Presence Of DCMU Research Articles

Interactions between inorganic phosphate (Pi) assimilation, photosynthesis and respiration in the Pi‐limited green alga Selenastrum minutum*

ABSTRACTInorganic phosphate (Pi)‐limited chemostat cultures of the green alga Selenastrum minutum were employed to investigate interactions between Pi assimilation, respiration and photosynthetic processes. Changes in net and gross gas exchange rates indicated that O2 evolution decreases during photosynthetic Pi assimilation. Room temperature and 77K Chi a fluorescence measurements revealed that this photosynthetic suppression is correlated with a transition from state 1 to state 2. Substantial photosynthetic Pi uptake rates occur in the presence of DCMU and KCN. Additionally, the cellular ratio of ATP:NADPH increases following Pi enrichment, suggesting that the ratio of cyclic to linear electron flow is enhanced in response to the high energy requirements of Pi uptake. Net starch degradation was observed during photosynthetic Pi assimilation and the cellular pool size of 3‐phosphoglycerate increased; however, gross gas exchange parameters and cellular metabolite pool sizes indicated that mitochondrial respiration plays a smaller role during Pi assimilation in the light than it does in the dark. These observations were used to formulate a model depicting possible interactions between photosynthetic electron flow, photosynthetic and respiratory carbon metabolism and metabolite exchange between the chloroplast, cytosol and mitochondrion during photosynthetic Pi assimilation.

Partial molecular analysis of the psbA gene in Euglena gracilis mutants exhibiting resistance to DCMU and atrazine.

Mutations conferring herbicide resistance have been detected in two new strains (ZR250 and ZR480) of Euglena gracilis Z by partial gene cloning and sequencing. These mutants were originally derived from Z cells grown in medium containing progressively increasing concentrations of DCMU. Each of these strains have been characterized by measuring their growth kinetics, O2 evolution, and resistance to DCMU and atrazine. Partial sequences of the psbA gene of these strains were compared to those published for strains Z and ZR25. The ZR250 and ZR480 strains were found to be double mutants. Besides the expected mutation S265A, they showed an additional point mutation at codon 219 (equivalent to codon 218 of other organisms). This mutation results in leucine being substituted by phenylalanine. For each of the ZR strains, two growth conditions (with or without DCMU in the medium) have been compared. The presence of the second mutation (at codon 219) leads to notable increase (20-fold) in resistance to DCMU, whereas the resistance to atrazine is only 2-fold. The presence of DCMU, as the selective agent, was responsible for an enhanced herbicide resistance, irrespective of the concentrations used. Substantial modifications in the rate of cell growth and O2 yields were observed when the maximal concentration (480 microM) of DCMU was used. These modifications were reversible on withdrawal of the DCMU. Thus the reversible adaptative modifications also adds to the mutational effect observed.

Suppression of flash‐induced PSII‐dependent electrogenesis caused by proton pumping in chloroplasts

Pre‐illumination of the thylakoid membrane of Peperomia metallica chloroplasts leads to a reversible suppression of the flash‐induced electrical potential as measured either with the electrochromic bandshift (P515), microelectrode impalement or patch‐clamp technique. The energization‐dependent potential suppression was not observed in the presence of 1 μM nigericin suggesting the involvement of proton and/or cation gradients. Energization in the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) and N,N,N′,N′‐tetramethylphenylenediamine (TMPD), i.e. cyclic electron flow around photosystem (PS) I, results in the accumulation of TMPD+ in the thylakoid lumen. The reversible suppression of the flash‐induced membrane potential was not observed in these conditions indicating that it is not a general cation‐induced increase of membrane capacitance. Cyclic electron flow around PSI in the presence of DCMU and phenazine methosulfate (PMS) results in the accumulation of PMS+ and H+ in the thylakoid lumen. The absence of reversible suppression of the flash‐induced membrane potential for this condition shows that accumulation of protons does not lead to (1) a reversible increase of membrane capacitance and (2) a reversible suppression of PSI‐dependent electrogenesis. Reversible inactivation of PSII by a low pH in the thylakoid lumen is therefore proposed to be the cause for the temporary suppression of the flash‐induced electrical potential. The flash‐induced PSII‐dependent membrane potential, as measured after major oxidation of P700 in far‐red background light, was indeed found to be suppressed at low assay pH (pH 5) in isolated spinach (Spinacia oleracea) chloroplasts.

