Growth and utilization of inorganic carbon were investigated in the acidophilic microalga Chlamydomonas acidophila, isolated from Tinto river, in the spanish southwest mining area, in the province of Huelva. According to the chemical composition of its natural habitat, an artificial culture medium containing 4 g l(-1) SO4K2 was designed at pH 2.5 and used to grow the acidophilic microalgae in batch cultures. The cultures were bubbled with air enriched with 5% (v/v) CO2 as unique carbon source. The culture reached the stationary growth phase 250 h after the experiment started up. The initial chlorophyll content was 5 mu g ml(-1) (initial value, t=0) and it increased up to 186 mu g ml(-1) (maximum value). The growth rate was 0.52 d(-1), low value when compared to other non-extremophilic microalgae. Carotenoids to chlorophyll ratio continuously increased from 0.1 to 0.2 due to fast production of carotenoids. HPLC analysis of the accumulated carotenoids showed lutein to play a significant role in the antioxidant response of the microalgae. The photosynthetic utilization of inorganic carbon by C. acidophila was studied in liquid medium cultures growing with air enriched with 5% (v/v) CO2. The maximum photosynthetic activity increased when increasing pH from 2.5 to 6. The apparent photosynthetic affinity constant for dissolved inorganic carbon (K-DIC) was 1.7 mu M at pH 2.5 and it hardly changed at pH 4.0 and 6.0. These results show the acidophilic microalgae to have high affinity by CO2 which seems not to depend on the presence of HCO3-. The growth of the acidophilic microalgae was found to be dependent on CO2. (c) 2006 Elsevier Inc. All rights reserved.