Dieter Wolf‐Gladrow
Buoyancy control in phytoplankton is a fundamental process for oceanic cycling of elements. Previous studies suggested that the ionic composition of cell saps (cytosol plus vacuole) contribute to buoyancy control by plankton cells: replacement of heavy ions in seawater by lighter ions in the cell saps could lead to enough positive buoyancy to keep the cells within the euphotic zone. This hypothesis has stimulated a number of experimental and theoretical investigations on the densities of cell saps. Unfortunately, these studies often neglected the importance of electric charge balance within cells which renders results largely useless for estimating mass densities. In this paper I review previous work with respect to charge balance and correct some of the ensuing errors in a well-cited ( > 100 citations) paper in this field. Rather than estimating cell sap densities from ionic compositions I propose an alternate approach based on salt compositions which allows more robust scaling for solutions of high ionic strength. This method will be applied to a data set that needs only slight adjustment (within measurement uncertainty) to obey charge balance.