Structure of a microbiol mat at Great Sippewissett Marsh, Cape Cod, Massachusetts

The microzination of phototrophic bacteria in a flat laminated microbiol mat at Great Sippewissett Salt Marsh on Cape Cod, Massachusetts, was studied using a combination of scanning and transmission electron microscopy, light microscopy and photosynthetic pigment analysis. Comparison of pigment content and ultrastructural information from electron microscopy of thin sections allowed us to determine the major groups of photosynthetic bacteria present. The approximately 1-cm-thick mat is located in sandy intertidal sediments of the marsh and comprised four to five distinctly colored layers. The uppermost brown layer contained Lyngbya, Nostoc, Phormidium (cyanobacteria) and Navicula (diatom) species. An intermediate bluish-green layer was dominated by Oscillatoria species. A central pink layer contained purple sulfur bacteria such as Amoebobacter, Thiocapsa, Chromatium and Thiocystis species, Below this was a distinctive orange layer, formed largely by one species of purple sulfur bacteria, Thiocapsa pfennigii. The lowermost and thinnest layer contained green sulfur bacteria of the genus Prosthecochloris, a very small prosthecate species with numerous knobby projections; this layer was not always present. Below this, where pigments were generally absent, were dark gray and black iron sulfide-rich sediments. Remnants of older decayed mats could be found deeper in the sediment. Extensive production of microbial extracellular polymers in all layers appeared to be responsible for attachment of cells to sand grains, for lamination of layers and for structural integrity of the mat as a whole. Below the layer of green sulfur bacteria, binding of sediment by microbial polymers ceased abruptly. Possibly in response to decreasing light penetration, the mean size of bacterial cells decreased in successively deeper layers. In the lowest layer where light penetration was very low, green sulfur bacteria with highly convoluted surfaces occurred. The increase in cell surface area-to-volume ratio may allow such organisms to survive at low light levels.     

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

Abstract 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.     

Vertical distribution of methane metabolism in microbial mats of the Great Sippewissett Salt Marsh

Methane metabolism was investigated with respect to depth in intertidal microbial mats of the Great Sippewissett Salt Marsh, Massachusetts. Although sulfate-reducing organisms dominate anaerobic carbon consumption in marine microbial mats, methanogens persist and their activity varies vertically and temporally in the mat system. In the Sippewissett mats, potential methane production for all mat layers was higher in the spring (17.2 ± 4.5 nmol CH₄ cm⁻² day⁻¹) than in the fall (3.0 ± 1.1 nmol CH₄ cm⁻² day⁻¹) and maximal rates were consistently observed in proximity to the chemocline (5–10 mm depth). The methane flux from the mat surface did not vary appreciably over time due to the ability of methanotrophic activity to limit net methane production. Evidence indicates that both aerobic and anaerobic oxidation of methane occurs in this system. The importance of H₂ as a substrate for methanogenesis appeared to be the greatest at the mat surface (0–10 mm), and the proportion of methylotrophic methanogens generally increased with depth. These results suggest that both non-equilibrium H₂ dynamics and the use of non-competitive substrates permit coexistence of methanogens and sulfate-reducing organisms in the mat system.     

沼泽红假单胞菌光合色素的分离、组成分析与光稳定性

【目的】探求光对不产氧光合细菌类胡萝卜素(Car)和细菌叶绿素a(BChl a)稳定性的影响规律。【方法】以沼泽红假单胞菌CQV97为材料,采用硅胶柱层析和HPLC方法进行Car和BChl a组分的纯化和成分分析,采用吸收光谱法研究Car和BChl a组分的光稳定性。【结果】在Car和BChl a组分分离过程中,Car组分回收率高且稳定,而BChl a回收率波动性较大。Car组分中含有6种螺菌黄质系Car和极少量(0.25%)的细菌脱镁叶绿素a。BChl a组分中包含BChl aGG、BChl aDHGG、BChl aTHGG和BChl aP4种成分。Car和BChl a组分在黑暗条件下非常稳定。2 000 lx白炽灯、日光灯和自然光照射时,Car在70 min内非常稳定,但对紫外光敏感,半衰期为11.15 min,BChl a组分对白炽灯、日光灯、自然光和紫外灯的光降解速率常数(min–1)分别为0.169 8、0.028 9、0.213 9和0.026 4,半衰期(min)分别为4.47、29.68、4.20和26.19。【结论】一步硅胶柱层析可同时得到Car和BChl a纯组分。Car对白光相对稳定,对紫外光不稳定。BChl光稳定性很差,分离过程中短期见光是导致BChl a回收率波动性较大的原因,光降解过程中产生了相对稳定的中间产物。该研究结果为光合色素的精制、功能研究和应用提供了理论依据。     

