Diversity and Evolutionary History of Iron Metabolism Genes in Diatoms

Short-term acidification promotes diverse iron acquisition and conservation mechanisms in upwelling-associated phytoplankton

Coastal upwelling regions are among the most productive marine ecosystems but may be threatened by amplified ocean acidification. Increased acidification is hypothesized to reduce iron bioavailability for phytoplankton thereby expanding iron limitation and impacting primary production. Here we show from community to molecular levels that phytoplankton in an upwelling region respond to short-term acidification exposure with iron uptake pathways and strategies that reduce cellular iron demand. A combined physiological and multi-omics approach was applied to trace metal clean incubations that introduced 1200 ppm CO2 for up to four days. Although variable, molecular-level responses indicate a prioritization of iron uptake pathways that are less hindered by acidification and reductions in iron utilization. Growth, nutrient uptake, and community compositions remained largely unaffected suggesting that these mechanisms may confer short-term resistance to acidification; however, we speculate that cellular iron demand is only temporarily satisfied, and longer-term acidification exposure without increased iron inputs may result in increased iron stress.

Genome analysis of Parmales, the sister group of diatoms, reveals the evolutionary specialization of diatoms from phago-mixotrophs to photoautotrophs

The order Parmales (class Bolidophyceae) is a minor group of pico-sized eukaryotic marine phytoplankton that contains species with cells surrounded by silica plates. Previous studies revealed that Parmales is a member of ochrophytes and sister to diatoms (phylum Bacillariophyta), the most successful phytoplankton group in the modern ocean. Therefore, parmalean genomes can serve as a reference to elucidate both the evolutionary events that differentiated these two lineages and the genomic basis for the ecological success of diatoms vs. the more cryptic lifestyle of parmaleans. Here, we compare the genomes of eight parmaleans and five diatoms to explore their physiological and evolutionary differences. Parmaleans are predicted to be phago-mixotrophs. By contrast, diatoms have lost genes related to phagocytosis, indicating the ecological specialization from phago-mixotrophy to photoautotrophy in their early evolution. Furthermore, diatoms show significant enrichment in gene sets involved in nutrient uptake and metabolism, including iron and silica, in comparison with parmaleans. Overall, our results suggest a strong evolutionary link between the loss of phago-mixotrophy and specialization to a silicified photoautotrophic life stage early in diatom evolution after diverging from the Parmales lineage.

Physiological responses of Humboldt current system diatoms to Fe and Cu co-limitation

Diatoms account for ∼20% of global primary production, often limited by the availability of Fe and other trace nutrients such as Cu. The present study examined the role of both metals in the physiology of two diatoms isolated from the Humboldt Currents System, the centric Chaetoceros c.f. dicipiens and the pennate Nitzschia c.f. draveillensis. Under Fe limitation, a decrease in specific growth rates and sizes of both species was observed, especially in Chaetoceros. However, regarding different photosynthetic parameters, Nitzschia was more impacted. The increase in Cu concentrations improved the physiology of both diatoms, mostly of Chaetoceros. When grown in mixed cultures and under co-limiting conditions, both species remained competive due to morphological advantages (i.e., lower cell size). These results may suggest that the increase of Cu under Fe limitation benefited C. c.f. dicipiens over N. c.f. draveillensis.

The Influence of Symbiosis on the Proteome of the Exaiptasia Endosymbiont Breviolum minutum

The cellular mechanisms responsible for the regulation of nutrient exchange, immune response, and symbiont population growth in the cnidarian-dinoflagellate symbiosis are poorly resolved. Here, we employed liquid chromatography-mass spectrometry to elucidate proteomic changes associated with symbiosis in Breviolum minutum, a native symbiont of the sea anemone Exaiptasia diaphana ('Aiptasia'). We manipulated nutrients available to the algae in culture and to the holobiont in hospite (i.e., in symbiosis) and then monitored the impacts of our treatments on host-endosymbiont interactions. Both the symbiotic and nutritional states had significant impacts on the B. minutum proteome. B. minutum in hospite showed an increased abundance of proteins involved in phosphoinositol metabolism (e.g., glycerophosphoinositol permease 1 and phosphatidylinositol phosphatase) relative to the free-living alga, potentially reflecting inter-partner signalling that promotes the stability of the symbiosis. Proteins potentially involved in concentrating and fixing inorganic carbon (e.g., carbonic anhydrase, V-type ATPase) and in the assimilation of nitrogen (e.g., glutamine synthase) were more abundant in free-living B. minutum than in hospite, possibly due to host-facilitated access to inorganic carbon and nitrogen limitation by the host when in hospite. Photosystem proteins increased in abundance at high nutrient levels irrespective of the symbiotic state, as did proteins involved in antioxidant defences (e.g., superoxide dismutase, glutathione s-transferase). Proteins involved in iron metabolism were also affected by the nutritional state, with an increased iron demand and uptake under low nutrient treatments. These results detail the changes in symbiont physiology in response to the host microenvironment and nutrient availability and indicate potential symbiont-driven mechanisms that regulate the cnidarian-dinoflagellate symbiosis.

