Cadmium Toxicity to Microcystis aeruginosa PCC 7806 and Its Microcystin-Lacking Mutant

Abstract

The adverse effects of microcystin (MC) produced by cyanobacteria have drawn considerable attention from the public. Yet it remains unclear whether MC confers any benefits to the cyanobacteria themselves. One suggested function of MC is complexation, which may influence the bioaccumulation and toxicity of trace metals. To test this hypothesis, we examined Cd toxicity to wild-type Microcystis aeruginosa PCC 7806 (WT) and its MC-lacking mutant (MT) under nutrient-enriched (+NP), phosphorus-limited (-P), and nitrogen-limited (-N) conditions. The accumulation of Cd and the biochemical parameters associated with its detoxification [total phosphorus (TP), inorganic polyphosphate (Poly-P), and glutathione (GSH) in the cells as well as intra- and extra-cellular carbohydrates] were quantified. Although the –P cyanobacteria accumulated less Cd than their +NP and –N counterparts, the different nutrient-conditioned cyanobacteria were similarly inhibited by similar free ion concentration of Cd in the medium ([Cd2+]F). Such good toxicity predictability of [Cd2+]F was ascribed to the synchronous decrease in the intracellular concentrations of Cd and TP. Nevertheless, Cd toxicity was still determined by the intracellular Cd to phosphorus ratio (Cd/P), in accordance with what has been reported in the literature. On the other hand, the concentrations of TP, Poly-P, and carbohydrates went up, but GSH concentration dropped down with the enhancement of [Cd2+]F, indicating their association with Cd detoxification. Although the inactivation of MC peptide synthetase gene had some nutrient and Cd concentration dependent effects on the parameters above, both cyanobacterial strains showed the same Cd accumulation ability and displayed similar Cd sensitivity. These results suggest that MC cannot affect metal toxicity either by regulating metal accumulation or by altering the detoxification ability of the cyanobacteria. Other possible functions of MC need to be further investigated.

A Review of the Effect of Trace Metals on Freshwater Cyanobacterial Growth and Toxin Production

“…Cyanotoxins may therefore be produced in response to high trace metal concentrations as a means of detoxification [125]. Huang et al [126] observed the effects of toxic levels of cadmium on Microcystis aeruginosa and found no evidence that microcystin can affect metal toxicity by regulating metal accumulation or by directly assisting in the detoxification.…”

Sequestration and Distribution Characteristics of Cd(II) byMicrocystis aeruginosaand Its Role in Colony Formation

“…Among the tested concentrations, 0.4 mg L −1 Cd(II) exhibited the strongest inhibitory effects, with a 4-day IR of 74.752% and a 10-day IR of 99.992% ( Figure 2 ). It is documented that M. aeruginosa PCC 7806 was susceptible to trace metal toxicity [ 17 ], while a M. aeruginosa strain isolated in the Czech Republic had high tolerance to 5.0–10.0 mg L −1 Cd(II) [ 18 ]. Therefore, it is possible that different strains of M. aeruginosa have different sensitivities to Cd(II)-induced toxicity.…”

“…Use of different strains of M. aeruginosa might lead to a longer adsorption process (10 min) [ 24 ]. In our experiment, there was also a slow adsorption process following the initial 5 min rapid adsorption, suggesting that Cd(II) was removed via interactions with functional groups on the cell surface [ 8 , 17 ].…”