Effects of Potassium-(picrate)-(18-crown-6) on the Photosynthetic Electron Transport

Abstract The effects of potassium-(picrate)-(18-crown-6) on the electron transport of photosystem II was investigated in isolated pea thylakoids. Low concentrations of the compound inhibited the fast decay of fluorescence yield associated with electron transfer between the primary (QA) and secondary (QB) quinone electron acceptor and increased the intermediary level of fluorescence to the Fmax level. The decay half-time of fluorescence yield measured in the presence of DCMU(S2QA - charge recombination) decreased from about 1.8 s to about 0.3 s in thylakoids treated with potassium-(picrate)-(18-crown-6). While the inhibition of electron transport by DCMU gave rise to the appearance of a thermoluminescence band at about + 10°C (S2QA - charge recombination) addition of potassium-(picrate)-(18-crown-6) resulted in a thermoluminescence band at about -10°C. Increasing concentrations of potassium-(picrate)-(18-crown-6) diminished the fluorescence yield and the -10°C TL band and abolished the Signal IIS and Signal IIf EPR signals of the tyrosine-D and tyrosine-Z electron donors, respectively. The phenolic-type inhibitor, potassium picrate had the same effect on thermoluminescence and on the tyrosine EPR signals. It is concluded that potassium-(picrate)-(18-crown-6) is a phenolic type inhibitor owing to its picrate constituent. At low concentrations picrate and potassium-(picrate)-18-crown) not only block the electron transport between QA and QB but they probably decrease the midpoint redox potential of QA, as well. At high concentrations they also inhibit the light-induced oxidation of the tyrosine-D and tyrosine-Z donors.

Alteration in the acceptor side of photosystem II of chloroplast by high light

The effect of high light on the acceptor side of photosystem II of chloroplasts and core particles of spinach was studied. BothVmax and apparentKm for DCIP were altered in photoinhibited photosystem II core particles. The double reciprocal plot analysis as a function of actinic light showed increased slope in chloroplasts photoinhibited in the presence of DCMU. Exposure of chloroplasts to high light in the presence of DCMU did not protect the chloroplast against high light induced decrease in Fm, level. Further the high light stress induced decrease inFm level was not restored by the addition of DCMU. These results suggest that the high light stress induced damage to chloroplast involves alteration in the binding site forQB on the DI protein on the acceptor side of photosystem II

BIOENERGETIC PROCESSES ARE MODIFIED DURING NITROGEN STARVATION AND RECOVERY IN PHORMIDIUM LAMINOSUM (CYANOPHYCEAE)1

ABSTRACTIn the non‐N2‐fixing cyanobacterium Phormidium laminosum (Agardh) Gomont (strain OH‐I‐pCl1), N starvation induced an increase in the rate of respiration and a decrease in the rate of O2 evolution. When NO3− was added to illuminated N‐starved cells, O2 evolution immediately increased to levels shown by NO3− grown cells, even though N‐starved cells had lost most of their in vitro photosynthetic activities. Stimulation of noncyclic electron flow was maximal under light‐saturating conditions and after 2–3 days of N starvation. The respiratory rate of N‐starved cells was stimulated by the addition of NO3− or NH4+ and partially inhibited at very low irradiances, even in the presence of DCMU (3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea). Results indicate that N‐starved cells obtain the energy supply for N assimilation through a process different from that used by N‐sufficient cells. N‐starved cells were able to take up NO3− in the dark and when illuminated in the presence of DCMU under anaerobiosis. Following NO3− addition, the photosynthetic yield of the in vivo noncyclic electron transport slightly increased, whereas it decreased after NH4+ addition. Addition of NO3− or NH4+ favored photoinhibition of photosystem II, the effect being faster after NH4+ addition.

