Environmental effects on the light-harvesting complex of cyanobacteria

Proteome trait regulation of marine Synechococcus elemental stoichiometry under global change

Recent studies have demonstrated regional differences in marine ecosystem C:N:P with implications for carbon and nutrient cycles. Due to strong co-variance, temperature and nutrient stress explain variability in C:N:P equally well. A reductionistic approach can link changes in individual environmental drivers with changes in biochemical traits and cell C:N:P. Thus, we quantified effects of temperature and nutrient stress on Synechococcus chemistry using laboratory chemostats, chemical analyses, and data-independent acquisition mass spectrometry proteomics. Nutrient supply accounted for most C:N:Pcell variability and induced tradeoffs between nutrient acquisition and ribosomal proteins. High temperature prompted heat-shock, whereas thermal effects via the "translation-compensation hypothesis" were only seen under P-stress. A Nonparametric Bayesian Local Clustering algorithm suggested that changes in lipopolysaccharides, peptidoglycans, and C-rich compatible solutes may also contribute to C:N:P regulation. Physiological responses match field-based trends in ecosystem stoichiometry and suggest a hierarchical environmental regulation of current and future ocean C:N:P.

Modeling and Optimizing the Effect of Light Color, Sodium Chloride and Glucose Concentration on Biomass Production and the Quality of Arthrospira platensis Using Response Surface Methodology (RSM)

Arthrospira platensis (Spirulina) biomass is a valuable source of sustainable proteins, and the basis for new food and feed products. State-of-the-art production of Spirulina biomass in open pond systems only allows limited control of essential process parameters, such as light color, salinity control, or mixotrophic growth, due to the high risk of contaminations. Closed photobioreactors offer a highly controllable system to optimize all process parameters affecting Spirulina biomass production (quantity) and biomass composition (quality). However, a comprehensive analysis of the impact of light color, salinity effects, and mixotrophic growth modes of Spirulina biomass production has not been performed yet. In this study, Response Surface Methodology (RSM) was employed to develop statistical models, and define optimal mixotrophic process conditions yielding maximum quantitative biomass productivity and high-quality biomass composition related to cellular protein and phycocyanin content. The individual and interaction effects of 0, 5, 15, and 30 g/L of sodium chloride (S), and 0, 1.5, 2, and 2.5 g/L of glucose (G) in three costume-made LED panels (L) where the dominant color was white (W), red (R), and yellow (Y) were investigated in a full factorial design. Spirulina was cultivated in 200 mL cell culture flasks in different treatments, and data were collected at the end of the log growth phase. The lack-of-fit test showed that the cubic model was the most suitable to predict the biomass concentration and protein content, and the two-factor interaction (2FI) was preferred to predict the cellular phycocyanin content (p > 0.05). The reduced models were produced by excluding insignificant terms (p > 0.05). The experimental validation of the RSM optimization showed that the highest biomass concentration (1.09, 1.08, and 0.85 g/L), with improved phycocyanin content of 82.27, 59.47, 107 mg/g, and protein content of 46.18, 39.76, 53.16%, was obtained under the process parameter configuration WL4.28S2.5G, RL10.63S1.33G, and YL1.00S0.88G, respectively.

Ecological impacts of photosynthetic light harvesting in changing aquatic environments: A systematic literature map

Underwater light is spatially as well as temporally variable and directly affects phytoplankton growth and competition. Here we systematically (following the guidelines of PRISMA-EcoEvo) searched and screened the published literature resulting in 640 individual articles. We mapped the conducted research for the objectives of (1) phytoplankton fundamental responses to light, (2) effects of light on the competition between phytoplankton species, and (3) effects of climate-change-induced changes in the light availability in aquatic ecosystems. Among the fundamental responses of phytoplankton to light, the effects of light intensity (quantity, as measure of total photon or energy flux) were investigated in most identified studies. The effects of the light spectrum (quality) that via species-specific light absorbance result in direct consequences on species competition emerged more recently. Complexity in competition arises due to variability and fluctuations in light which effects are sparsely investigated on community level. Predictions regarding future climate change scenarios included changes in in stratification and mixing, lake and coastal ocean darkening, UV radiation, ice melting as well as light pollution which affect the underwater light-climate. Generalization of consequences is difficult due to a high variability, interactions of consequences as well as a lack in sustained timeseries and holistic approaches. Nevertheless, our systematic literature map, and the identified articles within, provide a comprehensive overview and shall guide prospective research.

