Junko Nishio

Recovery processes of photosynthetic systems during rewetting were studied in detail in a terrestrial, highly drought-tolerant cyanobacterium, Nostoc commune. With absorption of water, the weight of N. commune colony increased in three phases with half-increase times of about 1 min, 2 h and 9 h. Fluorescence intensities of phycobiliproteins and photosystem (PS) I complexes were recovered almost completely within 1 min, suggesting that their functional forms were restored very quickly. Energy transfer from allophycocyanin to the core-membrane linker peptide (L(CM)) was recovered within 1 min, but not that from L(CM) to PSII. PSI activity and cyclic electron flow around PSI recovered within 2 min, while the PSII activity recovered in two phases after a time lag of about 5 min, with half times of about 20 min and 2 h. Photosynthetic CO(2) fixation was restored almost in parallel with the first recovery phase of the PSII reaction center activity. Although the amount of absorbed water became more than 20 times the initial dry weight of the N. commune colony in the presence of sufficient water, about twice the initial dry weight was enough for recovery and maintenance of the PSII activity.

Changes in photosynthetic activities under hypertonic conditions were studied in a terrestrial, highly desiccation-tolerant cyanobacterium, Nostoc commune, and in some desiccation-sensitive cyanobacteria. The amounts of water sustained in the colony matrix outside the N. commune cells and the cellular solute concentration were estimated by measuring the water potential, and the solute concentration was supposed to correspond to around 0.22 M sorbitol. Incubation of the colonies in 0.8 M sorbitol solution inhibited the energy transfer from the phycobilisome (PBS) anchor to PSII core complexes. At higher sorbitol concentrations, light energy absorbed by PSI, PSII, and PBS was dissipated to heat. PSI and cyclic electron flow around PSI was also deactivated by hypertonic treatment. Fv/Fm and (Fm'-F)/Fm' values started to decrease at 0.6 and 0.3 M sorbitol and reached zero at 1.0 and 0.8 M, respectively. Decreases in these two fluorescence parameters corresponded to the decreases in PSII fluorescence (F695) and photosynthetic CO2 fixation, respectively. The intensity of delayed light emission started to decrease at 1.0 M sorbitol and became negligible at 4.0 M. Comparing these changes in N. commune with those in desiccation-sensitive species, we found that N. commune cells actively deactivates photosynthetic systems on sensing water loss.

Green sulfur bacterial reaction centers (RCs) belong to an iron-sulfur type RC, and can photoreduce low potential iron-sulfur protein, ferredoxin, as does PSI. From the thermofilic green sulfur bacterium, C. tepidum, four 2[4Fe-4S] type Fd fractions were purified. The amount of each Fd fractions varied depending upon culture conditions. Absorbances at 385nm of these Fds decreased by 10-15% in 6 days at 4oC in aerobic condition, indicating that these Fds were rather stable in contrast with previous reports. Two of these Fds? N-terminal amino acid sequences (15 residues) were completely identical. All of these Fds? N-terminal amino acid sequences (15-25 residues) completely agreed to those deduced from the three fdx genes (Chung S., and Bryant D.A.). These Fds are photoreducible by purified RC from this bacterium, and support NADP+ photoreduction with spinach ferredoxin-NADP+ reductase. The Vmax and Km of electron transfer rates in the above reaction were significantly different among Fds.