The structure of cyanobacterial phycobilisomes: a model

Phycobilisomes, supramolecular complexes of water-soluble accessory pigments, serve as the major light-harvesting antennae in cyanobacteria and red algae. Regular arrays of these organelles are found on the surface of the thylakoid membranes of these organisms. In the present study, the hemi-discoidal phycobilisomes of several species of cyanobacteria were examined in thin sections of cells and by negative staining after isolation and fixation. Their fundamental structures were found to be the same. Isolated phycobilisomes possessed a triangular core assembled from three stacks of disc-shaped subunits. Each stack contained two discs which were 12 nm in diameter and 6–7 nm thick. Each of these discs was probably subdivided into halves 3–3.5 nm thick. Radiating from each of two sides of the triangular core were three rods 12 nm in diameter. Each rod consisted of stacks of 2 to 6 disc-shaped subunits 6 nm thick. These discs were subdivided into halves 3 nm thick.The average number of discs of 6 nm thickness forming the peripheral rods varied among the strains studied. For certain chromatically adapting strains, the average rod length was dependent upon the wavelength of light to which cells were exposed during growth. Analyses of phycobilisomes by spectroscopic techniques, polyacrylamide gel electrophoresis, and electron microscopy were compared. These analyses suggested that the triangular core was composed of allophycocyanin and that the peripheral rods contained phycocyanin and phycoerythrin (when present). A detailed model of the hemi-discoidal phycobilisome is proposed. This model can account for many aspects of phycobiliprotein assembly and energy transfer.Abbreviations PBS phycobilisome(s) - PBP phycobiliprotein(s) - AP allophycocyanin - PC phycocyanin - PE phycoerythrin - PEC phycoerythrocyanin - AP-B allophycocyanin B - C- cyanobacterial - R- rhodophytan - B- Bangiophycean - SDS sodium dodecyl sulfate - LPP Lyngbya-Plectonema-Phormidium group - Na-KPO₄ buffers NaH₂PO₄ titrated with a solution of KH₂PO₄ of equivalent molarity to a given pH     

Parameter identification of the calvin photosynthesis cycle

Summary This article is concerned with the determination of kinetic parameters of the Calvin photosynthesis cycle which is described by seventeen nonlinear ordinary differential equations. It is shown that the task requires dynamic data for several sets of initial conditions. The numerical technique is based upon an algorithm for non-linear optimization and Gear's numerical integration scheme for stiff systems of differential equations. The sensitivity of the parameters to noise in the data is tested with a method adapted from Rosenbrook and Storey. A preliminary set of parameters has been obtained from a preliminary set of experimental data. The numerical methods are then tested with synthetic data derived from these parameters. The mathematical model and the results obtained in the simulation are used as an aid in designing new experiments.     

Ribulose 1,5-diphosphate carboxylase and Chlorobium thiosulfatophilum

1. Cell-free extracts of the photosynthetic bacterium Chlorobium thiosulfatophilum, strains 8327 and Tassajara, were assayed for ribulose 1,5-diphosphate (RuDP) carboxylase and phosphoribulokinase-the two enzymes peculiar to the reductive pentose phosphate cycle. 2. RuDP carboxylase was consistently absent in strain 8327. The Tassajara strain showed a low RuDP-dependent CO₂ fixation activity that was somewhat higher in cells following transatlantic air shipment than in freshly grown cells. The stability and behaviour of this activity in sucrose density gradients were similar to those described by other workers. 3. The radioactive carboxylation products formed in the presence of RuDP by enzyme preparations from the Tassajara strain did not include 3-phosphoglycerate-the known product of the RuDP carboxylase reaction, but instead consisted of the unrelated acids glutamate, aspartate and malate. 4. Phosphoribulokinase was absent in all preparations of the two Chlorobium strains tested. By contrast, phosphoribulokinase as well as RuDP carboxylase were readily demonstrated in preparations from pea chloroplasts and the photosynthetic bacterium Rhodospirillum rubrum. 5. It is concluded that C. thiosulfatophilum appears to lack RuDP carboxylase, phosphoribulokinase, and hence, the reductive pentose phosphate cycle.Support of a J. S. Guggenheim Fellowship is gratefully acknowledged     

Characterization and structural properties of the major biliproteins of Anabaena sp.

