A branched β-d-(1→3,1→6)-glucan from the marine diatom Chaetoceros debilis (Bacillariophyceae) characterized by NMR

Physiochemical changes, metabolite discrepancies of brown seaweed-derived sulphated polysaccharides in the upper gastrointestinal tract and their effects on bioactive expression

Brown seaweed-derived polysaccharides, notably fucoidan and laminarin, are known for their extensive array of bioactivities and physicochemical properties. However, the effects of upper digestive tract modification on the bioactive performance of fucoidan and laminarin fractions (FLFs) sourced from Australian native species are largely unknown. Here, the digestibility and bioaccessibility of FLFs were evaluated by tracking the dynamic changes in reducing sugar content (CR), profiling the free monosaccharide composition using LC-MS, and comparing high-performance gel permeation chromatography profile variation via LC-SEC-RI. The effects of digestive progression on bioactive performance were assessed by comparing the antioxidant and antidiabetic potential of FLFs and FLF digesta. We observed that molecular weight (Mw) decreased during gastric digestion indicating that FLF aggregates were disrupted in the stomach. During intestinal digestion, Mw gradually decreased and CR increased indicating cleavage of glycosidic bonds releasing free sugars. Although the antioxidant and antidiabetic capacities were not eliminated by the digestion progression, the bioactive performance of FLFs under a digestive environment was reduced contrasting with the same concentration level of the undigested FLFs. These data provide comprehensive information on the digestibility and bioaccessibility of FLFs, and shed light on the effects of digestive progression on bioactive expression.

Metabolisms and multiple functions of laminaran in marine algae under global change: A critical review

Laminaran is a major storage of carbohydrate in marine algae. Its high content and potential functions draw increasing attention. However, our understanding of its metabolisms and functions is still fragmented. After reviewing, marine algae exhibit a spectacular capacity of laminaran accumulation especially in the diatom Odontella aurita (65 % DW). Marine particulate organic carbon (POC) also has high contents of laminaran (42 ± 21 % DW). Laminaran shows a diel variation trend in marine algae, the content of which increases in the day but decreases at night. Laminaran also significantly accumulates in the stationary phase of algal growth. Furthermore, the metabolic pathway of laminaran and the remolding carbon mechanism in response to marine nitrogen limitation are proposed and comprehensively discussed. Laminaran production in marine phytoplankton is predicted to increase in future warmer and CO2-enriched oceans. Laminaran has diverse biological functions, including antioxidant, antimicrobial, anti-cancer, immunomodulatory, wound healing, and prebiotics. In addition, laminaran is also a major carbon storage compound in marine algae, suggesting its significant ecological function in marine carbon cycle. This study provides new insight into algal laminaran functions and its response mechanisms to environmental and climate changes.

Functionality of Yeast β-Glucan Recovered from Kluyveromyces marxianus by Alkaline and Enzymatic Processes

β-Glucan (BG), one of the most abundant polysaccharides containing glucose monomers linked by β-glycosidic linkages, is prevalent in yeast biomass that needs to be recovered to obtain this valuable polymer. This study aimed to apply alkaline and enzymatic processes for the recovery of BG from the yeast strain Kluyveromyces marxianus TISTR 5925. For this purpose, the yeast was cultivated to produce the maximum yield of raw material (yeast cells). The effective recovery of BG was then established using either an alkaline or an enzymatic process. BG recovery of 35.45% was obtained by using 1 M NaOH at 90 °C for 1 h, and of 81.15% from 1% (w/v) hydrolytic protease enzyme at 55 °C for 5 h. However, BG recovered by the alkaline process was purer than that obtained by the enzymatic process. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy confirmed the purity, the functional groups, and the linkages of BG obtained from different recovery systems and different raw materials. The results of this study suggest that an alkaline process could be an effective approach for the solubilization and recovery of considerable purity of BG from the yeast cells. In addition, the obtained BG had comparable functional properties with commercially available BG. This study reveals the effectiveness of both chemical and biological recovery of BG obtained from yeast as a potential polymeric material.

