Diatom Metabolism Research Articles

Influence of Nutrient Deprivation on the Antioxidant Capacity and Chemical Profile of Two Diatoms from Genus Chaetoceros.

The limited availability of phosphate, nitrogen and silicon in the growth media affects the growth, cellular processes, and metabolism of diatoms. Silicon deficiency primarily affects diatom morphology, while phosphate deficiency reduces the production of nucleic acids and phospholipids. Differences in pigment and protein composition are mainly due to nitrogen deficiency. In this study, Chaetoceros socialis and Chaetoceros costatus were cultured under phosphate, nitrogen, and silicon deprivation conditions. The diatom biomass was collected during the stationary growth phase and extracted with 70% ethanol under ultrasonication. The chemical profiles of the extracts were analyzed by high-performance liquid chromatography with high-resolution mass spectrometry with electrospray ionisation (UHPLC-ESI-HRMS), while the antioxidant capacity was determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and oxygen radical absorbance capacity (ORAC) assays. Pigments, fatty acids, sterols, and derivatives were detected in both species. The total phenolic content in the extracts ranged from 46.25 ± 1.08 to 89.38 ± 6.21 mg of gallic acid equivalent (GAE)/L and from 29.58 ± 1.08 to 54.17 ± 1.18 mg GAE/L. for C. costatus and C. socialis, respectively. Antioxidant activity was higher in C. costatus extracts, especially those obtained from nitrogen-deprived media. The results of this study contribute to the existing knowledge and the ongoing efforts to overcome application and commercialization barriers of microalgae for wide-ranging potential in different industries.

Mercury species induce metabolic reprogramming in freshwater diatom Cyclotella meneghiniana

Mercury is a hazardous pollutant of global concern. While advances have been made in identifying the detrimental effects caused by Hg species in phytoplankton, knowledge gaps remain regarding the metabolomic perturbations induced by inorganic mercury (Hg(II)) and monomethylmercury (MeHg) in these organisms. Diatoms represent a major phytoplankton group essential in various global biogeochemical cycles. The current study combined targeted metabolomics, bioaccumulation, and physiological response assays to investigate metabolic perturbations in diatom Cyclotella meneghiniana exposed for 2 h to nanomolar concentrations of Hg(II) and MeHg. Our findings highlight that such exposures induce reprogramming of the metabolism of amino acids, nucleotides, fatty acids, carboxylic acids and antioxidants. These alterations were primarily mercury-species dependent. MeHg exposure induced more pronounced reprogramming of the metabolism of diatoms than Hg(II), which led to less pronounced effects on ROS generation, membrane permeability and chlorophyll concentrations. Hg(II) treatments presented distinct physiological responses, with more robust metabolic perturbations at higher exposures. The present study provides first-time insights into the main metabolic alterations in diatom C. meneghiniana during short-term exposure to Hg species, deepening our understanding of the molecular basis of these perturbations.

Oligotrophic waters of the Northwest Atlantic support taxonomically diverse diatom communities that are distinct from coastal waters.

Diatoms are important components of the marine food web and one of the most species-rich groups of phytoplankton. The diversity and composition of diatoms in eutrophic nearshore habitats have been well documented due to the outsized influence of diatoms on coastal ecosystem functioning. In contrast, patterns of both diatom diversity and community composition in offshore oligotrophic regions where diatom biomass is low have been poorly resolved. To compare the diatom diversity and community composition in oligotrophic and eutrophic waters, diatom communities were sampled along a 1,250 km transect from the oligotrophic Sargasso Sea to the coastal waters of the northeast US shelf. Diatom community composition was determined by amplifying and sequencing the 18S rDNA V4 region. Of the 301 amplicon sequence variants (ASVs) identified along the transect, the majority (70%) were sampled exclusively from oligotrophic waters of the Gulf Stream and Sargasso Sea and included the genera Bacteriastrum, Haslea, Hemiaulus, Pseudo-nitzschia, and Nitzschia. Diatom ASV richness did not vary along the transect, indicating that the oligotrophic Sargasso Sea and Gulf Stream are occupied by a diverse diatom community. Although ASV richness was similar between oligotrophic and coastal waters, diatom community composition in these regions differed significantly and was correlated with temperature and phosphate, two environmental variables known to influence diatom metabolism and geographic distribution. In sum, oligotrophic waters of the western North Atlantic harbor diverse diatom assemblages that are distinct from coastal regions, and these open ocean diatoms warrant additional study, as they may play critical roles in oligotrophic ecosystems.

