Origin of the 77 K variable fluorescence at 758 nm in the cyanobacterium Spirulina platensis

Cobalt chloride induced stimulation of Photosystem II electron transport in Synechocystis sp. PCC 6803 cells

Synechocystis sp. PCC 6803 when grown in the presence of sublethal (μM) levels of cobalt chloride shows an enhancement of Photosystem II (PS II) catalyzed Hill reaction. This stimulation seems to be induced by cobalt ions as other metal ions inhibit para-benzoquinone catalyzed Hill reaction. At saturating white light intensity, this enhancement is two times over that of the control cells on unit chlorophyll basis. Analysis of the PS II electron transport rate at varying intensities of white, blue or yellow light suggests an increased maximal rates but no change in the quantum yield or effective antenna size of CoCl2-grown cells. There were no structural and functional changes in the phycobilisome as judged by the absence of changes in the phycocyanin/allophycocyanin ratio, fluorescence emission spectra, second derivative absorption spectra at 77 K and SDS-PAGE analysis of isolated phycobilisomes. The 77 K fluorescence emission spectra of the cells showed a decrease in the ratio of Photosystem I emission (F725) to Photosystem II emission (F685) in CoCl2-grown cells compared to the control cells. These observations indicate three possibilities: (1) there is an increase in the number of Photosystem II units; (2) a faster turnover of Photosystem II centers; or (3) an alteration in energy redistribution between PS II and PS I in CoCl2-grown cells which causes stimulation of Photosystem II electron transport rate.

Isolation from Spirulina membranes of two photosystem I-type complexes, one of which contains chlorophyll responsible for the 77 K fluorescence band at 760 nm

Two types of chlorophyll-protein complexes of photosystem I (PSIa, PSIc) have been isolated from the membranes of Spirulina platensis using a Triton X-100 treatment and chromatography on DEAE-Toyopearl. The complexes are equally enriched with P700 (Chl: P700 = 100-110) but show different electrophoretic molecular masses--140 (PSIa) and 320 kDa (PSIc)--and differ in the content of long-wavelength absorbing Chl. PSIa has a typical PSI fluorescence band at 730 nm (F730) as the main band at 77 K, whereas PSIc is responsible for F760, the intensity of which depends on the redox state of P700. PSIc only shows 77 K light-induced variable fluorescence at 760 typical of Spirulina membranes and cells.

Fluorescence detected magnetic resonance of the primary donor and inner core antenna chlorophyll in Photosystem I reaction centre protein: Sign inversion and energy transfer

The Photosystem I reaction centre protein CP1, isolated from barley using polyacrylamide gel electrophoresis showed an EPR (Electron Paramgnetic Resonance) spectrum with the polarisation pattern AEEAAE, typical of the primary donor triplet state (3)P700, created via radical pair formation and recombination. (3)P700 could also be detected by Fluorescence Detected Magnetic Resonance (FDMR) at λf > 700 nm even in the presence of a large number of chlorophyll antennae. Its zero field splitting parameters, D=282.5×10(-4) cm(-1) and E=38.5×10(-4) cm(-1), were independent of the detection wavelength, and agreed with ADMR (Absorption Detected Magnetic Resonance) and EPR values. The signs of the (3)P700 D+E and D-E transitions were positive (increase in fluorescence intensity on applying a resonance microwave field). In contrast, in the emission band 685 < λf < 700 nm FDMR spectra with negative D+E and D-E transitions were detected, and the D value was wavelength-dependent. These FDMR results support an excitation energy transfer model for CP1, derived from time-resolved fluorescence studies, in which two chlorophyll antenna forms are distinguished, with fluorescence at 685 < λf < 700 nm (inner core antennae, F690), and λf > 700 nm (low energy antenna sites, F720), in addition to the P700. The FDMR spectrum in F690 emission can be interpreted as that of (3)P700, observed via reverse singlet excitation energy transfer and added to the FDMR spectrum of the antenna triplet states generated via intramolecular intersystem crossing. This would indicate that reversible energy transfer between F690 and P700 occurs even at 4.2 K.