Laminarin, a Major Polysaccharide in Stramenopiles

During the processes of primary and secondary endosymbiosis, different microalgae evolved to synthesis different storage polysaccharides. In stramenopiles, the main storage polysaccharides are β-1,3-glucan, or laminarin, in vacuoles. Currently, laminarin is gaining considerable attention due to its application in the food, cosmetic and pharmaceuticals industries, and also its importance in global biogeochemical cycles (especially in the ocean carbon cycle). In this review, the structures, composition, contents, and bioactivity of laminarin were summarized in different algae. It was shown that the general features of laminarin are species-dependence. Furthermore, the proposed biosynthesis and catabolism pathways of laminarin, functions of key genes, and diel regulation of laminarin were also depicted and comprehensively discussed for the first time. However, the complete pathways, functions of genes, and diel regulatory mechanisms of laminarin require more biomolecular studies. This review provides more useful information and identifies the knowledge gap regarding the future studies of laminarin and its applications.

Phagocytosis activity of three sulfated polysaccharides purified from a marine diatom cultured in a semi-continuous system

An efficient process has been developed for bioactive polysaccharide production and purification from a local diatom isolate, Halamphora sp. AQ4. First, a semi-continuous system with fixed harvesting frequency was employed to cultivate AQ4 for the production of cell mass and polysaccharides for more than 285 days with a high yield of biomass. Six cultivation sets are performed according to different harvesting volumes per 3 days with or without Na₂CO₃ supplement. The addition of Na₂CO₃ increases both cell mass and polysaccharide production. Furthermore, three different sulfated polysaccharides (PK1~PK3) were purified from the freshly-grown AQ4 diatoms following anion-exchange chromatography. Among them, polysaccharide PK3 not only has a high content of fucose and uronic acid, but also has a strong activity to stimulate murine macrophage cells and increase their phagocytosis rate up to 170%. This study demonstrates that diatom AQ4 is an important bioresource for the production of bioactive polysaccharides.

NMR characterization and evaluation of antibacterial and antiobiofilm activity of organic extracts from stationary phase batch cultures of five marine microalgae (Dunaliella sp., D. salina, Chaetoceros calcitrans, C. gracilis and Tisochrysis lutea)

The chemical composition of five marine microalgae (Dunaliella sp., Dunaliella salina, Chaetoceros calcitrans, Chaetoceros gracilis and Tisochrysis lutea) was investigated through nuclear magnetic resonance (NMR) spectroscopic study of the soluble material obtained by sequential extraction with hexane, ethyl acetate (AcOEt) and methanol of biomass from stationary phase cultures. Hexane extracted the major lipids present in the microalgae during the stationary phase of growth, which correspond to storage lipids. Triacylglycerols (TGs) were the only storage lipids produced by Dunaliella and Chaetoceros. In contrast, T. lutea predominantly stored polyunsaturated long-chain alkenones, with sterols also detected as minor components of the hexane extract. The molecular structure of brassicasterol was determined in T. lutea and the presence of squalene in this sample was also unequivocally detected. Monogalactosyldiacylglycerols (MGDGs) and pigments were concentrated in the AcOEt extracts. C. calcitrans and D. salina constituted an exception due to the high amount of TGs and glycerol produced, respectively, by these two strains. Chlorophylls a and b and β-carotene were the major pigments synthesized by Dunaliella and chlorophyll a and fucoxanthin were the only pigments detected in Chaetoceros and T. lutea. Information concerning the acyl chains present in TGs and MGDGs as well as the positional distribution of acyl chains on the glycerol moiety was obtained by NMR analysis of hexane and AcOEt extracts, with results consistent with those expected for the genera studied. Fatty acid composition of TGs in the two Dunaliella strains was different, with polyunsaturated acyl chains almost absent in the storage lipids produced by D. salina. Except in C. calcitrans, the polar nature of soluble compounds was inferred through the relative extraction yield using methanol as the extraction solvent. Glycerol was the major component of this fraction for the Dunaliella strains. In T. lutea 1,4/2,5-cyclohexanetetrol (CHT) and dimethylsulfoniopropionate (DMSP) preponderated. CHT was also the major polyol present in the Chaetoceros strains in which DMSP was not detected, but prominent signals of 2,3-dihydroxypropane-1-sulfonate (DHSP) were observed in the 1H NMR spectra of methanolic extracts. The presence of DHSP confirms the production of this metabolite by diatoms. In addition, several other minor compounds (digalactosyldiacyglycerols (DGDGs), sulphoquinovosyldiacylglycerols (SQDGs), amino acids, carbohydrates, scyllo-inositol, mannitol, lactic acid and homarine) were also identified in the methanolic extracts. The antibacterial and antibiofilm activities of the extracts were tested. The AcOEt extract from C. gracilis showed a moderate antibiofilm activity.