Selective enrichment and characterization of Roseospirillum parvum, gen. nov. and sp. nov., a new purple nonsulfur bacterium with unusual light absorption properties

A new type of phototrophic purple bacterium, strain 930I, was isolated from a microbial mat covering intertidal sandy sediments of Great Sippewissett Salt Marsh (Woods Hole, Mass., USA). The bacterium could only be enriched at a wavelength of 932 (± 10) nm. Cells were vibrioid- to spirilloid-shaped and motile by means of bipolar monotrichous flagellation. The intracytoplasmic membranes were of the lamellar type. Photosynthetic pigments comprised bacteriochlorophyll a and the carotenoids spirilloxanthin and lycopenal. The isolated strain exhibited an unusual, long-wavelength absorption maximum at 911 nm. Sulfide or thiosulfate served as electron donor for anoxygenic phototrophic growth. During growth on sulfide, elemental sulfur globules formed outside the cells. Elemental sulfur could not be further oxidized to sulfate. In the presence of sulfide plus bicarbonate, fructose, acetate, propionate, butyrate, valerate, 2-oxoglutarate, pyruvate, lactate, malate, succinate, fumarate, malonate, casamino acids, yeast extract, L(+)-alanine, and L(+)-glutamate were assimilated. Sulfide, thiosulfate, or elemental sulfur served as a reduced sulfur source for photosynthetic growth. Maximum growth rates were obtained at pH 7.9, 30 °C, 50 μmol quanta m^–2 s^–1 of daylight fluorescent tubes, and a salinity of 1–2% NaCl. The strain grew microaerophilically in the dark at a partial pressure of 1 kPa O₂. The DNA base composition was 71.2 mol% G + C. Sequence comparison of 16S rRNA genes indicated that the isolate is a member of the α-Proteobacteria and is most closely related to Rhodobium orientis at a similarity level of 93.5%. Because of the large phylogenetic distance to known phototrophic species of the α-Proteobacteria and of its unique absorption spectrum, strain 930I is described as a new genus and species, Roseospirillum parvum gen. nov. and sp. nov. Received: 29 December 1998 / Accepted: 17 March 1999     

Sulfur-metabolizing bacterial populations in microbial mats of the Nakabusa hot spring, Japan

At the Nakabusa hot spring, Japan, dense olive-green microbial mats develop in regions where the slightly alkaline, sulfidic effluent has cooled to 65 °C. The microbial community of such mats was analyzed by focusing on the diversity, as well as the in situ distribution and function of bacteria involved in sulfur cycling. Analyses of 16S rRNA and functional genes (aprA, pufM) suggested the importance of three thermophilic bacterial groups: aerobic chemolithotrophic sulfide-oxidizing species of the genus Sulfurihydrogenibium (Aquificae), anaerobic sulfate-reducing species of the genera Thermodesulfobacterium/Thermodesulfatator, and filamentous anoxygenic photosynthetic species of the genus Chloroflexus. A new oligonucleotide probe specific for Sulfurihydrogenibium was designed and optimized for catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). In situ hybridizations of thin mat sections showed a heterogeneous vertical distribution of Sulfurihydrogenibium and Chloroflexus. Sulfurihydrogenibium dominated near the mat surface (50% of the total mat biovolume), while Chloroflexus dominated in deeper layers (up to 64% of the total mat biovolume). Physiological experiments monitoring in vitro changes of sulfide concentration indicated slight sulfide production by sulfate-reducing bacteria under anoxic-dark conditions, sulfide consumption by photosynthetic bacteria under anoxic-light conditions and strong sulfide oxidation by chemolithotrophic members of Aquificae under oxic-dark condition. We therefore propose that Sulfurihydrogenibium spp. act as highly efficient scavengers of oxygen from the spring water, thus creating a favorable, anoxic environment for Chloroflexus and Thermodesulfobacterium/Thermodesulfatator in deeper layers.     