Arthrospira platensis as a Feasible Feedstock for Bioethanol Production

In recent decades and to deal with the scarcity of fossil fuels, many studies have been developed in order to set up a sustainable biofuel production sector. This new sector must be efficient (high productivity), economically profitable (low production costs and therefore acceptable fuel prices), and ethical (low carbon balance, no competition with food resources). The production of bioethanol is based on the fermentation of reserve sugars, accumulated in the form of starch in microalgae and glycogen in cyanobacteria. The advantage of this bioenergy production route lies in the fact that the post-crop fermentation process is at the industrial stage since it has already been tested for many years for the production of bioethanol from agricultural resources. One of the most cultivated cyanobacteria is Arthrospira (“Spirulina”) and its production is also already at industrial scale. Depending on the cultivation conditions, this cyanobacteria is able to accumulate up to 65% DW (dry weight) of glycogen, making it a feasible feedstock for bioethanol production. The aim of this review is to provide a clear overview of these operating conditions for glycogen accumulation.

Statistical optimization of process parameters for improvement of phycobiliproteins (PBPs) yield using ultrasound-assisted extraction and its kinetic study

The present study is mainly concerned for the development of an optimal ultrasound-assisted extraction (UAE) condition for phycobiliproteins (PBPs) from Oscillatoria sp. (BTA 170) using Taguchi methodology. Four process parameters viz. solid to liquid ratio, duty cycle, electrical acoustic intensity, and pH, for UAE were optimized using Taguchi methodology for enhanced PBPs extraction. The ratio of signal to noise (S/N) was used to compute the optimized condition required to attain a higher yield of PBPs, the average performance of individual parameter and corresponding interactive effects. The statistically significant parameters with their contribution were assessed using Analysis of variance (ANOVA). Results showed that duty cycle contributed the maximum influence (30.81%) on phycocyanin (PC) extraction followed by a solid liquid ratio (28.62%), pH (22.46%) and electrical acoustic intensity (18.10%). The highest contribution on the extraction of phycoerythrin (PE) was found from pH (33.16%), followed by duty cycle (31.57%), solid to liquid ratio (22.83%) and electrical acoustic intensity (12.45%). For extraction of allophycocyanin (APC), the duty cycle, solid to liquid ratio, pH and electrical acoustic intensity contributed 29.47, 29.07, 29.03, and 12.43% respectively. Results obtained from Taguchi methodology indicated that enhanced PC (94.10%), PE (95.20%) and APC (90.54%) can be achieved with solid-liquid ratio (0.2 g/ml), electrical acoustic intensity (16.99 w/cm²), duty cycle (75%), and pH 7 than the yield of PBPs obtained under unoptimized condition. In the present study, higher yield of PC (38.99%), PE (20.84%), and APC (11.93%) were attained with UAE compared to yield obtained from homogenized Oscillatoria sp. BTA 170 using 0.05 M phosphate buffer. Batch extraction data of PBPs under UAE was fitted well with the second order model. The values of second-order rate constant (k) were computed as 6.66 × 10^-4, 64.09 × 10^-4 and 1.49 × 10^-4 L/mg/min for extraction of PC, PE and APC respectively. The PBPs exhibited significant antioxidant property and hydrogen peroxide scavenging activity, which were increased with the enhancement of PBPs concentration.

Contrasting effects of monochromatic LED lighting on growth, pigments and photosynthesis in the commercially important cyanobacterium Arthrospira maxima

The aim of this study was to evaluatethe effects of different colored light emitting diodes (LEDs) on the growth, pigment yield, and photosynthetic performance of Arthrospira maxima, a commercially exploited species of cyanobacteria. The highest growth and chlorophyll a (Chl a) concentration were obtained under red LED and white fluorescent light, while the lowest growth and Chlaconcentration were observed under blue LED. However, blue LED produced the highest levels of phycobiliproteins (3.20 mg·g^-1phycocyanin [PC]; 0.19 mg·g^-1 allophycocyanin [APC]; 0.97 mg·g^-1for phycoerythrin [PE], effective quantum yield (Φ_PSII) and maximum relative electron transport rate (rETR_max) inA. maxima. The results of this study suggest that red and blue LEDs increase the biomass yield and pigment content of cyanobacteria, respectively, and the combined use of red and blue light may significantly improve algal biomass and biopigment yield.