Spatial and seasonal dynamics of biogenic silica in a eutrophic marginal sea, the East China Sea

Biogenic silica (BSi) is a key component of the marine silicon cycle, which is mainly driven by diatom metabolism notably contributing to primary production and export of particulate organic carbon (POC), particularly in ocean margins. Based on a large data set collected in the East China Sea (ECS), including BSi, fucoxanthin (Fuco), POC, and total suspended matter (TSM), we systematically explored the distribution and control of BSi in a typical eutrophic marginal sea. Spatially, BSi concentrations generally decreased from the shelf to the slope during all seasons, because the former is largely fed by river plumes and/or coastal currents enriched in nutrients favoring diatom growth. Abundant BSi was also observed in nearshore bottom waters probably influenced by sediment resuspension indicated by high TSM concentrations. Seasonally, BSi concentrations were on average higher in summer and autumn than in spring and winter, which reflects elevated diatom productivity during warm seasons. The BSi standing stock in the shallow water column of the ECS was significantly correlated with that of Fuco demonstrating diatoms’ dominant control on BSi dynamics. In addition, significant relationships between BSi and POC standing stocks were observed in summer and autumn, indicating the major role of diatoms in C fixation. By comparing with the northern South China Sea (NSCS), we suggested relatively small seasonal variability of BSi on the ECS shelf but significant decrease during cold seasons in the ECS slope. In the latter case, diatoms are ecologically and biogeochemically more important and more sensitive to the changing physical and/or chemical conditions than in the oligotrophic NSCS slope.

Thalassiosira pseudonana (Cyclotella nana) (Hustedt) Hasle et Heimdal (Bacillariophyceae): A genetically tractable model organism for studying diatom biology, including biological silica formation.

In 2004, Thalassiosira pseudonana was the first eukaryotic marine alga to have its genome sequenced. Since then, this species has quickly emerged as a valuable model species for investigating the molecular underpinnings of essentially all aspects of diatom life, particularly bio-morphogenesis of the cell wall. An important prerequisite for the model status of T. pseudonana is the ongoing development of increasingly precise tools to study the function of gene networks and their encoded proteins in vivo. Here, we briefly review the current toolbox for genetic manipulation, highlight specific examples of its application in studying diatom metabolism, and provide a peek into the role of diatoms in the emerging field of silica biotechnology.

Flavobacterial exudates disrupt cell cycle progression and metabolism of the diatom Thalassiosira pseudonana

Phytoplankton and bacteria form the base of marine ecosystems and their interactions drive global biogeochemical cycles. The effects of bacteria and bacteria-produced compounds on diatoms range from synergistic to pathogenic and can affect the physiology and transcriptional patterns of the interacting diatom. Here, we investigate physiological and transcriptional changes in the marine diatom Thalassiosira pseudonana induced by extracellular metabolites of a known antagonistic bacterium Croceibacter atlanticus. Mono-cultures of C. atlanticus released compounds that inhibited diatom cell division and elicited a distinctive morphology of enlarged cells with increased chloroplast content and enlarged nuclei, similar to what was previously observed when the diatom was co-cultured with live bacteria. The extracellular C. atlanticus metabolites induced transcriptional changes in diatom pathways that include recognition and signaling pathways, cell cycle regulation, carbohydrate and amino acid production, as well as cell wall stability. Phenotypic analysis showed a disruption in the diatom cell cycle progression and an increase in both intra- and extracellular carbohydrates in diatom cultures after bacterial exudate treatment. The transcriptional changes and corresponding phenotypes suggest that extracellular bacterial metabolites, produced independently of direct bacterial-diatom interaction, may modulate diatom metabolism in ways that support bacterial growth.

Application of focused ion beam-field emission scanning electron microscopy-X-ray microanalysis in the study of the surface alterations of archaeological tin-glazed ceramics

The historical evolution of tin-glazed ceramics is a subject that has attracted the attention of many scientists and, consequently, abundant archaeometrical studies can be found in the specialized literature. Nevertheless, a lesser number of studies aimed at the characterization of the alterations undergone by archaeological glazes has been reported. This work describes some unusual alteration processes found in tin-glazes depending on the environmental conditions surrounding the piece during centuries. For this purpose, focused ion beam-field emission scanning electron microscopy-X-ray microanalysis (FIB-FESEM-EDX), an advanced instrumental technique for surface analysis at the nanoscale, has been used for the first time, complementarily to optical microscopy (OM), FESEM-EDX and Fourier transform Infrared spectroscopy (FTIR). In the buried glaze the nanostructure of the outer lamellar corrosion layer due to humidity-dryness cycles is described. Unusual and selective erosion of the glaze matrix due to the silicification metabolism of diatoms is observed in a submarine glaze. A nanometric outer corrosion layer formed by precipitated corrosion products and unspecific organic matter that has been partially degraded to lead and calcium oxalates is found in a glazed tile subjected to an atmospheric environment.

