, may play a crucial role in regulating gene expression. Our study provides novel genetic resources, which are now accessible through the platform PhaeoEpiview (https://PhaeoEpiView.univ-nantes.fr), that offer both ease of use and advanced tools to further investigate microalgae biology and ecology, consequently enriching our current understanding of these organisms.

Dicer-dependent heterochromatic small RNAs in the model diatom species Phaeodactylum tricornutum

Diatoms are eukaryotic microalgae responsible for nearly half of the marine productivity. RNA interference (RNAi) is a mechanism of regulation of gene expression mediated by small RNAs (sRNAs) processed by the endoribonuclease Dicer (DCR). To date, the mechanism and physiological role of RNAi in diatoms are unknown. We mined diatom genomes and transcriptomes for key RNAi effectors and retraced their phylogenetic history. We generated DCR knockout lines in the model diatom species Phaeodactylum tricornutum and analyzed their mRNA and sRNA populations, repression-associated histone marks, and acclimatory response to nitrogen starvation. Diatoms presented a diversification of key RNAi effectors whose distribution across species suggests the presence of distinct RNAi pathways. P. tricornutum DCR was found to process 26-31-nt-long double-stranded sRNAs originating mostly from transposons covered by repression-associated epigenetic marks. In parallel, P. tricornutum DCR was necessary for the maintenance of the repression-associated histone marks H3K9me2/3 and H3K27me3. Finally, PtDCR-KO lines presented a compromised recovery post nitrogen starvation suggesting a role for P. tricornutum DCR in the acclimation to nutrient stress. Our study characterized the molecular function of the single DCR homolog of P. tricornutum suggesting an association between RNAi and heterochromatin maintenance in this model diatom species.

Phaeodactylum tricornutum: An established model species for diatom molecular research and an emerging chassis for algal synthetic biology

Diatoms are prominent and highly diverse microalgae in aquatic environments. Compared with other diatom species, Phaeodactylum tricornutum is an "atypical diatom" displaying three different morphotypes and lacking the usual silica shell. Despite being of limited ecological relevance, its ease of growth in the laboratory and well-known physiology, alongside the steady increase in genome-enabled information coupled with effective tools for manipulating gene expression, have meant it has gained increased recognition as a powerful experimental model for molecular research on diatoms. We here present a brief overview of how over the last 25 years P. tricornutum has contributed to the unveiling of fundamental aspects of diatom biology, while also emerging as a new tool for algal process engineering and synthetic biology.

The dynamic duo: how DNA methylation and gene transcription help diatoms thrive in modern oceans

This article comments on:

Wan J, Zhou Y, Beardall J, Raven JA, Lin J, Huang J, Lu Y, Liang S, Ye M, Xiao M, Zhao J, Dai X, Xia J, Jin P. 2023. DNA methylation and gene transcription act cooperatively in driving the adaptation of a marine diatom to global change. Journal of Experimental Botany74, 4259–4276.

Chromosome-Wide Distribution and Characterization of H3K36me3 and H3K27Ac in the Marine Model Diatom Phaeodactylum tricornutum

Histone methylation and acetylation play a crucial role in response to developmental cues and environmental changes. Previously, we employed mass spectrometry to identify histone modifications such as H3K27ac and H3K36me3 in the model diatom Phaeodactylum tricornutum, which have been shown to be important for transcriptional activation in animal and plant species. To further investigate their evolutionary implications, we utilized chromatin immunoprecipitation followed by deep sequencing (ChIP-Seq) and explored their genome-wide distribution in P. tricornutum. Our study aimed to determine their role in transcriptional regulation of genes and transposable elements (TEs) and their co-occurrence with other histone marks. Our results revealed that H3K27ac and H3K36me3 were predominantly localized in promoters and genic regions indicating a high conservation pattern with studies of the same marks in plants and animals. Furthermore, we report the diversity of genes encoding H3 lysine 36 (H3K36) trimethylation-specific methyltransferase in microalgae leveraging diverse sequencing resources including the Marine Microbial Eukaryote Transcriptome Sequencing Project database (MMETSP). Our study expands the repertoire of epigenetic marks in a model microalga and provides valuable insights into the evolutionary context of epigenetic-mediated gene regulation. These findings shed light on the intricate interplay between histone modifications and gene expression in microalgae, contributing to our understanding of the broader epigenetic landscape in eukaryotic organisms.

PhaeoEpiView: an epigenome browser of the newly assembled genome of the model diatom Phaeodactylum tricornutum

Recent advances in DNA sequencing technologies particularly long-read sequencing, greatly improved genomes assembly. However, this has created discrepancies between published annotations and epigenome tracks, which have not been updated to keep pace with the new assemblies. Here, we used the latest improved telomere-to-telomere assembly of the model pennate diatom Phaeodactylum tricornutum to lift over the gene models from Phatr3, a previously annotated reference genome. We used the lifted genes annotation and newly published transposable elements to map the epigenome landscape, namely DNA methylation and post-translational modifications of histones. This provides the community with PhaeoEpiView, a browser that allows the visualization of epigenome data and transcripts on an updated and contiguous reference genome, to better understand the biological significance of the mapped data. We updated previously published histone marks with a more accurate peak calling using mono instead of poly(clonal) antibodies and deeper sequencing. PhaeoEpiView ( https://PhaeoEpiView.univ-nantes.fr ) will be continuously updated with the newly published epigenomic data, making it the largest and richest epigenome browser of any stramenopile. In the upcoming era of molecular environmental studies, where epigenetics plays a significant role, we anticipate that PhaeoEpiView will become a widely used tool.

DNA methylation and gene transcription act cooperatively in driving the adaptation of a marine diatom to global change

Genetic changes together with epigenetic modifications such as DNA methylation have been demonstrated to regulate many biological processes and thereby govern the response of organisms to environmental changes. However, how DNA methylation might act cooperatively with gene transcription and thereby mediate the long-term adaptive responses of marine microalgae to global change is virtually unknown. Here we performed a transcriptomic analysis, and a whole-genome bisulfite sequencing, along with phenotypic analysis of a model marine diatom Phaeodactylum tricornutum adapted for two years to high CO2 and/or warming conditions. Our results show that the methylated islands (peaks of methylation) mCHH were positively correlated with expression of genes in the sub-region of the gene body when the populations were grown under high CO2 or its combination with warming for ~2 years. We further identified the differentially expressed genes (DEGs) and hence the metabolic pathways in which they function, at the transcriptomics level in differentially methylated regions (DMRs). Although DEGs in DMRs contributed only 18-24% of the total DEGs, we found that those DEGs acted cooperatively with DNA methylation and then regulated key processes such as central carbon metabolism, amino acid metabolism, ribosome biogenesis, terpenoid backbone biosynthesis, and degradation of misfolded proteins. Taken together, by integrating transcriptomic, epigenetic and phenotypic analysis, our study provides evidence for DNA methylation acting cooperatively with gene transcription to contribute to the adaptation of microalgae to global changes.