Comparative excitation-emission dependence of the FV /FM ratio in model green algae and cyanobacterial strains

The emission spectra collected under conditions of open (F₀ ) and closed (F_M ) photosystem II (PSII) reaction centres are close-to-independent from the excitation wavelength in Chlamydomonas reinhardtii and Chlorella sorokiniana, whereas a pronounced dependence is observed in Synechocystis sp. PCC6803 and Synechococcus PCC7942, instead. The differences in band-shape between the F₀ and F_M emission are limited in green algae, giving rise only to a minor trough in the F_V /F_M spectrum in the 705-720 nm range, irrespectively of the excitation. More substantial variations are observed in cyanobacteria, resulting in marked dependencies of the measured F_V /F_M ratios on both the excitation and the detection wavelengths. In cyanobacteria, the maximal F_V /F_M values (0.5-0.7), observed monitoring at approximately 684 nm and exciting Chl a preferentially, are comparable to those of green algae; however, F_V /F_M decreases sharply below approximately 660 nm. Furthermore, in the red emission tail, the trough in the F_V /F_M spectrum is more pronounced in cyanobacteria with respect to green algae, corresponding to F_V /F_M values of 0.25-0.4 in this spectral region. Upon direct phycobilisomes excitation (i.e. >520 nm), the F_V /F_M value detected at 684 nm decreases to 0.3-0.5 and is close-to-negligible (approximately 0.1) below 660 nm. At the same time, the F_V spectra are, in all species investigated, almost independent on the excitation wavelength. It is concluded that the excitation/emission dependencies of the F_V /F_M ratio arise from overlapped contributions from the three independent emissions of PSI, PSII and a fraction of energetically uncoupled external antenna, excited in different proportions depending on the respective optical cross-section and fluorescence yield.

A novel Ca2+-binding protein influences photosynthetic electron transport in Anabaena sp. PCC 7120

Ca²⁺ is a potent signalling molecule that regulates many cellular processes. In cyanobacteria, Ca²⁺ has been linked to cell growth, stress response and photosynthesis, and to the development of specialist heterocyst cells in certain nitrogen-fixing species. Despite this, the pathways of Ca²⁺ signal transduction in cyanobacteria are poorly understood, and very few protein components are known. The current study describes a previously unreported Ca²⁺-binding protein which was called the Ca²⁺ Sensor EF-hand (CSE), which is conserved in filamentous, nitrogen-fixing cyanobacteria. CSE is shown to bind Ca²⁺, which induces a conformational change in the protein structure. Poor growth of a strain of Anabaena sp. PCC 7120 overexpressing CSE was attributed to diminished photosynthetic performance. Transcriptomics, biophysics and proteomics analyses revealed modifications in the light-harvesting phycobilisome and photosynthetic reaction centre protein complexes.

NsrR1, a Nitrogen Stress-Repressed sRNA, Contributes to the Regulation of nblA in Nostoc sp. PCC 7120

Small regulatory RNAs (sRNAs) are currently considered as major post-transcriptional regulators of gene expression in bacteria. The interplay between sRNAs and transcription factors leads to complex regulatory networks in which both transcription factors and sRNAs may appear as nodes. In cyanobacteria, the responses to nitrogen availability are controlled at the transcriptional level by NtcA, a CRP/FNR family regulator. In this study, we describe an NtcA-regulated sRNA in the cyanobacterium Nostoc sp. PCC 7120, that we have named NsrR1 (nitrogen stress repressed RNA1). We show sequence specific binding of NtcA to the promoter of NsrR1. Prediction of possible mRNA targets regulated by NsrR1 allowed the identification of nblA, encoding a protein adaptor for phycobilisome degradation under several stress conditions, including nitrogen deficiency. We demonstrate specific interaction between NsrR1 and the 5'-UTR of the nblA mRNA, that leads to decreased expression of nblA. Because both NsrR1 and NblA are under transcriptional control of NtcA, this regulatory circuit constitutes a coherent feed-forward loop, involving a transcription factor and an sRNA.

Decomposition of cyanobacterial light harvesting complexes: NblA-dependent role of the bilin lyase homolog NblB

Phycobilisomes, the macromolecular light harvesting complexes of cyanobacteria are degraded under nutrient-limiting conditions. This crucial response is required to adjust light excitation to the metabolic status and avoid damage by excess excitation. Phycobilisomes are comprised of phycobiliproteins, apo-proteins that covalently bind bilin chromophores. In the cyanobacterium Synechococcus elongatus, the phycobiliproteins allophycocyanin and phycocyanin comprise the core and the rods of the phycobilisome, respectively. Previously, NblB was identified as an essential component required for phycocyanin degradation under nutrient starvation. This protein is homologous to bilin-lyases, enzymes that catalyze the covalent attachment of bilins to apo-proteins. However, the nblB-inactivated strain is not impaired in phycobiliprotein synthesis, but rather is characterized by aberrant phycocyanin degradation. Here, using a phycocyanin-deficient strain, we demonstrate that NblB is required for degradation of the core pigment, allophycocyanin. Furthermore, we show that the protein NblB is expressed under nutrient sufficient conditions, but during nitrogen starvation its level decreases about two-fold. This finding is in contrast to an additional component essential for degradation, NblA, the expression of which is highly induced under starvation. We further identified NblB residues required for phycocyanin degradation in vivo. Finally, we demonstrate phycocyanin degradation in a cell-free system, thereby providing support for the suggestion that NblB directly mediates pigment degradation by chromophore detachment. The dependence of NblB function on NblA revealed using this system, together with the results indicating presence of NblB under nutrient sufficient conditions, suggests a rapid mechanism for induction of pigment degradation, which requires only the expression of NblA.

