A streamlined and predominantly diploid genome in the tiny marine green alga Chloropicon primus

Chromosome-level genome provides novel insights into the starch metabolism regulation and evolutionary history of Tetraselmis helgolandica

INTRODUCTION

Tetraselmis helgolandica is a marine microalga belonging to the Chlorophyta phylum. It is widely distributed in the coastal waters of Asia and is commonly used as aquatic feed. T. helgolandica is characterized by its large size, preference for starch accumulation, low temperature tolerance, presence of flagella, and strong motility. However, research on T. helgolandica is limited, and its genome data remains unavailable.

OBJECTIVE

We generated a high-quality, chromosome-scale genome of T. helgolandica. Through comparative genomics, we uncovered the genome characteristics and evolutionary history of T. helgolandica. Additionally, by integrating transcriptome data, we elucidated how the light-dark rhythm enhances the high starch production.

METHODS

We utilized long-read sequencing data and high-throughput chromosome conformation capture data from the Oxford Nanopore platform to construct a high-quality genome of T. helgolandica. Genome annotation was performed using multiple databases, and comparative genomic analysis was conducted with nine species, including Arabidopsis thaliana, to reveal the evolutionary history. Finally, we combined transcriptome data to elucidate the molecular mechanisms underlying the high starch yield.

RESULTS

Circadian rhythm significantly promote starch accumulation and increase amylose content. The chromosome-scale genome revealed it shares a common ancestor with other green algae approximately 1,017 million years ago. This relatively ancient divergence underscores its evolutionary distinction within the green lineage. It may possess a more complex protein modification mechanism and a more fully developed Golgi apparatus. Circadian rhythm broadly up-regulates key enzymes involved in starch synthesis, including GBSS and Starch Synthase, while down-regulating SS IIIa. This regulation enhances starch accumulation and increases the amylose content.

CONCLUSION

This study provided a high-quality genome of T. helgolandica and revealed the potential mechanism by which the circadian rhythm promotes starch accumulation and increases the amylose ratio. The genome of T. helgolandica will serve as an important resource for evolutionary research and transgenic platform development.

Shallow Hydrothermal Fluids Shape Microbial Dynamics at the Tagoro Submarine Volcano (Canary Islands, Spain)

Shallow underwater hydrothermal systems are often overlooked despite their potential contribution to marine diversity and biogeochemistry. Over a decade after its eruption, the Tagoro submarine volcano continues to emit heat, reduced compounds, and nutrients into shallow waters, serving as a model system for studying the effects of diffuse hydrothermal fluids on surface microbial communities. The impact on both phytoplankton and bacterial communities was examined through experimental manipulations mimicking dilution levels up to ~100 m from the primary crater of Tagoro. Chlorophyll a concentration doubled in the presence of hydrothermal products, with peak levels detected about a day earlier than in controls. Picoeukaryotes and Synechococcus cell abundances moderately increased, yet small eukaryotic phytoplankton (≤ 5 μm) predominated in the hydrothermally enriched bottles. Dinoflagellates, diatoms, small green algae and radiolarians particularly benefited from the hydrothermal inputs, along with phototrophic and chemoautotrophic bacteria. Our results indicate that hydrothermal products in shallow waters enhance primary production driven by phototrophic microbes, potentially triggering a secondary response associated with increased organic matter availability. Additionally, protistan grazing and parasitism emerged as key factors modulating local planktonic communities. Our findings highlight the role of shallow submarine hydrothermal systems in enhancing local primary production and element cycling.