The model diatom Phaeodactylum tricornutum provides insights into the diversity and function of microeukaryotic DNA methyltransferases

Cytosine methylation is an important epigenetic mark involved in the transcriptional control of transposable elements in mammals, plants and fungi. The Stramenopiles-Alveolate-Rhizaria (SAR) lineages are a major group of ecologically important marine microeukaryotes, including the phytoplankton groups diatoms and dinoflagellates. However, little is known about their DNA methyltransferase diversity. Here, we performed an in-silico analysis of DNA methyltransferases found in marine microeukaryotes and showed that they encode divergent DNMT3, DNMT4, DNMT5 and DNMT6 enzymes. Furthermore, we found three classes of enzymes within the DNMT5 family. Using a CRISPR/Cas9 strategy we demonstrated that the loss of the DNMT5a gene correlates with a global depletion of DNA methylation and overexpression of young transposable elements in the model diatom Phaeodactylum tricornutum. The study provides a view of the structure and function of a DNMT family in the SAR supergroup using an attractive model species.

The global dynamic of DNA methylation in response to heat stress revealed epigenetic mechanism of heat acclimation in Saccharina japonica

Saccharina japonica is an ecologically and economically important kelp in cold-temperate regions. When it is cultivated on a large scale in the temperate and even subtropical zones, heat stress is a frequent abiotic stress. This study is the first attempt to reveal the regulatory mechanism of the response to heat stress from the perspective of DNA methylation in S. japonica. We firstly obtained the characteristics of variation in the methylome under heat stress, and observed that heat stress caused a slight increase in the overall methylation level and methylation rate, especially in the non-coding regions of the genome. Secondly, we noted that methylation was probably one of factors affecting the expression of genes, and that methylation within the gene body was positively correlated with the gene expression (rho = 0.0784). Moreover, it was found that among the differentially expressed genes regulated by methylation, many genes were related to heat stress response, such as HSP gene family, genes of antioxidant enzymes, genes related to proteasome-ubiquitination pathway, and plant cell signaling pathways. This study demonstrated that DNA methylation is involved in regulating the response to heat stress, laying a foundation for studying the acclimation and adaptation of S. japonica to heat stress from an epigenetic perspective.

DNA Methylation in Algae and Its Impact on Abiotic Stress Responses

Epigenetics, referring to heritable gene regulatory information that is independent of changes in DNA sequences, is an important mechanism involved both in organism development and in the response to environmental events. About the epigenetic marks, DNA methylation is one of the most conserved mechanisms, playing a pivotal role in organism response to several biotic and abiotic stressors. Indeed, stress can induce changes in gene expression through hypo- or hyper-methylation of DNA at specific loci and/or in DNA methylation at the genome-wide level, which has an adaptive significance and can direct genome evolution. Exploring DNA methylation in responses to abiotic stress could have important implications for improving stress tolerance in algae. This article summarises the DNA methylation pattern in algae and its impact on abiotic stress, such as heavy metals, nutrients and temperature. Our discussion provides information for further research in algae for a better comprehension of the epigenetic response under abiotic stress, which could favour important implications to sustain algae growth under abiotic stress conditions, often related to high biosynthesis of interesting metabolites.

A metabolic, phylogenomic and environmental atlas of diatom plastid transporters from the model species Phaeodactylum

Diatoms are an important group of algae, contributing nearly 40% of total marine photosynthetic activity. However, the specific molecular agents and transporters underpinning the metabolic efficiency of the diatom plastid remain to be revealed. We performed in silico analyses of 70 predicted plastid transporters identified by genome-wide searches of Phaeodactylum tricornutum. We considered similarity with Arabidopsis thaliana plastid transporters, transcriptional co-regulation with genes encoding core plastid metabolic pathways and with genes encoded in the mitochondrial genomes, inferred evolutionary histories using single-gene phylogeny, and environmental expression trends using Tara Oceans meta-transcriptomics and meta-genomes data. Our data reveal diatoms conserve some of the ion, nucleotide and sugar plastid transporters associated with plants, such as non-specific triose phosphate transporters implicated in the transport of phosphorylated sugars, NTP/NDP and cation exchange transporters. However, our data also highlight the presence of diatom-specific transporter functions, such as carbon and amino acid transporters implicated in intricate plastid-mitochondria crosstalk events. These confirm previous observations that substrate non-specific triose phosphate transporters (TPT) may exist as principal transporters of phosphorylated sugars into and out of the diatom plastid, alongside suggesting probable agents of NTP exchange. Carbon and amino acid transport may be related to intricate metabolic plastid-mitochondria crosstalk. We additionally provide evidence from environmental meta-transcriptomic/meta- genomic data that plastid transporters may underpin diatom sensitivity to ocean warming, and identify a diatom plastid transporter (J43171) whose expression may be positively correlated with temperature.

Telomere-to-telomere genome assembly of Phaeodactylum tricornutum

Phaeodactylum tricornutum is a marine diatom with a growing genetic toolbox available and is being used in many synthetic biology applications. While most of the genome has been assembled, the currently available genome assembly is not a completed telomere-to-telomere assembly. Here, we used Oxford Nanopore long reads to build a telomere-to-telomere genome for Phaeodactylum tricornutum. We developed a graph-based approach to extract all unique telomeres, and used this information to manually correct assembly errors. In total, we found 25 nuclear chromosomes that comprise all previously assembled fragments, in addition to the chloroplast and mitochondrial genomes. We found that chromosome 19 has filtered long-read coverage and a quality estimate that suggests significantly less haplotype sequence variation than the other chromosomes. This work improves upon the previous genome assembly and provides new opportunities for genetic engineering of this species, including creating designer synthetic chromosomes.

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).