Genome-scale metabolic model of the diatom Thalassiosira pseudonana highlights the importance of nitrogen and sulfur metabolism in redox balance

Diatoms are unicellular photosynthetic algae known to secrete organic matter that fuels secondary production in the ocean, though our knowledge of how their physiology impacts the composition of dissolved organic matter remains limited. Like all photosynthetic organisms, their use of light for energy and reducing power creates the challenge of avoiding cellular damage. To better understand the interplay between redox balance and organic matter secretion, we reconstructed a genome-scale metabolic model of Thalassiosira pseudonana strain CCMP 1335, a model for diatom molecular biology and physiology, with a 60-year history of studies. The model simulates the metabolic activities of 1,432 genes via a network of 2,792 metabolites produced through 6,079 reactions distributed across six subcellular compartments. Growth was simulated under different steady-state light conditions (5–200 μmol photons m-2 s-1) and in a batch culture progressing from exponential growth to nitrate-limitation and nitrogen-starvation. We used the model to examine the dissipation of reductants generated through light-dependent processes and found that when available, nitrate assimilation is an important means of dissipating reductants in the plastid; under nitrate-limiting conditions, sulfate assimilation plays a similar role. The use of either nitrate or sulfate uptake to balance redox reactions leads to the secretion of distinct organic nitrogen and sulfur compounds. Such compounds can be accessed by bacteria in the surface ocean. The model of the diatom Thalassiosira pseudonana provides a mechanistic explanation for the production of ecologically and climatologically relevant compounds that may serve as the basis for intricate, cross-kingdom microbial networks. Diatom metabolism has an important influence on global biogeochemistry; metabolic models of marine microorganisms link genes to ecosystems and may be key to integrating molecular data with models of ocean biogeochemistry.

Quantitative Proteomic Profiling of Marine Diatom Skeletonema dohrnii in Response to Temperature and Silicate Induced Environmental Stress.

Global warming is expected to reduce the nutrient concentration in the upper ocean and affect the physiology of marine diatoms, but the underlying molecular mechanisms controlling these physiological changes are currently unknown. To understand these mechanisms, here we investigated iTRAQ based proteomic profiling of diatom Skeletonema dohrnii in a multifactorial experimental with a combining change of temperature and silicate concentrations. In total, 3369 differently abundant proteins were detected in four different environmental conditions, and the function of all proteins was identified using Gene Ontology and KEGG pathway analysis. For discriminating the proteome variation among samples, multivariate statistical analysis (PCA, PLS-DA) was performed by comparing the protein ratio differences. Further, performing pathway analysis on diatom proteomes, we here demonstrated downregulation of photosynthesis, carbon metabolism, and ribosome biogenesis in the cellular process that leads to decrease the oxidoreductase activity and affects the cell cycle of the diatom. Using PLS-DA VIP score plot analysis, we identified 15 protein biomarkers for discriminating studied samples. Of these, five proteins or gene (rbcL, PRK, atpB, DNA-binding, and signal transduction) identified as key biomarkers, induced by temperature and silicate stress in diatom metabolism. Our results show that proteomic finger-printing of S. dohrnii with different environmental conditions adds biological information that strengthens marine phytoplankton proteome analysis.

A novel culture medium for freshwater diatoms promotes efficient photoautotrophic batch production of biomass, fucoxanthin, and eicosapentaenoic acid