Carotenoids and Reaction Center II‐D1 Protein in Light Regulation of the Photosynthetic Apparatus in Aphanocapsa*

Abstract:A new cyanobacterial isolate, morphologically closely resembling Aphanocapsa, was characterized for its growth requirements, as well as pigmentation, photosynthetic activity and dynamics of the D1 protein in the reaction center (RC) of photosystem II (PSII). It was shown to be able to grow on glucose in the dark in the presence of DCMU. The cyanobacterium turned light yellow at high light intensity in the absence, and dark emerald green in high light in the presence of sublethal concentrations of the DCMU‐type inhibitor atrazine. While total carotenoids per cell slightly decreased with increasing light intensity during growth, the cells still turned pale yellow due to decreased levels of chlorophyll and phycocyanin. In contrast to β‐carotene, zeaxanthin and echinenone which decreased with increasing light intensity during growth, the carotenoid glycoside, myxoxanthophyll, continuously increased in concentration.Extremely high rates of light‐saturated O2 evolution were recorded for the high light cultures after a 0.5 h recovery period in the dark. The recovery measured after 2.5 h was shown to be less effective in darkness than in dim light and was prohibited by chloramphenicol. The degree of recovery was dependent on the light intensity during growth. A fast light intensity‐dependent RC II‐D1 protein turnover was found for the bleached yellow cells rich in myxoxanthophyll. The half‐life of the RC II‐D1 protein, plotted against the light intensity during growth and experimentation, yielded a curve the slope of which was considerably steeper for Aphanocapsa than for Anacystis. Apparently, the isolated strain of Aphanocapsa reacts more vigorously to changes in the environment than other strains tested and may, therefore, turn out to be a suitable organism in the attempt to elucidate the molecular mechanism of light intensity adaptation.

Photosynthetic Characteristics of Three Strains of Cyanobacteria Grown under Low-or High-C02 Conditions

Abstract Quantum requirements of photosynthetic oxygen evolution at 679 nm, fluorescence emis­sion spectra at liquid nitrogen temperature (77 K) and fluorescence induction kinetics in the presence of DCM U, were measured in the cyanobacteria Anabaena variabilis M3, Anabaena variabilis ATCC 29413 and A nacystis nidulans R2, each grown under low-or high-CO2 conditions. Low -CO2 grown cells of the cyanobacteria showed a higher quantum requirement of photosynthetic oxygen evolution and a higher ratio o F710-740 to F680-700 fluorescence and a lower variable fluorescence in the presence of DCMU than high-CO2 grown cells. These findings indicate a change in excitation energy distribution in favour of photosystem I. The result might be an enhancement in ATP formation caused by cyclic electron flow which in turn provokes dissolved inorganic carbon (DIC) accumulation in these low-CO2 grown cells.

Effect of UV-B on Enzymes of Nitrogen Metabolism in the Cyanobacterium Nostoc calcicola

Summary The effects of ultraviolet-B (UV-B; 280–315 nm) irradiation on nitrogenase and nitrate reductase (NR) activity have been studied in the fdamentous and heterocystous N2-fixing cyanobacterium Nostoc calcicola . Exposure of cultures to UV-B (5W/m 2 ) for as little as 30 min caused complete inactivation of nitrogenase activity whereas nitrate reductase activity was stimulated twofold in comparison to one exposed to fluorescent white light. GS activity was also inhibited by UV-B treatment, but there was no total loss of activity even after 4 h. NR activity showed a gradual stimulation up to 4 h and thereafter it became constant. Stimulation was also obtained in reductant deficient cultures (12 h incubation in the dark) suggesting independence of NR of PS-II under UV-B. NR activity was also unaffected in the presence of DCMU, a known inhibitor of PS-II. However, both O 2 evolution and 14 CO 2 uptake were completely abolished following 30 min of UV-B treatment. Addition of the protein synthesis inhibitor chloramphenicol (25 μg/mL) to cultures did not show any inhibitory effect on NR activity. SDS-PAGE analysis of UV-B treated cultures elicited gradual loss of protein bands with increasing duration of exposure. Our findings suggest that UV-B irradiance has differential effects on the enzymes of the nitrogen metabolism in the cyanobacterium Nostoc calcicola . Further studies are needed to reveal the exact mechanism involved in the stimulation of NR activity by UV-B. Whether UV-B has a direct effect on NO 2 − accumulation in the cells needs detailed investigation.