The epigenetic origin of life history transitions in plants and algae

Plants and algae have a complex life history that transitions between distinct life forms called the sporophyte and the gametophyte. This phenomenon-called the alternation of generations-has fascinated botanists and phycologists for over 170 years. Despite the mesmerizing array of life histories described in plants and algae, we are only now beginning to learn about the molecular mechanisms controlling them and how they evolved. Epigenetic silencing plays an essential role in regulating gene expression during multicellular development in eukaryotes, raising questions about its impact on the life history strategy of plants and algae. Here, we trace the origin and function of epigenetic mechanisms across the plant kingdom, from unicellular green algae through to angiosperms, and attempt to reconstruct the evolutionary steps that influenced life history transitions during plant evolution. Central to this evolutionary scenario is the adaption of epigenetic silencing from a mechanism of genome defense to the repression and control of alternating generations. We extend our discussion beyond the green lineage and highlight the peculiar case of the brown algae. Unlike their unicellular diatom relatives, brown algae lack epigenetic silencing pathways common to animals and plants yet display complex life histories, hinting at the emergence of novel life history controls during stramenopile evolution.

Genome-Wide Mapping of Cytosine Methylation Revealed Dynamic DNA Methylation Patterns Associated with Sporophyte Development of Saccharina japonica

Cytosine methylation plays vital roles in regulating gene expression and plant development. However, the function of DNA methylation in the development of macroalgae remains unclear. Through the genome-wide bisulfite sequencing of cytosine methylation in holdfast, stipe and blade, we obtained the complete 5-mC methylation landscape of Saccharina japonica sporophyte. Our results revealed that the total DNA methylation level of sporophyte was less than 0.9%, and the content of CHH contexts was dominant. Moreover, the distribution of CHH methylation within the genes exhibited exon-enriched characteristics. Profiling of DNA methylation in three parts revealed the diverse methylation pattern of sporophyte development. These pivotal DMRs were involved in cell motility, cell cycle and cell wall/membrane biogenesis. In comparison with stipe and blade, hypermethylation of mannuronate C5-epimerase in holdfast decreased the transcript abundance, which affected the synthesis of alginate, the key component of cell walls. Additionally, 5-mC modification participated in the regulation of blade and holdfast development by the glutamate content respectively via glutamine synthetase and amidophosphoribosyl transferase, which may act as the epigenetic regulation signal. Overall, our study revealed the global methylation characteristics of the well-defined holdfast, stipe and blade, and provided evidence for epigenetic regulation of sporophyte development in brown macroalgae.

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.

Adaptive responses of free-living and symbiotic microalgae to simulated future ocean conditions

Marine microalgae are a diverse group of microscopic eukaryotic and prokaryotic organisms capable of photosynthesis. They are important primary producers and carbon sinks but their physiology and persistence are severely affected by global climate change. Powerful experimental evolution technologies are being used to examine the potential of microalgae to respond adaptively to current and predicted future conditions, as well as to develop resources to facilitate species conservation and restoration of ecosystem functions. This review synthesizes findings and insights from experimental evolution studies of marine microalgae in response to elevated temperature and/or pCO₂ . Adaptation to these environmental conditions has been observed in many studies of marine dinoflagellates, diatoms and coccolithophores. An enhancement in traits such as growth and photo-physiological performance and an increase in upper thermal limit have been shown to be possible, although the extent and rate of change differ between microalgal taxa. Studies employing multiple monoclonal replicates showed variation in responses among replicates and revealed the stochasticity of mutations. The work to date is already providing valuable information on species' climate sensitivity or resilience to managers and policymakers but extrapolating these insights to ecosystem- and community-level impacts continues to be a challenge. We recommend future work should include in situ experiments, diurnal and seasonal fluctuations, multiple drivers and multiple starting genotypes. Fitness trade-offs, stable versus plastic responses and the genetic bases of the changes also need investigating, and the incorporation of genome resequencing into experimental designs will be invaluable.

Long-Term m5C Methylome Dynamics Parallel Phenotypic Adaptation in the Cyanobacterium Trichodesmium

A major challenge in modern biology is understanding how the effects of short-term biological responses influence long-term evolutionary adaptation, defined as a genetically determined increase in fitness to novel environments. This is particularly important in globally important microbes experiencing rapid global change, due to their influence on food webs, biogeochemical cycles, and climate. Epigenetic modifications like methylation have been demonstrated to influence short-term plastic responses, which ultimately impact long-term adaptive responses to environmental change. However, there remains a paucity of empirical research examining long-term methylation dynamics during environmental adaptation in nonmodel, ecologically important microbes. Here, we show the first empirical evidence in a marine prokaryote for long-term m5C methylome modifications correlated with phenotypic adaptation to CO2, using a 7-year evolution experiment (1,000+ generations) with the biogeochemically important marine cyanobacterium Trichodesmium. We identify m5C methylated sites that rapidly changed in response to high (750 µatm) CO2 exposure and were maintained for at least 4.5 years of CO2 selection. After 7 years of CO2 selection, however, m5C methylation levels that initially responded to high-CO2 returned to ancestral, ambient CO2 levels. Concurrently, high-CO2 adapted growth and N2 fixation rates remained significantly higher than those of ambient CO2 adapted cell lines irrespective of CO2 concentration, a trend consistent with genetic assimilation theory. These data demonstrate the maintenance of CO2-responsive m5C methylation for 4.5 years alongside phenotypic adaptation before returning to ancestral methylation levels. These observations in a globally distributed marine prokaryote provide critical evolutionary insights into biogeochemically important traits under global change.

Genome-wide analysis of allele-specific expression of genes in the model diatom Phaeodactylum tricornutum

Recent advances in next generation sequencing technologies have allowed the discovery of widespread autosomal allele-specific expression (aASE) in mammals and plants with potential phenotypic effects. Extensive numbers of genes with allele-specific expression have been described in the diatom Fragilariopsis cylindrus in association with adaptation to external cues, as well as in Fistulifera solaris in the context of natural hybridization. However, the role of aASE and its extent in diatoms remain elusive. In this study, we investigate allele-specific expression in the model diatom Phaeodactylum tricornutum by the re-analysis of previously published whole genome RNA sequencing data and polymorphism calling. We found that 22% of P. tricornutum genes show moderate bias in allelic expression while 1% show nearly complete monoallelic expression. Biallelic expression associates with genes encoding components of protein metabolism while moderately biased genes associate with functions in catabolism and protein transport. We validated candidate genes by pyrosequencing and found that moderate biases in allelic expression were less stable than monoallelically expressed genes that showed consistent bias upon experimental validations at the population level and in subcloning experiments. Our approach provides the basis for the analysis of aASE in P. tricornutum and could be routinely implemented to test for variations in allele expression under different environmental conditions.

