Variability of the phenolic profile in the diatom Phaeodactylum tricornutum growing under copper and iron stress

Fourteen phenolic compounds were identified and quantified in extracts derived from in vitro culture of Phaeodactylum tricornutum diatom growing in natural seawater (control) and in natural seawater containing the following added copper (Cu) and iron (Fe) metals: Cu(II) (315 nmol L-1 and 790 nmol L-1) and Fe(III) (900 nmol L-1). The cell concentration was 471% for the last day of culture (expressed as a percentage of the control) for diatoms exposed to iron, while diatoms exposed to copper showed cell densities of 80% and 52.5% at concentrations of 315 nmol L-1 and 790 nmol L-1, respectively. Each extract revealed the presence of 14 phenolic compounds, with the exceptions of gallic acid, which was detected only in the iron-enriched diatoms, and quercetin, which was not detected in the control diatom exudates. Significant differences in the phenolic profiles were found depending on the metal added to the culture media. These differences seem to be the response to the different needs of diatoms exposed to copper and iron concentrations. These results show that increasing levels of metal result in a general increase in levels of total phenolic contents in the cells of P. tricornutum. The great increase in phenolic compounds in the cells at the highest copper concentration was 2.4 times higher than in the control, which may reflect the involvement of these compounds in protection against copper toxicity. All the extracts revealed radical scavenging activity against the stable radical 1,1-diphenyl-2-picrylhydrazyl, with the most active extracts from the copper enrichments.