Effects of different light source and media on growth and production of phycobiliprotein from freshwater cyanobacteria

The aim of this study was to determine the effect of different light sources and media (wastewater and BBM) on the growth of Pseudanabaena mucicola and its phycobiliprotein production. Results showed that P. mucicola grown in white light using wastewater as medium attributed higher biomass (0.55 g L^-1) and when extracted with water, also showed significantly higher (P < .05) production (237.01 mg g^-1) and purity (1.14) of phycobiliprotein. This study validated that phycobiliprotein extracted from P. mucicola using water can be food grade natural blue pigment. Moreover, cyanobacteria grown in wastewater could cut down the production cost of phycobiliprotein.

Availability and Utilization of Pigments from Microalgae

Microalgae are the major photosynthesizers on earth and produce important pigments that include chlorophyll a, b and c, β-carotene, astaxanthin, xanthophylls, and phycobiliproteins. Presently, synthetic colorants are used in food, cosmetic, nutraceutical, and pharmaceutical industries. However, due to problems associated with the harmful effects of synthetic colorants, exploitation of microalgal pigments as a source of natural colors becomes an attractive option. There are various factors such as nutrient availability, salinity, pH, temperature, light wavelength, and light intensity that affect pigment production in microalgae. This paper reviews the availability and characteristics of microalgal pigments, factors affecting pigment production, and the application of pigments produced from microalgae. The potential of microalgal pigments as a source of natural colors is enormous as an alternative to synthetic coloring agents, which has limited applications due to regulatory practice for health reasons.

A Metaproteomic Analysis of the Response of a Freshwater Microbial Community under Nutrient Enrichment

Eutrophication can lead to an uncontrollable increase in algal biomass, which has repercussions for the entire microbial and pelagic community. Studies have shown how nutrient enrichment affects microbial species succession, however details regarding the impact on community functionality are rare. Here, we applied a metaproteomic approach to investigate the functional changes to algal and bacterial communities, over time, in oligotrophic and eutrophic conditions, in freshwater microcosms. Samples were taken early during algal and cyanobacterial dominance and later under bacterial dominance. 1048 proteins, from the two treatments and two timepoints, were identified and quantified by their exponentially modified protein abundance index. In oligotrophic conditions, Bacteroidetes express extracellular hydrolases and Ton-B dependent receptors to degrade and transport high molecular weight compounds captured while attached to the phycosphere. Alpha- and Beta-proteobacteria were found to capture different substrates from algal exudate (carbohydrates and amino acids, respectively) suggesting resource partitioning to avoid direct competition. In eutrophic conditions, environmental adaptation proteins from cyanobacteria suggested better resilience compared to algae in a low carbon nutrient enriched environment. This study provides insight into differences in functional microbial processes between oligo- and eutrophic conditions at different timepoints and highlights how primary producers control bacterial resources in freshwater environments. The data have been deposited to the ProteomeXchange with identifier PXD004592.

Thylakoid membrane function in heterocysts

Multicellular cyanobacteria form different cell types in response to environmental stimuli. Under nitrogen limiting conditions a fraction of the vegetative cells in the filament differentiate into heterocysts. Heterocysts are specialized in atmospheric nitrogen fixation and differentiation involves drastic morphological changes on the cellular level, such as reorganization of the thylakoid membranes and differential expression of thylakoid membrane proteins. Heterocysts uphold a microoxic environment to avoid inactivation of nitrogenase by developing an extra polysaccharide layer that limits air diffusion into the heterocyst and by upregulating heterocyst-specific respiratory enzymes. In this review article, we summarize what is known about the thylakoid membrane in heterocysts and compare its function with that of the vegetative cells. We emphasize the role of photosynthetic electron transport in providing the required amounts of ATP and reductants to the nitrogenase enzyme. In the light of recent high-throughput proteomic and transcriptomic data, as well as recently discovered electron transfer pathways in cyanobacteria, our aim is to broaden current views of the bioenergetics of heterocysts. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Conrad Mullineaux.