Methylation Pattern and Expression Dynamics of Methylase and Photosystem Genes Under varying light Intensities in Fugacium kawagutii (Symbiodiniaceae)

Previous studies have documented extensive methylation of CpG islands and abundant methyltransferase gene (DNMT) in Fugacium kawagutii (formerly Symbiodinium kawagutii) genome. However, whether DNA methylation plays a role in regulating gene expression in this and other dinoflagellates remains unclear. Here, we characterized gene body methylation levels using methylation-specific PCR (MS-PCR) and bisulfite sequencing PCR (BSP) and measured transcriptional levels for three photosystem genes in F. kawagutii under different light conditions (20, 100, and 600 µE · m^-2  · s^-1 ). To explore the association of methylation with DNA methylase, the expression of DNA methyltransferase (Symbio-DIRS-Dnmt3) was also measured. Our results showed that peridinin-chlorophyll a-binding protein (PCP), light-harvesting complex (LHC), and chlorophyll a-c-binding protein complex (acpPC) gene expression was all significantly up-regulated under low light in which their methylation level was down-regulated, constant, and elevated, respectively. Symbio-DIRS-Dnmt3 exhibited elevated transcriptional level under increased light intensity. The results led us to hypothesize that DNA methylation level can be modulated by environmental conditions such as irradiance, probably through the regulation of Symbio-DIRS-Dnmt3 transcription level, and in turn may regulate the expression of genes in F. kawagutii. Further study is needed to determine whether the same gene methylation and expression characteristics reported here occur in other dinoflagellates and to explore their ecological implications.

Role of DNA methylation in the chromium tolerance of Scenedesmus acutus (Chlorophyceae) and its impact on the sulfate pathway regulation

DNA methylation is a very important epigenetic modification that participates in many biological functions. Although many researches on DNA methylation have been reported in various plant species, few studies have assessed the global DNA methylation pattern in algae. Even more the complex mechanisms by which DNA methylation modulates stress in algae are yet largely unresolved, mainly with respect to heavy metal stress, for which in plants, metal- and species- specific responses were instead evidenced. In this work, we performed a comparative Whole-Genome Bisulfite Sequencing (WGBS) on two strains of the green alga Scenedesmus acutus with different Cr(VI) sensitivity. The pattern of distribution of 5-mC showed significant differences between the two strains concerning both differentially methylated local contexts (CG, CHG and CHH) and Differentially Methylated Regions (DMRs) as well. We also demonstrated that DNA methylation plays an important role in modulating some genes for sulfate uptake/assimilation confirming the involvement of the sulfate pathway in the Cr-tolerance. Our results suggest that DNA methylation may be of particular importance in defining signal specificity associated with Cr-tolerance and in establishing new epigenetic marks which contribute to the adaptation to metal stress and also to transmit the epigenomic traits to the progeny.

The Aureochrome Photoreceptor PtAUREO1a Is a Highly Effective Blue Light Switch in Diatoms

Aureochromes represent a unique type of blue light photoreceptors that possess a blue light sensing flavin-binding LOV-domain and a DNA-binding bZIP domain, thus being light-driven transcription factors. The diatom Phaeodactylum tricornutum, a member of the essential marine primary producers, possesses four aureochromes (PtAUREO1a, 1b, 1c, 2). Here we show a dramatic change in the global gene expression pattern of P. tricornutum wild-type cells after a shift from red to blue light. About 75% of the genes show significantly changed transcript levels already after 10 and 60 min of blue light exposure, which includes genes of major transcription factors as well as other photoreceptors. Very surprisingly, this light-induced regulation of gene expression is almost completely inhibited in independent PtAureo1a knockout lines. Such a massive and fast transcriptional change depending on one single photoreceptor is so far unprecedented. We conclude that PtAUREO1a plays a key role in diatoms upon blue light exposure.

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Blue light induces a very fast transcriptional response in the diatom P. tricornutum

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This strong response is almost completely inhibited when Aureochrome 1a is absent

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The results imply a key role of PtAureo1a in blue light-induced responses in diatoms

Inhibition of DNA Methylation in Picochlorum soloecismus Alters Algae Productivity

Eukaryotic organisms regulate the organization, structure, and accessibility of their genomes through chromatin remodeling that can be inherited as epigenetic modifications. These DNA and histone protein modifications are ultimately responsible for an organism's molecular adaptation to the environment, resulting in distinctive phenotypes. Epigenetic manipulation of algae holds yet untapped potential for the optimization of biofuel production and bioproduct formation; however, epigenetic machinery and modes-of-action have not been well characterized in algae. We sought to determine the extent to which the biofuel platform species Picochlorum soloecismus utilizes DNA methylation to regulate its genome. We found candidate genes with domains for DNA methylation in the P. soloecismus genome. Whole-genome bisulfite sequencing revealed DNA methylation in all three cytosine contexts (CpG, CHH, and CHG). While global DNA methylation is low overall (∼1.15%), it occurs in appreciable quantities (12.1%) in CpG dinucleotides in a bimodal distribution in all genomic contexts, though terminators contain the greatest number of CpG sites per kilobase. The P. soloecismus genome becomes hypomethylated during the growth cycle in response to nitrogen starvation. Algae cultures were treated daily across the growth cycle with 20 μM 5-aza-2'-deoxycytidine (5AZA) to inhibit propagation of DNA methylation in daughter cells. 5AZA treatment significantly increased optical density and forward and side scatter of cells across the growth cycle (16 days). This increase in cell size and complexity correlated with a significant increase (∼66%) in lipid accumulation. Site specific CpG DNA methylation was significantly altered with 5AZA treatment over the time course, though nitrogen starvation itself induced significant hypomethylation in CpG contexts. Genes involved in several biological processes, including fatty acid synthesis, had altered methylation ratios in response to 5AZA; we hypothesize that these changes are potentially responsible for the phenotype of early induction of carbon storage as lipids. This is the first report to utilize epigenetic manipulation strategies to alter algal physiology and phenotype. Collectively, these data suggest these strategies can be utilized to fine-tune metabolic responses, alter growth, and enhance environmental adaption of microalgae for desired outcomes.