Molecular recognition of the beta-glucans laminarin and pustulan by a SusD-like glycan-binding protein of a marine Bacteroidetes

Marine bacteria catabolize carbohydrate polymers of algae, which synthesize these structurally diverse molecules in ocean surface waters. Although algal glycans are an abundant carbon and energy source in the ocean, the molecular details that enable specific recognition between algal glycans and bacterial degraders remain largely unknown. Here we characterized a surface protein, GMSusD from the planktonic Bacteroidetes-Gramella sp. MAR_2010_102 that thrives during algal blooms. Our biochemical and structural analyses show that GMSusD binds glucose polysaccharides such as branched laminarin and linear pustulan. The 1.8 Å crystal structure of GMSusD indicates that three tryptophan residues form the putative glycan-binding site. Mutagenesis studies confirmed that these residues are crucial for laminarin recognition. We queried metagenomes of global surface water datasets for the occurrence of SusD-like proteins and found sequences with the three structurally conserved residues in different locations in the ocean. The molecular selectivity of GMSusD underscores that specific interactions are required for laminarin recognition. In conclusion, our findings provide insight into the molecular details of β-glucan binding by GMSusD and our bioinformatic analysis reveals that this molecular interaction may contribute to glucan cycling in the surface ocean.

A β-glucan from Durvillaea Antarctica has immunomodulatory effects on RAW264.7 macrophages via toll-like receptor 4

We studied the mechanisms underlying the immunostimulatory effects of aβ-1,3/1,6-glucan (BG136) from Durvillaea Antarctica. Our data showed that BG136 promoted the activation of MAPKs and NF-κB signaling pathways and cytokines production. BG136 did not increase MCP-1 or NO production or phosphorylation of NF-κB and MAPK in TLR4 siRNA knockdown cells, indicating that BG136 activates macrophages through TLR4. Flow cytometry analysis and confocal experiment showed that BG136 bound to TLR4 expressed on RAW264.7 macrophage cells surface. The affinity of BG136 for TLR4 was determined using Surface Plasmon Resonance (SPR) (KD: 4.51 × 10-6M). Altogether, our results showed that BG136 activates RAW264.7 cells by binding to TLR4 and then triggering TLR4-mediated signaling pathways to promote cytokines secretion.

Accurate Quantification of Laminarin in Marine Organic Matter with Enzymes from Marine Microbes