The proteolysis adaptor, NblA, initiates protein pigment degradation by interacting with the cyanobacterial light-harvesting complexes

Degradation of the cyanobacterial protein pigment complexes, the phycobilisomes, is a central acclimation response that controls light energy capture. The small protein, NblA, is essential for proteolysis of these large complexes, which may reach a molecular mass of up to 4 MDa. Interactions of NblA in vitro supported the suggestion that NblA is a proteolysis adaptor that labels the pigment proteins for degradation. The mode of operation of NblA in situ, however, remained unresolved. Particularly, it was unclear whether NblA interacts with phycobilisome proteins while part of the large complex, or alternatively interaction with NblA, necessitates dissociation of pigment subunits from the assembly. Fluorescence intensity profiles demonstrated the preferential presence of NblA::GFP (green fluorescent protein) at the photosynthetic membranes, indicating co-localization with phycobilisomes. Furthermore, fluorescence lifetime imaging microscopy provided in situ evidence for interaction of NblA with phycobilisome protein pigments. Additionally, we demonstrated the role of NblA in vivo as a proteolysis tag based on the rapid degradation of the fusion protein NblA::GFP compared with free GFP. Taken together, these observations demonstrated in vivo the role of NblA as a proteolysis adaptor. Additionally, the interaction of NblA with phycobilisomes indicates that the dissociation of protein pigment subunits from the large complex is not a prerequisite for interaction with this adaptor and, furthermore, implicates NblA in the disassembly of the protein pigment complex. Thus, we suggest that, in the case of proteolysis of the phycobilisome, the adaptor serves a dual function: undermining the complex stability and designating the dissociated pigments for degradation.

LIGHT DEPENDENCY OF PHOTOSYNTHETIC RECOVERY DURING WETTING AND THE ACCLIMATION OF PHOTOSYNTHETIC APPARATUS TO LIGHT FLUCTUATION IN A TERRESTRIAL CYANOBACTERIUM NOSTOC COMMUNE(1)

The PSII photochemical activity in a terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault during rewetting was undetectable in the dark but was immediately recognized in the light. The maximum quantum yield of PSII (Fv /Fm ) during rewetting in the light rose to 85% of the maximum within ∼30 min and slowly reached the maximum within 6 h, while with rewetting in the darkness for 6 h and then exposure to light the recovery of Fv /Fm required only ∼3 min. These results suggested that recovery of photochemical activity might depend on two processes, light dependence and light independence, and the activation of photosynthetic recovery in the initial phase was severely light dependent. The inhibitor experiments showed that the recovery of Fv /Fm was not affected by chloramphenicol (CMP), but severely inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) in the light, suggesting that the light-dependent recovery of photochemical activity did not require de novo protein synthesis but required activation of PSII associated with electron flow to plastoquinone. Furthermore, the test indicated that the lower light intensity and the red light were of benefit to its activation of photochemical activity. In an outdoor experiment of diurnal changes of photochemical activity, our results showed that PSII photochemical activity was sensitive to light fluctuation, and the nonphotochemical quenching (NPQ) was rapidly enhanced at noon. Furthermore, the test suggested that the repair of PSII by de novo protein synthesis played an important role in the acclimation of photosynthetic apparatus to high light, and the heavily cloudy day was more beneficial for maintaining high photochemical activity.

Growth and biopigment accumulation of cyanobacterium Spirulina platensis at different light intensities and temperature

In order to find out optimum culture condition for algal growth, the effect of light irradiance and temperature on growth rate, biomass composition and pigment production of Spirulina platensis were studied in axenic batch cultures. Growth kinetics of cultures showed a wide range of temperature tolerance from 20 °C to 40 °C. Maximum growth rate, cell production with maximum accumulation of chlorophyll and phycobilliproteins were found at temperature 35 °C and 2,000 lux light intensity. But with further increase in temperature and light intensity, reduction in growth rate was observed. Carotenoid content was found maximum at 3,500 lux. Improvement in the carotenoid content with increase in light intensity is an adaptive mechanism of cyanobacterium S.platensis for photoprotection, could be a good basis for the exploitation of microalgae as a source of biopigments.