Sugarcane mosaic virus mediated changes in cytosine methylation pattern and differentially transcribed fragments in resistance-contrasting sugarcane genotypes

Sugarcane mosaic virus (SCMV) is the causal agent of sugarcane mosaic disease (SMD) in Brazil; it is mainly controlled by using resistant cultivars. Studies on the changes in sugarcane transcriptome provided the first insights about the molecular basis underlying the genetic resistance to SMD; nonetheless, epigenetic modifications such as cytosine methylation is also informative, considering its roles in gene expression regulation. In our previous study, differentially transcribed fragments (DTFs) were obtained using cDNA-amplified fragment length polymorphism by comparing mock- and SCMV-inoculated plants from two sugarcane cultivars with contrasting responses to SMD. In this study, the identification of unexplored DTFs was continued while the same leaf samples were used to evaluate SCMV-mediated changes in the cytosine methylation pattern by using methylation-sensitive amplification polymorphism. This analysis revealed minor changes in cytosine methylation in response to SCMV infection, but distinct changes between the cultivars with contrasting responses to SMD, with higher hypomethylation events 24 and 72 h post-inoculation in the resistant cultivar. The differentially methylated fragments (DMFs) aligned with transcripts, putative promoters, and genomic regions, with a preponderant distribution within CpG islands. The transcripts found were associated with plant immunity and other stress responses, epigenetic changes, and transposable elements. The DTFs aligned with transcripts assigned to stress responses, epigenetic changes, photosynthesis, lipid transport, and oxidoreductases, in which the transcriptional start site is located in proximity with CpG islands and tandem repeats. Real-time quantitative polymerase chain reaction results revealed significant upregulation in the resistant cultivar of aspartyl protease and VQ protein, respectively, selected from DMF and DTF alignments, suggesting their roles in genetic resistance to SMD and supporting the influence of cytosine methylation in gene expression. Thus, we identified new candidate genes for further validation and showed that the changes in cytosine methylation may regulate important mechanisms underlying the genetic resistance to SMD.

Improved Reference Genome for Cyclotella cryptica CCMP332, a Model for Cell Wall Morphogenesis, Salinity Adaptation, and Lipid Production in Diatoms (Bacillariophyta)

The diatom, Cyclotella cryptica, is a well-established model species for physiological studies and biotechnology applications of diatoms. To further facilitate its use as a model diatom, we report an improved reference genome assembly and annotation for C. cryptica strain CCMP332. We used a combination of long- and short-read sequencing to assemble a high-quality and contaminant-free genome. The genome is 171 Mb in size and consists of 662 scaffolds with a scaffold N50 of 494 kb. This represents a 176-fold decrease in scaffold number and 41-fold increase in scaffold N50 compared to the previous assembly. The genome contains 21,250 predicted genes, 75% of which were assigned putative functions. Repetitive DNA comprises 59% of the genome, and an improved classification of repetitive elements indicated that a historically steady accumulation of transposable elements has contributed to the relatively large size of the C. cryptica genome. The high-quality C. cryptica genome will serve as a valuable reference for ecological, genetic, and biotechnology studies of diatoms.

A genomics approach reveals the global genetic polymorphism, structure, and functional diversity of ten accessions of the marine model diatom Phaeodactylum tricornutum

Diatoms emerged in the Mesozoic period and presently constitute one of the main primary producers in the world's ocean and are of a major economic importance. In the current study, using whole genome sequencing of ten accessions of the model diatom Phaeodactylum tricornutum, sampled at broad geospatial and temporal scales, we draw a comprehensive landscape of the genomic diversity within the species. We describe strong genetic subdivisions of the accessions into four genetic clades (A-D) with constituent populations of each clade possessing a conserved genetic and functional makeup, likely a consequence of the limited dispersal of P. tricornutum in the open ocean. We further suggest dominance of asexual reproduction across all the populations, as implied by high linkage disequilibrium. Finally, we show limited yet compelling signatures of genetic and functional convergence inducing changes in the selection pressure on many genes and metabolic pathways. We propose these findings to have significant implications for understanding the genetic structure of diatom populations in nature and provide a framework to assess the genomic underpinnings of their ecological success and impact on aquatic ecosystems where they play a major role. Our work provides valuable resources for functional genomics and for exploiting the biotechnological potential of this model diatom species.

First Estimation of the Spontaneous Mutation Rate in Diatoms

Mutations are the origin of genetic diversity, and the mutation rate is a fundamental parameter to understand all aspects of molecular evolution. The combination of mutation-accumulation experiments and high-throughput sequencing enabled the estimation of mutation rates in most model organisms, but several major eukaryotic lineages remain unexplored. Here, we report the first estimation of the spontaneous mutation rate in a model unicellular eukaryote from the Stramenopile kingdom, the diatom Phaeodactylum tricornutum (strain RCC2967). We sequenced 36 mutation accumulation lines for an average of 181 generations per line and identified 156 de novo mutations. The base substitution mutation rate per site per generation is μbs = 4.77 × 10-10 and the insertion-deletion mutation rate is μid = 1.58 × 10-11. The mutation rate varies as a function of the nucleotide context and is biased toward an excess of mutations from GC to AT, consistent with previous observations in other species. Interestingly, the mutation rates between the genomes of organelles and the nucleus differ, with a significantly higher mutation rate in the mitochondria. This confirms previous claims based on indirect estimations of the mutation rate in mitochondria of photosynthetic eukaryotes that acquired their plastid through a secondary endosymbiosis. This novel estimate enables us to infer the effective population size of P. tricornutum to be Ne∼8.72 × 106.

Single-base methylome profiling of the giant kelp Saccharina japonica reveals significant differences in DNA methylation to microalgae and plants

Brown algae have convergently evolved plant-like body plans and reproductive cycles, which in plants are controlled by differential DNA methylation. This contribution provides the first single-base methylome profiles of haploid gametophytes and diploid sporophytes of a multicellular alga. Although only c. 1.4% of cytosines in Saccharina japonica were methylated mainly at CHH sites and characterized by 5-methylcytosine (5mC), there were significant differences between life-cycle stages. DNA methyltransferase 2 (DNMT2), known to efficiently catalyze tRNA methylation, is assumed to methylate the genome of S. japonica in the structural context of tRNAs as the genome does not encode any other DNA methyltransferases. Circular and long noncoding RNA genes were the most strongly methylated regulatory elements in S. japonica. Differential expression of genes was negatively correlated with DNA methylation with the highest methylation levels measured in both haploid gametophytes. Hypomethylated and highly expressed genes in diploid sporophytes included genes involved in morphogenesis and halogen metabolism. The data herein provide evidence that cytosine methylation, although occurring at a low level, is significantly contributing to the formation of different life-cycle stages, tissue differentiation and metabolism in brown algae.

