Toxic Effect of Metal Doping on Diatoms as Probed by Broadband Terahertz Time-Domain Spectroscopy

Abstract

The global marine environment is increasingly affected by human activities causing climate change, eutrophication, and pollution. These factors influence the metabolic mechanisms of phytoplankton species, such as diatoms. Among other pollutant agents, heavy metals can have dramatic effects on diatom viability. Detailed knowledge of the interaction of diatoms with metals is essential from both a fundamental and applicative point of view. To this aim, we assess terahertz time-domain spectroscopy as a tool for sensing the diatoms in aqueous systems which mimic their natural environment. Despite the strong absorption of terahertz radiation in water, we show that diatoms can be sensed by probing the water absorption enhancement in the terahertz range caused by the water-diatom interaction. We reveal that the addition of metal dopants affects this absorption enhancement, thus enabling the monitoring of the toxic effects of metals on diatoms using terahertz spectroscopy. We demonstrate that this technique can detect the detrimental effects of heavy metals earlier than conventional methods such as microscopy, enzymatic assays, and molecular analyses aimed at assessing the overexpression of genes involved in the heavy metal-stress response.

Keywords: bioremediation; broadband THz-TDS spectroscopy; diatoms; heavy metal pollution; marine ecotoxicology; solvating water.

Figure 1

The principle of THz-TDS: a sub-picosecond THz pulse passes through the sample, here formed of diatoms in water. The transmitted pulse is attenuated and delayed because of the absorption coefficient and the refractive index of the sample. Therefore, it carries information on these two parameters at all the frequencies composing the pulse bandwidth.

Figure 2

The THz absorption spectra of SBCM (blue curve) and diatoms in SBCM (red curve). In the inset, the corresponding refractive indexes are shown. The shaded areas indicate the measurement statistical error with a confidence level of 66% (one σ). The shaded areas overlap with the marker size up to 3.5 THz. On the upper x-axis we report the light wavenumber in cm⁻¹.

Figure 3

THz absorption variation as compared to the ‘baseline’ spectrum for diatom in SBCM, undoped (red line) and doped (green and purple lines). The dot-dashed blue line represents the zero level.

Figure 4

Panel (a) shows an example of the power spectrum of a THz pulse propagating in nitrogen, transmitted through the most absorbing sample, and detected by electro-optic sampling in LAPC (reported in the logarithm scale). Note that the signal is up to 5 THz greater than the background noise. In panel (b) the corresponding THz pulse is displayed. In panel (c) the scheme of our THz spectrometer is shown: BS, beam-splitter; CH, triggered chopper; L, lens; BBO, SHG crystal; PL, plasma; Si, silicon wafer; M, mirror; DL, delay line; VPLC, variable-path liquid cell; LAPC, electro-optic crystal; QW, quarter-waveplate; WP, Wollaston prism; BPs, balanced photodiodes.