Screening of cyanobacteria for phycobiliproteins and effect of different environmental stress on its yield

Among 18 strains, cyanobacterium Anabaena NCCU-9 contained the highest amount of phycobiliprotein (91 mg/g dry cell weights). Therefore, the effects of various environmental stresses were investigated on its phycobiliprotein production potential. The ideal conditions observed were 30 degrees C, 25 micromol photons/m2/s, white light, pH-8, 16:8 light and dark regimes, nitrogen free medium and 10 mM sodium chloride. Among three pesticides studied malathion showed highest toxicity. Under heavy metal stress the order of toxicity was chromium > cadmium > lead > nickel > copper > zinc.

sll1961 is a novel regulator of phycobilisome degradation during nitrogen starvation in the cyanobacterium Synechocystis sp. PCC 6803

The sll1961 gene was reported to encode a regulatory factor of photosystem stoichiometry in the cyanobacterium Synechocystis sp. PCC 6803. We here show that the sll1961 gene is also essential for the phycobilisome degradation during nitrogen starvation. The defect in phycobilisome degradation was observed in the sll1961 mutant despite the increased expression of nblA, a gene involved in phycobilisome degradation during nitrogen starvation. Photosystem stoichiometry is not affected by nitrogen starvation in the sll1961 mutant nor in the wild-type. The results indicate the presence of a novel pathway for phycobilisome degradation control independent of nblA expression.

Alanine dehydrogenase activity is required for adequate progression of phycobilisome degradation during nitrogen starvation in Synechococcus elongatus PCC 7942

Degradation of the cyanobacterial light-harvesting antenna, the phycobilisome, is a general acclimation response that is observed under various stress conditions. In this study we identified a novel mutant of Synechococcus elongatus PCC 7942 that exhibits impaired phycobilisome degradation specifically during nitrogen starvation, unlike previously described mutants, which exhibit aberrant degradation under nitrogen, sulfur, and phosphorus starvation conditions. The phenotype of the new mutant, AldOmega, results from inactivation of ald (encoding alanine dehydrogenase). AldOmega is deficient in transcription induction of a number of genes during nitrogen starvation. These genes include the "general nutrient stress-related" genes, nblA and nblC, the products of which are essential for phycobilisome degradation. Furthermore, transcripts of several specific nitrogen-responsive genes accumulate at lower levels in AldOmega than in the wild-type strain. In contrast, ald inactivation did not decrease the accumulation of transcripts during sulfur starvation. Transcription of ald is induced upon nitrogen starvation, which is consistent with the ability of wild-type cells to maintain a low cellular content of alanine under these conditions. Unlike wild-type cells, AldOmega accumulates alanine upon nitrogen starvation. Our analyses suggest that alanine dehydrogenase activity is necessary for an adequate cellular response to nitrogen starvation. Decomposition of alanine may be required to provide a sufficient amount of ammonia. Furthermore, the accumulated alanine, or a related metabolite, may interfere with the cues that modulate acclimation during nitrogen starvation. Taken together, our results provide novel information regarding cellular responses to nitrogen starvation and suggest that mechanisms related to nitrogen-specific responses are involved in modulation of a general acclimation process.

SipA, a novel type of protein from Synechococcus sp. PCC 7942, binds to the kinase domain of NblS

Cyanobacteria respond to nutrient stress conditions by degrading their light-harvesting complexes for photosynthesis, a process regulated in Synechococcus sp. PCC 7942 by the sensor histidine kinase non-bleaching sensor (NblS). In yeast two-hybrid screenings for proteins interacting with NblS we have identified a novel type of protein, named SipA for NblS interacting protein A. Specific binding between NblS and SipA is observed with both yeast and bacterial two-hybrid systems. Additional yeast two-hybrid screenings with SipA as bait further confirmed the specificity of the interaction and allowed us to map their determinants to the ATP-binding domain of NblS. Strong conservation and coevolution of both NblS and SipA in cyanobacteria further suggests the importance of SipA in the context of the NblS signal transduction network.

In vivo analysis of the roles of conserved aspartate and histidine residues within a complex response regulator

RcaC is the founding member of a group of large response regulators with complex domain combinations containing at least two receiver domains, an OmpR-class winged helix-turn-helix DNA binding domain, and a histidine phosphotransfer (HPt) domain. Within its two receiver and HPt domains, RcaC contains consensus phosphorylation sites at aspartates 51, 576 and histidine 316. RcaC operates in the pathway regulating transcription of genes encoding components of photosynthetic light harvesting antenna to changes in light colour. We show that phycocyanin gene expression requires RcaC. RcaC contributes to light regulation of phycoerythrin genes, but is not part of the second light regulation pathway controlling these genes. Substitutions at aspartate 51 or histidine 316 severely impaired light responsiveness while substitutions at aspartate 576 had little effect. Complete loss of light regulation, measured by phycocyanin gene expression, only occurred in the triple mutant. We conclude that aspartate 51 primarily controls light colour responsiveness and is regulated by histidine 316, and that these residues are likely phosphorylated in red light and dephosphorylated in green light. The carboxy-terminal receiver domain has a minor role in controlling this response. RcaC abundance is also light regulated and depends on aspartate 51 and histidine 316, but not aspartate 576.