Studies are presented of the biliproteins of Anabaena sp. This filamentous cyanobacterium contains three major biliproteins. Whereas two of these, C-phycocyanin and allophycocyanin, are common to all cyanobacteria, the third, phycoerythrocyanin (_max568nm) has hitherto not been described and its distribution among cyanobacteria appears to be limited. Anabaena variabilis and Anabaena sp. 6411 allophycocyanin, C-phycocyanin, and phycoerythrocyanin were purified to homogeneity and characterized with respect to molecular weight, isoelectric point, absorption spectrum and amino acid composition. The and subunits of each of these proteins were also purified to homogeneity and characterized in the same manner. The tetrapyrrole chromophore content was determined for each of the proteins and subunits. The subunit of phycoerythrocyanin carries a novel phycobiliviolin-like chromophore. This chromophore has not previously been detected in cyanobacterial biliproteins, but has been noted as a prosthetic group of a cryptophytan phycocyanin.Sedimentation equilibrium studies show that at pH 7.0, at protein concentrations of 0.2–0.6 mg/ml, allophycocyanin, C-phycocyanin and phycoerythrocyanin, each exists as a trimeric aggregate, ()₃, of molecular weight of approximately 105000. Structural studies of microcrystals of these three biliproteins by electron microscopy and X-ray diffraction reveal a common plan for the construction of higher assembly forms. The major building block appears to be the trimer ()₃. It is proposed that this is a dise-like structure about 3.0×12.0 nm. The individual or subunits are roughly spherical, 3 nm in diameter. Allophycocyanin trimers stack to form bundles of rods which form long needles. Both phycocyanin and phycoerythrocyanin form double dises ()₆ which are visible as ring-shaped structures by electron microscopy. The mode of assembly of the biliproteinstructures in the phycobilisome is, as yet, unknown.Abbreviation Used SDS sodium dodecyl sulfate Dedicated to Prof. Dr. Roger Y. Stanier on the occasion of his 60th birthday.     

Combinatorial optimization of cyanobacterial 2,3-butanediol production

A vital goal of renewable technology is the capture and re-energizing of exhausted CO₂ into usable carbon products. Cyanobacteria fix CO₂ more efficiently than plants, and can be engineered to produce carbon feedstocks useful for making plastics, solvents, and medicines. However, fitness of this technology in the economy is threatened by low yields in engineered strains. Robust engineering of photosynthetic microorganisms is lagging behind model microorganisms that rely on energetic carbon, such as Escherichia coli, due in part to slower growth rates and increased metabolic complexity. In this work we show that protein expression from characterized parts is unpredictable in Synechococcus elongatus sp. strain PCC 7942, and may contribute to slow development. To overcome this, we apply a combinatorial approach and show that modulation of the 5'-untranslated region (UTR) can produce a range of protein expression sufficient to optimize chemical feedstock production from CO₂.     

Conferring drought-tolerant wheat genotypes through morpho-physiological and chlorophyll indices at seedling stage

The decrease in water resources due to the excessive use of water for irrigation purpose and climatic changes represents a serious world-wide threat to food security. In this regards, 50 wheat accessions were analyzed, using completely random factorial design at the seedlings stage under normal and drought stress conditions. Significant variation was detected among all accessions under both conditions. All characters studied showed variations in the mean values in water deficit environments in studied gemplasm at seedling stage. As seedling fresh weight, dry weight, relative water content, cell membrane thermo-stability, chlorophyll a & b were positively associated among themselves under drought conditions which showed the significance of these attribute for water deficit areas in future wheat breeding programs. Based on their performance, five accessions namely Aas-11, Chakwal-86, Pasban-90, Chakwal-97 and Kohistan-97 were selected as drought tolerant and three accessions namely Mairaj-08, Lasani-2008 and Gomal-2008 were selected as drought susceptible genotypes. The choice of wheat accessions based on the characteristics of the seedlings is informal, low-priced and less hassle. Likewise, the seedlings attributes exhibit moderate to high variation with an additive genetics effects on the environments. Best performance accessions under water deficit environment will be beneficial in future wheat breeding schemes and early screening for the attributes suggested in current experiment will be useful for producing best-yielded and drought-tolerance wheat genotypes to sustainable food security.     

Desiccation tolerance of the epilithic lichen Lecanora muralis (Schreb.) Rabenh. in the temperate climate

Lecanora muralis (Schreb.) Rabenh. is a ubiquitous epilithic crustose lichen of the temperate climate. It is well studied in terms of diel and annual carbon budget and productivity with continuous long-term observations in the field in 1995/96 by Otto L. Lange, Würzburg, and collaborators. However, these earlier studies left open the question to which extent the lichen is desiccation tolerant and if desiccation tolerance might possibly limit photosynthetic activity. In present study measurements of chlorophyll fluorescence parameters were performed to assess photosynthetic activity under various daily weather conditions throughout the year and recovery from desiccation after various periods of dryness in ambient air. Under any weather conditions, including strong frost for several days with night-temperatures around −15 °C and strong heat of several days with day-temperatures around 35 °C, the lichen was fully photosynthetically competent after wetting the samples for 15 min by submersion in water when they were dry in the field in the absence of actual incident precipitation. Chlorophyll fluorescence parameters were identical under all weather conditions sampled. A sample kept dry in ambient air for 37 days showed full recovery of chlorophyll fluorescence parameters after wetting for 30 min. Samples desiccated for longer periods up to 155 days took longer wetting times of about 300 min and recovered only partially but nevertheless showed active photosynthetic electron transport. Of 17 samples desiccated for 177–178 days only three recovered after rewetting for several days. It is concluded that the desiccation tolerance of L. muralis is sufficient to overcome dry spells of duration as it normally occurs in its natural environment. Desiccation tolerance is not likely to limit carbon budget and productivity.     

