The Characterization of Metal Biotransformation Capabilities of Photosynthetic Microorganisms

Abstract

Industrial activity over the last two centuries have increased heavy metal contamination worldwide and has led to greater human interaction with these contaminants. Cadmium, copper and zinc are particularly common industrial effluents and are highly toxic in elevated concentrations. Due to the hazards of heavy metal pollution, there is a growing demand for cost effective and efficient means to remove heavy metals. Photoautotrophic microbes can biotransform heavy metals into the relatively biologically unavailable and insoluble metal-sulfides. By characterizing the sulfur assimilation pathways’ role in heavy metal biotransformation, more effective heavy metal bioremediation processes may be developed. Chlamydomonas reinhardtii, Synechoccocus leopoliensis and Cyanidioschyzon merolae were exposed to cadmium, copper, and zinc and their abilities to synthesize metal-sulfides were assessed. Sulfate, sulfite and cysteine were enriched in the respective media of each species to determine the role of sulfur metabolites in enhancing metal tolerance and metal-sulfide formation. The activities of cysteine desulfhydrase, serine acetyltransferase and O-acetylserine(thiol)lyase activity were analyzed under the stress of Cd(II), Cu(II) and Zn(II). C. merolae supplemented with sulfate yielded a significant increase in Cd(II), Cu(II), and Zn(II) tolerance, higher sulfide formation, and increased enzyme activity. C. reinhardtii and S. leopoliensis supplemented with sulfate yielded significantly more biomass than the sulfite and cysteine treatments when exposed to Cd(II), Cu(II) and Zn(II). In all species the addition of sulfite and cysteine was found to be detrimental to cell growth. C. reinhardtii had an average increase of 5-8 times the initial cysteine desulfhydrase and serine acetyltransferase activity for each metal treatment. Cd(II), Cu(II) and Zn(II) proved to have a limiting effect on S. leopoliensis’ enzyme activity. The potential limitations in the sulfur assimilation pathway will be discussed. C. merolae may prove to be a successful candidate for metal bioremediation. Utilizing supplemental sulfate nutrition may improve the rate of biotransformation in aerobic microbes.