The mechanisms of arsenic detoxification by the green microalgae chlorella vulgaris

The mechanisms of arsenic interaction with the green microalga Chlorella vulgaris (C. vulgaris) and the potential for its bio-remediation from water were investigated. This was made possible by the development of an improved arsenic extraction from C. vulgaris, leading to successful glutathione and phytochelatins (GSH/PC) complex speciation analysis with 71.1% efficiency. The response of C. vulgaris when challenged by As(III), As(V) and dimethylarsinic acid (DMA) was assessed through experiments on adsorption, efflux and speciation of arsenic (reduction, oxidation, methylation and chelation with GSH/PC). At high phosphate concentration (1.62 mM of PO4-3), poor adsorption of As(V) led to low intracellular uptake; at low phosphate concentration (3.2 μM of PO4-3), an increase in the level of free thiols was observed as well as a moderate decrease in intracellular pH with no evidence for signals of oxidative stress. Chlorella vulgaris cells did not produce any As-GS/PC complex when exposed to As(V). This may indicate that a reduction step is needed for As(V) complexation with GSH/PC. Chlorella vulgaris cells formed DMASV-GS upon exposure to DMA. The formation of this complex in vivo has only been reported once in Brassica oleracea plants. This complex is perhaps a fragment of a bigger molecule and thus part of another detoxification mechanism since its formation was not related to the concentration of DMA in media or the exposure time. It was found that As(III) triggers the formation of arsenic complexes with PC and homophytochelatins (hPC) and their compartmentalisation to vacuoles. It is the first time that, as a result of the newly developed extraction method using sonication, such intact complexes have been identified in C. vulgaris exposed to arsenic and their hPC complexes have been reported in any organism. The potential of C. vulgaris to bio-remediate arsenic from water is highly selective and effective for the more toxic As(III) (for human life) without the potential hazard to reduce As(V) to As(III). This was possible to assess because of the following empirical observations:
• At low phosphate (3.2 μM of PO4-3) and in the presence of As(V), C. vulgaris are not likely to grow and be efficient at bio-remediating arsenic.
• At high phosphate (1.62 mM of PO4-3) and in the presence of As(V), C. vulgaris are highly likely to grow but are not likely to be efficient at bio-remediating arsenic. However the potential to transform As(V) into more toxic As(III) is very low.
• Under any phosphate concentration and in the presence of As(III), C. vulgaris has high potential to bio-remediate arsenic, by storing As(III) into the cell biomass while retaining significantly high growth rates.