Genomic capacities for Reactive Oxygen Species metabolism across marine phytoplankton

Marine phytoplankton produce and scavenge Reactive Oxygen Species, to support cellular processes, while limiting damaging reactions. Some prokaryotic picophytoplankton have, however, lost all genes encoding scavenging of hydrogen peroxide. Such losses of metabolic function can only apply to Reactive Oxygen Species which potentially traverse the cell membrane outwards, before provoking damaging intracellular reactions. We hypothesized that cell radius influences which elements of Reactive Oxygen Species metabolism are partially or fully dispensable from a cell. We therefore investigated genomes and transcriptomes from diverse marine eukaryotic phytoplankton, ranging from 0.4 to 44 μm radius, to analyze the genomic allocations encoding enzymes metabolizing Reactive Oxygen Species. Superoxide has high reactivity, short lifetimes and limited membrane permeability. Genes encoding superoxide scavenging are ubiquitous across phytoplankton, but the fractional gene allocation decreased with increasing cell radius, consistent with a nearly fixed set of core genes for scavenging superoxide pools. Hydrogen peroxide has lower reactivity, longer intracellular and extracellular lifetimes and readily crosses cell membranes. Genomic allocations to both hydrogen peroxide production and scavenging decrease with increasing cell radius. Nitric Oxide has low reactivity, long intracellular and extracellular lifetimes and readily crosses cell membranes. Neither Nitric Oxide production nor scavenging genomic allocations changed with increasing cell radius. Many taxa, however, lack the genomic capacity for nitric oxide production or scavenging. The probability of presence of capacity to produce nitric oxide decreases with increasing cell size, and is influenced by flagella and colony formation. In contrast, the probability of presence of capacity to scavenge nitric oxide increases with increasing cell size, and is again influenced by flagella and colony formation.

In vivo localization of iron starvation induced proteins under variable iron supplementation regimes in Phaeodactylum tricornutum

The model pennate diatom Phaeodactylum tricornutum is able to assimilate a range of iron sources. It therefore provides a platform to study different mechanisms of iron processing concomitantly in the same cell. In this study, we follow the localization of three iron starvation induced proteins (ISIPs) in vivo, driven by their native promoters and tagged by fluorophores in an engineered line of P. tricornutum. We find that the localization patterns of ISIPs are dynamic and variable depending on the overall iron status of the cell and the source of iron it is exposed to. Notwithstanding, a shared destination of the three ISIPs both under ferric iron and siderophore-bound iron supplementation is a globular compartment in the vicinity of the chloroplast. In a proteomic analysis, we identify that the cell engages endocytosis machinery involved in the vesicular trafficking as a response to siderophore molecules, even when these are not bound to iron. Our results suggest that there may be a direct vesicle traffic connection between the diatom cell membrane and the periplastidial compartment (PPC) that co-opts clathrin-mediated endocytosis and the "cytoplasm to vacuole" (Cvt) pathway, for proteins involved in iron assimilation. Proteomics data are available via ProteomeXchange with identifier PXD021172.

HIGHLIGHT

The marine diatom P. tricornutum engages a vesicular network to traffic siderophores and phytotransferrin from the cell membrane directly to a putative iron processing site in the vicinity of the chloroplast.

Luxury iron uptake and storage in pennate diatoms from the equatorial Pacific Ocean

Iron is a key micronutrient for ocean phytoplankton, and the availability of iron controls primary production and community composition in large regions of the ocean. Pennate diatoms, a phytoplankton group that responds to iron additions in low-iron areas, can have highly variable iron contents, and some groups such as Pseudo-nitzschia are known to use ferritin to store iron for later use. We quantified and mapped the intracellular accumulation of iron by a natural population of Pseudo-nitzschia from the Fe-limited equatorial Pacific Ocean. Forty-eight hours after iron addition, nearly half of accumulated iron was localized in storage bodies adjacent to chloroplasts believed to represent ferritin. Over the subsequent 48 h, stored iron was distributed to the rest of the cell through subsequent growth and division, partially supporting the iron contents of the daughter cells. This study provides a first quantitative view into the cellular trafficking of iron in a globally relevant phytoplankton group and demonstrates the unique capabilities of synchrotron-based element imaging approaches.

New Insights into the Evolution of the Electron Transfer from Cytochrome f to Photosystem I in the Green and Red Branches of Photosynthetic Eukaryotes

In cyanobacteria and most green algae of the eukaryotic green lineage, the copper-protein plastocyanin (Pc) alternatively replaces the heme-protein cytochrome c6 (Cc6) as the soluble electron carrier from cytochrome f (Cf) to photosystem I (PSI). The functional and structural equivalence of 'green' Pc and Cc6 has been well established, representing an example of convergent evolution of two unrelated proteins. However, plants only produce Pc, despite having evolved from green algae. On the other hand, Cc6 is the only soluble donor available in most species of the red lineage of photosynthetic organisms, which includes, among others, red algae and diatoms. Interestingly, Pc genes have been identified in oceanic diatoms, probably acquired by horizontal gene transfer from green algae. However, the mechanisms that regulate the expression of a functional Pc in diatoms are still unclear. In the green eukaryotic lineage, the transfer of electrons from Cf to PSI has been characterized in depth. The conclusion is that in the green lineage, this process involves strong electrostatic interactions between partners, which ensure a high affinity and an efficient electron transfer (ET) at the cost of limiting the turnover of the process. In the red lineage, recent kinetic and structural modeling data suggest a different strategy, based on weaker electrostatic interactions between partners, with lower affinity and less efficient ET, but favoring instead the protein exchange and the turnover of the process. Finally, in diatoms the interaction of the acquired green-type Pc with both Cf and PSI may not yet be optimized.

Probing the Bioavailability of Dissolved Iron to Marine Eukaryotic Phytoplankton Using In Situ Single Cell Iron Quotas

We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (k_in-app) computed by combining data on: (a) Fe content of individual in situ phytoplankton cells; (b) concurrently determined seawater dFe concentrations; and (c) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002 and 2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high-affinity Fe uptake system, so data were screened, yielding 560 single cell k _in-app values from 31 low-Fe stations. We normalized k _in-app to cell surface area (S.A.) to account for cell-size differences. The resulting bioavailability proxy (k _in-app/S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe-siderophore complexes and often approaches that of highly available inorganic Fe'. Station averaged k _in-app/S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency of k _in-app/S.A. among stations (ca. five-fold variation), we computed a grand-averaged dFe availability, which upon normalization to cell carbon (C) yields k _in-app/C of 42,200 ± 11,000 L mol C^-1 d^-1. We utilize k _in-app/C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability of k _in-app/C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe-limited Equatorial Pacific.

