Measurement of carbon dioxide compensation points of freshwater algae.

A technique is described for the measurement of total dissolved inorganic carbon by acid release as CO(2) followed by its conversion to methane and detection by flame ionization in a modified gas chromatograph. This method was used to determine the dissolved inorganic carbon concentration reached at compensation point when algae were allowed to photosynthesize in a closed system in a buffer at known pH, and the CO(2) compensation point was calculated from this concentration. The CO(2) compensation points of 16 freshwater algae were measured at acid and alkaline pH in air-saturated medium: at acid pH the CO(2) compensation points ranged from 4.8 to 41.5 microliters per liter while at alkaline pH they ranged from 0.2 to 7.2 microliters per liter. Removal of O(2) from the medium caused a slight lowering of compensation point at acid pH but had little effect at alkaline pH. These low, O(2)-insensitive compensation points are characteristic of C(4) plants. It is suggested that these low CO(2) compensation points are maintained by an active bicarbonate uptake by algae especially at alkaline pH.