To investigate the influence of HCO3- alkalinity on calcification in thalli of the genus Chara, we studied the effects of increased light level and additional HCO3- on calcification rate in C. vulgaris at various hours (30 min, 8 h, 12 h, 12.5 h, 20 h and 24 h after treatment) in a 24 h experiment (12 h light: 12 h dark). We identified a significant Pearson's correlation between exogenous dissolved inorganic carbon (DIC) concentrations and the utilization of DIC (mu mol C g(-1) fresh weight). Plotting the daily rhythm of DIC utilization produced negative quadratic curves. Furthermore, calcification rate (mu mol Ca h(-1) g(-1)) was linearly related to DIC utilization rate (DICuptake; mu mol C h(-1) g(-1)), indicating that the calcification rate is dependent on DICuptake. However, ratios of calcification to the utilization of DIC were decreased at high light intensity and increased with HCO3- addition, which was mainly ascribed to changes in the ratio of calcification to photosynthesis. Chlorophyll fluorescence results provided direct evidence for the promotion of photosynthesis in Chara thalli by both high light and DIC addition and their positive influence on maximum relative electron transport rate. These results suggest that calcification in calcareous C. vulgaris is mainly restrained by HCO3- alkalinity, which could explain the correlation between calcification of Chara thalli and alkalinity of water bodies in the field.