Comparative Analysis of Microalgae's Physiological Responses to Fibrous and Layered Clay Minerals

Abstract

Microalgae interact with mineral particles in an aqueous environment, yet how clay minerals affect physiological processes in algal cells remains unexplored. In this study, we compared the effects of palygorskite (Pal) and montmorillonite (Mt), which respectively represent fibrous and layered clay minerals, on the physiological processes of Chlamydomonas reinhardtii. It was observed that C. reinhardtii responded differently to the treatments of Pal and Mt. The Pal particles bound tightly to and even inserted themselves into cells, resulting in a significant decrease in cell numbers from 27.35 to 21.02 × 10⁷ mL^-1. However, Mt was only loosely attached to the cell surface. The photosynthesis in the algal cells was greatly inhibited by Pal, with the rETR_max significantly reduced from 103.80 to 56.67 μmol electrons m^-2s^-1 and the downregulation of IF2CP, psbH and OHP1, which are key genes involved in photosynthesis. In addition, Pal reduced the quantities of proteins and polysaccharides in extracellular polymeric substances (EPSs) and the P uptake by C. reinhardtii when the P level in the culture was 3.15 mg/L. However, no significant changes were found regarding the above EPS components or the amount of P in algal cells upon the addition of Mt. Together, the impacts of fibrous Pal on C. reinhardtii was more profound than those of layered Mt.

Keywords: 3D-EEM; RNA-Seq; clay minerals; microalgae; physiology.

Figure 1

The effects of montmorillonite (Mt) and palygorskite (Pal) on (A) the maximum quantum yields of the photosystem II reaction center and (B) electron transport rate. The vertical bar represents the standard deviation of triplicate samples.

Figure 2

(A) P uptake of Chlamydomonas reinhardtii cultured at 3.15 mg/L and 31.5 mg/L P concentrations. (B) The number of counts (per 10⁷ cells) of C. reinhardtii after four days’ incubation. * p < 0.05. (C) CO₂ emission (per 10¹⁰ cells) of C. reinhardtii. The error bars represent the standard deviation based on three parallel experiments. * p < 0.05; NS, not significant. Different uppercase letters indicate significant (p < 0.05) differences among various mineral concentrations at 3.15 mg/L P concentrations while different lowercase letters indicate significant (p < 0.05) differences at 31.5 mg/L P concentrations.

Figure 3

Three-dimensional excitation–emission matrix fluorescence spectra of the supernatant under different treatments. (A) The four fluorescent components (C1–C4) identified by PARAFAC analysis; (B): relative abundances of fluorescent components under different treatments.

Figure 4

The content of proteins (A), polysaccharides (B) and photosynthetic pigments (C) after four days of incubation. Different lower case letters indicate significant (p < 0.05) differences among various mineral treatments.

Figure 5

ATR−IR spectra of the C. reinhardtii cells incubated under 200, 500 Mt/Pal.

Figure 6

(A,D) SEM images of Mt (A) and Pal (D). (B,C,E,F): SEM imaging on the algae cells under 200 (B,E), 500 (C,F) Mt/Pal.

Figure 7

TEM images of C. reinhardtii cells incubated with 200 (A), 500 (B) mg/L Mt and 200 (C), 500 (D) mg/L Pal of the medium in all the treatments.

Figure 8

Main metabolic pathway network related to differentially expressed proteins (DEPs). The color bar indicates the degree of up–down adjustment of DEPs (C for CK; P for Pal (200 mg/L); M for Mt (200 mg/L)). IF2CP, CPLS1 genes: related to the transformation of precursor substances into chloroplasts; CAB4, CAB7 and CHLG genes: related to chlorophyll synthesis; SAMC1 and odhA genes: related to cellular respiration; psbH and OHP1 genes: related to Photosystem II synthesis.