Characterization of zinc stress response in Cyanobacterium Synechococcus sp. IU 625

The ability of cyanobacteria to survive many environmental stress factors is a testament to their resilience in nature. Of these environmental stress factors, overexposure to zinc is important to study since excessive zinc intake can be a severe hazard. Zinc toxicity in freshwater has been demonstrated to affects organisms such as invertebrates, algae and cyanobacteria. Cyanobacteria which possess increased resistance to zinc have been isolated. It is therefore important to elucidate the mechanism of survival and response to determine what factors allow their survival; as well as any remediation implications they may have. To characterize the effects of zinc in freshwater cyanobacteria, we investigated the response of Synechococcus sp. IU 625 (S. IU 625) over 29 days to various concentrations (10, 25, and 50 mg/L) of ZnCl₂. S. IU 625 was shown to be tolerant up to 25 mg/L ZnCl₂ exposure, with 10 mg/L ZnCl₂ having no outward physiological change and 50 mg/L ZnCl₂ proving lethal to the cells. To determine a potential mechanism Inductive Coupled Plasma–Mass Spectrometry (ICP–MS) and RNA-seq analysis were performed on zinc exposed cells. Analysis performed on days 4 and 7 indicated that response is dose-dependent, with 10 mg/L ZnCl₂ exhibiting nearly all zinc extracellular, corresponding with upregulation of cation transport response. Whereas the 25 mg/L ZnCl₂ exhibited half of total zinc sequestered by the cells, which corresponds with the upregulation of sequestering proteins such as metallothionein and the downregulation of genes involved with ATP synthesis and phycobilisome assembly. These analyses were combined with growth monitoring, microscopy, quantitative polymerase chain reaction (qPCR) and flow cytometry to present a full spectrum of mechanisms behind zinc response in S. IU 625.