Marine algae produce a variety of glycans, which fulfill diverse biological functions and fuel the carbon and energy demands of heterotrophic microbes. A common approach to analysis of marine organic matter uses acid to hydrolyze the glycans into measurable monosaccharides. The monosaccharides may be derived from different glycans that are built with the same monosaccharides, however, and this approach does not distinguish between glycans in natural samples. Here we use enzymes to digest selectively and thereby quantify laminarin in particulate organic matter. Environmental metaproteome data revealed carbohydrate-active enzymes from marine flavobacteria as tools for selective hydrolysis of the algal β-glucan laminarin. The enzymes digested laminarin into glucose and oligosaccharides, which we measured with standard methods to establish the amounts of laminarin in the samples. We cloned, expressed, purified, and characterized three new glycoside hydrolases (GHs) of Formosa bacteria: two are endo-β-1,3-glucanases, of the GH16 and GH17 families, and the other is a GH30 exo-β-1,6-glucanase. Formosa sp. nov strain Hel1_33_131 GH30 (FbGH30) removed the β-1,6-glucose side chains, and Formosa agariphila GH17A (FaGH17A) and FaGH16A hydrolyzed the β-1,3-glucose backbone of laminarin. Specificity profiling with a library of glucan oligosaccharides and polysaccharides revealed that FaGH17A and FbGH30 were highly specific enzymes, while FaGH16A also hydrolyzed mixed-linked glucans with β-1,4-glucose. Therefore, we chose the more specific FaGH17A and FbGH30 to quantify laminarin in two cultured diatoms, namely, Thalassiosira weissflogii and Thalassiosira pseudonana, and in seawater samples from the North Sea and the Arctic Ocean. Combined, these results demonstrate the potential of enzymes for faster, stereospecific, and sequence-specific analysis of select glycans in marine organic matter.IMPORTANCE Marine algae synthesize substantial amounts of the glucose polymer laminarin for energy and carbon storage. Its concentrations, rates of production by autotrophic organisms, and rates of digestion by heterotrophic organisms remain unknown. Here we present a method based on enzymes that hydrolyze laminarin and enable its quantification even in crude substrate mixtures, without purification. Compared to the commonly used acid hydrolysis, the enzymatic method presented here is faster and stereospecific and selectively cleaves laminarin in mixtures of glycans, releasing only glucose and oligosaccharides, which can be easily quantified with reducing sugar assays.

Diatom-Specific Oligosaccharide and Polysaccharide Structures Help to Unravel Biosynthetic Capabilities in Diatoms

Diatoms are marine organisms that represent one of the most important sources of biomass in the ocean, accounting for about 40% of marine primary production, and in the biosphere, contributing up to 20% of global CO₂ fixation. There has been a recent surge in developing the use of diatoms as a source of bioactive compounds in the food and cosmetic industries. In addition, the potential of diatoms such as Phaeodactylum tricornutum as cell factories for the production of biopharmaceuticals is currently under evaluation. These biotechnological applications require a comprehensive understanding of the sugar biosynthesis pathways that operate in diatoms. Here, we review diatom glycan and polysaccharide structures, thus revealing their sugar biosynthesis capabilities.

Preliminary characterization, antioxidant properties and production of chrysolaminarin from marine diatom Odontella aurita

A new chrysolaminarin, named CL2, with a molecular mass of 7.75 kDa, was purified from the marine diatom, Odontella aurita, using DEAE-52 cellulose anion-exchange chromatography and Sephadex G-200 gel-filtration chromatography. The monosaccharide and structural analysis revealed that CL2 was a glucan mainly composed of glucose, which was linked by the β-d-(1→3) (main chain) and β-d-(1→6) (side chain) glycosidic bond, demonstrated by infrared spectroscopy (IR) and nuclear magnetic resonance (NMR). The antioxidant activity tests revealed that the CL2 presented stronger hydroxyl radical scavenging activity with increasing concentrations, but less was effective on reducing power analysis and scavenging 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical. The influences of nitrogen concentration and light intensity on chrysolaminarin production of O. aurita were further investigated in a glass column photobioreactor, and a record high chrysolaminarin productivity of 306 mg L-1 day-1 was achieved. In conclusion, the chrysolaminarin CL2 from O. aurita may be explored as a natural antioxidant agent for application in aquaculture, food and pharmaceutical areas.