Engineering Climate-Change-Resilient Crops: New Tools and Approaches

Environmental adversities, particularly drought and nutrient limitation, are among the major causes of crop losses worldwide. Due to the rapid increase of the world's population, there is an urgent need to combine knowledge of plant science with innovative applications in agriculture to protect plant growth and thus enhance crop yield. In recent decades, engineering strategies have been successfully developed with the aim to improve growth and stress tolerance in plants. Most strategies applied so far have relied on transgenic approaches and/or chemical treatments. However, to cope with rapid climate change and the need to secure sustainable agriculture and biomass production, innovative approaches need to be developed to effectively meet these challenges and demands. In this review, we summarize recent and advanced strategies that involve the use of plant-related cyanobacterial proteins, macro- and micronutrient management, nutrient-coated nanoparticles, and phytopathogenic organisms, all of which offer promise as protective resources to shield plants from climate challenges and to boost stress tolerance in crops.

Iron metabolism strategies in diatoms

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

The heterologous expression of a plastocyanin in the diatom Phaeodactylum tricornutum improves cell growth under iron-deficient conditions

We have investigated if the heterologous expression of a functional green alga plastocyanin in the diatom Phaeodactylum tricornutum can improve photosynthetic activity and cell growth. Previous in vitro assays showed that a single-mutant of the plastocyanin from the green algae Chlamydomonas reinhardtii is effective in reducing P. tricornutum photosystem I. In this study, in vivo assays with P. tricornutum strains expressing this plastocyanin indicate that even the relatively low intracellular concentrations of holo-plastocyanin detected (≈4 μM) are enough to promote an increased growth (up to 60%) under iron-deficient conditions as compared with the WT strain, measured as higher cell densities, content in pigments and active photosystem I, global photosynthetic rates per cell, and even cell volume. In addition, the presence of plastocyanin as an additional photosynthetic electron carrier seems to decrease the over-reduction of the plastoquinone pool. Consequently, it promotes an improvement in the maximum quantum yield of both photosystem II and I, together with a decrease in the acceptor side photoinhibition of photosystem II-also associated to a reduced oxidative stress-a decrease in the peroxidation of membrane lipids in the choroplast, and a lower degree of limitation on the donor side of photosystem I. Thus the heterologous plastocyanin appears to act as a functional electron carrier, alternative to the native cytochrome c₆ , under iron-limiting conditions.

Dinoflagellates alter their carbon and nutrient metabolic strategies across environmental gradients in the central Pacific Ocean

Marine microeukaryotes play a fundamental role in biogeochemical cycling through the transfer of energy to higher trophic levels and vertical carbon transport. Despite their global importance, microeukaryote physiology, nutrient metabolism and contributions to carbon cycling across offshore ecosystems are poorly characterized. Here, we observed the prevalence of dinoflagellates along a 4,600-km meridional transect extending across the central Pacific Ocean, where oligotrophic gyres meet equatorial upwelling waters rich in macronutrients yet low in dissolved iron. A combined multi-omics and geochemical analysis provided a window into dinoflagellate metabolism across the transect, indicating a continuous taxonomic dinoflagellate community that shifted its functional transcriptome and proteome as it extended from the euphotic to the mesopelagic zone. In euphotic waters, multi-omics data suggested that a combination of trophic modes were utilized, while mesopelagic metabolism was marked by cytoskeletal investments and nutrient recycling. Rearrangement in nutrient metabolism was evident in response to variable nitrogen and iron regimes across the gradient, with no associated change in community assemblage. Total dinoflagellate proteins scaled with particulate carbon export, with both elevated in equatorial waters, suggesting a link between dinoflagellate abundance and total carbon flux. Dinoflagellates employ numerous metabolic strategies that enable broad occupation of central Pacific ecosystems and play a dual role in carbon transformation through both photosynthetic fixation in the euphotic zone and remineralization in the mesopelagic zone.

An Insight into the Algal Evolution and Genomics

With the increase in biotechnological, environmental, and nutraceutical importance of algae, about 100 whole genomic sequences of algae have been published, and this figure is expected to double in the coming years. The phenotypic and ecological diversity among algae hints at the range of functional capabilities encoded by algal genomes. In order to explore the biodiversity of algae and fully exploit their commercial potential, understanding their evolutionary, structural, functional, and developmental aspects at genomic level is a pre-requisite. So forth, the algal genomic analysis revealed us that algae evolved through endosymbiotic gene transfer, giving rise to around eight phyla. Amongst the diverse algal species, the unicellular green algae Chlamydomonas reinhardtii has attained the status of model organism as it is an ideal organism to elucidate the biological processes critical to plants and animals, as well as commercialized to produce range of bio-products. For this review, an overview of evolutionary process of algae through endosymbiosis in the light of genomics, as well as the phylogenomic, studies supporting the evolutionary process of algae was reviewed. Algal genomics not only helped us to understand the evolutionary history of algae but also may have an impact on our future by helping to create algae-based products and future biotechnological approaches.