PhaeoNet: A Holistic RNAseq-Based Portrait of Transcriptional Coordination in the Model Diatom Phaeodactylum tricornutum

Transcriptional coordination is a fundamental component of prokaryotic and eukaryotic cell biology, underpinning the cell cycle, physiological transitions, and facilitating holistic responses to environmental stress, but its overall dynamics in eukaryotic algae remain poorly understood. Better understanding of transcriptional partitioning may provide key insights into the primary metabolism pathways of eukaryotic algae, which frequently depend on intricate metabolic associations between the chloroplasts and mitochondria that are not found in plants. Here, we exploit 187 publically available RNAseq datasets generated under varying nitrogen, iron and phosphate growth conditions to understand the co-regulatory principles underpinning transcription in the model diatom Phaeodactylum tricornutum. Using WGCNA (Weighted Gene Correlation Network Analysis), we identify 28 merged modules of co-expressed genes in the P. tricornutum genome, which show high connectivity and correlate well with previous microarray-based surveys of gene co-regulation in this species. We use combined functional, subcellular localization and evolutionary annotations to reveal the fundamental principles underpinning the transcriptional co-regulation of genes implicated in P. tricornutum chloroplast and mitochondrial metabolism, as well as the functions of diverse transcription factors underpinning this co-regulation. The resource is publically available as PhaeoNet, an advanced tool to understand diatom gene co-regulation.

Multi-generational impacts of organic contaminated stream water on Daphnia magna: A combined proteomics, epigenetics and ecotoxicity approach

The present study aimed to elucidate the mechanisms of organismal sensitivity and/or physiological adaptation in the contaminated water environment. Multigenerational cultures (F0, F1, F2) of Daphnia magna in collected stream water (OCSW), contaminated with high fecal coliform, altered the reproductive scenario (changes in first brood size timing, clutch numbers, clutch size etc.), compromised fitness (increase hemoglobin, alteration in behavior), and affected global DNA methylation (hypermethylation) without affecting survival. Using proteomics approach, we found 288 proteins in F0 and 139 proteins in F2 that were significantly differentially upregulated after OCSW exposure. The individual protein expressions, biological processes and molecular functions were mainly related to metabolic processes, development and reproduction, transport (protein/lipid/oxygen), antioxidant activity, increased globin and S-adenosylmethionine synthase protein level etc., which was further found to be connected to phenotype-dependent endpoints. The proteomics pathway analysis evoked proteasome, chaperone family proteins, neuronal disease pathways (such as, Parkinson's disease) and apoptosis signaling pathways in OCSW-F0, which might be the cause of behavioral and developmental alterations in OCSW-F0. Finally, chronic multigenerational exposure to OCSW exhibited slow physiological adaptation in most of the measured effects, including proteomics analysis, from the F0 to F2 generations. The common upregulated proteins in both generations (F0 & F2), such as, globin, vitellinogen, lipid transport proteins etc., were possibly play the pivotal role in the organism's physiological adaptation. Taken together, our results, obtained with a multilevel approach, provide new insight of the molecular mechanism in fecal coliform-induced phenotypic plasticity in Daphnia magna.

A Potential Role for Epigenetic Processes in the Acclimation Response to Elevated pCO2 in the Model Diatom Phaeodactylum tricornutum

Understanding of the molecular responses underpinning diatom responses to ocean acidification is fundamental for predicting how important primary producers will be shaped by the continuous rise in atmospheric CO2. In this study, we have analyzed global transcriptomic changes of the model diatom Phaeodactylum tricornutum following growth for 15 generations in elevated pCO2 by strand-specific RNA sequencing (ssRNA-seq). Our results indicate that no significant effects of elevated pCO2 and associated carbonate chemistry changes on the physiological performance of the cells were observed after 15 generations whereas the expression of genes encoding histones and other genes involved in chromatin structure were significantly down-regulated, while the expression of transposable elements (TEs) and genes encoding histone acetylation enzymes were significantly up-regulated. Furthermore, we identified a series of long non-protein coding RNAs (lncRNAs) specifically responsive to elevated pCO2, suggesting putative regulatory roles for these largely uncharacterized genome components. Taken together, our integrative analyses reveal that epigenetic elements such as TEs, histone modifications and lncRNAs may have important roles in the acclimation of diatoms to elevated pCO2 over short time scales and thus may influence longer term adaptive processes in response to progressive ocean acidification.

Genome-wide DNA methylation and transcriptomic profiles in the lifestyle strategies and asexual development of the forest fungal pathogen Heterobasidion parviporum

Heterobasidion parviporum is the most devastating fungal pathogen of conifer forests in Northern Europe. The fungus has dual life strategies, necrotrophy on living trees and saprotrophy on dead woods. DNA cytosine methylation is an important epigenetic modification in eukaryotic organisms. Our presumption is that the lifestyle transition and asexual development in H. parviporum could be driven by epigenetic effects. Involvements of DNA methylation in the regulation of aforementioned processes have never been studied thus far. RNA-seq identified lists of highly induced genes enriched in carbohydrate-active enzymes during necrotrophic interaction with host trees and saprotrophic sawdust growth. It also highlighted signaling- and transcription factor-related genes potentially associated with the transition of saprotrophic to necrotrophic lifestyle and groups of primary cellular activities throughout asexual development. Whole-genome bisulfite sequencing revealed that DNA methylation displayed pronounced preference in CpG dinucleotide context across the genome and mostly targeted transposable element (TE)-rich regions. TE methylation level demonstrated a strong negative correlation with TE expression, reinforcing the protective function of DNA methylation in fungal genome stability. Small groups of genes putatively subject to methylation transcriptional regulation in response to saprotrophic and necrotrophic growth in comparison with free-living mycelia were also explored. Our study reported on the first methylome map of a forest pathogen. Analysis of transcriptome and methylome variations associated with asexual development and different lifestyle strategies provided further understanding of basic biological processes in H. parviporum. More importantly, our work raised additional potential roles of DNA methylation in fungi apart from controlling the proliferation of TEs.