Diatoms have recently emerged as new promising sources of fucoxanthin and eicosapentaenoic acid (EPA). Most applied studies in this area have focused on marine diatoms whereas the characterization of freshwater diatom metabolism has been restricted to ecophysiological studies. Here, we describe a novel culture medium for freshwater diatoms, the FDMed medium, and we demonstrate that it supports high biomass, fucoxanthin, and EPA production yields in photoautotrophic batch cultures of Sellaphora minima and Nitzschia palea. Growth was monitored for 21 days, fucoxanthin and EPA concentrations were assayed all along cultivation processes, and results were compared to those obtained for two other media (WC and Modified COMBO (MCOMBO)). The FDMed medium was found to increase biomass yields comparatively to MCOMBO medium (5–7-fold higher algal concentration at the stationary phase in S. minima and N. palea, respectively) while WC medium enables nearly no growth. The evolution of nitrate, silicate, and phosphate concentrations in culture media indicated a much better balance of macroelements in the FDMed medium. Fucoxanthin contents within the biomass remained very stable in the FDMed medium compared to the other media, in which it gradually decreases, and the best results (around 7.5 mg g-1 dry weight ) were obtained for S. minima in the FDMed medium. EPA could only be measured in the FDMed medium and the highest content (14.0 mg EPA g-1 dry weight) was recorded in N. palea. This work demonstrates the great industrial potential of freshwater diatoms and provides an efficient culture medium for those organisms.

Genome-Scale Metabolic Reconstruction and in Silico Perturbation Analysis of the Polar Diatom Fragilariopsis cylindrus Predicts High Metabolic Robustness.

Diatoms are major primary producers in polar environments where they can actively grow under extremely variable conditions. Integrative modeling using a genome-scale model (GSM) is a powerful approach to decipher the complex interactions between components of diatom metabolism and can provide insights into metabolic mechanisms underlying their evolutionary success in polar ecosystems. We developed the first GSM for a polar diatom, Fragilariopsis cylindrus, which enabled us to study its metabolic robustness using sensitivity analysis. We find that the predicted growth rate was robust to changes in all model parameters (i.e., cell biochemical composition) except the carbon uptake rate. Constraints on total cellular carbon buffer the effect of changes in the input parameters on reaction fluxes and growth rate. We also show that single reaction deletion of 20% to 32% of active (nonzero flux) reactions and single gene deletion of 44% to 55% of genes associated with active reactions affected the growth rate, as well as the production fluxes of total protein, lipid, carbohydrate, DNA, RNA, and pigments by less than 1%, which was due to the activation of compensatory reactions (e.g., analogous enzymes and alternative pathways) with more highly connected metabolites involved in the reactions that were robust to deletion. Interestingly, including highly divergent alleles unique for F. cylindrus increased its metabolic robustness to cellular perturbations even more. Overall, our results underscore the high robustness of metabolism in F. cylindrus, a feature that likely helps to maintain cell homeostasis under polar conditions.

Levels of Diatom Minor Sterols Respond to Changes in Temperature and Salinity

Diatoms are a broadly distributed and evolutionarily diversified group of microalgae that produce a diverse range of sterol compounds. Sterols are triterpenoids that play essential roles in membrane-related processes in eukaryotic cells. Some sterol compounds possess bioactivities that promote human health and are currently used as nutraceuticals. The relationship between sterol diversity in diatoms and their acclimation to different environments is not well understood. In this study, we investigated the occurrence of different sterol types across twelve diatom species, as well as the effect of temperature reduction and changes in salinity on the sterol contents of three model diatom species. In the diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum and Chaetoceros muelleri, we found that changes in the relative contents of minor sterols accompanied shifts in temperature and salinity. This may be indicative of acquired adaptive traits in diatom metabolism.

Ecophysiological linkage of nitrogen enrichment to heavily silicified diatoms in winter

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 604:51-63 (2018) - DOI: https://doi.org/10.3354/meps12747 Ecophysiological linkage of nitrogen enrichment to heavily silicified diatoms in winter Dongyan Liu1,*, Patricia M. Glibert2 1State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, PR China 2University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, MD 21613, USA *Corresponding author: dyliu@sklec.ecnu.edu.cn ABSTRACT: Over recent decades, increased anthropogenic nitrogen and reduced land-based loading of silica in many coastal waters have asymmetrically changed the nitrogen:silica ratios. These changes have contributed to shifts in phytoplankton assemblages from diatoms to non-diatoms, as well as increases in the frequency and magnitude of non-diatom harmful algal blooms. Here we show a subtle and counterintuitive change in diatom assemblage, i.e. heavily silicified diatoms significantly increased in winter after nitrogen enrichment, based on paleoecological and contemporary seasonal water column data from 2 eutrophic bays in the Yellow Sea. The heavily silicified diatom Paralia sulcata showed an increasing trend over time in parallel with nitrogen enrichment, which was associated with low temperature, and low dissolved inorganic nitrogen:phosphorus, high dissolved nitrogen:silica, and low ammonium:nitrate ratios on a seasonal basis. Applying recent insights regarding diatom nitrogen metabolism and its putative urea cycle, a physiological mechanism linking nitrogen, carbon, and silica metabolism is suggested to explain the phenomenon of increased silicification under winter conditions at the cellular level. Winter sequestration of silica in P. sulcata valves also has biogeochemical consequences, including a weakening of the silica pump and a slowing of biogenic silica dissolution, thereby reducing the availability of silica for further diatom growth in subsequent seasons and increasing the window of opportunity for summer growth of non-diatom harmful algal bloom species. KEY WORDS: Paralia sulcate · Nitrification · Diatom metabolism · Photosynthesis · Phytoplankton shift · Si cycling Full text in pdf format PreviousNextCite this article as: Liu D, Glibert PM (2018) Ecophysiological linkage of nitrogen enrichment to heavily silicified diatoms in winter. Mar Ecol Prog Ser 604:51-63. https://doi.org/10.3354/meps12747 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 604. Online publication date: October 04, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Quantification of chrysolaminarin from the model diatom Phaeodactylum tricornutum