Metagenome-assembled genome of the glacier alga Ancylonema yields insights into the evolution of streptophyte life on ice and land

Contemporary glaciers are inhabited by streptophyte algae that balance photosynthesis and growth with tolerance of low temperature, desiccation and UV radiation. These same environmental challenges have been hypothesised as the driving force behind the evolution of land plants from streptophyte algal ancestors in the Cryogenian (720-635 million years ago). We sequenced, assembled and analysed the metagenome-assembled genome of the glacier alga Ancylonema nordenskiöldii to investigate its adaptations to life in ice, and whether this represents a vestige of Cryogenian exaptations. Phylogenetic analysis confirms the placement of glacier algae within the sister lineage to land plants, Zygnematophyceae. The metagenome-assembled genome is characterised by an expansion of genes involved in tolerance of high irradiance and UV light, while lineage-specific diversification is linked to the novel screening pigmentation of glacier algae. We found no support for the hypothesis of a common genomic basis for adaptations to ice and to land in streptophytes. Comparative genomics revealed that the reductive morphological evolution in the ancestor of Zygnematophyceae was accompanied by reductive genome evolution. This first genome-scale data for glacier algae suggests an Ancylonema-specific adaptation to the cryosphere, and sheds light on the genome evolution of land plants and Zygnematophyceae.

Plastid HSP90C C-terminal extension region plays a regulatory role in chaperone activity and client binding

HSP90Cs are essential molecular chaperones localized in the plastid stroma that maintain protein homeostasis and assist the import and thylakoid transport of chloroplast proteins. While HSP90C contains all conserved domains as an HSP90 family protein, it also possesses a unique feature in its variable C-terminal extension (CTE) region. This study elucidated the specific function of this HSP90C CTE region. Our phylogenetic analyses revealed that this intrinsically disordered region contains a highly conserved DPW motif in the green lineages. With biochemical assays, we showed that the CTE is required for the chaperone to effectively interact with client proteins PsbO1 and LHCB2 to regulate ATP-independent chaperone activity and to effectuate its ATP hydrolysis. The CTE truncation mutants could support plant growth and development reminiscing the wild type under normal conditions except for a minor phenotype in cotyledon when expressed at a level comparable to wild type. However, higher HSP90C expression was observed to correlate with a stronger response to specific photosystem II inhibitor DCMU, and CTE truncations dampened the response. Additionally, when treated with lincomycin to inhibit chloroplast protein translation, CTE truncation mutants showed a delayed response to PsbO1 expression repression, suggesting its role in chloroplast retrograde signaling. Our study therefore provides insights into the mechanism of HSP90C in client protein binding and the regulation of green chloroplast maturation and function, especially under stress conditions.

MarFERReT, an open-source, version-controlled reference library of marine microbial eukaryote functional genes

Metatranscriptomics generates large volumes of sequence data about transcribed genes in natural environments. Taxonomic annotation of these datasets depends on availability of curated reference sequences. For marine microbial eukaryotes, current reference libraries are limited by gaps in sequenced organism diversity and barriers to updating libraries with new sequence data, resulting in taxonomic annotation of about half of eukaryotic environmental transcripts. Here, we introduce Marine Functional EukaRyotic Reference Taxa (MarFERReT), a marine microbial eukaryotic sequence library designed for use with taxonomic annotation of eukaryotic metatranscriptomes. We gathered 902 publicly accessible marine eukaryote genomes and transcriptomes and assessed their sequence quality and cross-contamination issues, selecting 800 validated entries for inclusion in MarFERReT. Version 1.1 of MarFERReT contains reference sequences from 800 marine eukaryotic genomes and transcriptomes, covering 453 species- and strain-level taxa, totaling nearly 28 million protein sequences with associated NCBI and PR2 Taxonomy identifiers and Pfam functional annotations. The MarFERReT project repository hosts containerized build scripts, documentation on installation and use case examples, and information on new versions of MarFERReT.

