This study investigated the role of a novel metal-dependent catalase (Npun_R4582) that reduces hydrogen peroxide in the cyanobacterium Nostoc punctiforme. Quantitative real-time PCR showed that npun_R4582 relative mRNA levels were upregulated by over 16-fold in cells treated with either 2 mu M added Co, 0.5 mu M added Cu, 500 mu M Mn, 1 mu M Ni, or 18 mu M Zn. For cells treated with 60 mu M H2O2, no significant alteration in Npun_R4582 relative mRNA levels was detected, while in cells treated with Co, Cu, Mn, Ni, or Zn and 60 mu M peroxide, relative mRNA levels were generally above control or peroxide only treated cells. Disruption or overexpression of npun_R4582 altered sensitivity to cells exposed to 60 mu M H2O2 and metals for treatments beyond the highest viable concentrations, or in a mixed metal solution for Npun_R4582(-) cells. Moreover, overexpression of npun_R4582 increased cellular peroxidase activity in comparison with wild-type and Npun_R4582(-) cells, and reduced peroxide levels by over 50%. The addition of cobalt, manganese, nickel, and zinc increased the capacity of Npun_R4582 to reduce the rate or total levels of peroxide produced by cells growing under photooxidative conditions. The work presented confirms the function of NpunR4582 as a catalase and provides insights as to how cells reduce potentially lethal peroxide levels produced by photosynthesis. The findings also show how trace elements play crucial roles as enzymatic cofactors and how the role of Npun_R4582 in hydrogen peroxide breakdown is dependent on the type of metal and the level available to cells.