We have established a protocol that allows for parallel quantification of three carbohydrate pools in the marine diatom Phaeodactylum tricornutum. This method utilizes a series of extraction and digestion steps followed by the employment of the 3-methyl-2-benzothiazolinone hydrazone (MBTH) reducing sugar assay. Comparing carbohydrate content between hydrolyzed and non-hydrolyzed soluble extracts enables quantification of soluble, nonreducing carbohydrate. The latter fraction contains chrysolaminarin as verified by 1H NMR and monomer composition of the purified glucan. We applied this method to investigate carbon partitioning in two experiments. We observed the accumulation of chrysolaminarin during the day and near complete consumption in the dark, supporting its role for fueling heterotrophic metabolism at night. We then observed little change in chrysolaminarin accumulation or consumption during nitrogen starvation, a condition that is known to increase the cellular content of the biofuel precursor triacylglycerol. Overall, this method improves the resolution of major carbohydrate pools in diatoms, complementing physiological and systems biology studies interested in exploring diatom metabolism.

Molecular characterization of a glycerol-3-phosphate acyltransferase reveals key features essential for triacylglycerol production in Phaeodactylum tricornutum.

BackgroundThe marine diatom, Phaeodactylum tricornutum, has become a model for studying lipid metabolism and its triacylglycerol (TAG) synthesis pathway makes it an ideal target for metabolic engineering to improve lipid productivity. However, the genetic background and metabolic networks of fatty acid biosynthesis in diatoms are not well understood. Glycerol-3-phosphate acyltransferase (GPAT) is the critical enzyme that catalyzes the first step of TAG formation. So far, characterization of GPAT in marine microalgae has not been reported, especially at the level of comprehensive sequence-structure and functional analysis.ResultsA GPAT was cloned from P. tricornutum and overexpressed in P. tricornutum. Volumes of oil bodies were produced and the neutral lipid content was increased by twofold determined by Nile red fluorescence staining. Fatty acid composition was analyzed by GC–MS, which showed significantly higher proportion of unsaturated fatty acids compared to wild type.ConclusionThese results suggested that the identified GPAT could upregulate TAG biosynthesis in P. tricornutum. Moreover, this study offers insight into the lipid metabolism of diatoms and supports the role of microalgal strains for biofuels production.

Flux balance analysis of primary metabolism in the diatom Phaeodactylum tricornutum.

Diatoms (Bacillarophyceae) are photosynthetic unicellular microalgae that have risen to ecological prominence in oceans over the past 30 million years. They are of interest as potential feedstocks for sustainable biofuels. Maximizing production of these feedstocks will require genetic modifications and an understanding of algal metabolism. These processes may benefit from genome-scale models, which predict intracellular fluxes and theoretical yields, as well as the viability of knockout and knock-in transformants. Here we present a genome-scale metabolic model of a fully sequenced and transformable diatom: Phaeodactylum tricornutum. The metabolic network was constructed using the P. tricornutum genome, biochemical literature, and online bioinformatic databases. Intracellular fluxes in P. tricornutum were calculated for autotrophic, mixotrophic and heterotrophic growth conditions, as well as knockout conditions that explore the in silico role of glycolytic enzymes in the mitochondrion. The flux distribution for lower glycolysis in the mitochondrion depended on which transporters for TCA cycle metabolites were included in the model. The growth rate predictions were validated against experimental data obtained using chemostats. Two published studies on this organism were used to validate model predictions for cyclic electron flow under autotrophic conditions, and fluxes through the phosphoketolase, glycine and serine synthesis pathways under mixotrophic conditions. Several gaps in annotation were also identified. The model also explored unusual features of diatom metabolism, such as the presence of lower glycolysis pathways in the mitochondrion, as well as differences between P. tricornutum and other photosynthetic organisms.

