Biochemical basis of heavy metal induced stress tolerance in the N₂ fixing cyanobacterium <i>Anabaena doliolum</i>

Abstract

The effect of heavy metals (Cd and Cu) on the nitrogen fixing cyanbacterium, Anabaena doliolum was observed in the present study. To explore the survival strategy of the test cyanobacterium, Chl/CAR content, protein content, antioxidative defense system ( SOD, APX and GR) as well as biochemical fractionation (carbohydrates, lipids, protein, DNA and RNA) were studied. Increasing concentrations of metals inhibited the growth and survival significantly; chlorophyll and carotenoid content were found inhibited with increase in concentration of metals. Among the antioxidative enzymes, SOD and APX were increased with the increase in concentration of both the metals, whereas Catalase and Glutathione reductase were decreased at higher dose of Cd and Cu. APX played a major role for scavenging H₂O₂ rather than CAT. Results revealed decrease in all parameters with the duration of time. The role of metal induced PC in offering tolerance to UV-B was confirmed by measuring lipid peroxidation and antioxidant defense system of the cyanobacterium treated with Cd and UV-B as well as in the Cd pretreated cells of A.dolium exposed to UV-B. Lipid peroxidation (measured in terms of MDA content) as well as SOD and APX were found to be less induced, thus showing less oxidative damage incase of interactive treatment when applied separately. However, CAT and GR which showed sensitivity at higher dose of both the stresses were found to be induced. Thus Cd appears to antagonize the effect of UV-B in test cyanobacterium. To know the actual reason for the antagonism, PC concentration was measured in the cells with and without BSO (a potent inhibitor of phytochelatin synthase) pretreatment. The results emphasized that the extent of antagonism was reverted in the BSO pretreated cells than the normal BSO non- treated cells. Nevertheless, the PC content was found to be more in case of Cd + UV-B than the individually treated cells, but the PC was more or less completely inhibited after BSO pretreatment in all the cases. The above finding was also visualized on the SDS-PAGE. Therefore this study showed that Cd induced PC has role in UV-B tolerance.

African Journal of Clinical and Experimental Microbiology Vol. 8 (1) 2007: pp. 8-22