Photosynthetic response of Chara braunii towards different bicarbonate concentrations.

A variety of inorganic carbon acquisition modes have been proposed in Characean algae, however, a broadly applicable inorganic carbon uptake mechanism is unknown for the genus Chara. In the present study, we analyzed if C. braunii can efficiently use HCO 3 - as a carbon source for photosynthesis. For this purpose, C. braunii was exposed to different concentrations of NaHCO 3 - at different time scales. The photosynthetic electron transport through photosystem I (PSI) and II (PSII), the maximum electron transport rate (ETR max ), the efficiency of the electron transport rate (α, the initial slope of the ETR), and the light saturation point of photosynthesis (E k ) were evaluated. Additionally, pigment contents (chlorophyll a, chlorophyll b, and carotenoids) were determined. Bicarbonate addition positively affected ETR max , after direct HCO 3 - application, of both PSII and PSI, but this effect seems to decrease after 1 h and 24 h. Similar trends were seen for E k , but no significant effect was observed for α. Pigment contents showed no significant changes in relation to different HCO 3 - concentrations. To evaluate if cyclic electron flow around PSI was involved in active HCO 3 - uptake, the ratio of PSI ETR max /PSII ETR max was calculated but did not show a distinctive trend. These results suggest that C. braunii can utilize NaHCO 3 - in short-term periods as a carbon source but could rely on other carbon acquisition mechanisms over prolonged time periods. These observations suggest that the minor role of HCO 3 - as a carbon source for photosynthesis in this alga might differentiate C. braunii from other examined Chara spp.