Response of cell surface pH to pCO₂ and iron limitation in the marine diatom Thalassiosira weissflogii

The physicochemical conditions immediately adjacent to the phytoplankton cell membrane, within the diffusion boundary layer, determine the speciation and reaction rates of biologically important nutrients such as iron and carbon dioxide. Over the past decade, several modeling efforts have attempted to calculate the supply of nutrients to phytoplankton surfaces using diffusion-reaction models. In these modeling efforts the pH within the cellular boundary layer is an important unknown and a direct measure of the cell surface pH is needed. We prepared and calibrated a new cell-bound, pH sensitive dye (a lectin-SNARF conjugate) that can be used to determine surface pH in living diatoms and applied the method to cultures grown under iron and carbon stress; conditions that are likely to elicit a pH response. We found that Thalassiosira weissflogii responded to iron limitation by reducing growth rates and increasing both the activity of phosphoenolpyruvate carboxylase (PEPCase) and the efflux of the organic acid malate, consistent with the iron-limited response of vascular plants termed Strategy I. Despite our predictions there is no proton efflux from iron or carbon stressed T. weissflogii and that the cell surface pH can be higher than the bulk seawater pH by as much as 0.4 units. High surface pH and increased PEPCase activity in low-pCO₂, high-iron grown cells is consistent with an efficient C4 type CO₂ concentrating mechanism. An increase in surface pH may impact cell surface precipitated iron and associated trace metals such as barium and may be an important factor to consider in paleo-export flux proxies when carbon dioxide is depleted.