Biological Oceanography, Volume 6, pp. 443-461 Printed in the UK. All rights reserved. 0196-5581/88/89 $3.00 + .00 Copyright © 198811989 Taylor & Francis Preparation and Chemistry of the Artificial Algal Culture Medium Aquil NEIL M. PRICE GAIL I. HARRISON JANET G. HERING ROBERTJ.HUDSON PASCALE M. V. NIREL BRIAN PALENIK FRAN<;OIS M. M. MOREL R. M. Parsons Laboratory Massachusetts Institute of Technology Cambridge, MA 02139 Abstract The culture medium Aquil has been designed for studying trace metal physiology in algae. »f? describe recent modifications in the preparation of Aquil and discuss processes that affect its trace metals and their physiological effects. The major changes in Aquil preparation are purification of the Chelex column to avoid contamination by chelating agents, use of alternative sterilization procedures, and increases in the concentration of trace metal buffers. During growth, phytoplankton take up trace metals, thus continuously reducing their concentrations in the medium. Algae can also modify the redox state and degree of organic complexation of trace metals through the direct and indirect activity of cell surface enzymes and the release of metabolites. Illumination of the culture medium necessary to promote photosynthesis also promotes a variety of photochemical reactions that alter the chemistry of the medium and maintain it in a state of disequilibrium. In particular, light absorption by FeEDTA leads to reduction of the iron and oxidation of the EDTA. Rapid reoxidation of Fe(II) leads to a high steady-state inorganic Fe(l/1) concentration. Slow coordination kinetics with chelating agents contribute to maintaining the disequilibrium conditions promoted by cellular and photochemical processes. Kinetic processes rather than pseudo-equilibrium conditions are now the focus in the study of trace metal-phytoplankton interactions. Keywords Aquil, phytoplankton culture, seawater medium. Gail I. Harrison's current address is the Harvard-MIT Division of Health Sciences and Tech- nology, MIT, Cambridge, MA. Janet G. Hering's current address is Institute for Water Resources and Water Pollution Con- trol (EAWAG), Dubendorf, Switzerland. Robert J. Hudson's current address is Tetra Tech, Inc., Hadley, MA. Pascale M. V. Nirel's current address is Trace Analytic, Buvelot 9, Switzerland. Brian Palenik's current address is the Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL. 443