RHO of plant signaling was established early in streptophyte evolution

The algal ancestors of land plants underwent a transition from a unicellular to a multicellular body plan.1 This transition likely took place early in streptophyte evolution, sometime after the divergence of the Chlorokybophyceae/Mesostigmatophyceae lineage, but before the divergence of the Klebsormidiophyceae lineage.2 How this transition was brought about is unknown; however, it was likely facilitated by the evolution of novel mechanisms to spatially regulate morphogenesis. In land plants, RHO of plant (ROP) signaling plays a conserved role in regulating polarized cell growth and cell division orientation to orchestrate morphogenesis.3,4,5,6,7,8 ROP constitutes a plant-specific subfamily of the RHO GTPases, which are more widely conserved throughout eukaryotes.9,10 Although the RHO family originated in early eukaryotes,11,12 how and when the ROP subfamily originated had remained elusive. Here, we demonstrate that ROP signaling was established early in the streptophyte lineage, sometime after the divergence of the Chlorokybophyceae/Mesostigmatophyceae lineage, but before the divergence of the Klebsormidiophyceae lineage. This period corresponds to when the unicellular-to-multicellular transition likely took place in the streptophytes. In addition to being critical for the complex morphogenesis of extant land plants, we speculate that ROP signaling contributed to morphological evolution in early streptophytes.

Phylotranscriptomics unveil a Paleoproterozoic-Mesoproterozoic origin and deep relationships of the Viridiplantae

The Viridiplantae comprise two main clades, the Chlorophyta (including a diverse array of marine and freshwater green algae) and the Streptophyta (consisting of the freshwater charophytes and the land plants). Lineages sister to core Chlorophyta, informally refer to as prasinophytes, form a grade of mainly planktonic green algae. Recently, one of these lineages, Prasinodermophyta, which is previously grouped with prasinophytes, has been identified as the sister lineage to both Chlorophyta and Streptophyta. Resolving the deep relationships among green plants is crucial for understanding the historical impact of green algal diversity on marine ecology and geochemistry, but has been proven difficult given the ancient timing of the diversification events. Through extensive taxon and gene sampling, we conduct large-scale phylogenomic analyses to resolve deep relationships and reveal the Prasinodermophyta as the lineage sister to Chlorophyta, raising questions about the necessity of classifying the Prasinodermophyta as a distinct phylum. We unveil that incomplete lineage sorting is the main cause of discordance regarding the placement of Prasinodermophyta. Molecular dating analyses suggest that crown-group green plants and crown-group Prasinodermophyta date back to the Paleoproterozoic-Mesoproterozoic. Our study establishes a plausible link between oxygen levels in the Paleoproterozoic-Mesoproterozoic and the origin of Viridiplantae.

Genomic analysis of an ultrasmall freshwater green alga, Medakamo hakoo

Ultrasmall algae have attracted the attention of biologists investigating the basic mechanisms underlying living systems. Their potential as effective organisms for producing useful substances is also of interest in bioindustry. Although genomic information is indispensable for elucidating metabolism and promoting molecular breeding, many ultrasmall algae remain genetically uncharacterized. Here, we present the nuclear genome sequence of an ultrasmall green alga of freshwater habitats, Medakamo hakoo. Evolutionary analyses suggest that this species belongs to a new genus within the class Trebouxiophyceae. Sequencing analyses revealed that its genome, comprising 15.8 Mbp and 7629 genes, is among the smallest known genomes in the Viridiplantae. Its genome has relatively few genes associated with genetic information processing, basal transcription factors, and RNA transport. Comparative analyses revealed that 1263 orthogroups were shared among 15 ultrasmall algae from distinct phylogenetic lineages. The shared gene sets will enable identification of genes essential for algal metabolism and cellular functions.

The Land-Sea Connection: Insights Into the Plant Lineage from a Green Algal Perspective

The colonization of land by plants generated opportunities for the rise of new heterotrophic life forms, including humankind. A unique event underpinned this massive change to earth ecosystems-the advent of eukaryotic green algae. Today, an abundant marine green algal group, the prasinophytes, alongside prasinodermophytes and nonmarine chlorophyte algae, is facilitating insights into plant developments. Genome-level data allow identification of conserved proteins and protein families with extensive modifications, losses, or gains and expansion patterns that connect to niche specialization and diversification. Here, we contextualize attributes according to Viridiplantae evolutionary relationships, starting with orthologous protein families, and then focusing on key elements with marked differentiation, resulting in patchy distributions across green algae and plants. We place attention on peptidoglycan biosynthesis, important for plastid division and walls; phytochrome photosensors that are master regulators in plants; and carbohydrate-active enzymes, essential to all manner of carbohydratebiotransformations. Together with advances in algal model systems, these areas are ripe for discovering molecular roles and innovations within and across plant and algal lineages.

