Thiol and exopolysaccharide production in a cyanobacterium under heavy metal stress

The effects of Ni, Cu and Hg on thiol and exopolysaccharide synthesis in Nostoc spongineforme has been investigated. LD(50)s for Ni, Cu and Hg based on the specific growth rate, were 8, 4 and 0.2 mu M, respectively. The presence of Ni was most favourable to thiol production (6d) at 1, 2 and 5 mu M as the average value reached 43.37 mu mol/g dry wt over the control cells (22.5 mu mol/g dry wt). Cu in the same concentration range, favoured the process (average 33.64 mu mol/g dry wt). The thiol level in cells dosed with Hg (0.2 mu M, LD50) was similar to that in the control sets. Nevertheless, lower concentrations of Hg (0.15 mu M) produced a rise in thiol to 31.12 mu mol on 6d. A time course study on thiol biosynthesis at the respective LD50 concentrations of Ni, Cu or Hg indicated that with the less toxic cations more time was required to induce thiol biosynthesis with ultimate hyperproduction while the more toxic cations reduced the lag as well as the final thiol level. The effectiveness of cations, therefore, followed the order Ni > Cu > Hg while that of toxicity was Hg > Cu > Ni. Exopolymer systhesis was also metal-specific as the maxima corresponded to 80, 26 and 48 mu g glucose equivalent/mg dry wt for Cu (2 mu M), Ni (5 mu M) and Hg (0.05 mu M), respectively relative to the control (25 mu g glucose equivalent). The sequence Cu > Hg > Ni, therefore, presents a contrast with those for thiol production. These data form the first report on the metal-specific nature of thiol and exopolymer production in a cyanobacterium. (C) 1999 Elsevier Science Ltd. All rights reserved.