Alkalinity changes generated by phytoplankton growth1

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, and NHa+ uptake. Uptake of NO,caused an increase in alkalinity, whereas uptake of NH,+ 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, uptake is balanced by OHproduction and NHa+ 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. An understanding of the oceanic CO2 system is important in many branches of marine chemistry and biolo,gy. Experimentally, this may be approached through measurement of any two of the four variablcs pH, alkalinity, pCOa, and total COz, and from any two of these properties the others can be calculated (Skirrow 1965). Many recent data have been obtained by the potentiometric titration technique ( Dyrsscn 1965; Edmond 1970) that enables the alkalinity and total COz to be measured in a single sample. The alkalinity of scawater is defined as the number of equivalents of strong acid required to titrate 1 liter of seawater to the C02-HC03end point. Analytically, this is equivalent to the sum of the concentrations of the anions of carbonic and other weaker acids. Alkalinity is thus inherently associated with charge balance ( Stumm and Morgan 1970), and the addition or removal of neutral CO2 during respiration or photosynthesis has no cffcct on this property. All this is frequently taken to mean that the photosynthetic or decomposition processes occurring in aquatic environments do not generate alkalinity changes. This is ’ Contribution No. 3595 from the Woods IIole Oceanographic Institution. This research was supported by NSF grants GA-22292 and GX-33295 (P.G.B. ) and ERDA contract AT( 11-l )2532 ( J.C.G. ) . by no means correct. Goldman c t al. (1972) have pointed out that uptake of N03during photosynthesis generates strong base, whereas NH4+ assimilation leads to acid production. Brewer et al. ( 1975) have discussed the reverse effect in which the dccomposition of marine organic matter csscntially leads to the liberation of nitric acid. This strong acid reacts with carbonate ion derived from calcium carbonate dissolution, thus leaving an excess of calcium over that predicted from observed alkalinity changes. These effects have been inferred for many years (Trelease and Trelease 1935; Cramer and Myers 1948; Rcdfield ct al. 1963; Richards 1965; Berner et al. 1970; Gaines and Pilson 1972) and indeed are implicit in the Redficld-Ketchum-Richards phytoplankton equations. Yet, as far as we are aware, these phenomena have not been adequately documented and no cxpcrimental prooE cxis ts. Here, we report our mcasurcmcnts of alkalinity changes during the growth of phytoplankton on both NOaand NI-IJ+ as nitrogen sources. We thank J. M. Edmond and F. Morel for their critical review and R. Smith, S. Kadar, A. Fleer, and R. Carbon for their technical assistance.