Alkalinity changes generated by phytoplankton growth1

Abstract

Continuous cultures of three marine phytoplankton species, Phaeodactylum tricornutum, Dunaliella tertiolecta, and Monochrysis lutheri, were monitored for changes in alkalinity of the culture medium resulting from NO 3 − and NH 4 + uptake. Uptake of NO 3 − caused an increase in alkalinity, whereas uptake of NH 4 + produced a decrease. These results are consistent with the type of schematic equation proposed by Redfield, Ketchum, and Richards for photosynthetic assimilation of inorganic nitrogen, in which NO 3 − uptake is balanced by OH − production and NH 4 + uptake leads to H + generation. These reactions suggest active uptake of nitrogen species by microbes. We have been unable to demonstrate the exact stoichiometry of this relationship, and the role of P uptake in the alkalinity change is unclear. An offset in the data, functionally equivalent to the production of some strong acid, may be due to reactions on the walls of the vessel, active uptake of cations, or extrusion of H + ions by the growing cells.