The evolution of diatoms and their biogeochemical functions

In contemporary oceans diatoms are an important group of eukaryotic phytoplankton that typically dominate in upwelling regions and at high latitudes. They also make significant contributions to sporadic blooms that often occur in springtime. Recent surveys have revealed global information about their abundance and diversity, as well as their contributions to biogeochemical cycles, both as primary producers of organic material and as conduits facilitating the export of carbon and silicon to the ocean interior. Sequencing of diatom genomes is revealing the evolutionary underpinnings of their ecological success by examination of their gene repertoires and the mechanisms they use to adapt to environmental changes. The rise of the diatoms over the last hundred million years is similarly being explored through analysis of microfossils and biomarkers that can be traced through geological time, as well as their contributions to seafloor sediments and fossil fuel reserves. The current review aims to synthesize current information about the evolution and biogeochemical functions of diatoms as they rose to prominence in the global ocean.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Integrative analysis of large scale transcriptome data draws a comprehensive landscape of Phaeodactylum tricornutum genome and evolutionary origin of diatoms

Diatoms are one of the most successful and ecologically important groups of eukaryotic phytoplankton in the modern ocean. Deciphering their genomes is a key step towards better understanding of their biological innovations, evolutionary origins, and ecological underpinnings. Here, we have used 90 RNA-Seq datasets from different growth conditions combined with published expressed sequence tags and protein sequences from multiple taxa to explore the genome of the model diatom Phaeodactylum tricornutum, and introduce 1,489 novel genes. The new annotation additionally permitted the discovery of extensive alternative splicing in diatoms, including intron retention and exon skipping, which increase the diversity of transcripts generated in changing environments. In addition, we have used up-to-date reference sequence libraries to dissect the taxonomic origins of diatom genes. We show that the P. tricornutum genome is enriched in lineage-specific genes, with up to 47% of the gene models present only possessing orthologues in other stramenopile groups. Finally, we have performed a comprehensive de novo annotation of repetitive elements showing novel classes of transposable elements such as SINE, MITE and TRIM/LARD. This work provides a solid foundation for future studies of diatom gene function, evolution and ecology.

Biogeographic conservation of the cytosine epigenome in the globally important marine, nitrogen-fixing cyanobacterium Trichodesmium

Cytosine methylation has been shown to regulate essential cellular processes and impact biological adaptation. Despite its evolutionary importance, only a handful of bacterial, genome-wide cytosine studies have been conducted, with none for marine bacteria. Here, we examine the genome-wide, C⁵ -Methyl-cytosine (m5C) methylome and its correlation to global transcription in the marine nitrogen-fixing cyanobacterium Trichodesmium. We characterize genome-wide methylation and highlight conserved motifs across three Trichodesmium isolates and two Trichodesmium metagenomes, thereby identifying highly conserved, novel genomic signatures of potential gene regulation in Trichodesmium. Certain gene bodies with the highest methylation levels correlate with lower expression levels. Several methylated motifs were highly conserved across spatiotemporally separated Trichodesmium isolates, thereby elucidating biogeographically conserved methylation potential. These motifs were also highly conserved in Trichodesmium metagenomic samples from natural populations suggesting them to be potential in situ markers of m5C methylation. Using these data, we highlight predicted roles of cytosine methylation in global cellular metabolism providing evidence for a 'core' m5C methylome spanning different ocean regions. These results provide important insights into the m5C methylation landscape and its biogeochemical implications in an important marine N₂ -fixer, as well as advancing evolutionary theory examining methylation influences on adaptation.

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.

Genome and methylome of the oleaginous diatom Cyclotella cryptica reveal genetic flexibility toward a high lipid phenotype

BACKGROUND

Improvement in the performance of eukaryotic microalgae for biofuel and bioproduct production is largely dependent on characterization of metabolic mechanisms within the cell. The marine diatom Cyclotella cryptica, which was originally identified in the Aquatic Species Program, is a promising strain of microalgae for large-scale production of biofuel and bioproducts, such as omega-3 fatty acids.

RESULTS

We sequenced the nuclear genome and methylome of this oleaginous diatom to identify the genetic traits that enable substantial accumulation of triacylglycerol. The genome is comprised of highly methylated repetitive sequence, which does not significantly change under silicon starved lipid induction, and data further suggests the primary role of DNA methylation is to suppress DNA transposition. Annotation of pivotal glycolytic, lipid metabolism, and carbohydrate degradation processes reveal an expanded enzyme repertoire in C. cryptica that would allow for an increased metabolic capacity toward triacylglycerol production. Identification of previously unidentified genes, including those involved in carbon transport and chitin metabolism, provide potential targets for genetic manipulation of carbon flux to further increase its lipid phenotype. New genetic tools were developed, bringing this organism on a par with other microalgae in terms of genetic manipulation and characterization approaches.

CONCLUSIONS

Functional annotation and detailed cross-species comparison of key carbon rich processes in C. cryptica highlights the importance of enzymatic subcellular compartmentation for regulation of carbon flux, which is often overlooked in photosynthetic microeukaryotes. The availability of the genome sequence, as well as advanced genetic manipulation tools enable further development of this organism for deployment in large-scale production systems.

Bridging the gap between omics and earth system science to better understand how environmental change impacts marine microbes

The advent of genomic-, transcriptomic- and proteomic-based approaches has revolutionized our ability to describe marine microbial communities, including biogeography, metabolic potential and diversity, mechanisms of adaptation, and phylogeny and evolutionary history. New interdisciplinary approaches are needed to move from this descriptive level to improved quantitative, process-level understanding of the roles of marine microbes in biogeochemical cycles and of the impact of environmental change on the marine microbial ecosystem. Linking studies at levels from the genome to the organism, to ecological strategies and organism and ecosystem response, requires new modelling approaches. Key to this will be a fundamental shift in modelling scale that represents micro-organisms from the level of their macromolecular components. This will enable contact with omics data sets and allow acclimation and adaptive response at the phenotype level (i.e. traits) to be simulated as a combination of fitness maximization and evolutionary constraints. This way forward will build on ecological approaches that identify key organism traits and systems biology approaches that integrate traditional physiological measurements with new insights from omics. It will rely on developing an improved understanding of ecophysiology to understand quantitatively environmental controls on microbial growth strategies. It will also incorporate results from experimental evolution studies in the representation of adaptation. The resulting ecosystem-level models can then evaluate our level of understanding of controls on ecosystem structure and function, highlight major gaps in understanding and help prioritize areas for future research programs. Ultimately, this grand synthesis should improve predictive capability of the ecosystem response to multiple environmental drivers.

