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.
Highlights
Public health and environmental concerns from heavy metal contamination have increased in recent times
Cell size was determined as a function of the forward scatter signal (FSC)
Determination of cell size and chlorophyll content in cc4425 and cc5013 A statistically significant difference in chlorophyll fluorescence was observed between cc4425 and cc5013 (6,127 and 3,858 counts respectively) at 10 μM Cd (Table 1, Figure 1)
Summary
Public health and environmental concerns from heavy metal contamination have increased in recent times. Cadmium (Cd), chromium, lead, and mercury are among the priority metals of public health significance; this, in part, is due to their high degree of toxicity. These metals are systemic toxicants known to induce multiple organ damage, even at lower levels of exposure (Sevin et al, 2020; Tchounwou et al, 2012). 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. The cia7mutant, 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
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