Oxygenic photosynthetic organisms perform solar energy conversion of water and CO2 to O2 and sugar at a broad range of wavelengths and light intensities. These cells also metabolize sugars using a respiratory system that functionally overlaps the photosynthetic apparatus. In this study, we describe the harvesting of photocurrent used for hydrogen production from live cyanobacteria. A non-harmful gentle physical treatment of the cyanobacterial cells enables light-driven electron transfer by an endogenous mediator to a graphite electrode in a bio-photoelectrochemical cell, without the addition of sacrificial electron donors or acceptors. We show that the photocurrent is derived from photosystem I and that the electrons originate from carbohydrates digested by the respiratory system. Finally, the current is utilized for hydrogen evolution on the cathode at a bias of 0.65 V. Taken together, we present a bio-photoelectrochemical system where live cyanobacteria produce stable photocurrent that can generate hydrogen.

Summary Previous studies have shown that live cyanobacteria can produce photocurrent in bio-photoelectrochemical cells (BPECs) that can be exploited for clean renewable energy production. Electron transfer from cyanobacteria to the electrochemical cell was proposed to be facilitated by small molecule(s) mediator(s) whose identity (or identities) remain unknown. Here, we elucidate the mechanism of electron transfer in the BPEC by identifying the major electron mediator as NADPH in three cyanobacterial species. We show that an increase in the concentration of NADPH secreted into the external cell medium (ECM) is obtained by both illumination and activation of the BPEC. Elimination of NADPH in the ECM abrogates the photocurrent while addition of exogenous NADP + significantly increases and prolongs the photocurrent production. NADP + is thus the first non-toxic, water soluble electron mediator that can functionally link photosynthetic cells to an energy conversion system and may serve to improve the performance of future BPECs.

The mechanism of RNA degradation in Escherichia coli involves endonucleolytic cleavage, polyadenylation of the cleavage product by poly(A) polymerase, and exonucleolytic degradation by the exoribonucleases, polynucleotide phosphorylase (PNPase) and RNase II. The poly(A) tails are homogenous, containing only adenosines in most of the growth conditions. In the chloroplast, however, the same enzyme, PNPase, polyadenylates and degrades the RNA molecule; there is no equivalent for the E. coli poly(A) polymerase enzyme. Because cyanobacteria is a prokaryote believed to be related to the evolutionary ancestor of the chloroplast, we asked whether the molecular mechanism of RNA polyadenylation in the Synechocystis PCC6803 cyanobacteria is similar to that in E. coli or the chloroplast. We found that RNA polyadenylation in Synechocystis is similar to that in the chloroplast but different from E. coli. No poly(A) polymerase enzyme exists, and polyadenylation is performed by PNPase, resulting in heterogeneous poly(A)-rich tails. These heterogeneous tails were found in the amino acid coding region, the 5 and 3 untranslated regions of mRNAs, as well as in rRNA and the single intron located at the tRNA fmet . Furthermore, unlike E. coli, the inactivation of PNPase or RNase II genes caused lethality. Together, our results show that the RNA polyadenylation and degradation mechanisms in cyanobacteria and chloroplast are very similar to each other but different from E. coli.The molecular mechanism of RNA degradation in prokaryotes and organelles includes a series of sequential steps. The degradation starts with the initial endonucleolytic cleavage carried out primarily by the endoribonuclease E (RNase E). The cleavage products are then polyadenylated at their 3Ј ends by the poly(A) polymerase (PAP) 1 enzyme in Escherichia coli and the polynucleotide phosphorylase (PNPase) in the chloroplast. The polyadenylated molecules are then rapidly degraded exonucleolytically by PNPase and ribonuclease II (RNase II) (1-3). Finally, the remaining short oligoribonucleotides are degraded by the oligoribonuclease enzyme (4). Although the inhibition of polyadenylation in the chloroplast inhibited exonucleolytic degradation, implying that this is the only mechanism in the RNA degradation process, two RNA degradation mechanisms, a polyadenylation-dependent and a polyadenylation-independent one, were suggested to take place in E. coli (5, 6). Polyadenylation in E. coli is carried out primarily by PAP (7). Also in this bacterium, the RNase E enzyme, part of the PNPase population, an RNA helicase, some RNA molecules, and the glycolytic enzyme enolase are associated in a high molecular weight complex called a degradosome (8).In the chloroplast, the photosynthetic organelle of the plant cell is believed to have an evolutionary prokaryotic origin; many characteristics of the gene expression system resemble those of bacteria. When the RNA degradation mechanism was analyzed in the chloroplast, it was found to be very similar to that of E. coli (1-3). However...

