The Uptake of Organic Solutes in Lake Water1

Basic changes are made in an existing method using radioactive organic compounds for measuring the uptake of organic solutes by planktonic microorganisms. The changes give greater experimental flexibility and enable the maximum uptake velocity, V, to be found. Measurements over a range of substrate concentrations appear to show uptake by at least two mechanisms, by a transport system that becomes saturated at very low substrate concentrations and by simple diffusion, which shows a fairly constant increase in velocity of uptake with increasing substrate concentration. Because of this, the technique of measuring uptake at a single substrate concentration to give “relative heterotrophic potential” is questioned. The best estimate of heterotrophic potential may be V, the maximum uptake velocity attainable by planktonic populations with a transport system functioning according to the Michaelis laws. It is suggested that bacterial populations are responsible for uptake by means of transport systems and that algal populations show uptake by diffusion.