A new microsporidian parasite, Ordospora pajunii sp. nov (Ordosporidae), of Daphnia longispina highlights the value of genomic data for delineating species boundaries

Speciation is a complex and continuous process that makes the delineation of species boundaries a challenging task in particular in species with little morphological differentiation, such as parasites. In this case, the use of genomic data is often necessary, such as for the intracellular Microsporidian parasites. Here we characterize the genome of a gut parasite of the cladoceran Daphnia longispina (isolate FI-F-10), which we propose as a new species within the genus Ordospora: O. pajunii sp. nov (Ordosporidae). FI-F-10 closest relative, O. colligata is only found in D. magna. Both microsporidian species share several morphological features. Although it is not possible to estimate divergence times for Microsporidia due to the lack of fossil records and accelerated evolutionary rates, we base our proposal on the phylogenomic and genomic distances between both microsporidian lineages. Phylogenomic reconstruction shows that FI-F-10 forms an early diverging branch basal to the cluster that contains all known O. colligata strains. Whole-genome comparisons show that FI-F-10 presents a greater divergence at the sequence level than observed among O. colligata strains, and its genomic Average Nucleotide Identity (ANI) values against O. colligata are beyond the intra-specific range previously established for yeast and prokaryotes. Our data confirm that the ANI metrics are useful for fine genetic divergence calibration across Microsporidia taxa. In combination with phylogenetic and ecological data, genome-based metrics provide a powerful approach to delimitate species boundaries.

Nuclear genome of a pedinophyte pinpoints genomic innovation and streamlining in the green algae

The genomic diversity underpinning high ecological and species diversity in the green algae (Chlorophyta) remains little known. Here, we aimed to track genome evolution in the Chlorophyta, focusing on loss and gain of homologous genes, and lineage-specific innovations of the core Chlorophyta. We generated a high-quality nuclear genome for pedinophyte YPF701, a sister lineage to others in the core Chlorophyta and incorporated this genome in a comparative analysis with 25 other genomes from diverse Viridiplantae taxa. The nuclear genome of pedinophyte YPF701 has an intermediate size and gene number between those of most prasinophytes and the remainder of the core Chlorophyta. Our results suggest positive selection for genome streamlining in the Pedinophyceae, independent from genome minimisation observed among prasinophyte lineages. Genome expansion was predicted along the branch leading to the UTC clade (classes Ulvophyceae, Trebouxiophyceae and Chlorophyceae) after divergence from their last common ancestor with pedinophytes, with genomic novelty implicated in a range of basic biological functions. Results emphasise multiple independent signals of genome minimisation within the Chlorophyta, as well as the genomic novelty arising before diversification in the UTC clade, which may underpin the success of this species-rich clade in a diversity of habitats.

Perfluorinated compounds (PFCs) in regional industrial rivers: Interactions between pollution flux and eukaryotic community phylosymbiosis

