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The green alga, Chlamydomonas reinhardtii, can photoproduce molecular H 2 via ferredoxin and the reversible [Fe]hydrogenase enzyme under anaerobic conditions. Recently, a novel approach for sustained H 2 gas photoproduction was discovered in cell cultures subjected to S-deprived conditions (A. Melis, L. Zhang, M. Forestier, M.L. Ghirardi, M. Seibert [2000] Plant Physiol 122: 127-135). The close relationship between S and Fe in the H 2 -production process is of interest because Fe-S clusters are constituents of both ferredoxin and hydrogenase. In this study, we used Mö ssbauer spectroscopy to examine both the uptake of Fe by the alga at different CO 2 concentrations during growth and the influence of anaerobiosis on the accumulation of Fe. Algal cells grown in media with 57 Fe(III) at elevated (3%, v/v) CO 2 concentration exhibit elevated levels of Fe and have two comparable pools of the ion: (a) Fe(III) with Mö ssbauer parameters of quadrupole splitting ϭ 0.65 mm s Ϫ1 and isomeric shift ϭ 0.46 mm s Ϫ1 and (b) Fe(II) with quadrupole splitting ϭ 3.1 mm s Ϫ1 and isomeric shift ϭ 1.36 mm s Ϫ1 . Disruption of the cells and use of the specific Fe chelator, bathophenanthroline, have demonstrated that the Fe(II) pool is located inside the cell. The amount of Fe(III) in the cells increases with the age of the algal culture, whereas the amount of Fe(II) remains constant on a chlorophyll basis. Growing the algae under atmospheric CO 2 (limiting) conditions, compared with 3% (v/v) CO 2 , resulted in a decrease in the intracellular Fe(II) content by a factor of 3. Incubating C. reinhardtii cells, grown at atmospheric CO 2 for 3 h in the dark under anaerobic conditions, not only induced hydrogenase activity but also increased the Fe(II) content in the cells up to the saturation level observed in cells grown aerobically at high CO 2 . This result is novel and suggests a correlation between the amount of Fe(II) cations stored in the cells, the CO 2 concentration, and anaerobiosis. A comparison of Fe-uptake results with a cyanobacterium, yeast, and algae suggests that the intracellular Fe(II) pool in C. reinhardtii may reside in the cell vacuole.Light energy conversion by algae, higher plants, and cyanobacteria is accompanied by water oxidation on the donor side of photosystem II (PSII) with the resultant evolution of molecular O 2 . The electrons extracted from water by PSII are transported to ferredoxin and NAPD ϩ via photosystem I (PSI), where they are normally used to fix CO 2 . However, after anaerobic incubation in the dark, illumination of Chlamydomonas reinhardtii (Greenbaum, 1982), Chlorella fusca (Kessler, 1974), Scenedesmus obliquus (Gaffron and Rubin, 1942), and some other species of algae leads to the expression of H 2 -evolution function. Molecular H 2 is produced as a result of ferredoxin-mediated electron transport to an induced, reversible [Fe]hydrogenase (rather than to NAPD ϩ and the Benson-Calvin Cycle) where the enzyme catalyzes the reduction of protons to H 2 gas.There are several types of hydrogenases tha...

Light-induced interaction of Fe(II) cations with the donor side of Mn-depleted photosystem II (PS II(-Mn)) results in the binding of iron cations and blocking of the high-affinity (HAZ) Mn-binding site. The pH dependence of the blocking was measured using the diphenylcarbazide/2,6-dichlorophenolindophenol test. The curve of the pH dependence is bell-shaped with pK1 = 5.8 and pK2 = 8.0. The pH dependence of the O2-evolution mediated by PS II membranes is also bell-shaped (pK2 = 7.6). The pH dependence of the process of electron donation from exogenous donors in PS II(-Mn) was studied to determine the location of the alkaline pH sensitive site of the electron transport chain. The data of the study showed that the decrease in the iron cation binding efficiency at pH > 7.0 during blocking was determined by the donor side of the PS II(-Mn). Mössbauer spectroscopy revealed that incubation of PS II(-Mn) membranes in a buffer solution containing 57Fe(II) + 57Fe(III) was accompanied by binding only Fe(III) cations. The pH dependence of the nonspecific Fe(III) cation binding is also described by the same bell-shaped curve with pK2 = 8.1. The treatment of the PS II(-Mn) membranes with the histidine modifier diethylpyrocarbonate resulted in an increase in the iron binding strength at alkaline pH. It is suggested that blocking efficiency at alkaline pH is determined by competition between OH- and histidine ligand for Fe(III). Because the high-affinity Mn-binding site contains no histidine residue, this fact can be regarded as evidence that histidine is located at another (other than high-affinity) Fe(III) binding site. In other words, this means that the blockage of the high-affinity Mn-binding site is determined by at least two iron cations. We assume that inactivation of oxygen-evolving complex and inhibition of photoactivation in the alkaline pH region are also determined by competition between OH- and a histidine residue involved in coordination of manganese cation outside the high-affinity site.

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