Similarities and Differences in the Effects of Toxic Concentrations of Cadmium and Chromium on the Structure and Functions of Thylakoid Membranes in Chlorella variabilis

Abstract

Trace metal contaminations in natural waters, wetlands, and wastewaters pose serious threats to aquatic ecosystems—mainly via targeting microalgae. In this work, we investigated the effects of toxic amounts of chromium and cadmium ions on the structure and function of the photosynthetic machinery of Chlorella variabilis cells. To halt the propagation of cells, we used high concentrations of Cd and Cr, 50–50 mg L −1 , in the forms of CdCl 2 x 2.5 H 2 O and K 2 Cr 2 O 7 , respectively. Both treatments led to similar, about 50% gradual diminishment of the chlorophyll contents of the cells in 48 h, which was, however, accompanied by a small (~10%) but statistically significant enrichment (Cd) and loss (Cr) of ß-carotene. Both Cd and Cr inhibited the activity of photosystem II (PSII)—but with more severe inhibitions with Cr. On the contrary, the PsbA (D1) protein of PSII and the PsbO protein of the oxygen-evolving complex were retained more in Cr-treated cells than in the presence of Cd. These data and the higher susceptibility of P700 redox transients in Cr-treated cells suggest that, unlike with Cd, PSII is not the main target in the photochemical apparatus. These differences at the level of photochemistry also brought about dissimilarities at higher levels of the structural complexity of the photosynthetic apparatus. Circular dichroism (CD) spectroscopy measurements revealed moderate perturbations in the macro-organization of the protein complexes—with more pronounced decline in Cd-treated cells than in the cells with Cr. Also, as reflected by transmission electron microscopy and small-angle neutron scattering, the thylakoid membranes suffered shrinking and were largely fragmented in Cd-treated cells, whereas no changes could be discerned with Cr. The preservation of integrity of membranes in Cr-treated cells was most probably aided by high proportion of the de-epoxidized xanthophylls, which were absent with Cd. It can thus be concluded that beside strong similarities of the toxic effects of Cr and Cd, the response of the photosynthetic machinery of C. variabilis to these two trace metal ions substantially differ from each other—strongly suggesting different inhibitory and protective mechanisms following the primary toxic events.

Salt Stress Induces Paramylon Accumulation and Fine-Tuning of the Macro-Organization of Thylakoid Membranes in Euglena gracilis Cells

“…There was a difference between the RD values of the SANS measurements and that of the TEM images. A similar difference was also observed in the Cr-treated Chlorella variabilis cells (Zsiros et al, 2020 ). The difference can be explained by the different sample preparation processes; SANS provided information about the periodic organization of the thylakoid membranes gained in vivo without any fixation or staining but in D 2 O instead of H 2 O (Nagy et al, 2011 ).…”

“…The pigment composition of the cells was determined by high-performance liquid chromatography (HPLC), using a Shimadzu Prominence HPLC (Shimadzu Corp., Japan) system as described by Zsiros et al ( 2020 ). The pigments were identified according to their retention times and absorption spectra and quantified by the integrated chromatographic peak area recorded at the wavelength of maximum absorbance for each kind of pigments using the corresponding molar decadic absorption coefficient (Wright et al, 2005 ).…”

Image_2.TIF

“…There was a difference between the RD values of the SANS measurements and that of the TEM images. A similar difference was also observed in the Cr-treated Chlorella variabilis cells (Zsiros et al, 2020). The difference can be explained by the different sample preparation processes; SANS provided information about the periodic organization of the thylakoid membranes gained in vivo without any fixation or staining but in D 2 O instead of H 2 O (Nagy et al, 2011).…”

“…The pigment composition of the cells was determined by highperformance liquid chromatography (HPLC), using a Shimadzu Prominence HPLC (Shimadzu Corp., Japan) system as described by Zsiros et al (2020). The pigments were identified according to their retention times and absorption spectra and quantified by the integrated chromatographic peak area recorded at the wavelength of maximum absorbance for each kind of pigments using the corresponding molar decadic absorption coefficient (Wright et al, 2005).…”

Microalgae Improve the Photosynthetic Performance of Rice Seedlings (Oryza sativa L.) under Physiological Conditions and Cadmium Stress

“…Cadmium-induced changes on both the donor and the acceptor sides of PSII complex lead to an increase in the amount of inactive PSII centers [6,12]. The inhibition of the PSII activity is due to degradation of D1 protein of PSII complex, PsbO proteins of the oxygenevolving complex (OEC) and damage the light-harvesting complex of PSII [13,14]. It has been proposed that the Cd competitive binding to the essential Ca 2+ cofactor in the Mn 4 Ca cluster of OEC also leads to inhibition of the oxygen evolution [15].…”