nutritional status, and the particular phase of the cell cycle during which the 

 experiment is conducted - to name but a few obvious determinants of 

 physiological status — must affect its sensitivity to trace metals. Batch culture 

 experiments which, so far, have been used principally for metal toxicity 

 studies, have inherent restrictions to resolve the importance of these 

 physiological factors. Toxicity studies in continuous phytoplankton cultures 

 promise to be enlightening in this respect; they also promise to accentuate the 

 difficulties in controlling precisely the chemistry of the system. 



It is hoped that the conceptual framework presented here will help in 

 designing and interpreting experiments where physiological responses to trace 

 metal toxicity are clearly assessed, distinctly from purely chemical effects in 

 the growth medium. It is also hoped that this study will help to increase 

 phytoplankton physiologists' awareness of the important chemical processes 

 which can affect their studies. It is, for example, surprising that so little 

 attention has been paid to the possible importance of phosphate speciation in 

 nutrient uptake experiments. Understanding the ecology of phytoplankton 

 requires detailed resolution of the cells' physiological responses to the total 

 aquatic chemistry of their environment. 



ACKNOWLEDGlVIENTS 



We thank S.W. Chisholm for her critical review of the manuscript and R.C. 

 Selman for her excellent job in typing the manuscript. This work was funded 

 by National Science Foundation grant no. DES75-15023, Environmental 

 Protection Agency grant no. R-803738 and the office of Sea Grant in the 

 National Oceanic and Atmospheric Administration grant no. 04-6-158-4407. 



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