The Transfer of the Ferredoxin Gene From the Chloroplast to the Nuclear Genome Is Ancient Within the Paraphyletic Genus Thalassiosira

Ferredoxins are iron–sulfur proteins essential for a wide range of organisms because they are an electron transfer mediator involved in multiple metabolic pathways. In phytoplankton, these proteins are active in the mature chloroplasts, but the petF gene, encoding for ferredoxin, has been found either to be in the chloroplast genome or transferred to the nuclear genome as observed in the green algae and higher plant lineage. We experimentally determined the location of the petF gene in 12 strains of Thalassiosira covering three species using DNA sequencing and qPCR assays. The results showed that petF gene is located in the nuclear genome of all confirmed Thalassiosira oceanica strains (CCMP0999, 1001, 1005, and 1006) tested. In contrast, all Thalassiosira pseudonana (CCMP1012, 1013, 1014, and 1335) and Thalassiosira weissflogii (CCMP1010, 1049, and 1052) strains studied retained the gene in the chloroplast genome, as generally observed for Bacillariophyceae. Our evolutionary analyses further extend the dataset on the localization of the petF gene in the Thalassiosirales. The realization that the petF gene is nuclear-encoded in the Skeletonema genus allowed us to trace the petF gene transfer back to a single event that occurred within the paraphyletic genus Thalassiosira. Phylogenetic analyses revealed the need to reassess the taxonomic assignment of the Thalassiosira strain CCMP1616, since the genes used in our study did not cluster within the T. oceanica lineage. Our results suggest that this strains’ diversification occurred prior to the ferredoxin gene transfer event. The functional transfer of petF genes provides insight into the evolutionary processes leading to chloroplast genome reduction and suggests ecological adaptation as a driving force for such chloroplast to nuclear gene transfer.

The Importance of Protein Phosphorylation for Signaling and Metabolism in Response to Diel Light Cycling and Nutrient Availability in a Marine Diatom

Diatoms are major contributors to global primary production and their populations in the modern oceans are affected by availability of iron, nitrogen, phosphate, silica, and other trace metals, vitamins, and infochemicals. However, little is known about the role of phosphorylation in diatoms and its role in regulation and signaling. We report a total of 2759 phosphorylation sites on 1502 proteins detected in Phaeodactylum tricornutum. Conditionally phosphorylated peptides were detected at low iron (n = 108), during the diel cycle (n = 149), and due to nitrogen availability (n = 137). Through a multi-omic comparison of transcript, protein, phosphorylation, and protein homology, we identify numerous proteins and key cellular processes that are likely under control of phospho-regulation. We show that phosphorylation regulates: (1) carbon retrenchment and reallocation during growth under low iron, (2) carbon flux towards lipid biosynthesis after the lights turn on, (3) coordination of transcription and translation over the diel cycle and (4) in response to nitrogen depletion. We also uncover phosphorylation sites for proteins that play major roles in diatom Fe sensing and utilization, including flavodoxin and phytotransferrin (ISIP2A), as well as identify phospho-regulated stress proteins and kinases. These findings provide much needed insight into the roles of protein phosphorylation in diel cycling and nutrient sensing in diatoms.

Deciphering Patterns of Adaptation and Acclimation in the Transcriptome of Phaeocystis antarctica to Changing Iron Conditions1

The haptophyte Phaeocystis antarctica is endemic to the Southern Ocean, where iron supply is sporadic and its availability limits primary production. In iron fertilization experiments, P. antarctica showed a prompt and steady increase in cell abundance compared to heavily silicified diatoms along with enhanced colony formation. Here we utilized a transcriptomic approach to investigate molecular responses to alleviation of iron limitation in P. antarctica. We analyzed the transcriptomic response before and after (14 h, 24 h and 72 h) iron addition to a low-iron acclimated culture. After iron addition, we observed indicators of a quick reorganization of cellular energetics, from carbohydrate catabolism and mitochondrial energy production to anabolism. In addition to typical substitution responses from an iron-economic toward an iron-sufficient state for flavodoxin (ferredoxin) and plastocyanin (cytochrome c₆ ), we found other genes utilizing the same strategy involved in nitrogen assimilation and fatty acid desaturation. Our results shed light on a number of adaptive mechanisms that P. antarctica uses under low iron, including the utilization of a Cu-dependent ferric reductase system and indication of mixotrophic growth. The gene expression patterns underpin P. antarctica as a quick responder to iron addition.

Diatom Molecular Research Comes of Age: Model Species for Studying Phytoplankton Biology and Diversity

Diatoms are the world's most diverse group of algae, comprising at least 100,000 species. Contributing ∼20% of annual global carbon fixation, they underpin major aquatic food webs and drive global biogeochemical cycles. Over the past two decades, Thalassiosira pseudonana and Phaeodactylum tricornutum have become the most important model systems for diatom molecular research, ranging from cell biology to ecophysiology, due to their rapid growth rates, small genomes, and the cumulative wealth of associated genetic resources. To explore the evolutionary divergence of diatoms, additional model species are emerging, such as Fragilariopsis cylindrus and Pseudo-nitzschia multistriata Here, we describe how functional genomics and reverse genetics have contributed to our understanding of this important class of microalgae in the context of evolution, cell biology, and metabolic adaptations. Our review will also highlight promising areas of investigation into the diversity of these photosynthetic organisms, including the discovery of new molecular pathways governing the life of secondary plastid-bearing organisms in aquatic environments.

Re-assembly, quality evaluation, and annotation of 678 microbial eukaryotic reference transcriptomes

BACKGROUND

De novo transcriptome assemblies are required prior to analyzing RNA sequencing data from a species without an existing reference genome or transcriptome. Despite the prevalence of transcriptomic studies, the effects of using different workflows, or "pipelines," on the resulting assemblies are poorly understood. Here, a pipeline was programmatically automated and used to assemble and annotate raw transcriptomic short-read data collected as part of the Marine Microbial Eukaryotic Transcriptome Sequencing Project. The resulting transcriptome assemblies were evaluated and compared against assemblies that were previously generated with a different pipeline developed by the National Center for Genome Research.

