Cyclic electron flow and chlorophyll fluorescence in thermophilic and acidophilic red algae

摘要

Red algae, Cyanidium caldarium and Cyanidioschyzon merolae, live in acidic hot springs; in a temperature range of 35 approx 45 deg C and in a pH range from 2.0 to 3.0. Under these conditions,the algae should be producing a lot of ATP molecules by respiration and by cyclic electron flow via Photosystem (PS) I because ATP must be needed to transport H~+, which constantly penetrates into the cells (Enami et al. 1986). Furthermore, environmental CO_2 concentration must be very low; dissolved CO_2 molecules can not be converted to HCO_3~- at low pH, and solubility of CO_2 in water is very low at high temperatures. In cyanobacteria, cyclic electron flow through complex I-type NAD(P)H-plastoquinone oxidoreductase (NDH) has been reported to be necessary for a CO_2-concentration mechanism in cells (Ogawa 1991). However, the cyclic electron transport system in red algae is not well known. Therefore, thermophilic and acidophilic red algae are interesting materials for the study of cyclic electron flow through PSI, activity of which is supposedly very high.