Green light drives photosynthesis in mosses

Temperate forests are characterised by variable light quality (i.e. spectral composition of light) at or near the forest floor. These understory environments have a high concentration of green light, as red and blue light are preferentially absorbed by upper canopy leaves. Understory species may be well-adapted for using green light to drive photosynthesis. Angiosperms have been shown to use green light for photosynthesis, but this ability has not been demonstrated in shade-dwelling bryophytes. In this study, net photosynthetic rate (P_N) of three temperate understory species of moss (Dichodontium pellucidum (Hedw.) Schimp., Leucobryum albidum (Brid. ex P.Beauv) Lindb. and Amblystegium serpens (Hedw.) Schimp.) was measured under green, red?+?blue, and red?+?blue?+?green light to assess green light use efficiency. All three species were capable of photosynthesising beyond their respiratory demands using solely green light, with higher green light use efficiency measured in plants collected from areas with greater canopy cover, suggesting growth in a green light concentrated environment increases green light use efficiency. Each species was also collected from sites differing in their degree of canopy cover and grown under three light treatments (high light, low light, and green light). Photosynthetic efficiency (chlorophyll fluorescence), tissue nitrogen and carbon isotope concentrations were assessed after a short growth period. Growth conditions had little effect on leaf chemistry and monochromatic green light did not significantly degrade photosynthetic efficiency. This study provides the first evidence to date of positive net ‘green light photosynthesis’ in mosses.     

Modeling the effects of light and temperature on algae growth: State of the art and critical assessment for productivity prediction during outdoor cultivation

The ability to model algal productivity under transient conditions of light intensity and temperature is critical for assessing the profitability and sustainability of full-scale algae cultivation outdoors. However, a review of over 40 modeling approaches reveals that most of the models hitherto described in the literature have not been validated under conditions relevant to outdoor cultivation. With respect to light intensity, we therefore categorized and assessed these models based on their theoretical ability to account for the light gradients and short light cycles experienced in well-mixed dense outdoor cultures. Type I models were defined as models predicting the rate of photosynthesis of the entire culture as a function of the incident or average light intensity reaching the culture. Type II models were defined as models computing productivity as the sum of local productivities within the cultivation broth (based on the light intensity locally experienced by individual cells) without consideration of short light cycles. Type III models were then defined as models considering the impacts of both light gradients and short light cycles. Whereas Type I models are easy to implement, they are theoretically not applicable to outdoor systems outside the range of experimental conditions used for their development. By contrast, Type III models offer significant refinement but the complexity of the inputs needed currently restricts their practical application. We therefore propose that Type II models currently offer the best compromise between accuracy and practicability for full scale engineering application. With respect to temperature, we defined as “coupled” and “uncoupled” models the approaches which account and do not account for the potential interdependence of light and temperature on the rate of photosynthesis, respectively. Due to the high number of coefficients of coupled models and the associated risk of overfitting, the recommended approach is uncoupled models. Most of models do not include the modeling of endogenous respiration and the modeling of light and temperature acclimation in spite of their potential effect on productivity.     

Evolutionary Changes in Chlorophyllide a Oxygenase (CAO) Structure Contribute to the Acquisition of a New Light-harvesting Complex in Micromonas

Chlorophyll b is found in photosynthetic prokaryotes and primary and secondary endosymbionts, although their light-harvesting systems are quite different. Chlorophyll b is synthesized from chlorophyll a by chlorophyllide a oxygenase (CAO), which is a Rieske-mononuclear iron oxygenase. Comparison of the amino acid sequences of CAO among photosynthetic organisms elucidated changes in the domain structures of CAO during evolution. However, the evolutionary relationship between the light-harvesting system and the domain structure of CAO remains unclear. To elucidate this relationship, we investigated the CAO structure and the pigment composition of chlorophyll-protein complexes in the prasinophyte Micromonas. The Micromonas CAO is composed of two genes, MpCAO1 and MpCAO2, that possess Rieske and mononuclear iron-binding motifs, respectively. Only when both genes were introduced into the chlorophyll b-less Arabidopsis mutant (ch1-1) was chlorophyll b accumulated, indicating that cooperation between the two subunits is required to synthesize chlorophyll b. Although Micromonas has a characteristic light-harvesting system in which chlorophyll b is incorporated into the core antennas of reaction centers, chlorophyll b was also incorporated into the core antennas of reaction centers of the Arabidopsis transformants that contained the two Micromonas CAO proteins. Based on these results, we discuss the evolutionary relationship between the structures of CAO and light-harvesting systems.