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. 
ACKNOWLEDGMENTS 
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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