Salt stress and fluctuating light have separate effects on photosynthetic acclimation,but interactively affect biomass

In nature, soil salinity and fluctuating light (FL) often occur concomitantly. However, it is unknown whether salt stress interacts with FL on leaf photosynthesis, architecture, biochemistry, pigmentation, mineral concentrations, as well as whole-plant biomass. To elucidate this, tomato (Solanum lycopersicum) seedlings were grown under constant light (C, 200 μmol m⁻² s⁻¹) or FL (5–650 μmol m⁻² s⁻¹), in combination with no (0 mM NaCl) or moderate (80 mM NaCl) salinity, for 14 days, at identical photoperiods and daily light integrals. FL and salt stress had separate effects on leaf anatomy, biochemistry and photosynthetic capacity: FL reduced leaf thickness as well as nitrogen, chlorophyll and carotenoid contents per unit leaf area, but rarely affected steady-state and dynamic photosynthetic properties along with abundance of key proteins in the electron transport chain. Salt stress, meanwhile, mainly disorganized chloroplast grana stacking, reduced stomatal density, size and aperture as well as photosynthetic capacity. Plant biomass was affected interactively by light regime and salt stress: FL reduced biomass in salt stressed plants by 17%, but it did not affect biomass of non-stressed plants. Our results stress the importance of considering FL when inferring effects of salt-stress on photosynthesis and productivity under fluctuating light intensities.     

Thermal properties and oxygenase activity of ribulose-1,5-bisphosphate carboxylase from the thermophilic purple bacterium, Chromatium tepidum

Abstract Ribulose-1,5-biphosphate carboxylase (RuBPCase) partially purified from the thermophilic purple bacterium Chromatium tepidum displayed maximum carboxylase activity at 50°C, while enzyme from a related mesophilic species, Chromatium vinosum , was completely inactive at 50°C. RuBPCase from C. tepidum showed ribulose-1,5- bisphosphate-dependent oxygenase activity, and, in addition, O₂ was found to partially destroy carboxylase activity. It is concluded that thermophilic purple bacteria produce heat-stable RuBPCase and that all RuBPCases, even those from an obligate anaerobe such as C. tepidum , have associated oxygenase activity.     

Organization of intracytoplasmic membranes in a novel thermophilic purple photosynthetic bacterium as revealed by absorption, circular dichroism and emission spectra

The chromatophore of a novel thermophilic purple photosynthetic bacterium, Chromatium tepidum, had light-harvesting BChl proteins which gave absorption maxima at 917, 855 and 800 nm at 20°C. These antenna complexes were found to have BChl of the a type [4]. This is, therefore, the first example of a BChl a antenna complex which shows a long-wavelength absorption up to 917 nm. Treatment by Triton X-100 and successive sodium dodecyl sulfate polyacrylamide gel electrophoresis separated these antenna complexes into two groups. One of them has one antenna component which absorbs around 917 nm (B917). The other contains at least an antennae which absorb maximally at 800 and 855 nm (B800–855). The temperature-dependent changes of absorption, circular dichroism and emission spectra were reversible up to 70°C in the intact chromatophore and in the isolated B800–855 complex. On the contrary, the isolated complex B917 lost its absorption irreversibly over the temperature of 50°C. These results suggest a membrane structure which is essential for the thermostability of chromatophores from C. tepidum.     