Dietary polysaccharide extracts of Agaricus brasiliensis fruiting bodies: chemical characterization and bioactivities at different levels of purification

Polysaccharides of the European strain of A. brasiliensis were obtained by hot water extraction and ethanol precipitation (HWPE I) of fruiting bodies, and further purified by dialysis (HWPE II) and pronase incubation (PPE). These polysaccharides consisted mainly of (1→6)-β-d-glucans. PPE was free of proteins and polyphenols as demonstrated by quantitative assays and NMR profiling. They showed a clear IFN-γ inducing activity in human PBMCs, which suggests these polysaccharides to have proinflammatory effects. Treatment by β-glucosidase caused the polysaccharides to be degraded into smaller fragments and at the same time increased their IFN-γ inducing activity in PBMCs fourfold. In vitro, PPE showed a dose-dependent inhibition of the proliferation of the human leukemia Jurkat cell. At 100μg/mL the cells' viability was decreased by appr. 51% compared to the control. EPR spin trapping demonstrated a high antioxidative activity against ^•OH and ^•O2^- radicals of HWPE I and PPE. Further, the results of the antioxidant assays indicated that antioxidant activity against ^•OH radicals in the Fenton system was achieved through scavenging or through chelating iron mechanisms. The good immunomodulating and antioxidative properties of A. brasiliensis polysaccharide extract obtained by hot water extraction and ethanol precipitation make it suitable for everyday use as an inexpensive dietary supplement.

Marennine, promising blue pigments from a widespread Haslea diatom species complex

In diatoms, the main photosynthetic pigments are chlorophylls a and c, fucoxanthin, diadinoxanthin and diatoxanthin. The marine pennate diatom Haslea ostrearia has long been known for producing, in addition to these generic pigments, a water-soluble blue pigment, marennine. This pigment, responsible for the greening of oysters in western France, presents different biological activities: allelopathic, antioxidant, antibacterial, antiviral, and growth-inhibiting. A method to extract and purify marennine has been developed, but its chemical structure could hitherto not be resolved. For decades, H. ostrearia was the only organism known to produce marennine, and can be found worldwide. Our knowledge about H. ostrearia-like diatom biodiversity has recently been extended with the discovery of several new species of blue diatoms, the recently described H. karadagensis, H. silbo sp. inedit. and H. provincialis sp. inedit. These blue diatoms produce different marennine-like pigments, which belong to the same chemical family and present similar biological activities. Aside from being a potential source of natural blue pigments, H. ostrearia-like diatoms thus present a commercial potential for aquaculture, cosmetics, food and health industries.

Solid-State 13C CP MAS NMR Spectroscopy as a Tool for Detection of (1→3, 1→6)-β-D-Glucan in Products Prepared from Pleurotus ostreatus

The 13C CP/MAS NMR solid-state NMR technique was used to detect the presence of β-D-glucan and trace compounds in samples prepared from dried, naturally grown oyster mushroom (Pleurotus ostreatus) and commercially available products of dried, specially cultivated oyster mushroom and β-D-glucan isolated from this mushroom. The NMR spectra of all samples displayed signals typical for (1→3, 1→6)-β-D-glucan; however, signals which could be assigned to other trace compounds—(1→3)-α-glucan, chitin, and proteins—were also observed in the spectra. The amount of trace compounds was negligible in the commercially available products.

Soluble β-1,3/1,6-glucan in seaweed from the southern hemisphere and its immunomodulatory effect

Five types of macroalgae from the southern hemisphere were analysed for the presence of β-1,3/1,6-glucan and its immunostimulant properties. We were able to extract soluble β-1,3/1,6-D-glucan from Durvillaea antarctica (Chamisso) Hariot (DA). The morphology of the brown algae influenced extraction, and the highest percentage of β-glucan was found in the fronds. The content of β-glucan in the stipes and holdfast was on average 33% and <5%, respectively, of that in the fronds. A simple laboratory extraction process was developed. A highly pure water-soluble polysaccharide, mainly composed of glucose residues, was obtained with a dominant average molecular weight of 6.9 kDa. NMR spectroscopy confirmed the polysaccharide structure to be of β-1,3/1,6-glucan type, comprising a β-1,3-glucan backbone and 21% degree of branching of β-1,6-glucan side chains. Mouse cells were exposed to four DA extract concentrations in water (50, 100, 250 and 500 μg/mL) and no adverse effects on survival were noted. Remarkably, the β-glucan induced a 16.9% increase in activated CD19+ B lymphocytes compared with the control sample. The optimal concentration for maximum activity was 100 μg DA extract/mL.