EPR study of trapped tyrosine Z + in Ca-depleted photosystem II

The dependence of the light-induced transient EPR signal intensity of Y Z + on the illumination temperature was measured in Ca-depleted PS II in the presence of DCMU. The maximum yield of Y Z +, about 70% of Y D + EPR intensity, was obtained by illumination at 245 K, and trapped by immediate freezing in liquid nitrogen. Analysis of the relative intensities of Y Z + and modified multiline EPR signals under various illumination periods and temperatures shows that Y Z + has been trapped in the S 1 state of the oxygen evolving center, and the trapped Y Z + EPR converted to the multiline signal through the advancement from the S 1 to S 2 state in the dark at 273 K. Spin-lattice relaxation times of Y D + and Y Z + in the Ca-depleted and Mn-depleted PS II were measured by pulsed EPR. From the relaxation enhancements of these radicals due to the dipole interaction with the Mn cluster, the distance from Y Z + to the Mn cluster was estimated to be 15–20 Å. Using the 2 + 1 ESE method, the distance between Y Z + and Y D + was determined to be 29–30 Å.

PH-dependent photoreactions of the high- and low-potential forms of cytochrome b559 in spinach PS II-enriched membranes

Cytochrome b559 (Cyt b559) is a well-known intrinsic component of Photosystem II (PS II) reaction center in all photosynthetic oxygen-evolving organisms, but its physiological role remains unclear. This work reports the response of the two redox forms of Cyt b559 (i.e. the high- (HP) and low-potential (LP) forms) to inhibition of the donor or acceptor side of PS II. The photooxidation of HP Cyt b559 induced by red light at room temperature was pH-dependent under conditions in which electron flow from water was diminished. This photooxidation was observed only at pH values higher than 7.5. However, in the presence of 1 μM CCCP, a limited oxidation of HP Cyt b559 was observed at acidic pH, At pH 8.5 and in the presence of the protonophore, this photooxidation of the HP form was accompanied by its partial transformation into the LP form. On the other hand, a partial photoreduction of LP Cyt b559 was induced by red light under aerobic conditions when electron transfer through the primary quinone acceptor QA was impaired by strong irradiation in the presence of DCMU. This photoreduction was enhanced at acidic pH values. To the best of our knowledge, this is the first time that both photoreduction and photooxidation of Cyt b559 is described under inhibitory conditions using the same kind of membrane preparations. A model accommodating these findings is proposed.

Thermoluminescence as a probe of Photosystem II in intact leaves: Non-photochemical fluorescence quenching in peas grown in an intermittent light regime

We have measured thermoluminescence (TL) and chlorophyll fluorescence from leaves of peas grown under an intermittent light regime (IML) and followed changes in those leaves during greening. IML peas show low variable fluorescence and a certain capacity for reversible non-photochemical quenching. It has been suggested that reversible quenching may be caused by pH-dependent release of Ca(2+) from Photosystem II (PS II) (Krieger and Weis (1992) Photosynthetica 27: 89-98). Under conditions in which reversible non-photochemical quenching occurs, a TL band at around 50 °C is observed, in the presence of DCMU, in IML leaves. A band in this temperature range has previously been observed in PS II depleted of Ca(2+) (Ono and Inoue (1989) Biochimica et Biophysica Acta 973: 443-449). The 50 °C band disappears upon dark adaptation. In mature leaves, no significant band is seen at 50 °C. It is concluded that, in IML leaves, reversible quenching may be related to the release of Ca(2+) from Photosystem II. However, it seems that in the mature system, under most conditions, such release does not contribute significantly to quenching.