Adjustment of photosynthetic activity to drought and fluctuating light in wheat

Drought is a major cause of losses in crop yield. Under field conditions, plants exposed to drought are usually also experiencing rapid changes in light intensity. Accordingly, plants need to acclimate to both, drought and light stress. Two crucial mechanisms in plant acclimation to changes in light conditions comprise thylakoid protein phosphorylation and dissipation of light energy as heat by non-photochemical quenching (NPQ). Here, we analyzed the acclimation efficacy of two different wheat varieties, by applying fluctuating light for analysis of plants, which had been subjected to a slowly developing drought stress as it usually occurs in the field. This novel approach allowed us to distinguish four drought phases, which are critical for grain yield, and to discover acclimatory responses which are independent of photodamage. In short-term, under fluctuating light, the slowdown of NPQ relaxation adjusts the photosynthetic activity to the reduced metabolic capacity. In long-term, the photosynthetic machinery acquires a drought-specific configuration by changing the PSII-LHCII phosphorylation pattern together with protein stoichiometry. Therefore, the fine-tuning of NPQ relaxation and PSII-LHCII phosphorylation pattern represent promising traits for future crop breeding strategies.     

Formation of photosynthetic pigments and quinones and development of photosynthetic activity in barley etioplasts during greening in intermittent and continuous white light

The appearance and development of photosynthetic activity, and the accumulation of chlorophylls, carotenoids and quinones, was investigated in etiolated barley shoots (Hordeum vulgare L. cv. Villa) during greening in flash light, periodic light-dark cycles, and continuous white light. Greening and the development of photosynthetic activity was delayed in flash and periodic light compared to continuous white light. Photosystem II activity occurred after 6 light-dark cycles and increased continuously during greening. After 3 h greening in continuous white light, photosystem II activity appeared with a very high rate and decreased to that of a green leaf after 50 h greening. Parallel to the development of photosynthetic activity, light stimulated the biosynthesis of prenyllipids. Moreover, chlorophylls and those carotenoids and quinones that are contained in etioplasts in relatively small amounts, were particularly enhanced in their biosynthesis. Chlorophyll a was synthesized without a lag phase during greening in flash light, whereas a 2 h lag phase occurred in continuous white light. In all three modes of illumination the formation of chlorophyll a exceeded that of chlorophyll b. After 4 flashes and 2 light-dark cycles, chlorophyll b could be detected with a very high initial a/b ratio. Higher chlorophyll a/b ratios were reached after 200 flashes (a/b=10.9) and 50 light-dark cycles (a/b=6.6) than after 50 h continuous white light (a/b=3.3). The formation of carotenes, lutein, violaxanthin and neoxanthin was also enhanced by light. This was also confirmed for plast-ouinone-9. ?-tocopherol,α-tocoquinone and phylloquinone. A comparison of the carotenoid and quinone composition of the differentiating thylakoid membrane before and after onset of photosynthesis, reveals that the photosynthetic membrane is already equipped with photosynthetic pigments and quinones before the appearance of photosystem II activity. It is concluded that during development of the photo-synthetic apparatus the thylakoid membrane with its structural and functional constituents is formed first. In a second and slower process the water splitting enzyme system and enzymes of the Calvin cycle are activated.     

Absorbance changes of carotenoids and bacteriochlorophylls responding to the change of local electrical field induced by hydrophobic anion,tetraphenylborate in membranes of photosynthetic bacteria

Large blue-shifts of carotenoid absorption bands were induced by dark addition of a hydrophobic anion, tetraphenylborate, in chromatophores and cell membranes of photosynthetic bacteria, Rhodopseudomonas sphaeroides and Rhodopseudomonas capsulata. Tetraphenylborate also induced a red-shift of the 850 nm absorption band and a blue-shift and broadening of the 800 nm band of bacteriochlorophyll. From the analysis of the relation between the magnitude and isosbestic wavelength of the absorbance changes the tetraphenylborate-induced carotenoid band shift were assumed to reflect the change of local electrical field close to each carotenoid molecule which exists as a minor pool on the light-harvesting pigment-protein complex II (LHC II). Absorbance changes of carotenoid and chlorophylls were also induced by tetraphenylborate in membranes of spinach chloroplasts.     

