Thiol and exopolysaccharide production in a cyanobacterium under heavy metal stress

The effects of Ni, Cu and Hg on thiol and exopolysaccharide synthesis in No stoc spongineforme has been investigated. LD(50)s for Ni, Cu and Hg based o n the specific growth rate, were 8, 4 and 0.2 mu M, respectively. The prese nce of Ni was most favourable to thiol production (6d) at 1, 2 and 5 mu M a s the average value reached 43.37 mu mol/g dry wt over the control cells (2 2.5 mu mol/g dry wt). Cu in the same concentration range, favoured the proc ess (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, lo wer concentrations of Hg (0.15 mu M) produced a rise in thiol to 31.12 mu m ol 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 m ore time was required to induce thiol biosynthesis with ultimate hyperprodu ction while the more toxic cations reduced the lag as well as the final thi ol level. The effectiveness of cations, therefore, followed the order Ni > Cu > Hg while that of toxicity was Hg > Cu > Ni. Exopolymer systhesis was a lso 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), resp ectively relative to the control (25 mu g glucose equivalent). The sequence Cu > Hg > Ni, therefore, presents a contrast with those for thiol producti on. These data form the first report on the metal-specific nature of thiol and exopolymer production in a cyanobacterium. (C) 1999 Elsevier Science Lt d. All rights reserved.