Perfluorinated compounds (PFCs) pose serious threats to aquatic ecosystems, especially their microbial communities. However, little is known about the phylosymbiosis of aquatic fungal and viridiplantae communities in response to PFC accumulation. We quantified the distribution of 14 PFCs in rivers and found that PFBA was dominant in the transition from water to sediment. High through-put sequencing revealed that phyla Ascomycota, Basidiomycota, Anthophyta, and Chlorophyta were the predominant in eukaryotic community. The effects of PFCs on spatial community coalescence at taxonomic and phylogenetic levels (p < 0.05) were revealed. Fungal community coalescence triggered the spatial assembly of fungal and viridiplantae communities in riverine environments (p < 0.05). Null modeling indicated that PFBA, PFTrDA and PFOS, etc, mediated phylogenetic assembly (p < 0.05) and stochastic processes (86.67-100%) maintain phylogenetic turnover in the fungal community. Meanwhile, variable selection (27.78-54.44%) explained the viridiplantae community assemblage. Finally, we identified fungal genera Hannaella, Naganishia, Purpureocillium and Stachybotrys as indicators for PFC pollution (p < 0.001). These results help explain the effects of PFCs on riverine ecological remediation.

The Unicellular Red Alga Cyanidioschyzon merolae-The Simplest Model of a Photosynthetic Eukaryote

Several species of unicellular eukaryotic algae exhibit relatively simple genomic and cellular architecture. Laboratory cultures of these algae grow faster than plants and often provide homogeneous cellular populations exposed to an almost equal environment. These characteristics are ideal for conducting experiments at the cellular and subcellular levels. Many microalgal lineages have recently become genetically tractable, which have started to evoke new streams of studies. Among such algae, the unicellular red alga Cyanidioschyzon merolae is the simplest organism; it possesses the minimum number of membranous organelles, only 4,775 protein-coding genes in the nucleus, and its cell cycle progression can be highly synchronized with the diel cycle. These properties facilitate diverse omics analyses of cellular proliferation and structural analyses of the intracellular relationship among organelles. C. merolae cells lack a rigid cell wall and are thus relatively easily disrupted, facilitating biochemical analyses. Multiple chromosomal loci can be edited by highly efficient homologous recombination. The procedures for the inducible/repressive expression of a transgene or an endogenous gene in the nucleus and for chloroplast genome modification have also been developed. Here, we summarize the features and experimental techniques of C. merolae and provide examples of studies using this alga. From these studies, it is clear that C. merolae-either alone or in comparative and combinatory studies with other photosynthetic organisms-can provide significant insights into the biology of photosynthetic eukaryotes.

Evolution of plant telomerase RNAs: farther to the past, deeper to the roots

The enormous sequence heterogeneity of telomerase RNA (TR) subunits has thus far complicated their characterization in a wider phylogenetic range. Our recent finding that land plant TRs are, similarly to known ciliate TRs, transcribed by RNA polymerase III and under the control of the type-3 promoter, allowed us to design a novel strategy to characterize TRs in early diverging Viridiplantae taxa, as well as in ciliates and other Diaphoretickes lineages. Starting with the characterization of the upstream sequence element of the type 3 promoter that is conserved in a number of small nuclear RNAs, and the expected minimum TR template region as search features, we identified candidate TRs in selected Diaphoretickes genomes. Homologous TRs were then used to build covariance models to identify TRs in more distant species. Transcripts of the identified TRs were confirmed by transcriptomic data, RT-PCR and Northern hybridization. A templating role for one of our candidates was validated in Physcomitrium patens. Analysis of secondary structure demonstrated a deep conservation of motifs (pseudoknot and template boundary element) observed in all published TRs. These results elucidate the evolution of the earliest eukaryotic TRs, linking the common origin of TRs across Diaphoretickes, and underlying evolutionary transitions in telomere repeats.

Experimental identification and in silico prediction of bacterivory in green algae

