Photosynthesis Research (v.100, #1)

Phycobiliprotein diffusion in chloroplasts of cryptophyte Rhodomonas CS24 by Tihana Mirkovic; Krystyna E. Wilk; Paul M. G. Curmi; Gregory D. Scholes (7-17).
Unicellular cryptophyte algae employ antenna proteins with phycobilin chromophores in their photosynthetic machinery. The mechanism of light harvesting in these organisms is significantly different than the energy funneling processes in phycobilisomes utilized by cyanobacteria and red algae. One of the most striking features of cryptophytes is the location of the water-soluble phycobiliproteins, which are contained within the intrathylakoid spaces and are not on the stromal side of the lamellae as in the red algae and cyanobacteria. Studies of mobility of phycobiliproteins at the lumenal side of the thylakoid membranes and how their diffusional behavior may influence the energy funneling steps in light harvesting are reported. Confocal microscopy and fluorescence recovery after photobleaching (FRAP) are used to measure the diffusion coefficient of phycoerythrin 545 (PE545), the primary light harvesting protein of Rhodomonas CS24, in vivo. It is concluded that the diffusion of PE545 in the lumen is inhibited, suggesting possible membrane association or aggregation as a potential source of mobility hindrance.
Keywords: Cryptophyte; Phycobiliprotein; Diffusion; Fluorescence recovery after photobleaching; Thylakoid membrane; Energy transfer

Singlet oxygen formation and chlorophyll a triplet excited state deactivation in the cytochrome b 6 f complex from Bryopsis corticulans by Fei Ma; Xiao-Bo Chen; Min Sang; Peng Wang; Jian-Ping Zhang; Liang-Bi Li; Ting-Yun Kuang (19-28).
We have attempted to investigate the correlation between the detergent-perturbed structural integrity of the Cyt b 6 f complex from the marine green alga Bryopsis corticulans and its photo-protective properties, for which the nonionic detergents n-octyl-β-d-glucopyranoside (β-OG) and n-dodecyl-β-d-maltoside (β-DM), respectively, were used for the preparation of Cyt b 6 f, and the singlet oxygen (1O2*) production as well as the triplet excited-state chlorophyll a (3Chl a*) formation and deactivation were examined by spectroscopic means. Near-infrared luminescence of 1O2 * (~1,270 nm) on photo-irradiation was detected for the β-OG preparation where the complex is mainly in oligomeric state, but not for the β-DM one in which the complex exists in dimeric form. Under anaerobic condition, photo-excitation of Chl a in the β-DM preparation generated 3Chl a* with a lower quantum yield of ΦT ~ 0.02 and a longer lifetime of ~600 μs with respect to those as in the case of β-OG preparation, ΦT ~ 0.12 and 200–300 μs. These results prove that the enzymatically active and intact Cyt b 6 f complex on photo-excitation tends to produce little 3Chl a* or 1O2 *, which implies that the pigment–protein assembly of Cyt b 6 f complex per se is crucial for photo-protection.
Keywords: Cytochrome b 6 f ; Photo protection; Singlet oxygen; Triplet excited state; Excitation energy transfer

Moderate heat stress has been reported to increase PSI cyclic electron flow (CEF). We subjected leaves of Arabidopsis (Arabidopsis thaliana) mutants disrupted in the regulation of one or the other pathway of CEF flow—crr2 (chlororespiratory reduction, deficient in regulation of chloroplast NAD(P)H dehydrogenase-dependent CEF) and pgr5 (proton gradient regulation, proposed to have reduced efficiency of antimycin-A-sensitive-CEF regulation) to moderate heat stress. Light-adapted leaves were switched from 23 to 40°C in 2 min. Gas exchange, chlorophyll fluorescence, the electrochromic shift (ECS), and P700 were measured. Photosynthesis of crr2 and pgr5 was more sensitive to heat and had less ability to recover than the genetic background gl. The proton conductance in light was increased by heat and it was twice as much in pgr5, which had much smaller light-induced proton motive force. We confirmed that P700 becomes more reduced at high temperature and show that, in contrast, the proportion of PSII open centers (with Q A oxidized) increases. The two mutants had much slower P700+ reduction rate during and after heat than gl. The proportion of light absorbed by PSI versus PSII was increased in gl and crr2 during and after heat treatment, but not in pgr5. We propose that heat alters the redox balance away from PSII and toward PSI and that the regulation of CEF helps photosynthesis tolerate heat stress.
Keywords: Chlorophyll fluorescence; Cyclic electron transport around PSI; Electrochromic shift; Gas exchange; Moderate heat stress; P700 measurement

The functional Mn content of intact photosystem II membrane fragments was measured as 4.06 ± 0.13 Mn/reaction center when determined using a simple, sensitive colorimetric assay that will also work with thylakoids and core complexes. This procedure requires minimal sample material, does not need expensive assay equipment, requires four simple steps, and only takes 20–30 min to perform. These include (a) removal of the adventitious Mn ions by CaCl2 treatment of the membranes, (b) extraction of the Mn from the O2-evolving complex with hydrochloric acid, (c) purification of the extract by centrifugation followed by filtration of the supernatant through an Acrodisc syringe filter (0.2 μm nylon membrane), and (d) colorimetric determination of Mn in the extract using the reaction of the chromogenic agent, 3,3′,5,5′-tetramethylbenzidine, with previously oxidized Mn(II) cations carried out at high pH. The colorimetric assay itself has been used previously by Serrat (Mikrochim Acta 129:77–80, 1998) for assaying Mn concentrations in sea water and drinking water.
Keywords: Manganese assay; Colorimetric assay; Manganese content; Photosystem II; Oxygen-evolving complex