New insights into the structure of (1→3,1→6)-β-D-glucan side chains in the Candida glabrata cell wall

β-Glucan is a (1→3)-β-linked glucose polymer with (1→6)-β-linked side chains and a major component of fungal cell walls. β-Glucans provide structural integrity to the fungal cell wall. The nature of the (1-6)-β-linked side chain structure of fungal (1→3,1→6)-β-D-glucans has been very difficult to elucidate. Herein, we report the first detailed structural characterization of the (1→6)-β-linked side chains of Candida glabrata using high-field NMR. The (1→6)-β-linked side chains have an average length of 4 to 5 repeat units spaced every 21 repeat units along the (1→3)-linked polymer backbone. Computer modeling suggests that the side chains have a bent curve structure that allows for a flexible interconnection with parallel (1→3)-β-D-glucan polymers, and/or as a point of attachment for proteins. Based on these observations we propose new approaches to how (1→6)-β-linked side chains interconnect with neighboring glucan polymers in a manner that maximizes fungal cell wall strength, while also allowing for flexibility, or plasticity.

New insight into silica deposition in horsetail (Equisetum arvense)

BACKGROUND

The horsetails (Equisetum sp) are known biosilicifiers though the mechanism underlying silica deposition in these plants remains largely unknown. Tissue extracts from horsetails grown hydroponically and also collected from the wild were acid-digested in a microwave oven and their silica 'skeletons' visualised using the fluor, PDMPO, and fluorescence microscopy.

RESULTS

Silica deposits were observed in all plant regions from the rhizome through to the stem, leaf and spores. Numerous structures were silicified including cell walls, cell plates, plasmodesmata, and guard cells and stomata at varying stages of differentiation. All of the major sites of silica deposition in horsetail mimicked sites and structures where the hemicellulose, callose is known to be found and these serendipitous observations of the coincidence of silica and callose raised the possibility that callose might be templating silica deposition in horsetail. Hydroponic culture of horsetail in the absence of silicic acid resulted in normal healthy plants which, following acid digestion, showed no deposition of silica anywhere in their tissues. To test the hypothesis that callose might be templating silica deposition in horsetail commercially available callose was mixed with undersaturated and saturated solutions of silicic acid and the formation of silica was demonstrated by fluorimetry and fluorescence microscopy.

CONCLUSIONS

The initiation of silica formation by callose is the first example whereby any biomolecule has been shown to induce, as compared to catalyse, the formation of silica in an undersaturated solution of silicic acid. This novel discovery allowed us to speculate that callose and its associated biochemical machinery could be a missing link in our understanding of biosilicification.

Polysaccharides present in cultivated Teloschistes flavicans symbiosis: comparison with those of the thallus

The chemical structures of polysaccharides present in aposymbiotically cultured myco- and photobionts of the lichen Teloschistes flavicans were determined, in order to compare them with those previously found in the intact thallus. The mycobiont was cultured on a solid Lilly and Barnett medium and the resulting colonies were freeze dried, defatted, and their polysaccharides were extracted successively with 2%, 10% and 30% aq. KOH, each at 100 degrees C. The extracts were neutralized (HOAc) and fractionated, giving rise to three homogeneous fractions, PFSK2 from 2% KOH, which contained a (1-->4),(1-->6)-linked alpha-glucan (1:1 ratio, pullulan), fraction PK10 from 10% KOH extraction, which was a linear (1-->3)-linked linear beta-glucan (laminaran), and fraction PK30 from 30% KOH extraction, being a branched (1-->3),(1-->6)-linked beta-glucan. The photobiont (Trebouxia sp. de Puymaly) was cultured in liquid nutrient medium, and after purification, a linear (1-->5)-linked beta-galactofuranan was characterized. The galactofuranan and the laminaran were not present in the symbiotic thallus, in contrast to the glucans, showing that the mycobiont alone produces them without participation of the photobiont.

