2021 Volume 60 Issue 1 Pages 235-240
Reactive oxygen species (ROS) formation and subsequent damage in Synechococcus elongatus PCC 7942 (S. elongatus) cells after contact with metal ions in aqueous solutions were studied. Percentage ratios of divalent metal ions in aqueous solutions were used to mimic the metal ratios in pure copper (Cu), brass (C2600) and nickel silver (C7701 and C7521). The interactions between the generated ROS and cell components such as DNA and polyphosphate bodies (PPBs) in S. elongatus cells were observed by fluorescence microscopy, and the localization of ROS was confirmed at the ultra–structural level by staining cells with 3,3’–diaminobenzidine hydrochloric acid (DAB), which was oxidized by ROS and subsequently DAB–osmium black deposits were produced during post fixation with osmium tetroxide. Co–localization of elements including Cu, Zn, and P was observed by EDX elemental mapping. TEM observation of DAB treated cells successfully showed the locations of ROS accumulation on and in metal ion treated cyanobacterial cells. 0.3% of divalent Cu ions (Cu2+) caused excessive production of ROS on the cell wall, penetration of ROS into the cell, and DNA damage. When 0.13% of divalent Zn ions (Zn2+) were present in the solution in addition to 0.3% Cu2+, DNA structures were protected in a compacted shape, although ROS formed inside the cells to some extent. That phenomenon also occurred in the presence of both Zn2+ and divalent Ni ions (Ni2+) in the solution in addition to Cu2+, but only when the percentage of Zn2+ was higher than a certain level. When cyanobacteria came in contact with 0.09% of both Zn2+ and Ni2+ and 0.3% Cu2+, DNA was severely damaged although ROS formation was reduced. This leads to the hypothesis that DNA in cyanobacteria cells is possibly damaged by two different mechanisms under metal ion stress : 1) by metal ions inducing an excessive amount of ROS and/or 2) by some direct effect of the metal ions with less production of ROS. After the metal ion treatment, Cu and Zn co–localized with P at some sites. This observation indicates the ability of inorganic PPBs to attract metal ions. The presence of PPBs in bacteria cells may thus reduce the antibacterial effect of treatment by metal ion solutions and on certain Cu–based alloy surfaces.
