| Summary: | The significance of Mn and Fe accumulation by Phaeocystis colonies and its control by photosynthesis were investigated by performing incubation assays with radiotracers ( 54 Mn, 59 Fe and 14 C). Experiments were conducted on pure cultures of Phaeocystis globosa and on natural communities collected during a P. pouchetii bloom in the Balsfjord (subarctic Norwegian fjord) and a P. globosa bloom in the Southern Bight of the North Sea. Results indicate significant accumulation of Mn and Fe in the cultured colonies, as previously shown for Mn. Most of the Mn and Fe accumulation occurred in the mucilaginous matrix of the colonies, and the intracellular assimilation represented only ~10% of the total uptake of these trace elements. These experiments demonstrated that photosynthesis largely governed the uptake of Mn by the colonies but only slightly affected the accumulation of Fe. The positive 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 could significantly decrease Mn solubility and enhance Mn oxidation rate. However, 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 and organic suspended particles, with no significant effect of photosynthesis on the overall uptake.
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