Destroys superoxide anion radicals which are normally produced within the cells and which are toxic to biological systems. Catalyzes the dismutation of superoxide anion radicals into O2 and H2O2 by successive reduction and oxidation of the transition metal ion at the active site. May play a role in maintaining cell viability during the late-exponential and stationary phases of growth since it becomes a major source of activity under oxidative stress. Has a role in resisting external superoxide stress. Involved in acid tolerance and the acid-adaptive response. Mediates the derepression of perR regulon in the response to HOCl stress at low level of SOD activity (By similarity). 2 H(+) + 2 superoxide = H2O2 + O2 Binds 1 Mn(2+) or Fe(3+) ion per subunit. Homodimer. Can also form a heterodimer with SodA. Transcriptionally induced by externally generated superoxide stress in a manganese-dependent way. The presence of manganese increases SodA homodimer activity and simultaneously decreases SodM homodimer activity. This occurs primarily due to post-transcriptional effects, since the expression of the gene is independent of manganese availability in the absence of superoxide generating compounds. According to PubMed:11344148 the levels of SodM activity and sodM expression are growth-phase dependent, occurring most during the late-exponential and stationary phases. This response is also dependent on the level of aeration with highest activity occurring under high aeration. SodM expression under low-aeration growth conditions is most abundant during the late-exponential phase while under high-aeration growth conditions is highest during the stationary phase. Transcribed from a single sigmaA-type promoter (PM). Transcriptional data show an indirect repression of PM promoter by sigmaB which can also be involved in the post-transcriptional regulation of SodM homodimer activity. Belongs to the iron/manganese superoxide dismutase family.