Green alga closes wounds by means of enzymatic activation of a cross-linking protein

A small wound is painful, but is normally not a problem. Like all multicellular organisms, our bodies form new tissue around the damaged cells. Single-celled organisms cannot do this. In order to survive an injury, they have to quickly repair the cell itself. Large single-celled organisms, such as the green alga Caulerpa taxifolia, which can be several meters long, are especially at risk for injuries. This alga is a master of wound repair; ever since this invasive alga was brought from tropical waters to the Mediterranean and to the Pacific off the coast of North America, it has crowded out native species and caused great ecological damage.

"The success of this alga depends, among other things, on its unusual asexual reproductive strategy, which depends on its rapid cell healing," explains Georg Pohnert. "If the alga is torn, cell components are mixed. Just like a two- component glue, neutral components make up a quickly polymerizing mixture that seals off the resulting fragments by a gelatinous wound covering within seconds. Each cell fragment later acts as the starting point for new algal colonies."

Pohnert and his team at the Max Planck Institute for Chemical Ecology in Jena have now solved the puzzle of how this alga closes its wounds so quickly and effectively. Essentially, the only things Caulerpa needs for this feat are caulerpenin, a metabolite from the sesquiterpene class, and an esterase, an enzyme that splits ester compounds. When the alga is wounded, the esterase immediately springs into action, splitting off three molecular fragments and converting caulerpenin into a reactive 1,4-dialdehyde called oxytoxin 2 (An aldehyde group consists of a carbon atom bound to an oxygen atom by a double bond, and a hydrogen atom. The numbers indicate the positions of the aldehyde functional groups relative to one another).

Reactive dialdehydes, such as oxytoxin 2, attack proteins; they can use their two aldehyde groups to bind proteins into a regular polymeric network. "This is precisely what happens in the alga's wound-healing process," says Pohnert. "That this mechanism functions so reliably for the alga is a result of its very reactive esterase and the high concentration of caulerpenin, which may account for over 1.3% of its wet mass."

Other species of green algae also contain metabolites that resemble caulerpenin in their structure. Says Pohnert: "The principle we have described for wound repair in Caulerpa could thus be widespread among macroalgae.

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