While algal phago-mixotrophs play a major role in aquatic microbial food webs, their diversity remains poorly understood. Recent studies have indicated several species of prasinophytes, early diverging green algae, to be able to consume bacteria for nutrition. To further explore the occurrence of phago-mixotrophy in green algae, we conducted feeding experiments with live fluorescently labeled bacteria stained with CellTracker Green CMFDA, heat-killed bacteria stained with 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF), and magnetic beads. Feeding was detected via microscopy and/or flow cytometry in five strains of prasinophytes when provided with live bacteria: Pterosperma cristatum NIES626, Pyramimonas parkeae CCMP726, Pyramimonas parkeae NIES254, Nephroselmis pyriformis RCC618, and Dolichomastix tenuilepis CCMP3274. No feeding was detected when heat-killed bacteria or magnetic beads were provided, suggesting a strong preference for live prey in the strains tested. In parallel to experimental assays, green algal bacterivory was investigated using a gene-based prediction model. The predictions agreed with the experimental results and suggested bacterivory potential in additional green algae. Our observations underline the likelihood of widespread occurrence of phago-mixotrophy among green algae, while additionally highlighting potential biases introduced when using prey proxy to evaluate bacterial ingestion by algal cells.

The Histone Acetyltransferase GCN5 and the Associated Coactivators ADA2: From Evolution of the SAGA Complex to the Biological Roles in Plants

Transcription of protein-encoding genes starts with forming a pre-initiation complex comprised of RNA polymerase II and several general transcription factors. To activate gene expression, transcription factors must overcome repressive chromatin structure, which is accomplished with multiprotein complexes. One such complex, SAGA, modifies the nucleosomal histones through acetylation and other histone modifications. A prototypical histone acetyltransferase (HAT) known as general control non-repressed protein 5 (GCN5), was defined biochemically as the first transcription-linked HAT with specificity for histone H3 lysine 14. In this review, we analyze the components of the putative plant SAGA complex during plant evolution, and current knowledge on the biological role of the key components of the HAT module, GCN5 and ADA2b in plants, will be summarized.

A putative origin of the insect chemosensory receptor superfamily in the last common eukaryotic ancestor

The insect chemosensory repertoires of Odorant Receptors (ORs) and Gustatory Receptors (GRs) together represent one of the largest families of ligand-gated ion channels. Previous analyses have identified homologous 'Gustatory Receptor-Like' (GRL) proteins across Animalia, but the evolutionary origin of this novel class of ion channels is unknown. We describe a survey of unicellular eukaryotic genomes for GRLs, identifying several candidates in fungi, protists and algae that contain many structural features characteristic of animal GRLs. The existence of these proteins in unicellular eukaryotes, together with ab initio protein structure predictions, provide evidence for homology between GRLs and a family of uncharacterized plant proteins containing the DUF3537 domain. Together, our analyses suggest an origin of this protein superfamily in the last common eukaryotic ancestor.

Phylogeny and Structure of Fatty Acid Photodecarboxylases and Glucose-Methanol-Choline Oxidoreductases

Glucose-methanol-choline (GMC) oxidoreductases are a large and diverse family of flavin-binding enzymes found in all kingdoms of life. Recently, a new related family of proteins has been discovered in algae named fatty acid photodecarboxylases (FAPs). These enzymes use the energy of light to convert fatty acids to the corresponding Cn-1 alkanes or alkenes, and hold great potential for biotechnological application. In this work, we aimed at uncovering the natural diversity of FAPs and their relations with other GMC oxidoreductases. We reviewed the available GMC structures, assembled a large dataset of GMC sequences, and found that one active site amino acid, a histidine, is extremely well conserved among the GMC proteins but not among FAPs, where it is replaced with alanine. Using this criterion, we found several new potential FAP genes, both in genomic and metagenomic databases, and showed that related bacterial, archaeal and fungal genes are unlikely to be FAPs. We also identified several uncharacterized clusters of GMC-like proteins as well as subfamilies of proteins that lack the conserved histidine but are not FAPs. Finally, the analysis of the collected dataset of potential photodecarboxylase sequences revealed the key active site residues that are strictly conserved, whereas other residues in the vicinity of the flavin adenine dinucleotide (FAD) cofactor and in the fatty acid-binding pocket are more variable. The identified variants may have different FAP activity and selectivity and consequently may prove useful for new biotechnological applications, thereby fostering the transition from a fossil carbon-based economy to a bio-economy by enabling the sustainable production of hydrocarbon fuels.