RESULTS

New transcriptome assemblies contained the majority of previous contigs as well as new content. On average, 7.8% of the annotated contigs in the new assemblies were novel gene names not found in the previous assemblies. Taxonomic trends were observed in the assembly metrics. Assemblies from the Dinoflagellata showed a higher number of contigs and unique k-mers than transcriptomes from other phyla, while assemblies from Ciliophora had a lower percentage of open reading frames compared to other phyla.

CONCLUSIONS

Given current bioinformatics approaches, there is no single "best" reference transcriptome for a particular set of raw data. As the optimum transcriptome is a moving target, improving (or not) with new tools and approaches, automated and programmable pipelines are invaluable for managing the computationally intensive tasks required for re-processing large sets of samples with revised pipelines and ensuring a common evaluation workflow is applied to all samples. Thus, re-assembling existing data with new tools using automated and programmable pipelines may yield more accurate identification of taxon-specific trends across samples in addition to novel and useful products for the community.

Reduction-dependent siderophore assimilation in a model pennate diatom

Diatoms can access inorganic iron with remarkable efficiency, but this process is contingent on carbonate ion concentration. As ocean acidification reduces carbonate concentration, inorganic iron uptake may be discouraged in favor of carbonate-independent uptake. We report details of an iron assimilation process that needs no carbonate but requires exogenous compounds produced by cooccurring organisms. We show this process to be critical for diatom growth at high siderophore concentrations, but ineffective at acquiring iron from low-affinity organic chelators or lithogenic particulates. Understanding the caveats associated with iron source preference in diatoms will help predict the impacts of climate change on microbial community structure in high-nitrate low-chlorophyll ecosystems.

Iron uptake by diatoms is a biochemical process with global biogeochemical implications. In large regions of the surface ocean diatoms are both responsible for the majority of primary production and frequently experiencing iron limitation of growth. The strategies used by these phytoplankton to extract iron from seawater constrain carbon flux into higher trophic levels and sequestration into sediments. In this study we use reverse genetic techniques to target putative iron-acquisition genes in the model pennate diatom Phaeodactylum tricornutum. We describe components of a reduction-dependent siderophore acquisition pathway that relies on a bacterial-derived receptor protein and provides a viable alternative to inorganic iron uptake under certain conditions. This form of iron uptake entails a close association between diatoms and siderophore-producing organisms during low-iron conditions. Homologs of these proteins are found distributed across diatom lineages, suggesting the significance of siderophore utilization by diatoms in the marine environment. Evaluation of specific proteins enables us to confirm independent iron-acquisition pathways in diatoms and characterize their preferred substrates. These findings refine our mechanistic understanding of the multiple iron-uptake systems used by diatoms and help us better predict the influence of iron speciation on taxa-specific iron bioavailability.

Metabolic Innovations Underpinning the Origin and Diversification of the Diatom Chloroplast

Of all the eukaryotic algal groups, diatoms make the most substantial contributions to photosynthesis in the contemporary ocean. Understanding the biological innovations that have occurred in the diatom chloroplast may provide us with explanations to the ecological success of this lineage and clues as to how best to exploit the biology of these organisms for biotechnology. In this paper, we use multi-species transcriptome datasets to compare chloroplast metabolism pathways in diatoms to other algal lineages. We identify possible diatom-specific innovations in chloroplast metabolism, including the completion of tocopherol synthesis via a chloroplast-targeted tocopherol cyclase, a complete chloroplast ornithine cycle, and chloroplast-targeted proteins involved in iron acquisition and CO₂ concentration not shared between diatoms and their closest relatives in the stramenopiles. We additionally present a detailed investigation of the chloroplast metabolism of the oil-producing diatom Fistulifera solaris, which is of industrial interest for biofuel production. These include modified amino acid and pyruvate hub metabolism that might enhance acetyl-coA production for chloroplast lipid biosynthesis and the presence of a chloroplast-localised squalene synthesis pathway unknown in other diatoms. Our data provides valuable insights into the biological adaptations underpinning an ecologically critical lineage, and how chloroplast metabolism can change even at a species level in extant algae.

Towards the development of a new generation of whole-cell bioreporters to sense iron bioavailability in oceanic systems-learning from the case of Synechococcus sp. PCC7002 iron bioreporter

Whole-cell bioreporters are genetically modified micro-organisms designed to sense bioavailable forms of nutrients or toxic compounds in aquatic systems. As they represent the most promising cost-efficient tools available for such purpose, engineering and use of bioreporters is rapidly growing in association with wide applicability. Bioreporters are urgently needed to determine phytoplankton iron (Fe) limitation, which has been reported in up to 30% of the ocean, with consequences affecting Earth's global carbon cycle and climate. This study presents a critical evaluation and optimization of the only Cyanobacteria bioreporter available to sense Fe limitation in marine systems (Synechococcus sp. PCC7002). The nonmonotonic biphasic dose-response curve between the bioreporters' signal and Fe bioavailability impairs an appropriate data interpretation, highlighting the need for new carefully designed bioreporters. Here, limitations under low Fe concentrations were related to cellular energy stress, nonlinear expression of the targeted promoter and siderophore expression. Furthermore, we provide critical standard criteria for the development of new Fe bioreporters. Finally, based on gene expression data under a range of marine Fe concentrations, we propose novel sensor genes for the development of new Cyanobacteria Fe bioreporters for distinct marine regions.

