How a Taxonomically-Ambiguous Cyanobiont and Vanadate Assist in the Phytoremediation of Cadmium by Azolla pinnata: Implications for CKDu

We employed scientific tools to investigate the ex situ phytoremediation of cadmium by Azolla pinnata. Azolla pinnata was capable of efficient sequestration of cadmium up to a concentration of 1 ppm, though with a visibly high “physiological cost”. The sequestration of cadmium (1 ppm) was strongly reduced after 24 hours, in Azolla plants pre-treated with the gram-negative antibiotic erythromycin (60 µg/l), suggesting that the cyanobacterial population was important for phytoremediation. Only the co-treatment of 1 ppm cadmium with 1 ppm vanadate, showed significantly higher phytoremediation (P<0.05) compared to the “cadmium+erythromycin” treatment. The phytoremediation of Cadmium by the Azolla-Nostoc symbiosis was significantly (p<0.05) improved by the addition of citrate at 10 ppm in the presence of 1 ppm vanadate, compared to the 1 ppm cadmium only treatment. We hypothesize that citrate acting either as “vanadophores” or working as a cofactor in the Homocitrate Synthase enzyme, facilitates remediation of cadmium. When phylogeny was inferred using Homocitrate Synthases, the cyanobiont was approximated to a taxonomical twilight zone between Nostoc and Anabaena, although showing more proximity to the Anabaena cluster. It is proposed here that the cyanobacterial contribution appears to be crucial for the ability of Azolla pinnata to efficiently remediate cadmium and a “helping hand” appears to be provided by a vanadate dependent mechanism, which is likely to be nitrogen fixation. The association between vanadate-assisted phytoremediation by Azolla pinnata and the heightened bioavailability of vanadium in CKDu endemic areas, could serve as a vital stepping stone in developing a biological solution to CKDu.