Characterization, seasonal occurrence, and diel fluctuation of poly(hydroxyalkanoate) in photosynthetic microbial mats

In situ poly(hydroxyalkanoate) (PHA) levels and repeating-unit compositions were examined in stratified photosynthetic microbial mats from Great Sippewissett Salt Marsh, Mass., and Ebro Delta, Spain. Unlike what has been observed in pure cultures of phototrophic bacteria, the prevalence of hydroxyvalerate (HV) repeating units relative to hydroxybutyrate (HB) repeating units was striking. In the cyanobacteria-dominated green material of Sippewissett mats, the mole percent ratio of repeating units was generally 1HB:1HV. In the purple sulfur bacteria-dominated pink material the relationship was typically 1HB:2HV. In Sippewissett mats, PHA contributed about 0.5 to 1% of the organic carbon in the green layer and up to 6% in the pink layer. In Ebro Delta mats, PHA of approximately 1HB:2HV-repeating-unit distribution contributed about 2% of the organic carbon of the composite photosynthetic layers (the green and pink layers were not separated). Great Sippewissett Salt Marsh mats were utilized for more extensive investigation of seasonal, diel, and exogenous carbon effects. When the total PHA content was normalized to organic carbon, there was little seasonal variation in PHA levels. However, routine daily variation was evident at all sites and seasons. In every case, PHA levels increased during the night and decreased during the day. This phenomenon was conspicuous in the pink layer, where PHA levels doubled overnight. The daytime declines could be inhibited by artificial shading. Addition of exogenous acetate, lactate, and propionate induced two- to fivefold increases in the total PHA levels when applied in the daylight but had no effect when applied at night. The distinct diel pattern of in situ PHA accumulation at night appears to be related, in some phototrophs, to routine dark energy metabolism and is not influenced by the availability of organic nutrients.     

Evidence for the functioning of photosynthetic CO2-concentrating mechanisms in lichens containing green algal and cyanobacterial photobionts

The photosynthetic properties of a range of lichens containing both green algal (11 species) and cyanobacterial (6 species) photobionts were examined with the aim of determining if there was clear evidence for the operation of a CO₂-concentrating mechanism (CCM) within the photobionts. Using a CO₂-gas-exchange system, which allowed resolution of fast transients, evidence was obtained for the existence of an inorganic carbon pool which accumulated in the light and was released in the dark. The pool was large (500–1000 nmol · mg Chl) in cyanobacterial lichens and about tenfold smaller in green algal lichens. In Hypogymnia physodes (L.) Nyl., which contains the green alga Trebouxia jamesii, a small inorganic carbon pool was rapidly formed in the light. Carbon dioxide was released from this pool into the gas phase upon darkening within about 20 s when photosynthesis was inhibited by the carbon-reduction-cycle inhibitor glycolaldehyde. In the absence of this inhibitor, release appeared to be obscured by carboxylation of ribulose bisphosphate. The kinetics of CO₂ uptake and release were monophasic. The operation of an active CCM could be distinguished from passive accumulation and release accompanying the reversible light-dependent alkalization of the stroma by the presence of saturation characteristics with respect to external CO₂. In Peltigera canina (L.) Willd., which contains the cyanobacterium Nostoc sp., a larger CO₂ pool was taken up over a longer period in the light and the release of this pool in the dark was slow, lasting 3–5 min. This pool also accumulated in the presence of glycolaldehyde, and under these conditions the CO₂ release was biphasic. In both species, photosynthesis at low CO₂ was inhibited by the carbonic-anhydrase inhibitor ethoxyzolamide (EZ). Inhibition could be reversed fully or to a considerable extent by high CO₂. In Peltigera, EZ decreased both the accumulation of the CO₂ pool by the CCM and the rate of photosynthesis. Free-living cultures of Nostoc sp. showed a similar effect of EZ on photosynthesis, although it was more dramatic than that seen with the lichen thalli. In contrast, in Hypogymnia, EZ actually increased the size of the CO₂ pool, although it inhibited photosynthesis. This effect was also seen when glycolaldehyde was present together with EZ. Surprisingly, EZ did not alter the kinetics of either CO₂ uptake or release. Taken together, the evidence indicates the operation in cyanobacterial lichens of a CCM which is capable of considerable elevation of internal CO₂ and is similar to that reported for free-living cyanobacteria. The CCM of green algal lichens accumulates much less CO₂ and is probably less effective than that which operates in cyanobacterial lichens.     

pH regulation of growth, photosynthesis, glutamine synthetase activity and micronutrient transport in the cyanobacterium Hapalosiphon welwitschii

Abstract Optima for growth, oxygenic photosynthesis and glutamine synthetase activity occurred at pH 10, thus suggesting that the cyanobacterium Hapalosiphon welwitschii is an alkalophile. It produced a Cu-Zn efflux system at pH 9 or 10, but not at pH 7 or 8, to relieve photosynthesis from Cu or Zn inhibition. This finding has a bearing on the ecophysiological competence of the cyanobacterium under natural conditions.     

