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

Abstract

Red algae, Cyanidium caldarium and Cyanidioschyzon merolae, live in acidic hot springs; in a temperature range 35~45°C and in a pH range from 2.0 to 3.0. In general, solubility of CO2 in water decreases as the temperature increases, and a lot of ATP molecules are used to transport H+, which constantly penetrates into the cells in acidophilic algae. Cyclic electron flow is known to be needed for CO2 concentration in cyanobacteria. Therefore, in thermophilic and acidophilic algae, the activity of cyclic electron transport system is supposed to be very high. In the present work, we studied various inhibitors of cyclic electron transport systems on fluorescence changes and redox changes of P700 in the presence or absence of DCMU in thermophilic and acidophilic red algae, C. caldarium and C. merolae. The inhibitors used are rotenone, antimycinA, HgCl2, phenylmercuric acetate, N-ethylmaleimide, and diphenyleneiodonium chloride. C. caldarium cells showed a distinct post-illumination transient fluorescence increase and transient fluorescence quenching in the presence of DCMU under illumination. The inhibitors decreased or increased the transient fluorescence increase and the transient fluorescence quenching. The inhibitors also differently affected time courses of photooxidation and dark re-reduction of P700. All the results suggest that there are several routes of cyclic electron flow in the thermophilic and acidophilic red algae and that their activities are high.