Photoelectrochemical water splitting uses solar power to decompose water to hydrogen and oxygen. Here we show how the photocatalytic activity of thylakoid membranes leads to overall water splitting in a bio-photo-electro-chemical (BPEC) cell via a simple process. Thylakoids extracted from spinach are introduced into a BPEC cell containing buffer solution with ferricyanide. Upon solar-simulated illumination, water oxidation takes place and electrons are shuttled by the ferri/ferrocyanide redox couple from the thylakoids to a transparent electrode serving as the anode, yielding a photocurrent density of 0.5 mA cm−2. Hydrogen evolution occurs at the cathode at a bias as low as 0.8 V. A tandem cell comprising the BPEC cell and a Si photovoltaic module achieves overall water splitting with solar to hydrogen efficiency of 0.3%. These results demonstrate the promise of combining natural photosynthetic membranes and man-made photovoltaic cells in order to convert solar power into hydrogen fuel.

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SUMMARY Although light is the driving force of photosynthesis, excessive light can be harmful. One of the main processes that limits photosynthesis is photoinhibition, the process of light‐induced photodamage. When the absorbed light exceeds the amount that is dissipated by photosynthetic electron flow and other processes, damaging radicals are formed that mostly inactivate photosystem II (PSII). Damaged PSII must be replaced by a newly repaired complex in order to preserve full photosynthetic activity. Chlorella ohadii is a green microalga, isolated from biological desert soil crusts, that thrives under extreme high light and is highly resistant to photoinhibition. Therefore, C. ohadii is an ideal model for studying the molecular mechanisms underlying protection against photoinhibition. Comparison of the thylakoids of C. ohadii cells that were grown under low light versus extreme high light intensities found that the alga employs all three known photoinhibition protection mechanisms: (i) massive reduction of the PSII antenna size; (ii) accumulation of protective carotenoids; and (iii) very rapid repair of photodamaged reaction center proteins. This work elucidated the molecular mechanisms of photoinhibition resistance in one of the most light‐tolerant photosynthetic organisms, and shows how photoinhibition protection mechanisms evolved to marginal conditions, enabling photosynthesis‐dependent life in severe habitats.

A chloroplast (nuclear-encoded) RNA-binding protein (28RNP) was previously purified from spinach (Spinacia oleracea). This 28RNP was found to be the major RNA-binding protein co-purified during the isolation scheme of 3' end RNA-processing activity of severa1 chloroplastic genes. To learn more about the possible involvement of 28RNP in the 3' end RNA-processing event, we investigated the RNA-binding properties and the location of the protein in the chloroplast. We found that recombinant Escherichia coliexpressed 28RNP binds with apparently the same affinity to every chloroplastic 3' end RNA that was analyzed, as well as to RNAs derived from the 5' end or the coding region of some chloroplastic genes. Differences in the RNA-binding affinities for some chloroplastic 3' end RNAs were observed when the recombinant 28RNP was compared with the "native" 28RNP in the chloroplast-soluble protein extract. In addition, we found that the 28RNP i s not associated with either thylakoid-bound or soluble polysomes in which a great portion of the chloroplast rRNA and mRNA are localized. These results suggest that the native 28RNP binds specifically to certain RNA molecules in the chloroplast in which other components (possibly proteins) and/or posttranslational modifications are involved i n determining RNA-binding specificity of the 28RNP.

Photosystem I reaction center was isolated from the cyanobacterium Mastigocladus laminosus. It contained four different subunits with molecular masses (as determined by sodium dodecyl-sulfate gels) of about 70,000 (subunit I), 16,000 (subunit II), 11,000 (subunit Ill), and 10,000 (subunit IV) daltons. The purified reaction center contained about 100 chlorophyll a moleoules per P700; however, they could be readily depleted down to about 50 chlorophyll a per P700 without loss in the photochemical activities. The reaction center was active in cytochrome c photooxidation, but the photooxidation of an acidic cytochrome, like the Euglena cytochrome 552, required the presence of cations. The purified reaction center was found to be similar in several respects to the photosystem I reaction centers from higher plants and, especially, to the one isolated from green algae. Subunit I appeared -on sodium dodecyl sulfate gels in the same position and possessed the same shape of an apparent double band as the corresponding subunits I of green plants and of algae. Subunits I and II of photosystem I reaction centers from Mastigocladus, higher plants, and green algae showed immunological crossreactivity. This observation might serve as biochemical evidence for the common evolution of the photosystem I reaction centers. In higher plants and green algae subunit II is a product of cytoplasmic ribosomes and therefore, a high degree of homology should have been preserved upon transfer of its gene from the prokaryote to the nucleus of the eukaryotes.