MEASUREMENT OF CO2 AND HCO3- FLUXES IN CYANOBACTERIA AND MICROALGAE DURING STEADY-STATE PHOTOSYNTHESIS

A mass spectrometric procedure is described which allows the simultane ous estimation of both CO2 and HCO3- fluxes associated with cyanobacte ria and green algae during steady-state photosynthesis. This technique utilizes the chemical disequilibrium which exists between CO2 and HCO 3- during photosynthesis in cell suspensions which lack carbonic anhyd rase activity. The kinetic equations which are derived for flux determ inations are based on models of photosynthesis in both cyanobacteria a nd green algae which seem most reasonable given our present level of u nderstanding, together with direct measurement of [CO2], estimation of [HCO3-] and application of the kinetic rate constants for the interco nversion Of CO2 and HCO3-. From measurements made in the light, net up take of both CO2 and HCO3- can be readily determined. In addition, ana lysis of the dark phase immediately following light-off provides the p ossibility of also determining the CO2 evolution which is occurring du ring photosynthesis, and thus also the gross CO2 uptake rates in the l ight. Results are presented for the response of dissolved inorganic ca rbon (C(i)) flux rates to external C(i) in low-C(i) grown cells of bot h Synechococcus PCC7942 and Chlamydomonas reinhardtii and these are co nsistent with previous studies showing that such cells possess capacit ies to utilize both CO2 and HCO3- for photosynthesis. The advantages a nd potential errors which are inherent in this technique are discussed together with its potential for future studies on C(i) transport unde r various experimental conditions.