Modelling metabolism of the diatom Phaeodactylum tricornutum.

Marine diatoms have potential as a biotechnological production platform, especially for lipid-derived products, including biofuels. Here we introduce some features of diatom metabolism, particularly with respect to photosynthesis, photorespiration and lipid synthesis and their differences relative to other photosynthetic eukaryotes. Since structural metabolic modelling of other photosynthetic organisms has been shown to be capable of representing their metabolic capabilities realistically, we briefly review the main approaches to this type of modelling. We then propose that genome-scale modelling of the diatom Phaeodactylum tricornutum, in response to varying light intensity, could uncover the novel aspects of the metabolic potential of this organism.

Chemoselective silicification of synthetic peptides and polyamines.

Biosilicification sets the standard for the localized in vitro precipitation of silica at low orthosilicate concentrations in aqueous environment under ambient conditions. Numerous parameters must be controlled for the development of new technologies in designing inventive nanosilica structures, which are able to challenge the biological templates. A long neglected requirement that came into focus in the recent years are the cellular techniques of preventing unintentional lithification of cellular structures since numerous cellular components such as membranes, DNA, and proteins are known to precipitate nanosilica. The diatom metabolism makes use of techniques that restrict silicification to an armor of silica around the cell wall while avoiding the petrifying gaze of Medusa, which turns the whole cell into stone. Step by step, biochemistry unveils the hierarchical interplay of an arsenal of low-molecular weight molecules, proteins, and the cytoskeletal architecture and it becomes clearer why the organisms invest much metabolic effort for an obviously simple chemical reaction like the precipitation of amorphous silica. The discrimination between different soluble components in the silicification process (chemoselective silicification) is not only vitally important for the diatom but poses an interesting challenge for in vitro experiments. Until now, silica precipitation studies were mainly focused on the amount, the morphology, and composition of the precipitate while disregarding a quantitative analysis of the remaining soluble components. Here, we turn the tables and quantify the soluble components by 1H NMR in the progress of precipitation and present experiments which quantify the additivity, and potential cooperativity of long chain polyamines (LCPAs) and cationic peptides in the silicification process.

Matrix‐free single‐cell LDI‐MS investigations of the diatoms Coscinodiscus granii and Thalassiosira pseudonana

Single-cell investigations of the diatoms Coscinodsicus granii and Thalassiosira pseudonana were performed using laser desorption/ionization (LDI)-MS without the addition of chemical matrices. The unique cell wall architecture of these microalgae, more precisely the biomineralized nanostructured surface, supported the ionization of cellular as well as surface-related metabolites. In model experiments with purified diatom cell walls of eight species C. granii and T. pseudonana proved to promote the ionization of the polymer polyethylene glycol most efficiently. These species were therefore chosen for further experiments. Without any additional workup, living diatom cells can be washed, can be placed on the LDI target and can immediately be profiled using LDI-MS. Characteristic signals arising from the two species were assigned to common metabolites known from diatom metabolism. Among others, chlorophyll, phospholipids and amino acids were detected. Using these fingerprint signals, we were able to perform species-specific MS imaging down to a single-cell resolution of 20 by 20 µm. The larger C. granii cells can be directly visualized, while more than one of the smaller T. pseudonana cells is needed to generate high-quality images. The introduced technique will pave the way toward a chemotyping of phytoplankton that will enable the automated annotation of microalgal species. But also, an assignment of metabolic plasticity on a single-cell level that could answer fundamental questions about plankton diversity is now in reach.

Biochemical and genetic engineering of diatoms for polyunsaturated fatty acid biosynthesis.

The role of diatoms as a source of bioactive compounds has been recently explored. Diatom cells store a high amount of fatty acids, especially certain polyunsaturated fatty acids (PUFAs). However, many aspects of diatom metabolism and the production of PUFAs remain unclear. This review describes a number of technical strategies, such as modulation of environmental factors (temperature, light, chemical composition of culture medium) and culture methods, to influence the content of PUFAs in diatoms. Genetic engineering, a newly emerging field, also plays an important role in controlling the synthesis of fatty acids in marine microalgae. Several key points in the biosynthetic pathway of PUFAs in diatoms as well as recent progresses are also a critical part and are summarized here.