A New Mechanism for Adaptation to Changes in Light Intensity and Quality in the Red Alga Porphyra perforata

Abstract

In the red alga Pwphyra perforata, the level of chlorophyll fluorescence in the presence of 3-(3,4-dichrophenyl)-1,1-dimethylurea (DCMU) decreased during illumination of the thaflus. The results showed that: (a) this decay was related to the photooxidative activity of photosystem I; (b) Q, the primary electron acceptor of photosystem II, became oxidize during the decay of the fluorescence; (c) reagents which inhibit the back reaction of photosystem II inhibited the decay.From these results, it is suggested that, when conditions in the chiloroplasts of this red alga become too oxidative, excess light energy can be converted to heat as a result of an accelerated back reaction ofphotosystem II. This may be one of the mechanisms by which this alga can cope with the high salt and high libt condiftons that can occur in its natural hatat.In the photosynthesis of red algae, two light-harvesting pigment proteins are involved. Phycobilisomes are mainly connected to PSII and most of Chl a belongs to PSI (11). To maintain a suitable ratio of activities of the two photosystems, red algae have evolved a mechanism to control the transfer of light energy from pigment system II to I, called state 12, 18). The red alga Porphyra perforata is an intertidal alga that is periodically exposed to high light and/or high salt conditions. Therefore, it might be expected that this alga has other mechanisms besides the state I-state II transitions to adapt it to live under high light conditions. Fork and Oquist have reported (5) that the morphology of the pigment systems in Porphyra changed after air drying so that light energy absorbed by PSII was preferentially transferred to PSI. Recently, we found another mechanism in P. peforata that was termed state II-state III transitions (Satoh and Fork [13,14]). After a state II to III transition, the light energy reaching the reaction centers of PSII was decreased without any significant change in the PSI activity (Satoh and Fork [13]). All these mechanisms may be useful to avoid the photoinhibition of photosynthesis in a plant such as Porphyra which is often exposed to high light intensity. However, in high salt concentrations where the water potential is very low, PSII activity would be expected to be strongly inhibited. Under these conditions, an additional mechanism(s) would be needed to avoid photodamage to this alga.In this study, we found in P. perforata a large light-induced fluorescence decrease in the presence of DCMU. It is proposed that this fluorescence decrease is related to one of the adaptation mechanisms of this alga to conditions of high light and low water potential. Fluorescence spectra at 77K and time courses at room temperature of fluorescence at 685 nm were measured using a fiber optic system to excite and collect the fluorescent light (6). PSI (433 nm) and PSII (550 nm) light, which are mainly absorbed by Chl a and phycoerythrin, respectively, were -obtained by passing the white light from a 150-w, 21.5-v projector lamp (type DLS) through Balzers interference filte...

Photosystem II Complex in Vivo Is a Monomer

“…6). Again, only monophasic increases of fluorescence were observed in these materials, although the increasing rates and the fluorescence F o levels were variable, which is consistent with the previously published similar measurements in red algae and cyanobacteria (32,(52)(53)(54).…”

Photochemical and Nonphotochemical Fluorescence Quenching Processes in the Diatom Phaeodactylum tricornutum

“…3). These results are qualitatively similar to what has previously been reported for diatoms (Owens, 1986b;Caron et al, 1987) and red algae (Satoh and Fork, 1982). However, our data also indicate that Fluorescence values were normalized to F,, = O.…”

“…For instance, quenching of fluorescence from F, was also observed in the presence of DCMU and actinic illumination in red algae (Satoh and Fork, 1982). However, antimycin A addition resulted in an enhancement and not an inhibition of this fluorescence quenching.…”

“…This quenching in the presence of DCMU has been reported in diatoms (Owens, 1986b;Caron et al, 1987), and a red alga (Satoh and Fork, 1982), but not in higher plants and green algae. Although our results are qualitatively similar to what was previously observed in diatoms, we have reexamined these processes in greater detail to resolve ambiguities present in the earlier studies.…”

“…However, antimycin A addition resulted in an enhancement and not an inhibition of this fluorescence quenching. These authors attributed this quenching to a PSII back reaction, which dissipated excess excitation energy via thermal emission (Satoh and Fork, 1982).…”