Comparison of cell size, chlorophyll fluorescence and cadmium (Cd2+) bioaccumulation between wild-type and mutant strains of Chlamydomonas reinhardtii upon exposure to Cd2+

Abstract

Background: Heavy metal contamination presents a constant threat to biological systems. Simultaneously, heavy metals have become one of the major contaminants in the aquatic ecosystems. In this regard, the investigation of heavy metal-tolerance genes in algae is relevant. Chlamydomonas reinhardtii is a unicellular green alga, and an excellent model organism used in heavy metal studies. In C. reinhardtii, a novel gene designated as Cia7, was hypothesized to play a role in heavy metal homeostasis due to CIA7’s conserved cysteine-residue motif. This study compared two strains of C. reinhardtii, cc4425, the wild-type with the functional CIA7 protein and cc5013, the mutant strain with the disrupted cia7- gene. The hypothesis was that the expression of Cia7 contributes to an increased cadmium (Cd)-tolerance in C. reinhardtii. The Cd-tolerance would be described by physiological markers of microalgae health, and by intracellular accumulation of the metal. Methods: The objectives of this study were (1) to compare chlorophyll fluorescence and cell size in cc4425 and cc5013 exposed to Cd2+, and (2) to compare Cd2+ bioaccumulation in cc4425 and cc5013 strains in different growth media. Flow cytometry, and inductively coupled plasma optical emission spectrometry (ICP-OES) analysis were performed. Results: There was no significant statistical difference in Cd2+ bioaccumulation between the two strains, cc4425 and cc5013, regardless of growth media. However, a statistically significant difference in Cd2+ bioaccumulation (p<0.0001) was determined between the media (with acetate and without acetate).  The cia7- mutant, cc5013 was found to be more susceptible to a Cd2+-induced decrease in chlorophyll fluorescence and had a reduced cell size compared to cc4425, the wild-type strain. Conclusions: These observed differences between the strains suggest that CIA7’s biological activity could play a direct or indirect role in increasing Cd tolerance in C. reinhardtii.