Diatoms synthesize sterols by inclusion of animal and fungal genes in the plant pathway

Abstract

Diatoms are ubiquitous microalgae that have developed remarkable metabolic plasticity and gene diversification. Here we report the first elucidation of the complete biosynthesis of sterols in the lineage. The study has been carried out on the bloom-forming species Skeletonema marinoi and Cyclotella cryptica that synthesise an ensemble of sterols with chemotypes of animals (cholesterol and desmosterol), plants (dihydrobrassicasterol and 24-methylene cholesterol), algae (fucosterol) and marine invertebrates (clionasterol). In both species, sterols derive from mevalonate through cyclization of squalene to cycloartenol by cycloartenol synthase. The pathway anticipates synthesis of cholesterol by enzymes of the phytosterol route in plants, as recently reported in Solanaceae. Major divergences stem from reduction of Δ24(28) and Δ24(25) double bonds which, in diatoms, are apparently dependent on sterol reductases of fungi, algae and animals. Phylogenetic comparison revealed a good level of similarity between the sterol biosynthetic genes of S. marinoi and C. cryptica with those in the genomes of the other diatoms sequenced so far. Sterols are vital components of all eukaryotic cells where they modulate structure and function of membranes and, as precursors of signaling molecules, they control growth and development. Plant sterols, commonly named phytosterols, are involved in morphogenesis, development, reproduction and stress response 1-3. Analogously, microalgal sterols showed critical physiological roles related to photosynthesis, growth, light response and fatty acid metabolism 4. Sterols are terpenes deriving from a complex process of polymerization of six isoprene units. In animals and fungi, the mevalonic acid (MVA) pathway is the only route for the biosynthesis of the isoprene units isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). In higher plants and algae, IPP and DMAPP derive either from the MVA pathway in the cytoplasm or the methylerythritol phosphate (MEP) pathway in the plastids 5. Without a clear cellular compartmentalization, both biochemical routes have been also reported in mosses and streptomyces. Sterol biosynthesis from IPP and DMAPP is generally taxa-specific and proceeds via lanosterol by lanosterol synthase (LSS) in nonphotosynthetic organisms (e.g. animals and fungi) or cycloartenol by cycloartenol synthase (CAS) in photosynthetic lineages (e.g. plants and algae) 6. Cholesterol is the major animal sterol but its biosynthesis in tomato has been recently shown to derive from a cycloartenol-dependent pathway composed of enzymes either shared with phytosterols or evolved from phytosterol biosynthetic genes 7. Furthermore, insects and lower eukaryotes, including marine invertebrates and a few microalgae, are suggested to convert phytosterols to cholesterol and vice versa 8-11. Diatoms represent an important component of the aquatic ecosystem 12,13 and form the largest biological group of marine phytoplankton 14,15. These microalgae are major global producers by contributing to...

Natural Products from Octocorals of the Genus Dendronephthya (Family Nephtheidae)

“…Moreover, a fatty acid mixture, showing the presence of saturated and unsaturated fatty acids with a chain length of C 12 to C 22 , being rich in C 16 and C 18 acids as active constituents in antifouling activity [97]. (Figure 5), β-sitosterol (stigmasta-5-en-3β-ol) (39) [48], and β-cholestanol (5α-cholestan-3βol) (107) [99] were identified and sterol 39 in this study [97] was found to contain 35% of a 24S epimer (clionasterol) (108) [100,101] (Figure 17). Sterol 39 had the highest antifouling activity among sterols 26, 39, and 107 [97].…”

“…Purification of the extract gave mixtures of sterols and fatty acids as active components. In the sterol mixture, there are several sterols, (24 S )-24-methylcholesta-5( E ),22- dien-3β-ol (= pincsterol) or (24 R )-24-methylcholesta-5( E ),22-dien-3β-ol (= brassicasterol) ( 106 ) [ 98 ], cholesterol ( 26 ) [ 30 ] ( Figure 5 ), β-sitosterol (stigmasta-5-en-3β-ol) ( 39 ) [ 48 ], and β-cholestanol (5α- cholestan-3β-ol) ( 107 ) [ 99 ] were identified and sterol 39 in this study [ 97 ] was found to contain 35% of a 24 S epimer (clionasterol) ( 108 ) [ 100 , 101 ] ( Figure 17 ). Sterol 39 had the highest antifouling activity among sterols 26 , 39 , and 107 [ 97 ] .…”

Semi-Quantitative Targeted Gas Chromatography-Mass Spectrometry Profiling Supports a Late Side-Chain Reductase Cycloartenol-to-Cholesterol Biosynthesis Pathway in Brown Algae

“…The biosynthetic pathway from cycloartenol to 24-methylcholest-22-enol shares many steps with the classical land plant biosynthesis pathway from cycloartenol to campesterol ( Desmond and Gribaldo, 2009 ), the main difference being that as for the pathway from cycloartenol to cholesterol, the second sterol-4-demethylation is inferred here to occur before the sterol-Δ 8 -Δ 7 -isomerization. The pathway leading to fucosterol may end up in a diatom-like way with a sterol-5-desaturation and a sterol-Δ 7 -reduction following a sterol-28-methylation leading to different epimers than in land plants ( Gallo et al, 2020 ). However, since the main biochemical requirement for sterol side chain alkylation at carbons 24 and 28 is the presence of a Δ 24(25) or Δ 24(28) double bond ( Nes, 2003 ), alternative routes may be possible from other precursors having those double bonds (dotted arrows on Figure 2 ).…”

“…The associated biosynthetic pathways are good models to study metabolic pathway evolution because the enzymes involved are well-known across eukaryotes and relatively conserved despite some variation ( Desmond and Gribaldo, 2009 ). Recently, several studies have enabled to better understand the history of this pathway in land plants ( Sonawane et al, 2016 ), diatoms ( Gallo et al, 2020 ), or red algae ( Belcour et al, 2020 ). However, a general model for sterol biosynthesis in brown algae – a vast diversified and independent eukaryotic lineage ( Bringloe et al, 2020 ) – is still lacking.…”

Bioinformatic Characterization of Sulfotransferase Provides New Insights for the Exploitation of Sulfated Polysaccharides in Caulerpa

“…Indeed, in Fiji, Samoa and Tonga the market value reached by the harvested alga is estimated about USD 270,000 per year resulting in a production of over 120 tonnes of fresh Caulerpa , which plays a crucial role in the economy of these countries [ 13 ]. In recent years, Caulerpa species may be highly economically useful as they can be readily used for the production of bio-compounds for nutraceutical, pharmaceutical and cosmetic purposes [ 4 , 5 , 14 , 15 , 16 , 17 ]. Caulerpa produces interesting metabolites, namely caulerpin, sulfated polysaccharides (SP), racemosin, alkaloids, xyloglucans and fatty acid derived products, which are highly desirable for commercial purposes, and are not fully commercially exploited yet [ 4 , 5 ].…”