Ectocarpus siliculosus growing on the marine plant Zostera  © Akira Peters, Station Biologique Roscoff

Ectocarpus siliculosus growing on the marine plant Zostera
© Akira Peters, Station Biologique Roscoff

Brown algae contain phlorotannins, aromatic (phenolic) compounds that are unique in the plant kingdom. As natural antioxidants, phlorotannins are of great interest for the treament and prevention of cancer and inflammatory, cardiovascular and neurodegenerative diseases.

Researchers at France’s Végétaux marins et biomolécules (CNRS/UPMC) laboratory at the Station biologique de Roscoff, in collaboration with two colleagues at the Laboratoire des sciences de l’Environnement MARin (Laboratory of Marine Environment Sciences) in Brest (CNRS/UBO/IFREMER/IRD) have recently described the key step in the production of these compounds in Ectocarpus siliculosus, a small brown alga model species. The study also revealed the specific mechanism of an enzyme that synthesizes phenolic compounds with commercial applications.

These findings have been patented and should make it easier to produce the phlorotannins presently used as natural extracts in the pharmaceutical and cosmetic industries. The results have also been published online on the site of the journal The Plant Cell (“Structure/Function Analysis of a Type III Polyketide Synthase in the Brown Alga Ectocarpus siliculosus Reveals a Biochemical Pathway in Phlorotannin Monomer Biosynthesis”).

Until now, extracting phlorotannins from brown algae for use in industry was a complex process, and the biosynthesis pathways of these compounds were unknown. By studying the first genome sequenced from a brown alga, the team in Roscoff identified several genes homologous to those involved in phenolic compound biosynthesis in terrestrial plants.

Among these genes, the researchers found that at least one was directly involved in the synthesis of phlorotannins in brown algae. They then inserted these genes into a bacterium, which thus produced a large quantity of the enzymes that could synthesize the desired phenolic compounds.

One of these enzymes, a type III polyketide synthase (PKS III), was studied in detail and revealed how it produces phenolic compounds. PKS III is able, for example, to synthesize phloroglucinol (notably used in antispasmodic drugs and in explosives) and other phenolic compounds with commercial applications.

Besides this mechanism, results revealed that the compounds had other biological functions in the acclimation and adaptation of brown algae to salinity stress. Knowledge of these biosynthesis pathways would allow researchers to uncover the signaling mechanisms that regulate this metabolism. It would also be useful for understanding the biological and ecological functions of these compounds in other brown algae that are already used commercially.

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