Heavy metals introduced to aquatic habitats through industry, agriculture, and medical waste have been found to be toxic in many species. Two metal ions, Cu2+ and Ni2+ are being studied for their effects on Synechocystis PCC 6803. It was expected that decreased culture density and pigmentation would be observed just before the lethal concentration was reached. It has been known that these ions interfere with normal functions of essential nutritional ions, such as Calcium, however the combined effects of these metals have not been extensively studied and information on the topic is scarce. This study will focus on the mechanisms of metal toxicity and whether the combined effects are additive, antagonistic, or synergistic.
The Laccase- mutant (Lac-), which is missing a laccase enzyme, was exposed to increasing concentrations of CuCl2 and NiCl2. The mutant was maintained with using a Spectinomycin resistance gene. Cultures were placed in BG-11 liquid culture and on BG-11 plates treated with incrementally increasing concentrations of two metals of interest, CuCl2 and NiCl2. At 2µM CuCl2 and NiCl2 in liquid culture, methanol extraction was used to determine variation in chlorophyll and carotenoid levels.
Results/Conclusions
On solid media, CuCl2 and NiCl2 concentrations became lethal between 30 µM and 90 µM for all strains. In liquid media, the lethal concentration of both metals is just above 3 µM. Lac- appeared to show a higher tolerance for a mixture of the two metals than for either CuCl2 or NiCl2 alone. It also appears more susceptible to the toxic effects in the initial stages of growth. Lac- showed decreased chlorophyll and carotenoid level when exposed to each metal individually. However, when exposed to a combination of the metals, the pigmentation levels showed a less severe decrease.
Other mutants currently being examined for Cu2+ and Ni2+ tolerance are HT3 (mutant with a modified PS II protein), PfsR (strain lacking PfsR, a regulatory protein), DspA (sensory kinase mutant), isiA-PfsR (double mutant lacking both the iron stress inducible protein and PfsR), and PfsR-HIS-HT3 (double mutant with a His-tagged PfsR and HT3). Further study will be conducted on changes in protein expression and photosynthetic activity due to near-lethal metal treatment. The effects of heavy metal toxicity studies on Synechocystis PCC 6803 may help to determine the strain’s efficacy in bioremediation or as a bioindicator in water quality analysis. These studies can also benefit the evaluation of primary production in marine environments.