Emission of Sulfur Dioxide from Sulfite-Treated Birch Leaves

Summary In the present paper, we report the emission of S02 from 10 mM sulfite-treated birch (Betula papyrifera Marsh) leaves both in light and in the dark. The emission reaches maximum at around 60 min during treatment. In some leaves, maximum emission is noticed even after 5 to 10 min of initiation of sulfitetreatment. It is also observed in the presence of 3{3,4-dichlorophenyl)-1-1 dimethylurea (DCMU). However, the emission rates are higher in light than in the dark or in the presence of DCMU. This emission is not observed with sulfate-treated leaves.

Different niches of the Halophila stipulacea seagrass bed harbor distinct populations of nitrogen fixing bacteria

Halophila stipulacea beds in the northern Gulf of Elat (Red Sea) may be subdivided into three different sub-habitats, each harboring a distinct population of diazotrophs. Nitrogen (N2) fixation in the phyllosphere and in the rhizomes/upper-sediment niche was light dependent, suggesting its dependence on photosynthesis. N2 fixation in the phyllosphere was not affected by the addition of either glucose or 3-3,4-dichloro-phenyl-1,1-dimethyl-urea (DCMU), indicating that the diazotrophs involved carried out non-oxygenic photosynthesis. They may, thus, have been photosynthetic bacteria. N2 fixation in the rhizomes/upper-sediment niche, however, was greatly enhanced by the addition of glucose, but was suppressed in the presence of DCMU. This indicates that the diazotrophs involved here probably possess two photosystems (I and II) and may be Cyanobacteria. The anaerobic rhizosphere, in which fixation rates in light were very slow but were greatly enhanced by the addition of glucose, is probably populated by heterotrophic diazotrophs. Plant and sediment samples used in the present study were collected from the Gulf of Elat between 1990 and 1992.

Influence of DCMU and ferricyanide on photodamage in photosystem II

The effect of strong illumination of thylakoid membranes was studied under a range of conditions. Under anaerobic conditions, the relatively small quenching of the maximum fluorescence (Fmax) is accompanied by a large increase of the initial fluorescence (F0), which is partially reversible. Changes in the extent of the QA-Fe2+ and chlorophyll triplet EPR signal during anaerobic photoinhibition were consistent with double reduction of QA [as reported by Vass et al. ((1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1408-1412)]. When illumination was done in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or ferricyanide, no change occurred in F0 while Fmax was quenched. The quenching of Fmax occurred more rapidly than the loss of oxygen evolution, and they were both irreversible. In the presence of ferricyanide, the percentage of inhibition of oxygen evolution was larger than the decrease in the extent of the QA-Fe2+ signal, indicating that damage of the donor side occurred. In the presence of DCMU, a decrease of the QA-Fe2+ EPR signal occurred which corresponded to the inhibition of oxygen evolution and to an increase of the triplet EPR signal, indicating a possible overreduction of QA. However, these changes were less marked in the DCMU-treated samples than in the sample without additions and occurred despite the quenching of Fmax. These results suggest that strong illumination of thylakoids, in the presence of DCMU, results in a slower formation of stable forms of reduced QA, thereby allowing the occurrence of side-path reactions leading to Fmax quenching.(ABSTRACT TRUNCATED AT 250 WORDS)