Metabolic Consequences of Cobalamin Scarcity in the Diatom Thalassiosira pseudonana as Revealed Through Metabolomics

Diatoms perform an estimated 20% of global photosynthesis, form the base of the marine food web, and sequester carbon into the deep ocean through the biological pump. In some areas of the ocean, diatom growth is limited by the micronutrient cobalamin (vitamin B₁₂), yet the biochemical ramifications of cobalamin limitation are not well understood. In a laboratory setting, we grew the diatom Thalassiosira pseudonana under replete and low cobalamin conditions to elucidate changes in metabolite pools. Using metabolomics, we show that the diatom experienced a metabolic cascade under cobalamin limitation that affected the central methionine cycle, transsulfuration pathway, and composition of osmolyte pools. In T. pseudonana, 5'-methylthioadenosine decreased under low cobalamin conditions, suggesting a disruption in the diatom's polyamine biosynthesis. Furthermore, two acylcarnitines accumulated under low cobalamin, suggesting the limited use of an adenosylcobalamin-dependent enzyme, methylmalonyl CoA mutase. Overall, these changes in metabolite pools yield insight into the metabolic consequences of cobalamin limitation in diatoms and suggest that cobalamin availability may have consequences for microbial interactions that are based on metabolite production by phytoplankton.

Ubiquitin fusion proteins in algae: implications for cell biology and the spread of photosynthesis

Background

The process of gene fusion involves the formation of a single chimeric gene from multiple complete or partial gene sequences. Gene fusion is recognized as an important mechanism by which genes and their protein products can evolve new functions. The presence-absence of gene fusions can also be useful characters for inferring evolutionary relationships between organisms.

Results

Here we show that the nuclear genomes of two unrelated single-celled algae, the cryptophyte Guillardia theta and the chlorarachniophyte Bigelowiella natans, possess an unexpected diversity of genes for ubiquitin fusion proteins, including novel arrangements in which ubiquitin occupies amino-terminal, carboxyl-terminal, and internal positions relative to its fusion partners. We explore the evolution of the ubiquitin multigene family in both genomes, and show that both algae possess a gene encoding an ubiquitin-nickel superoxide dismutase fusion protein (Ubiq-NiSOD) that is widely but patchily distributed across the eukaryotic tree of life – almost exclusively in phototrophs.

Conclusion

Our results suggest that ubiquitin fusion proteins are more common than currently appreciated; because of its small size, the ubiquitin coding region can go undetected when gene predictions are carried out in an automated fashion. The punctate distribution of the Ubiq-NiSOD fusion across the eukaryotic tree could serve as a beacon for the spread of plastids from eukaryote to eukaryote by secondary and/or tertiary endosymbiosis.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5412-4) contains supplementary material, which is available to authorized users.

Different iron storage strategies among bloom-forming diatoms

Diatoms are prominent eukaryotic phytoplankton despite being limited by the micronutrient iron in vast expanses of the ocean. As iron inputs are often sporadic, diatoms have evolved mechanisms such as the ability to store iron that enable them to bloom when iron is resupplied and then persist when low iron levels are reinstated. Two iron storage mechanisms have been previously described: the protein ferritin and vacuolar storage. To investigate the ecological role of these mechanisms among diatoms, iron addition and removal incubations were conducted using natural phytoplankton communities from varying iron environments. We show that among the predominant diatoms, Pseudo-nitzschia were favored by iron removal and displayed unique ferritin expression consistent with a long-term storage function. Meanwhile, Chaetoceros and Thalassiosira gene expression aligned with vacuolar storage mechanisms. Pseudo-nitzschia also showed exceptionally high iron storage under steady-state high and low iron conditions, as well as following iron resupply to iron-limited cells. We propose that bloom-forming diatoms use different iron storage mechanisms and that ferritin utilization may provide an advantage in areas of prolonged iron limitation with pulsed iron inputs. As iron distributions and availability change, this speculated ferritin-linked advantage may result in shifts in diatom community composition that can alter marine ecosystems and biogeochemical cycles.

Pseudo-nitzschia, Nitzschia, and domoic acid: New research since 2011

Some diatoms of the genera Pseudo-nitzschia and Nitzschia produce the neurotoxin domoic acid (DA), a compound that caused amnesic shellfish poisoning (ASP) in humans just over 30 years ago (December 1987) in eastern Canada. This review covers new information since two previous reviews in 2012. Nitzschia bizertensis was subsequently discovered to be toxigenic in Tunisian waters. The known distribution of N. navis-varingica has expanded from Vietnam to Malaysia, Indonesia, the Philippines and Australia. Furthermore, 15 new species (and one new variety) of Pseudo-nitzschia have been discovered, bringing the total to 52. Seven new species were found to produce DA, bringing the total of toxigenic species to 26. We list all Pseudo-nitzschia species, their ability to produce DA, and show their global distribution. A consequence of the extended distribution and increased number of toxigenic species worldwide is that DA is now found more pervasively in the food web, contaminating new marine organisms (especially marine mammals), affecting their physiology and disrupting ecosystems. Recent findings highlight how zooplankton grazers can induce DA production in Pseudo-nitzschia and how bacteria interact with Pseudo-nitzschia. Since 2012, new discoveries have been reported on physiological controls of Pseudo-nitzschia growth and DA production, its sexual reproduction, and infection by an oomycete parasitoid. Many advances are the result of applying molecular approaches to discovering new species, and to understanding the population genetic structure of Pseudo-nitzschia and mechanisms used to cope with iron limitation. The availability of genomes from three Pseudo-nitzschia species, coupled with a comparative transcriptomic approach, has allowed advances in our understanding of the sexual reproduction of Pseudo-nitzschia, its signaling pathways, its interactions with bacteria, and genes involved in iron and vitamin B₁₂ and B₇ metabolism. Although there have been no new confirmed cases of ASP since 1987 because of monitoring efforts, new blooms have occurred. A massive toxic Pseudo-nitzschia bloom affected the entire west coast of North America during 2015-2016, and was linked to a 'warm blob' of ocean water. Other smaller toxic blooms occurred in the Gulf of Mexico and east coast of North America. Knowledge gaps remain, including how and why DA and its isomers are produced, the world distribution of potentially toxigenic Nitzschia species, the prevalence of DA isomers, and molecular markers to discriminate between toxigenic and non-toxigenic species and to discover sexually reproducing populations in the field.

