Earthworms produce phytochelatins in response to arsenic

Phytochelatins are small cysteine-rich non-ribosomal peptides that chelate soft metal and metalloid ions, such as
cadmium and arsenic. They are widely produced by plants and microbes; phytochelatin synthase genes are also
present in animal species from several different phyla, but there is still little known about whether these genes are
functional in animals, and if so, whether they are metal-responsive. We analysed phytochelatin production by direct
chemical analysis in Lumbricus rubellus earthworms exposed to arsenic for a 28 day period, and found that arsenic
clearly induced phytochelatin production in a dose-dependent manner. It was necessary to measure the
phytochelatin metabolite concentrations directly, as there was no upregulation of phytochelatin synthase gene
expression after 28 days: phytochelatin synthesis appears not to be transcriptionally regulated in animals. A further
untargetted metabolomic analysis also found changes in metabolites associated with the transsulfuration pathway,
which channels sulfur flux from methionine for phytochelatin synthesis. There was no evidence of biological
transformation of arsenic (e.g. into methylated species) as a result of laboratory arsenic exposure. Finally, we
compared wild populations of earthworms sampled from the field, and found that both arsenic-contaminated and
cadmium-contaminated mine site worms had elevated phytochelatin concentrations.