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

Abstract

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.

Keywords: EPS; diatom; exopolysaccharides; glycan; microalgae; nucleotide sugars; polysaccharide.

Figure 1

Applications of diatom active compounds in human health and food supplements.

Figure 2

Ultrastructural organization of a diatom cell: transmission electron micrographs of Phaeodactylum tricornutum oval morphotype. The cells were embedded in LRW resin with 0.5% uranyl acetate in a methanol/Reynold’s lead citrate solution. (A) general overview of a P. tricornutum oval cell. Scale bar = 0.4 µm; (B) zoom of the cell wall. Scale bar = 50 nm. N: nucleus; V: vacuole; C: chloroplast; py: pyrenoid; EPS: exopolysaccharides.

Figure 3

Drawings of hypothetical structures of oligosaccharides found in insoluble cell wall polysaccharides after mild acid hydrolysis of Phaeodactylum tricornutum cell wall extracts. (A) 1,3-linked mannopyranose chains; (B) oligosaccharide fragments. Although hypothetical alpha-linkages are shown here, there is no clear evidence for either alpha- or beta-linkages [65].

Figure 4

β-chitin fibers of Thalassiosira sp. (A) Transmission electron micrograph shadowed with tanta-lum/tungsten (Ta/W). (B): N-acetylglucosamine sequence of the chemical structure of chitin. The image was recorded and kindly provided by Dr. H. Chanzy, CERMAV-CNRS, France.

Figure 5

Structural analysis of β(1,3) glucan, a food storage polysaccharide. ¹H NMR spectra of (A) laminarin from Saccharina latissima and (B) β(1,3) glucan (chrysolaminarin) extracted from Phaeodactylum tricornutum (400 MHz, 353 K). Chrysolaminarin contains fewer β(1,6) branching signals (4.5–4.6 ppm) than laminarin. The slightly higher reducing end signal at 5.26 ppm (α-anomer) in the chrysolaminarin spectrum can be attributed to a lower molecular weight or the absence of a mannitol residue at the reducing end.

Figure 6

Proposed N-glycosylation pathway in Phaeodactylum tricornutum. Sequences predicted in the P. tricornutum genome are shown in bold. ALG10 and glucosidase I genes have not been identified so far. The N-glycan structures presented in this figure are as given in Varki et al. [121]. ER: endoplasmic reticulum; DPM1: dolichol-phosphate mannosyl transferase; ALG: asparagine-linked glycosylation; PP-Dol: pyrophosphate dolichol; P-Dol: dolichol phosphate; OST: oligosaccharyl transferase; Asn: asparagine; UGGT: UDP-glucose glycoprotein glucosyl transferase; GnT: N-acetylglucosaminyl transferase; α-Man: α-Mannosidase; FuT: fucosyl transferase; Man-5 to Man-9: oligomannoside bearing 5 to 9 mannose residues.

Figure 7

Predicted nucleotide sugar metabolism in diatoms based on bioinformatics analyses of the genomes from Phaeodactylum tricornutum [129], Thalassiosira pseudonana [130], Fragilariopsis cylindrus [131] and Aureococcus anophagefferens [132].