Phycobilisome linker proteins are phosphorylated in Synechocystis sp. PCC 6803

The controversial issue of protein phosphorylation from the photosynthetic apparatus of Synechocystis sp. PCC 6803 has been reinvestigated using new detection tools that include various immunological and in vivo labeling approaches. The set of phosphoproteins detected with these methods includes ferredoxin-NADPH reductase and the linker proteins of the phycobilisome antenna. Using mutants that lack a specific set of linker proteins and are affected in phycobilisome assembly, we show that the phosphoproteins from the phycobilisomes correspond to the membrane, rod, and rod-core linkers. These proteins are in a phosphorylated state within the assembled phycobilisomes. Their dephosphorylation requires partial disassembly of the phycobilisomes and further contributes to their complete disassembly in vitro. In vivo we observed linker dephosphorylation upon long-term exposure to higher light intensities and under nitrogen limitation, two conditions that lead to remodeling and turnover of phycobilisomes. We conclude that this phosphorylation process is instrumental in the regulation of assembly/disassembly of phycobilisomes and should participate in signaling for their proteolytic cleavage and degradation.

Chlorosis during nitrogen starvation is altered by carbon dioxide and temperature status and is mediated by the ClpP1 protease in Synechococcus elongatus

The interactive effects of inorganic carbon status, temperature and light on chlorosis induced by nitrogen deficiency, and the roles of Clp proteases in this process were investigated. In wild-type cultures grown in high or ambient CO(2), following transfer to media lacking combined nitrogen, phycocyanin per cell dropped primarily through dilution of the pigment through cell division, and also suffered variable degrees of net degradation. When grown at high CO(2) (5%), chlorophyll (Chl) suffered net degradation to a greater extent than phycocyanin. In marked contrast, growth at ambient CO(2) resulted in Chl per cell dropping through dilution. Conditions that drove net Chl degradation in the wild-type resulted in little or no net Chl degradation in a clpPI inactivation mutant, with Chl content dropping largely through growth dilution in the mutant. The chlorotic response of a clpPII inactivation strain was nearly the same as that of wild-type, although phycocyanin degradation may have been slightly accelerated in the former.

Revealing the molecular secrets of marine diatoms

Diatoms are unicellular photosynthetic eukaryotes that contribute close to one quarter of global primary productivity. In spite of their ecological success in the world's oceans, very little information is available at the molecular level about their biology. Their most well-known characteristic is the ability to generate a highly ornamented silica cell wall, which made them very popular study organisms for microscopists in the last century. Recent advances, such as the development of a range of molecular tools, are now allowing the dissection of diatom biology, e.g., for understanding the molecular and cellular basis of bioinorganic pattern formation of their cell walls and for elucidating key aspects of diatom ecophysiology. Making diatoms accessible to genomics technologies will potentiate greatly these efforts and may lead to the use of diatoms to construct submicrometer-scale silica structures for the nanotechnology industry.

Effect of the respiratory activity on photoinhibition of the cyanobacterium Synechocystis sp

The influence of respiratory activity on photosynthesis in Synechocystis cells that had been exposed to high light intensity was studied using distinct conditions of nitrogen supply. The photoinhibitory rate of N-sufficient cells was not influenced by the presence of different nitrogen sources. In contrast, when N-starved cells were resupplied with ammonium, they were protected from photoinhibition. Although N-starved cells presented a higher rate of dark O(2) uptake than N-sufficient ones, the photoinhibitory rate increased in both cases after addition of sodium azide or sodium azide plus salicylhydroxamic acid in the photoinhibitory treatment. In the absence of the D(1) protein repair mechanism, photodamage to Photosystem II was faster in N-sufficient cells than in N-starved ones. Mitigation of photodamage disappeared when the respiratory activity of N-starved cells was partially suppressed by the addition of sodium azide or sodium azide and salicylhydroxamic acid. Our results suggest that electron flow through cyanobacterial terminal oxidases can assist Photosystem I in removing electrons from the reduced plastoquinone pool, thus contributing to both reopening of Photosystem II reaction centers and avoiding photogeneration of reactive oxygen species under photoinhibitory conditions.

