A role for manganese in superoxide dismutases and growth of iron-deficient diatoms

We have discovered that coastal and oceanic diatoms require more manganese (Mn) to grow in iron (Fe) -deficient than in Fe-sufficient seawater. At low inorganic concentrations, like those of the open sea, Fe and Mn can thus colimit Thalassiosira pseudonana and T. oceanica so that maximum rates of cell division are achieved only when both resources are added simultaneously to cultures. Colimited diatoms amended with either Fe or Mn alone show unique physiological responses, which implies that the observed interaction between Fe and Mn is not caused by a substitution of one metal for the other. Iron deficiency increases the Mn quota of T. pseudonana by three times compared with controls and enhances the production of reactive oxygen species by 1.7 times in T. weissflogii. Both diatoms respond to this oxidative stress by increasing the activities of the antioxidant enzyme superoxide dismutase (SOD). The Mn content of the SODs increases by 1.8 to 2.8 times when Fe is limiting, which suggests that the SODs contain Mn and may account for part of the observed increase in the Mn quota. Such an increased biochemical requirement may elevate the Mn content of low Fe diatoms, and possibly other phytoplankton, resulting in high Mn: Fe ratios in surficial particulate matter in Fe-limited regions of the sea.