Regulating cellular trace metal economy in algae

As indispensable protein cofactors, Fe, Mn, Cu and Zn are at the center of multifaceted acclimation mechanisms that have evolved to ensure extracellular supply meets intracellular demand. Starting with selective transport at the plasma membrane and ending in protein metalation, metal homeostasis in algae involves regulated trafficking of metal ions across membranes, intracellular compartmentalization by proteins and organelles, and metal-sparing/recycling mechanisms to optimize metal-use efficiency. Overlaid on these processes are additional circuits that respond to the metabolic state as well as to the prior metal status of the cell. In this review, we focus on recent progress made toward understanding the pathways by which the single-celled, green alga Chlamydomonas reinhardtii controls its cellular trace metal economy. We also compare these mechanisms to characterized and putative processes in other algal lineages. Photosynthetic microbes continue to provide insight into cellular regulation and handling of Cu, Fe, Zn and Mn as a function of the nutritional supply and cellular demand for metal cofactors. New experimental tools such as RNA-Seq and subcellular metal imaging are bringing us closer to a molecular understanding of acclimation to supply dynamics in algae and beyond.

The influence of light on copper-limited growth of an oceanic diatom, Thalassiosira oceanica (Coscinodiscophyceae)

Thalassiosira oceanica (CCMP 1005) was grown over a range of copper concentrations at saturating and subsaturating irradiance to test the hypothesis that Cu and light were interacting essential resources. Growth was a hyperbolic function of irradiance in Cu-replete medium (263 fmol Cu' · L^-1 ) with maximum rates achieved at 200 μmol photons · m^-2  · s^-1 . Lowering the Cu concentration at this irradiance to 30.8 fmol Cu' · L^-1 decreased cellular Cu quota by 7-fold and reduced growth rate by 50%. Copper-deficient cells had significantly slower (P < 0.0001) rates of maximum, relative photosynthetic electron transport (rETR_max ) than Cu-sufficient cells, consistent with the role of Cu in photosynthesis in this diatom. In low-Cu medium (30.8 fmol Cu' · L^-1 ), growth rate was best described as a positive, linear function of irradiance and reached the maximum value measured in Cu-replete cells when irradiance increased to 400 μmol photons · m^-2  · s^-1 . Thus, at high light, low-Cu concentration was no longer limiting to growth: Cu concentration and light interacted strongly to affect growth rate of T. oceanica (P < 0.0001). Relative ETR_max and Cu quota of cells grown at low Cu also increased at 400 μmol photons · m^-2  · s^-1 to levels measured in Cu-replete cells. Steady-state uptake rates of Cu-deficient and sufficient cells were light-dependent, suggesting that faster growth of T. oceanica under high light and low Cu was a result of light-stimulated Cu uptake.

Development of a molecular-based index for assessing iron status in bloom-forming pennate diatoms

Iron availability limits primary productivity in large areas of the world's oceans. Ascertaining the iron status of phytoplankton is essential for understanding the factors regulating their growth and ecology. We developed an incubation-independent, molecular-based approach to assess the iron nutritional status of specific members of the diatom community, initially focusing on the ecologically important pennate diatom Pseudo-nitzschia. Through a comparative transcriptomic approach, we identified two genes that track the iron status of Pseudo-nitzschia with high fidelity. The first gene, ferritin (FTN), encodes for the highly specialized iron storage protein induced under iron-replete conditions. The second gene, ISIP2a, encodes an iron-concentrating protein induced under iron-limiting conditions. In the oceanic diatom Pseudo-nitzschia granii (Hasle) Hasle, transcript abundance of these genes directly relates to changes in iron availability, with increased FTN transcript abundance under iron-replete conditions and increased ISIP2a transcript abundance under iron-limiting conditions. The resulting ISIP2a:FTN transcript ratio reflects the iron status of cells, where a high ratio indicates iron limitation. Field samples collected from iron grow-out microcosm experiments conducted in low iron waters of the Gulf of Alaska and variable iron waters in the California upwelling zone verify the validity of our proposed Pseudo-nitzschia Iron Limitation Index, which can be used to ascertain in situ iron status and further developed for other ecologically important diatoms.

Iron incorporation in biosilica of the marine diatom Stephanopyxis turris: dispersed or clustered?

Iron incorporation into diatom biosilica was investigated for the species Stephanopyxis turris. It is known that several "foreign" elements (e.g., germanium, titanium, aluminum, zinc, iron) can be incorporated into the siliceous cell walls of diatoms in addition to silicon dioxide (SiO₂). In order to examine the amount and form of iron incorporation, the iron content in the growth medium was varied during cultivation. Fe:Si ratios of isolated cell walls were measured by ICP-OES. SEM studies were performed to examine of a possible influence of excess iron during diatom growth upon cell wall formation. The chemical state of biosilica-attached iron was characterized by a combination of infrared, ²⁹Si MAS NMR, and EPR spectroscopy. For comparison, synthetic silicagels of variable iron content were studied. Our investigations show that iron incorporation in biosilica is limited. More than 95% of biosilica-attached iron is found in the form of iron clusters/nanoparticles. In contrast, iron is preferentially dispersedly incorporated within the silica framework in synthetic silicagels leading to Si-O-Fe bond formation.