The origin of 40–50°C thermoluminescence bands in Photosystem II

We have used thermoluminescence (TL) and EPR measurements of Photosystem II (PS II) from spinach in order to identify charge pairs responsible for TL bands in the region of 40–50°C including the ‘C-band’ (peak V) and the TL band from PS II depleted of calcium. In intact PS II membrane preparations, in the presence of DCMU, a TL band at 50°C is induced following illumination at 77 K. This band decays, at 30°C, with a half-time of 10 min. This decay corresponds to the disappearance of the EPR signal arising from Q A − and an accelerated decay of the organic free radical Tyr D +. It is concluded that recombination of this charge pair is probably responsible for the thermoluminescence emission. In PS II preparations that have been depleted of calcium using a salt/EGTA wash followed by rebinding of the extrinsic polypeptides, a TL band is produced at around 45–50°C following 198 K illumination. In such samples a stable S 2 state of the water-splitting complex is present, giving rise to a modified form of the EPR multiline signal. During incubation at 30°C in the dark this signal decays with a half-time around 20–25 min. This decay is not accelerated by the presence of Q A − induced by low-temperature illumination of the sample. In contrast, low-temperature illumination does result in an acceleration in the decay of Tyr D +, indicating that Tyr D +/Q − A recombination is again the dominant origin of thermoluminescence. In PS II depleted of calcium by incubation at pH 4.0, the possibility that TL emission temperature is determined by a change in the mid-point redox potential of Q A (Krieger, A. and Weis, E. (1992) Photosynthetica 27, 89–98) was investigated by comparing TL from equivalent samples of control and Ca 2+-depleted PS II. It was shown that the emission temperature of the high temperature TL band induced by illumination at 77 K did not differ significantly between control and treated samples, suggesting that, under the conditions used, the potential of Q A does not change significantly.

Characterization of temperature‐induced changes in the photosynthetic properties of Anacystis nidulans grown at elevated temperature: A differential response to heat shock

AbstractPhotosynthetic properties of high temperature (38 °C) grown (HTG)1 and low temperature (25 °C) grown (LTG) cells of Anacystis were examined. The results revealed several characteristic changes in the HTG cells which included light absorption property of the pigments, DCMU binding affinity of the photosynthetic apparatus and photosynthetic yield. There was a time course dependent heat shock (47 ± 1 °C) induced inhibition of the O2 evolution in both the types of cells. However, the response of the HTG cells was sluggish as compared to the LTG cells. Unlike the LTG cells, the heat shock treatment (47 ± 1 °C, 5 min) of the HTG cells did not result into a change in the pigment absorption spectra. The rise kinetics of fluorescence induction curve of chlorophyll a in the presence of DCMU and DCMU + NH2OH suggested for heat‐induced damage to the oxidizing side of photosystem II in both the HTG and LTG cells. Further, results on the Hill activity in the heat shocked HTG cells showed that the exogenous electron donors DPC and NH2OH not only retrieved the heat shock induced inactivation of the Hill activity, but stimulated the rate of activity. On the other hand, the rate of Hill activity in the heat shocked LTG cells was restored by only NH2OH an electron donor to PS II reaction centre, while DPC‐an electron donor to water oxidation complex was ineffective against the heat shock induced inactivation of the electron transport. These results, together, suggested for the presence of less number of heat susceptible sites in the HTG cells than that in the LTG cells, which might be accounting for the high degree of thermostability to the former type of cells.

PsaE Is Required for in Vivo Cyclic Electron Flow around Photosystem I in the Cyanobacterium Synechococcus sp. PCC 7002.

Electron transfer rates to P700+ have been determined in wild-type and three interposon mutants (psaE-, ndhF-, and psaE- ndhF-) of Synechococcus sp. PCC 7002. All three mutants grew significantly more slowly than wild type at low light intensities, and each failed to grow photoheterotrophically in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and a metabolizable carbon source. The kinetics of P700+ reduction were similar in the wild-type and mutant whole cells in the absence of DCMU. In the presence of DCMU, the P700+ reduction rate in the psaE mutant was significantly slower than in the wild type. In the presence of DCMU and potassium cyanide, added to inhibit the outflow of electrons through cytochrome oxidase, P700+ reduction rates increased for both the psaE- and ndhF- strains. The reduction rates for these two mutants were nonetheless slower than that observed for the wild-type strain. The further addition of methyl viologen caused the rate of P700+ reduction in the wild type to become as slow as that for the psaE mutant in the absence of methyl viologen. Given the ability of methyl viologen to intercept electrons from the acceptor side of photosystem I, this response reveals a lesion in cyclic electron flow in the psaE mutant. In the presence of DCMU, the rate of P700+ reduction in the psaE ndhF double mutant was very slow and nearly identical with that for the wild-type strain in the presence of 2,4-dibromo-3-methyl-6-isopropyl-p-benzoquinone, a condition under which physiological electron donation to P700+ should be completely inhibited. These results suggest that NdhF- and PsaE-dependent electron donation to P700+ occurs only via plastoquinone and/or cytochrome b6/f and indicate that there are three major electron sources for P700+ reduction in this cyanobacterium. We conclude that, although PsaE is not required for linear electron flow to NADP+, it is an essential component in the cyclic electron transport pathway around photosystem I.