Carbon isotope ratio and photosynthetic activity of phytoplankton in the eutrophic Mikawa Bay,Japan

Primary production by phytoplankton in the eutrophic Mikawa Bay, Japan, was studied by simultaneous measurements of natural carbon isotope ratio (δ ¹³C) and short-term carbon uptake rates (¹³C tracer study) of size-fractionated nannoplankton (<10 μm) and net plankton (>10 μm) samples. Short-term photosynthetic rates, which represent the physiological state of algae, were variable regardless of standing stock sizes. Theδ ¹³C values of particulate organic carbon (POC) in June and July displayed horizontal variations for both the net plankton fraction (−19.8 to −12.7‰) and the nannoplankton fraction (−22.0 to −12.8‰). For both fractions, low concentrations of POC had more negativeδ ¹³C values (−22 to −18‰). Highδ ¹³C values for the net plankton were found when POC concentrations were much higher, due to red tide. This suggests that the increase in algal standing crop for the net plankton fraction resulted from accelerated photosynthetic activity. However the nannoplankton fractions with higher POC values have relatively lowδ ¹³C values.     

Localisation of reaction centre and light harvesting complexes in the photosynthetic unit of Rhodopseudomonas viridis

The photosynthetic unit of Rhodopseudomonas viridis contains a reaction centre (P960) and a light harvesting complex (B1015). Immune electron microscopy combined with image processing has allowed the central core of the photosynthetic unit to be identified as the reaction centre and the surrounding protein ring as the light harvesting complex. This light harvesting complex, subdivided into twelve subunits was shown to contain 24 bacteriochlorophyll b molecules. A model is presented which may account for the far red shift of the Qy absorption of the bacteriochlorophyll b molecules in vivo.     

Acclimation of barley to changes in light intensity: photosynthetic electron transport activity and components

Barley seedlings (Hordeum vulgare L. Boone) were grown at 20°C with 16 h/8 h light/dark cycle of either high (H) intensity (500 mole m^-2 s^-1) or low (L) intensity (55 mole m^-2 s^-1) white light. Plants were transferred from high to low (H L) and low to high (L H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod. Thylakoid membranes were isolated from 3 cm apical segments and assayed for photosynthetic electron transport, Photosystem II (PS II) atrazine-binding sites (Q_B), cytochrome f(Cytf) and the P-700 reaction center of Photosystem I (PS I). Whole chain, PS I and PS II electron transport activities were higher in H than in L controls. Q_B and Cytf were elevated in H plants compared with L plants. The acclimation of H L plants to low light occurred slowly over a period of 7 days and resulted in decreased whole chain and PS II electron transport with variable effects on PS I activity. The decrease in electron transport of H L plants was associated with a decrease in both Q_B and Cytf. In L H plants, acclimation to high light occurred slowly over a period of 7 days with increased whole chain, PS I and PS II activities. The increase in L H electron transport was associated with increased levels of Q_B and Cytf. In contrast to the light intensity effects on Q_B levels, the P-700 content was similar in both control and transferred plants. Therefore, PS II/PS I ratios were dependent on light environment.Abbreviations Asc ascorbate - BQ 2,5-dimethyl-p-benzoquinone - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichlorophenolindophenol - H control plants grown under high light intensity - H L plants transferred from high to low light intensity - L low control plants grown under low light intensity - L H plants transferred from low to high light intensity - MV methyl viologen - P-700 photoreaction center of Photosystem I - Q_B atrazine binding site - TMPD N,N,N,N-tetramethyl-p-phenylenediamine Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 11990 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.