A redox-responsive regulator of photosynthesis gene expression in the cyanobacterium Synechocystis sp. Strain PCC 6803

We have identified genes in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 that are involved with redox control of photosynthesis and pigment-related genes. The genes, rppA (sll0797) and rppB (sll0798), represent a two-component regulatory system that controls the synthesis of photosystem II (PSII) and PSI genes, in addition to photopigment-related genes. rppA (regulator of photosynthesis- and photopigment-related gene expression) and rppB exhibit strong sequence similarity to prokaryotic response regulators and histidine kinases, respectively. In the wild type, the steady-state mRNA levels of PSII reaction center genes increased when the plastoquinone (PQ) pool was oxidized and decreased when the PQ pool was reduced, whereas transcription of the PSI reaction center genes was affected in an opposite fashion. Such results suggested that the redox poise of the PQ pool is critical for regulation of the photosystem reaction center genes. In Delta rppA, an insertion mutation of rppA, the PSII gene transcripts were highly up-regulated relative to the wild type under all redox conditions, whereas transcription of phycobilisome-related genes and PSI genes was decreased. The higher transcription of the psbA gene in Delta rppA was manifest by higher translation of the D1 protein and a concomitant increase in O(2) evolution. The results demonstrated that RppA is a regulator of photosynthesis- and photopigment-related gene expression, is involved in the establishment of the appropriate stoichiometry between the photosystems, and can sense changes in the PQ redox poise.

Electrophoretic applications of phycobiliproteins

Phycobiliproteins are homologous chromoproteins which constitute the phycobilisomes, the light harvesting complexes of the photosynthetic apparatus in cyanobacteria, rhodophyta and cryptophyta. In the present work, phycocyanin (PC) and phycoerythrin (PE) from a Nostoc species are proposed as protein markers for electrophoretic techniques. Phycocyanin is a blue-colored phycobiliprotein; it carries phycocyanobilin as chromophoric group and is composed of two subunits, alpha and beta, with Mr of 14000 and 17000, respectively. In contrast, the PE subunits, having a similar Mr of 21000, are deep rose chromoproteins and carry phycoerythrobilin residues. Both low molecular weight phycobiliproteins are also suitable for monitoring protein blotting and the focusing time of protein samples during isoelectric focusing as internal markers. The PE subunits which form a single broad band after sodium dodecyl sulfate-polyacrylamide gel electrophoresis have different isoelectric points, and they form two visible bands when they reach their isoelectric point. The phycobilisomes constitute up to 50% of the total protein in cyanobacteria and their content in PC or PE can be up- or down-regulated by using different light conditions (chromatic adaptation).

Acclimation of the Photosynthetic Apparatus to Growth Irradiance in a Mutant Strain of Synechococcus Lacking Iron Superoxide Dismutase

The acclimation of the photosynthetic apparatus to growth irradiance in a mutant strain of Synechococcus sp. PCC 7942 lacking detectable iron superoxide dismutase activity was studied. The growth of the mutant was inhibited at concentrations of methyl viologen 4 orders of magnitude smaller than those required to inhibit the growth of the wild-type strain. An increased sensitivity of photosynthetic electron transport near photosystem I (PSI) toward photooxidative stress was also observed in the mutant strain. In the absence of methyl viologen, the mutant exhibited similar growth rates compared with those of the wild type, even at high growth irradiance (350 [mu]E m-2 s-1) where chronic inhibition of photosystem II (PSII) was observed in both strains. Under high growth irradiance, the ratios of PSII to PSI and of [alpha]-phycocyanin to chlorophyll a were less than one-third of the values for the wild type. In both strains, cellular contents of chlorophyll a, [alpha]-phycocyanin, and [beta]-carotene, as well as the length of the phycobilisome rods, declined with increasing growth irradiance. Only the cellular content of the carotenoid zeaxanthin seemed to be independent of growth irradiance. These results suggest an altered acclimation to growth irradiance in the sodB mutant in which the stoichiometry between PSI and PSII is adjusted to compensate for the loss of PSI efficiency occurring under high growth irradiance. Similar shortening of the phycobilisome rods in the sodB mutant and wild-type strain suggest that phycobilisome rod length is regulated independently of photosystem stoichiometry.

Insertional inactivation of genes to isolate mutants of Synechococcus sp. strain PCC 7942: isolation of filamentous strains

We have developed a simple procedure for generating mutants of the cyanobacterium Synechococcus sp. strain PCC 7942 in which the site of the lesion can be readily identified. This procedure involves transforming Synechococcus sp. strain PCC 7942 with a library of its own DNA that was fully digested with Sau3A and ligated into the plasmid vector pUC8. The homologous integration of the recombinant plasmid into the genome will often result in the disruption of a gene and the loss of gene function. We have used this method to generate many mutants of Synechococcus sp. strain PCC 7942 which grow as multicellular filaments rather than as unicells. Since the gene harboring the lesion was tagged with pUC8, it was easily isolated. In this paper, we discuss the usefulness of this procedure for the generation of mutants, and we characterize one mutant in which the lesion may be in an operon involved in the assembly of lipopolysaccharides.