ENERGY TRANSFER FROM PHYCOBILINS TO CHLOROPHYLL a IN HEAT-STRESSED CELLS OF Anacystis nidulans: CHARACTERIZATION OF THE LOW TEMPERATURE 683 nm FLUORESCENCE EMISSION BAND

David (Dave) Charles Fork (1929-2020): a gentle human being, a great experimenter, and a passionate researcher

We provide here an overview of the remarkable life and outstanding research of David (Dave) Charles Fork (March 4, 1929-December 13, 2021) in oxygenic photosynthesis. In the words of the late Jack Edgar Myers, he was a top 'photosynthetiker'. His research dealt with novel findings on light absorption, excitation energy distribution, and redistribution among the two photosystems, electron transfer, and their relation to dynamic membrane change as affected by environmental changes, especially temperature. David was an attentive listener and a creative designer of experiments and instruments, and he was also great fun to work with. He is remembered here by his family, coworkers, and friends from around the world including Australia, France, Germany, Japan, Sweden, Israel, and USA.

Prasanna K. Mohanty (1934-2013): a great photosynthetiker and a wonderful human being who touched the hearts of many

Prasanna K. Mohanty, a great scientist, a great teacher and above all a great human being, left us more than a year ago (on March 9, 2013). He was a pioneer in the field of photosynthesis research; his contributions are many and wide-ranging. In the words of Jack Myers, he would be a "photosynthetiker" par excellence. He remained deeply engaged with research almost to the end of his life; we believe that generations of researchers still to come will benefit from his thorough and enormous work. We present here his life and some of his contributions to the field of Photosynthesis Research. The response to this tribute was overwhelming and we have included most of the tributes, which we received from all over the world. Prasanna Mohanty was a pioneer in the field of "Light Regulation of Photosynthesis", a loving and dedicated teacher-unpretentious, idealistic, and an honest human being.

Structural and functional alterations of cyanobacterial phycobilisomes induced by high-light stress

Exposure of cyanobacterial or red algal cells to high light has been proposed to lead to excitonic decoupling of the phycobilisome antennae (PBSs) from the reaction centers. Here we show that excitonic decoupling of PBSs of Synechocystis sp. PCC 6803 is induced by strong light at wavelengths that excite either phycobilin or chlorophyll pigments. We further show that decoupling is generally followed by disassembly of the antenna complexes and/or their detachment from the thylakoid membrane. Based on a previously proposed mechanism, we suggest that local heat transients generated in the PBSs by non-radiative energy dissipation lead to alterations in thermo-labile elements, likely in certain rod and core linker polypeptides. These alterations disrupt the transfer of excitation energy within and from the PBSs and destabilize the antenna complexes and/or promote their dissociation from the reaction centers and from the thylakoid membranes. Possible implications of the aforementioned alterations to adaptation of cyanobacteria to light and other environmental stresses are discussed.

Heat- and light-induced reorganizations in the phycobilisome antenna of Synechocystis sp. PCC 6803. Thermo-optic effect

By using absorption and fluorescence spectroscopy, we compared the effects of heat and light treatments on the phycobilisome (PBS) antenna of Synechocystis sp. PCC 6803 cells. Fluorescence emission spectra obtained upon exciting predominantly PBS, recorded at 25 degrees C and 77 K, revealed characteristic changes upon heat treatment of the cells. A 5-min incubation at 50 degrees C, which completely inactivated the activity of photosystem II, led to a small but statistically significant decrease in the F(680)/F(655) fluorescence intensity ratio. In contrast, heat treatment at 60 degrees C resulted in a much larger decrease in the same ratio and was accompanied by a blue-shift of the main PBS emission band at around 655 nm (F(655)), indicating an energetic decoupling of PBS from chlorophylls and reorganizations in its internal structure. (Upon exciting PBS, F(680) originates from photosystem II and from the terminal emitter of PBS.). Very similar changes were obtained upon exposing the cells to high light (600-7500 micromol photons m(-2) s(-1)) for different time periods (10 min to 3 h). In cells with heat-inactivated photosystem II, the variations caused by light treatment could clearly be assigned to a similar energetic decoupling of the PBS from the membrane and internal reorganizations as induced at around 60 degrees C. These data can be explained within the frameworks of thermo-optic mechanism [Cseh et al. 2000, Biochemistry 39, 15250]: in high light the heat packages originating from dissipation might lead to elementary structural changes in the close vicinity of dissipation in heat-sensitive structural elements, e.g. around the site where PBS is anchored to the membrane. This, in turn, brings about a diminishment in the energy supply from PBS to the photosystems and reorganization in the molecular architecture of PBS.

Ultrastructural and biochemical characterization of a Synechocystis 6803 mutant with inactivated psbA genes

A constructed Synechocystis 6803 mutant with a deletion of the three psbA genes was subjected to ultrastructural and biochemical characterization. This D1-depleted mutant also lacks the D2 protein and the chlorophyll a-binding protein CP-47. A general ultrastructural comparison between the wild type and the mutant did not reveal any major changes in cell appearance. We found by freeze-fracture analysis that approximately 60% of the endoplasmic face particles found in the wild-type thylakoids were missing in the mutant. A corresponding increase in protoplasmic face particles in the mutant thylakoids may represent a subcomplex of those photosystem II (PS II) polypeptides which accumulate in the absence of the D1 protein. Correlation of the PS I:PS II ratio with freeze-fracture data indicates that there is only one reaction center in each PS II freeze-fracture particle. Fluorescence measurements show that the CP-43 polypeptide in the mutant binds chlorophyll and that it may be connected to the phycobilisomes. Excitation energy can be transferred from the phycobilisomes to photosystem I in the absence of the photosystem II reaction center heterodimer and CP-47. This suggests that exciton transfer to photosystem I is mediated either directly by a terminal phycobilisome transmitter or via CP-43.

The relationship between heat-stress and photobleaching in green and blue-green algae

Two characteristic temperatures were identified from measurements of the temperature dependence of O2 evolution by Chlorella vulgaris and Anacystis nidulans: T1, the threshold temperature for inhibition of O2 evolution under saturating light conditions, and T2, the upper temperature limit for O2 evolution. Measurement of delayed light emission from photosystem II (PSII) showed that it passed through a maximum at T1 and was virtually eliminated on heating the samples to T2. Related changes were observed in low-temperature (77K) fluoresence emission spectra. Heat-stress had little effect on the absorption properties of the cells at temperatures below T1 but incubation at higher temperatures, particularly under high-light conditions, resulted in extensive absorption losses. An analysis of these measurements suggests that this increased susceptibility to photobleaching is triggered by an inhibition of the flow of reducing equivalents from PSII that normally serves to protect the light-harvesting apparatus of the cells from photo-oxidation. Adaptation to higher growth temperatures resulted in increases in the values of T1 and T2 for Anacystis nidulans but not for Chlorella vulgaris.