Photosynthetic pigment laser-induced fluorescence indicators for the detection of changes associated with trace element stress in the diatom model species Phaeodactylum tricornutum

This work reports changes on cell number, growth rate, trace element content, chlorophyll a (Chl a) and carotenoid concentrations, and laser-induced fluorescence (LIF) spectra of Phaeodactylum tricornutum exposed to Co, Ni, Cu, Zn, Cd, Hg, Pb, and a mixture of all elements combined (Mix). The total levels of trace elements associated with the cells were significantly higher in the exposed than in control ones. Concomitantly, specific cell growth was significantly lower in exposed P. tricornutum, suggesting that trace elements affected the microalgae physiology. The LIF emission spectra showed two typical emission bands in red (683698 nm) and far-red (725-730 nm) regions. Deviations in LIF spectra and changes in F685/F735 ratio were investigated as indicators of trace element-induced changes. Fluorescence intensity emitted by exposed microalgae decreased in far-red region when compared to control cells, suggesting Chl a damage and impairment of pigment biosynthesis pathways by trace elements, confirmed by Chl a and carotenoid concentration decrease. Significant increase in F685/F735 ratio was detected for all elements except Zn and more accentuated for Co, Hg, and Mix. Significant deviations in wavelength emission maxima in red region were also more significant (between 8 and 13 nm) for Co, Hg, and Mix. Growth changes agreed with deviations in LIF spectra and F685/F735 ratio, supporting their applicability as indicators. This study clearly shows F685/F735 ratio and the deviations in wavelength emission maxima as adequate trace element stress indicators and P. tricornutum as a promising biomonitor model species. LIF-based techniques can be used as time-saving, highly sensitive, and effective alternative tool for the detection of trace element stress, with potential for remote sensing and trace element contamination screening in marine coastal areas.