Dissolved inorganic carbon speciation in aquatic environments and its application to monitor algal carbon uptake

Abstract

Dissolved inorganic carbon (DIC) speciation is an important parameter that enables chemical and ecological changes in aquatic environments, such as the aquatic environmental impact of increasing atmospheric CO2 levels, to be monitored. We have examined and developed a sensitive and cost-effective ‘back-titration’ method to determine the DIC species and abundance in aqueous environments that is more accurate and reproducible than existing methods and is applicable in a range of fresh, brackish and sea waters. We propose the use of pHHCO3 (bicarbonate-dominant pH) and pH3.5 as the titration end points in the back-titration technique to accurately determine carbonate alkalinity. The proposed method has a higher accuracy and precision than other modified Gran's methods that are currently in use. The detection limit was found to be ~5μmolkg−1 with an accuracy within 1% and a precision (CV) within 0.2% and 0.5% at high and low level of carbonates, respectively. This method was successfully applied to monitor DIC in the aqueous medium of Nannochlopsis salina cultivation separately carried out with NaHCO3 and CO2 as the respective inorganic carbon source. The cells were able to grow in the NaHCO3 medium with a similar growth curve to cells with 0.039% CO2 (air). Increases in CO2 level stimulated lipid accumulation by diverting the fixed carbon from protein to lipids. The increased concentration of gaseous CO2 and the accompanying lower pH appears to significantly inhibit the growth of algae despite the presence of HCO3− when 20% CO2 was employed.