Physiological and Photosynthetic ToxiCity of Thallium in Synechocystis sp. PCC6803

Abstract

The physiological and photosynthetic toxiCity mechanism of monovalent thallium, Tl(I), in a cyanobacteria, Synechocystis sp. PCC6803, was examined based on a series of batch culture experiments, determination of the pigments content and measurements of photosynthetic activities under the metal exposure conditions. Results showed that micro-molar level of Tl(I) drastically inhibit its growth, then 50% inhibitory concentration (IC₅₀) was approximately 1 μM. An acclimating incubation with 0.5 μM Tl(I) for 72 h bring no significant changes in IC₅₀ of thallium for growth. Chlorophyll a and phycobiliproteins content per cell basis decreased by 71% and 94% during 72 h incubation with 2.5 μM Tl(I), respectively. Results from pigments determination suggested that metabolic defect was rose by thallium exposure in Synechocystis sp. PCC6803. Then, to investigate the effect of thallium on energy generation process, acute dose-response of Tl(I) on photosynthetic O₂ evolution activities were measured. No effect on net photosynthetic O₂ evolution activity per chlorophyll basis was observed in 1 mM and below Tl(I) exposure, while 20 mM Tl(I) decrease the activity by 60%. Furthermore, 20 mM thallium did not affect 1,4-benzoquinone dependent PSII activity. These photosynthetic 50% inhibitory doses of thallium were approximately 2,000-fold higher than IC₅₀ of growth. Thus, photosynthetic energy metabolism did not constitute a limiting factor of growth under the thallium exposure. These results suggested that substance metabolic defect and/or NADP reducing processes could be the main process involved in thallium toxiCity in Synechocystis sp. PCC6803.

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