Effects of photosynthesis on the accumulation of Mn and Fe by Phaeocystis colonies

The significance of Mn and Fe accumulation by Phaeocystis colonies and its control by photosynthesis were investigated by performing incubation assays with radiotracers (Mn-54, Fe-59 and C-14). Experiments were conducted on p ure cultures of Phaeocystis globosa and on natural communities collected du ring a P. pouchetii bloom in the Balsfjord (subarctic Norwegian fjord) and a P. globosa bloom in the Southern Eight of the North Sea. Results indicate significant accumulation of Mn and Fe in the cultured colonies, as previou sly shown for Mn. Most of the Mn and Fe accumulation occurred in the mucila ginous matrix of the colonies, and the intracellular assimilation represent ed only similar to 10% of the total uptake of these trace elements. These e xperiments demonstrated that photosynthesis largely governed the uptake of Mn by the colonies but only slightly affected the accumulation of Fe. The p ositive linear relationships observed for the Balsfjord samples between the Mn uptake and the C fixation in the light suggests photosynthetic control of dissolved Mn removal to the Phaeocystis colonies. As had been predicted in earlier studies, the increase in pH and dissolved oxygen observed around and inside the colonies during the photosynthetic activity of the cells co uld significantly decrease Mn solubility and enhance Mn oxidation rate. How ever, these changes would not affect significantly the precipitation of Fe according to the thermodynamic considerations. In the highly turbid waters of the North Sea, the removal of Mn and Fe is increased by both inorganic a nd organic suspended particles, with no significant effect of photosynthesi s on the overall uptake.