A model for carbohydrate metabolism in the diatom Phaeodactylum tricornutum deduced from comparative whole genome analysis

BACKGROUND

Diatoms are unicellular algae responsible for approximately 20% of global carbon fixation. Their evolution by secondary endocytobiosis resulted in a complex cellular structure and metabolism compared to algae with primary plastids.

METHODOLOGY/PRINCIPAL FINDINGS

The whole genome sequence of the diatom Phaeodactylum tricornutum has recently been completed. We identified and annotated genes for enzymes involved in carbohydrate pathways based on extensive EST support and comparison to the whole genome sequence of a second diatom, Thalassiosira pseudonana. Protein localization to mitochondria was predicted based on identified similarities to mitochondrial localization motifs in other eukaryotes, whereas protein localization to plastids was based on the presence of signal peptide motifs in combination with plastid localization motifs previously shown to be required in diatoms. We identified genes potentially involved in a C4-like photosynthesis in P. tricornutum and, on the basis of sequence-based putative localization of relevant proteins, discuss possible differences in carbon concentrating mechanisms and CO(2) fixation between the two diatoms. We also identified genes encoding enzymes involved in photorespiration with one interesting exception: glycerate kinase was not found in either P. tricornutum or T. pseudonana. Various Calvin cycle enzymes were found in up to five different isoforms, distributed between plastids, mitochondria and the cytosol. Diatoms store energy either as lipids or as chrysolaminaran (a beta-1,3-glucan) outside of the plastids. We identified various beta-glucanases and large membrane-bound glucan synthases. Interestingly most of the glucanases appear to contain C-terminal anchor domains that may attach the enzymes to membranes.

CONCLUSIONS/SIGNIFICANCE

Here we present a detailed synthesis of carbohydrate metabolism in diatoms based on the genome sequences of Thalassiosira pseudonana and Phaeodactylum tricornutum. This model provides novel insights into acquisition of dissolved inorganic carbon and primary metabolic pathways of carbon in two different diatoms, which is of significance for an improved understanding of global carbon cycles.

Mixed-linkage (1-->3,1-->4)-beta-D-glucan is a major hemicellulose of Equisetum (horsetail) cell walls

Mixed-linkage (1-->3,1-->4)-beta-d-glucan (MLG) is a hemicellulose reputedly confined to certain Poales. Here, the taxonomic distribution of MLG, and xyloglucan, especially in early-diverging pteridophytes, has been re-investigated. Polysaccharides were digested with lichenase and xyloglucan endoglucanase (XEG), which specifically hydrolyse MLG and xyloglucan, respectively. The oligosaccharides produced were analysed by thin-layer chromatography (TLC), high-pressure liquid chromatography (HPLC) and alkaline peeling. Lichenase yielded oligo-beta-glucans from all Equisetum species tested (Equisetum arvense, Equisetum fluviatile, Equisetum scirpoides, Equisetum sylvaticum and Equisetum xtrachyodon). The major product was the tetrasaccharide beta-glucosyl-(1-->4)-beta-glucosyl-(1-->4)-beta-glucosyl-(1-->3)-glucose (G4G4G3G), which was converted to cellotriose by alkali, confirming its structure. Minor products included G3G, G4G3G and a nonasaccharide. By contrast, poalean MLGs yielded G4G3G > G4G4G3G > nonasaccharide > dodecasaccharide. No other pteridophytes tested contained MLG, including Psilotum and eusporangiate ferns. No MLG was found in lycopodiophytes, bryophytes, Chara or Nitella. XEG digestion showed that Equisetum xyloglucan has unusual repeat units. Equisetum, an exceedingly isolated genus whose closest living relatives diverged > 380 million years ago, has evolved MLG independently of the Poales. Equisetum and poalean MLGs share basic structural motifs but also exhibit clear-cut differences. Equisetum MLG is firmly wall-bound, and may tether neighbouring microfibrils. It is also suggested that MLG acts as a template for silica deposition, characteristic of grasses and horsetails.