Physiology of purple sulfur bacteria forming macroscopic aggregates in Great Sippewissett Salt Marsh, Massachusetts

Purple bacterial aggregates found in tidal pools of Great Sippewissett Salt Marsh (Falmouth, Cape Cod, MA) were investigated in order to elucidate the ecological significance of cell aggregation. Purple sulfur bacteria were the dominant microorganisms in the aggregates which also contained diatoms and a high number of small rod-shaped bacteria. Urea in concentrations of ≥ 1 M caused disintegration of the aggregates while proteolytic enzymes, surfactants or chaotropic agents did not exhibit this effect. This suggests that polysaccharides in the embedding slime matrix stabilize the aggregate structure. In addition cell surface hydrophobicity is involved in aggregate formation. The concentration of dissolved oxygen decreased rapidly below the surface of aggregates while sulfide was not detected. The apparent respiration rate in the aggregates was high when the purple sulfur bacteria contained intracellular sulfur globules. In the presence of DCMU, respiration remained light-inhibited. Light inhibition disappeared in the presence of KCN. These results demonstrated that respiration in the aggregates is due mainly to purple sulfur bacteria. The concentration of bacteriochlorophyll (Bchl) a in the aggregates (0.205 mg Bchl a cm−3) was much higher than in the pool sediments but comparable to concentrations in microbial mats of adjacent sand flats. Purple aggregates may therefore originate in the microbial mats rather than in the pools themselves. Rapid sedimentation and high respiration rates of Chromatiaceae in the aggregates would prevent the inhibition of Bchl synthesis if aggregates were lifted off the sediment and up into the oxic pool water by tidal currents.

Properties of inactive Photosystem II centers

A fraction (usually in the range of 10-25%) of PS II centers is unable to transfer electrons from the primary quinone acceptor QA to the secondary acceptor QB. These centers are inactive with respect to O2 evolution since their reopening after photochemical charge separation to the S2OA (-) state involves predominantly a back reaction to S1QA in the few seconds time range (slower phases are also occurring). Several properties of these centers are analyzed by fluorescence and absorption change experiments. The initial rise phase Fo-Fpl of fluorescence induction under weak illumination reflects both the closure of inactive centers and the modulation of the fluorescence yield by the S-states of the oxygen-evolving system: We estimate typical relative amplitudes of these contributions as, respectively, 65 and 35% of the Fo-Fpl amplitude. The half-rise time of this phase is significantly shorter than for the fluorescence induction in the presence of DCMU (in which all centers are involved). This finding is shown to be consistent with inactive centers sharing the same light-harvesting antenna as normal centers, a view which is also supported by comparing the dependence of the fluorescence yield on the amount of closed active or inactive centers estimated through absorption changes. It is argued that the exponential kinetics of the Fo-Fpl phase does not indicate absence of excitation energy transfer between the antennas of inactive and active centers. We show that the acceptor dichlorobenzoquinone does not restore electron transfer in inactive centers, in disagreement with previous suggestions. We confirm, however, the enhancement of steady-state electron flow caused by this quinone and suggest that it acts by relieving a blocking step involved in the reoxidation of a fraction of the plastoquinone pool. Part of the discrepancies between the present results and those from previous literature may arise from the confusion of inactive centers characterized on a single turnover basis and PS II centers that become blocked under steady-state conditions because of deficient reoxidation of their secondary acceptors.