Specific Metabolites in a Phaeodactylum tricornutum Strain Isolated from Western Norwegian Fjord Water

We have searched for special characteristics in growth, protein expression, fatty acids and volatile organic compounds (VOCs) in a local Phaeodactylum tricornutum Bohlin strain (Bergen Marine Biobank), by comparing it with a common accession strain (CCAP). Differences in growth and expressed proteins were detected between the BMB strain and the CCAP strain, and the BMB strain reached the highest cell densities under the given growth conditions. Fatty acid (FA) analyses showed highest relative eicosapentaenoic acid (EPA) levels in the exponential phase (25.73% and 28.31%), and highest levels of palmitoleic acid (16:1 n-7) in the stationary phase (46.36% and 43.66%) in the BMB and CCAP strain, respectively. The most striking finding of the VOCs analyses was the relatively high levels of ectocarpene, 6-((1E)-butenyl)-1,4-cycloheptadiene, hormosirene, and desmarestene and structurally related compounds, which were exclusively detected in the BMB strain. Many of the VOCs detected in the CCAP and, in particular, in the BMB strain have been reported as antimicrobial agents. We suggest that the array of pheromones and antimicrobial substances could be part of an allelopathic strategy of the BMB strain, dominated by oval cells, thus reflecting the benthic life stage of this morphological form. These findings show the potential for bioactive metabolites in the BMB strain.

An epigenetic toolkit allows for diverse genome architectures in eukaryotes

Genome architecture varies considerably among eukaryotes in terms of both size and structure (e.g. distribution of sequences within the genome, elimination of DNA during formation of somatic nuclei). The diversity in eukaryotic genome architectures and the dynamic processes are only possible due to the well-developed epigenetic toolkit, which probably existed in the Last Eukaryotic Common Ancestor (LECA). This toolkit may have arisen as a means of navigating the genomic conflict that arose from the expansion of transposable elements within the ancestral eukaryotic genome. This toolkit has been coopted to support the dynamic nature of genomes in lineages across the eukaryotic tree of life. Here we highlight how the changes in genome architecture in diverse eukaryotes are regulated by epigenetic processes, such as DNA elimination, genome rearrangements, and adaptive changes to genome architecture. The ability to epigenetically modify and regulate genomes has contributed greatly to the diversity of eukaryotes observed today.

The upstream regulatory sequence of the light harvesting complex Lhcf2 gene of the marine diatom Phaeodactylum tricornutum enhances transcription in an orientation- and distance-independent fashion

Diatoms are a key phytoplankton group in the contemporary ocean, showing extraordinary adaptation capacities to rapidly changing environments. The recent availability of whole genome sequences from representative species has revealed distinct features in their genomes, like novel combinations of genes encoding distinct metabolisms and a significant number of diatom-specific genes. However, the regulatory mechanisms driving diatom gene expression are still largely uncharacterized. Considering the wide variety of fields of study orbiting diatoms, ranging from ecology, evolutionary biology to biotechnology, it is thus essential to increase our understanding of fundamental gene regulatory processes such as transcriptional regulation. To this aim, we explored the functional properties of the 5'-flanking region of the Phaeodatylum tricornutum Lhcf2 gene, encoding a member of the Light Harvesting Complex superfamily and we showed that this region enhances transcription of a GUS reporter gene in an orientation- and distance-independent fashion. This represents the first example of a cis-regulatory sequence with enhancer-like features discovered in diatoms and it is instrumental for the generation of novel genetic tools and diatom exploitation in different areas of study.

An integrative analysis of post-translational histone modifications in the marine diatom Phaeodactylum tricornutum

BACKGROUND

Nucleosomes are the building blocks of chromatin where gene regulation takes place. Chromatin landscapes have been profiled for several species, providing insights into the fundamental mechanisms of chromatin-mediated transcriptional regulation of gene expression. However, knowledge is missing for several major and deep-branching eukaryotic groups, such as the Stramenopiles, which include the diatoms. Diatoms are highly diverse and ubiquitous species of phytoplankton that play a key role in global biogeochemical cycles. Dissecting chromatin-mediated regulation of genes in diatoms will help understand the ecological success of these organisms in contemporary oceans.

RESULTS

Here, we use high resolution mass spectrometry to identify a full repertoire of post-translational modifications on histones of the marine diatom Phaeodactylum tricornutum, including eight novel modifications. We map five histone marks coupled with expression data and show that P. tricornutum displays both unique and broadly conserved chromatin features, reflecting the chimeric nature of its genome. Combinatorial analysis of histone marks and DNA methylation demonstrates the presence of an epigenetic code defining activating or repressive chromatin states. We further profile three specific histone marks under conditions of nitrate depletion and show that the histone code is dynamic and targets specific sets of genes.

CONCLUSIONS

This study is the first genome-wide characterization of the histone code from a stramenopile and a marine phytoplankton. The work represents an important initial step for understanding the evolutionary history of chromatin and how epigenetic modifications affect gene expression in response to environmental cues in marine environments.

Transposable element islands facilitate adaptation to novel environments in an invasive species

Adaptation requires genetic variation, but founder populations are generally genetically depleted. Here we sequence two populations of an inbred ant that diverge in phenotype to determine how variability is generated. Cardiocondyla obscurior has the smallest of the sequenced ant genomes and its structure suggests a fundamental role of transposable elements (TEs) in adaptive evolution. Accumulations of TEs (TE islands) comprising 7.18% of the genome evolve faster than other regions with regard to single-nucleotide variants, gene/exon duplications and deletions and gene homology. A non-random distribution of gene families, larvae/adult specific gene expression and signs of differential methylation in TE islands indicate intragenomic differences in regulation, evolutionary rates and coalescent effective population size. Our study reveals a tripartite interplay between TEs, life history and adaptation in an invasive species.

Genetic variation is key to species evolution. Here the authors sequence two phenotypically distinct populations of the ant Cardiocondyla obscurior, and find accumulations of transposable elements correlating with genetic variation that may have a role in differentiation, adaptation and speciation.