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.
Highlights
Microcystis is a freshwater cyanobacterium responsible for harmful algal blooms in rivers, lakes, and reservoirs
The bioavailability and toxicity of these metals to the freshwater green alga Chlamydomonas reinhardtii are still in accordance with the conventional free ion activity model (FIAM) [5]. The toxicity of these metals is determined by their free ion concentration in the medium
The case might be different for the MC-producing cyanobacteria, considering the fact that most MC remains inside the cells instead of being released into the environment [6]
Summary
Microcystis is a freshwater cyanobacterium responsible for harmful algal blooms in rivers, lakes, and reservoirs. One of the major concerns regarding Microcystis bloom is its ability to produce the toxin microcystin (MC) [1] This toxin is a cyclic nonribosomal heptapeptide and has at least 80 congeners. MC was found not to influence the speciation of four metal ions (Cd, Cr, Cu, and Zn) in the experimental medium [4]. Under this condition, the bioavailability and toxicity of these metals to the freshwater green alga Chlamydomonas reinhardtii are still in accordance with the conventional free ion activity model (FIAM) [5]. How metal toxicity (if any) may alter under this condition and whether their findings can be extrapolated to other metals need to be further investigated
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