Transcriptional Orchestration of the Global Cellular Response of a Model Pennate Diatom to Diel Light Cycling under Iron Limitation

Environmental fluctuations affect distribution, growth and abundance of diatoms in nature, with iron (Fe) availability playing a central role. Studies on the response of diatoms to low Fe have either utilized continuous (24 hr) illumination or sampled a single time of day, missing any temporal dynamics. We profiled the physiology, metabolite composition, and global transcripts of the pennate diatom Phaeodactylum tricornutum during steady-state growth at low, intermediate, and high levels of dissolved Fe over light:dark cycles, to better understand fundamental aspects of genetic control of physiological acclimation to growth under Fe-limitation. We greatly expand the catalog of genes involved in the low Fe response, highlighting the importance of intracellular trafficking in Fe-limited diatoms. P. tricornutum exhibited transcriptomic hallmarks of slowed growth leading to prolonged periods of cell division/silica deposition, which could impact biogeochemical carbon sequestration in Fe-limited regions. Light harvesting and ribosome biogenesis transcripts were generally reduced under low Fe while transcript levels for genes putatively involved in the acquisition and recycling of Fe were increased. We also noted shifts in expression towards increased synthesis and catabolism of branched chain amino acids in P. tricornutum grown at low Fe whereas expression of genes involved in central core metabolism were relatively unaffected, indicating that essential cellular function is protected. Beyond the response of P. tricornutum to low Fe, we observed major coordinated shifts in transcript control of primary and intermediate metabolism over light:dark cycles which contribute to a new view of the significance of distinctive diatom pathways, such as mitochondrial glycolysis and the ornithine-urea cycle. This study provides new insight into transcriptional modulation of diatom physiology and metabolism across light:dark cycles in response to Fe availability, providing mechanistic understanding for the ability of diatoms to remain metabolically poised to respond quickly to Fe input and revealing strategies underlying their ecological success.

Oceanic diatoms live in constantly fluctuating environments to which they must adapt in order to survive. During sunlit hours, photosynthesis occurs allowing diatoms to store energy used at night to sustain energy demands. Cellular and molecular mechanisms for regulation of phytoplankton growth are important to understand because of their environmental roles at the base of food webs and in regulating carbon flux out of the atmosphere. In ocean ecosystems, the availability of iron (Fe) commonly limits phytoplankton growth and diatoms typically outcompete other phytoplankton when Fe is added, indicating they have adaptations allowing them to both survive at low Fe and rapidly respond to Fe additions. These adaptations may be unique depending on isolation from coastal or oceanic locations. To identify adaptive strategies, we characterized the response of a model diatom, Phaeodactylum tricornutum, to limiting Fe conditions over day:night cycles using a combination of gene expression analyses, metabolite, and physiology measurements. Major coordinated shifts in metabolism and growth were documented over diel cycles, with peak expression of low Fe expressed genes in the dark phase. Diatoms respond to limiting Fe by increasing Fe acquisition, while decreasing growth rate through slowed cell cycle progression, reduced energy acquisition, and subtle metabolic remodeling.

Chronic Iron Limitation Confers Transient Resistance to Oxidative Stress in Marine Diatoms

Diatoms are single-celled, photosynthetic, bloom-forming algae that are responsible for at least 20% of global primary production. Nevertheless, more than 30% of the oceans are considered "ocean deserts" due to iron limitation. We used the diatom Phaeodactylum tricornutum as a model system to explore diatom's response to iron limitation and its interplay with susceptibility to oxidative stress. By analyzing physiological parameters and proteome profiling, we defined two distinct phases: short-term (<3 d, phase I) and chronic (>5 d, phase II) iron limitation. While at phase I no significant changes in physiological parameters were observed, molecular markers for iron starvation, such as Iron Starvation Induced Protein and flavodoxin, were highly up-regulated. At phase II, down-regulation of numerous iron-containing proteins was detected in parallel to reduction in growth rate, chlorophyll content, photosynthetic activity, respiration rate, and antioxidant capacity. Intriguingly, while application of oxidative stress to phase I and II iron-limited cells similarly oxidized the reduced glutathione (GSH) pool, phase II iron limitation exhibited transient resistance to oxidative stress, despite the down regulation of many antioxidant proteins. By comparing proteomic profiles of P. tricornutum under iron limitation and metatranscriptomic data of an iron enrichment experiment conducted in the Pacific Ocean, we propose that iron-limited cells in the natural environment resemble the phase II metabolic state. These results provide insights into the trade-off between optimal growth rate and susceptibility to oxidative stress in the response of diatoms to iron quota in the marine environment.

The importance of eukaryotic ferritins in iron handling and cytoprotection

Ferritins, the main intracellular iron storage proteins, have been studied for over 60 years, mainly focusing on the mammalian ones. This allowed the elucidation of the structure of these proteins and the mechanisms regulating their iron incorporation and mineralization. However, ferritin is present in most, although not all, eukaryotic cells, comprising monocellular and multicellular invertebrates and vertebrates. The aim of this review is to provide an update on the general properties of ferritins that are common to various eukaryotic phyla (except plants), and to give an overview on the structure, function and regulation of ferritins. An update on the animal models that were used to characterize H, L and mitochondrial ferritins is also provided. The data show that ferritin structure is highly conserved among different phyla. It exerts an important cytoprotective function against oxidative damage and plays a role in innate immunity, where it also contributes to prevent parenchymal tissue from the cytotoxicity of pro-inflammatory agonists released by the activation of the immune response activation. Less clear are the properties of the secretory ferritins expressed by insects and molluscs, which may be important for understanding the role played by serum ferritin in mammals.