MICRONUTRIENT UPTAKE AND TRANSLOCATION BY MACROCYSTIS PYRIFERA (PHAEOPHYTA)1

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Abstract
Radionuclides of the micronutrients (Mn, Co, Cu, Zn, Mo, and I) of Macrocystis pyrifera (L.) C. Ag. and the nonnutrient Ni were concentrated in sieve tube sap. Micronutrient uptake rates of mature blade tissue were determined using the appropriate radionuclide in artificial seawater. Cellular uptake was distinguished from free space uptake by the use of suitable wash solutions. The uptake rates were then transposed to conditions in the sea, with several assumptions being made. Micronutrient content was determined for mature blade tissue before and after treatment to remove extracellular micronutrients. A mathematical description of frond growth was developed from previously published data and used to derive equations relating micronutrient assimilation (tissue incorporation) and uptake by fronds, both expressed in mg·d−1, to frond length. Calculations revealed that at a certain frond length xi assimilation equaled uptake for Mn, Co, and Fe. Uptake was greater than assimilation for fronds longer than xi and less than assimilation for fronds shorter than xi. This indicated that Mn, Co, and Fe translocation was required from fronds longer than xi to fronds shorter than xi. The large assimilation values at xi for Mn and Co further indicate that these elements, at times, may limit Macrocystis growth. Calculations also showed that Cu, Zn, Mo, I, and Ni uptake was greater than their assimilation for all frond lengths, which suggested that these elements are transported to other tissues for storage (e.g. in holdfast) or release (e.g. from frond apices).