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 HCO₃ ^- as a carbon source for photosynthesis. For this purpose, C. braunii was exposed to different concentrations of NaHCO₃ ^- 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 HCO₃ ^- 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 HCO₃ ^- concentrations. To evaluate if cyclic electron flow around PSI was involved in active HCO₃ ^- 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 NaHCO₃ ^- 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 HCO₃ ^- as a carbon source for photosynthesis in this alga might differentiate C. braunii from other examined Chara spp.