Pb(II)-phycoremediation mechanism using Scenedesmus obliquus: cells physicochemical properties and metabolomic profiling

Abstract

This study highlights the mechanisms of Pb(II)-phycoremediation using the Pb(II) tolerant strain of Scenedesmus obliquus. First, monitoring of cell growth kinetics in control and Pb(II)-doped medium revealed significant growth inhibition, while the analyses through flow cytometry and Zetasizer revealed no difference in cell viability and size. Residual weights of control and Pb(II)-loaded cells assessed by thermogravimetric analysis were 31.34% and 57.8%, respectively, indicating the uptake of Pb(II) into S. obliquus cells. Next, the use of chemical extraction to distinguish between the intracellular and extracellular uptake indicated the involvement of both biosorption (85.5%) and bioaccumulation (14.5%) mechanisms. Biosorption interaction of Pb(II) ions and the cell wall was confirmed using SEM-EDX, FTIR, zeta potential, zero-charge pH, and contact angle analyses. Besides, the biochemical characterization of control and Pb(II)-loaded cells revealed that the bioaccumulation of Pb(II) induces significant increases in the carotenoids and lipids content, while it decreases in the chlorophyll, carbohydrates, and proteins content. Finally, the metabolomic analysis indicated an increase in the relative abundance of fatty acid methyl esters, alkanes, aromatic compounds, and sterols. However, the alkenes and monounsaturated fatty acids decreased. Such metabolic adjustment may represent an adaptive strategy that prevents high Pb(II)-bioaccumulation in cellular compartments.

Keywords: Biochemical composition; Metabolomic profiling; Phycoremediation; Physicochemical properties; Scenedesmus obliquus.

Figure 1

(A) plots the TGA and DTG profile of the decomposition of controlled and Pb(II)-loaded cells. (B) illustrates the rate of bioaccumulation and biosorption of Pb (II) ions by living cells of S. obliquus.

Figure 2

Displays SEM image of (A) control cells; (B) Pb(II)-loaded cells at x8000 magnification. The selected X-ray spectra of control (C), Pb(II)-loaded cells (D), FTIR spectra of control and Pb(II)-loaded cells (E), ζ-potential of control (F) and Pb(II)-loaded cells (G). Zero-charge pH loading (H) of S. obliquus biomass in the pHi range of 3–10.

Figure 3

Shows growth kinetics (mean ± S.D, n = 3) (A), (B) illustrate the cytogram of the cells viability: (Black) cells auto-fluorescence canceled (Green) the negative control (96.07%), (Red) after Pb(II)-phycoremediation (90.10 %) and (Pink) heat-inactivated cells. Population moved in the FL₂ channel corresponding to the positive control (0.07 %). UL: Upper Left, UR: Upper Right, LL: Lower Left, LR: Lower Right. (D) displays the Pb(II) effect on the intensity particle size distribution of S. obliquus cells. (E) displays the contents of Na⁺, K⁺ and the ratios of Na/K in control and Pb(II)-loaded cells, the error bar indicates the standard error of the mean (n = 3).

Figure 4

Shows the autofluorescence of native pigments of control cells (A, C, E and G) and (B, D, F and H) and Pb(II)-loaded cells. Figure 4I illustrate the concentration of chlorophyll a (Ch-a), b (Ch-b), and total carotenoids (C x + c) in control (CTR) and Pb(II)-loaded cells. Figure 4 (J) shows the compositions of the total lipid, carbohydrate and protein in control and Pb(II)-loaded cells. Figure 4 (K) and (L) present NR staining dot plots of control and Pb(II)-loaded cells.

Figure 5

Indicates the level of: Σ alkanes, Σ alkenes, Σ sterols, Fatty acid methyl ester (FAME); monounsaturated fatty acids (MUFA); saturated fatty acids (SFA); Polyunsaturated fatty acids (PUFA); Aromatic compounds (Arom.); divers compounds (Divers)) in control (A) and Pb(II)-loaded (B) cells. (C) shows the distribution of the fatty acid profile (fatty acid methyl ester (FAME); monounsaturated fatty acids (MUFA); saturated fatty acids (SFA); polyunsaturated fatty acids (PUFA)), and (D) illustrates the metabolomic profiling of alkanes and alkenes (very long-chain alkanes (V.L.C); long-chain alkanes (L.C)) in control and Pb(II)-loaded cells.