The relationship between the growth of Synura petersenii (Synurophyceae) and components of the dissolved inorganic carbon system

The relationship between the growth of the freshwater chrysophyte Synura pe tersenii Korshikov (Synurophyceae) and components of the dissolved inorgani c carbon (DIC) system was investigated by means of 'pH drift' studies. In g lass stoppered Vessels at light saturation, culture pH rose and DIC was dep leted until exponential growth ceased. Restoration of culture pH to the ini tial range by addition of HCl and consequent redistribution of DIC species produced a slight restimulation of growth, but growth was stimulated much m ore after restoring initial pH by addition of DIC as free carbon dioxide (C O2*). The maximum pH values reached (none above pH 7.6) and the concentrati ons of total DIC (DICT), HCO3-, and CO2* at the beginning and end of each c ycle of growth, pH drift, and C depletion indicated that growth was restric ted by the species of DIC available and not by PH or DICT per se. Under the conditions of low alkalinity and low DICT tested, S. petersenii apparently used CO2* and not HCO3- as a DIC source; it had a relatively high CO2* com pensation point of (21) 28-29 mu M and a relatively low DIC-extractive capa city compared with other freshwater phytoplankton. Support for these conclu sions also was provided by replacing the glass stoppers of the culture vess els at growth cessation with glass funnels packed with polystyrene pellets or soda lime. The polystyrene pellets allowed CO2 invasion from the atmosph ere and growth was restimulated. Zn contrast, no growth occurred when the g lass funnel was packed with soda lime. It is concluded that the apparent re striction of S. petersenii to waters of below pH 8.5-9.1, particularly in c onditions of low alkalinity, is due not to pH per se, but to the DIC utiliz ation characteristics of this species.