| Many eukaryotic microalgae have CO2 concentrating mechanisms (CCM) to maximize the concentration of CO2 at the active site of RubisCO as a response to the low CO2 concentrations in the external aquatic medium. The CCM generally consists of the active uptake of HCO 3- and CO2, and many have an external carbonic anhydrase (CA) to facilitate HCO3- use. However, some species rely on the diffusive uptake of CO2 to support photosynthesis; freshwater chrysophytes and synurophytes are thought to belong to this group because their ecological distribution is largely confined to neutral to slightly acidic waters.;Photosynthetic characteristics of four synurophytes, Mallomonas papillosa, Synura petersenii, Synura uvella and Tessellaria volvocina were investigated to determine the mechanism of inorganic carbon (Ci) uptake. All species were found to have no CA, no capacity for direct HCO3- uptake, low whole-cell affinity for Ci and high CO2 compensation concentrations. The internal pHs of M. papillosa and S. petersenii determined using 14C-benzoic acid and [2-14C]-DMO were pH 7.0--7.8, over an external pH range of 5.0--7.5. Thus, the pH difference between the cell interior of both species and the external medium was large enough, over their growth range, to allow the accumulation of Ci by the diffusive uptake of CO2. Monitoring CO2 evolution and CO 2 uptake by suspensions of M. papillosa, S. petersenii, and T. volvocina at pH 7.0 by mass spectrometry did not indicate a rapid uptake of CO2 and the final CO2 compensation concentrations reached were 16.6, 19.1, and 26.4 muM which are above CO 2 equilibrium concentration which indicated a lack of active CO 2 uptake. Furthermore, when cells were darkened, a brief burst of CO 2 occurred before a steady rate of dark respiration was established, suggesting a loss of CO2 by photorespiration. An examination of the kinetics of RubisCO in cells homogenates of M. papillosa, S. petersenii, S. uvella and T. volvocina showed that values of the Km(CO2) were 18.2, 28.4, 41.8, and 18.3 muM respectively. Overall, the results indicate that synurophytes lack the characteristics of cells with a CCM because the cell affinity for Ci appears to be determined by the relatively high CO2 affinity of the RubisCO. |
