Photophosphorylation in intact algae: Effects of inhibitors, intensity of light, electron acceptor and donors

Abstract

The luciferin-luciferase method was used to determine ATP extracted from darkmaintained and light-exposed samples of the green alga Chlorella pyrenoidosa and of the blue-green alga Anacystis nidulans. A few measurements on Synechococcus lividus (a bluegreen thermophile, clone 65°C) are also reported. The light-minus-dark ATP levels (δATP) from aerobic cells of Chlorella and Anacystis were negative; however, δATP from Synechococcus was positive. Large positive δATP was obtained in regularly grown (RG: moderate light) Chlorella treated with oligomycin; dark levels were reduced, light levels remained essentially unaffected. In high-light exposed (HLE) Chlorella, oligomycin reduced both light and dark ATP levels, but positive δ ATP was still obtained. However, in Anacystis, which has a different organization of thylakoid membrane, oligomycin severely reduced both the light and the dark ATP levels and the δATP remained negative. The oligomycin (12 μM) treated Chlorella and the untreated Anacystis and Synechococcus show the presence of cyclic photophosphorylation under conditions in which the non-cyclic electron flow from photosystem II to photosystem I is blocked by 10 μM 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU), or not allowed to operate by the absence of CO2. Cyclic photophosphorylation ranged from 10–30% of the maximum δATP in RG, to 40–50% in HLE Chlorella. In RG Chlorella, cyclic and non-cyclic (in the absence of DCMU) photophosphorylation (δATP) saturate at about 103 ergs cm−2 sec−1 and 104 ergs cm−2 sec−1 and 104 ergs cm−2 sec−1 red (>640 nm) light, respectively; a lag was observed in the light curve. In Chlorella, the addition of the photosystem I electron acceptor methyl viologen (MV; 1 mM) increased δATP by twofold. Further addition of DCMU (25 μm) reduced this to the level observed with DCMU alone. If 1 mM reduced dichlorophenol indophenol or phenazine methosulphate (DCPIPH2 or PMSH2, respectively) was added along with DCMU, the δATP level was 30–40% of the control. Further addition of MV increased the JATP to be 70–80% of that of the control. These and other results confirm the presence of both non-cyclic and cyclic photophosphorylation in vivo, the former predominating in Chlorella, and the latter in Anacystis and Synechococcus.