Light Wavelength as a Contributory Factor of Environmental Fitness in the Cyanobacterial Circadian Clock.

A circadian clock is an essential system that drives the 24-h expression rhythms for adaptation to day-night cycles. The molecular mechanism of the circadian clock has been extensively studied in cyanobacteria harboring the KaiC-based timing system. Nevertheless, our understanding of the physiological significance of the cyanobacterial circadian clock is still limited. In this study, we cultured wild-type Synechococcus elongatus PCC 7942 and circadian clock mutants in day-night cycles at different light qualities and found that the growth of the circadian clock mutants was specifically impaired during 12-h blue light/12-h dark (BD) cycles for the first time. The arrhythmic mutant kaiCAA was further analyzed by photosynthetic measurements. Compared with the wild type, the mutant exhibited decreases in the chlorophyll content, the ratio of photosystem I to II, net O2 evolution rate and efficiency of photosystem II photochemistry during BD cycles. These results indicate that the circadian clock is necessary for the growth and the maintenance of the optimum function of the photosynthetic apparatus in cyanobacteria under blue photoperiodic conditions.

Molecular bases of an alternative dual-enzyme system for light color acclimation of marine Synechococcus cyanobacteria.

Marine Synechococcus cyanobacteria owe their ubiquity in part to the wide pigment diversity of their light-harvesting complexes. In open ocean waters, cells predominantly possess sophisticated antennae with rods composed of phycocyanin and two types of phycoerythrins (PEI and PEII). Some strains are specialized for harvesting either green or blue light, while others can dynamically modify their light absorption spectrum to match the dominant ambient color. This process, called type IV chromatic acclimation (CA4), has been linked to the presence of a small genomic island occurring in two configurations (CA4-A and CA4-B). While the CA4-A process has been partially characterized, the CA4-B process has remained an enigma. Here we characterize the function of two members of the phycobilin lyase E/F clan, MpeW and MpeQ, in Synechococcus sp. strain A15-62 and demonstrate their critical role in CA4-B. While MpeW, encoded in the CA4-B island and up-regulated in green light, attaches the green light-absorbing chromophore phycoerythrobilin to cysteine-83 of the PEII α-subunit in green light, MpeQ binds phycoerythrobilin and isomerizes it into the blue light-absorbing phycourobilin at the same site in blue light, reversing the relationship of MpeZ and MpeY in the CA4-A strain RS9916. Our data thus reveal key molecular differences between the two types of chromatic acclimaters, both highly abundant but occupying distinct complementary ecological niches in the ocean. They also support an evolutionary scenario whereby CA4-B island acquisition allowed former blue light specialists to become chromatic acclimaters, while former green light specialists would have acquired this capacity by gaining a CA4-A island.

Involvement of glycogen metabolism in circadian control of UV resistance in cyanobacteria.

Most organisms harbor circadian clocks as endogenous timing systems in order to adapt to daily environmental changes, such as exposure to ultraviolet (UV) light. It has been hypothesized that the circadian clock evolved to prevent UV-sensitive activities, such as DNA replication and cell division, during the daytime. Indeed, circadian control of UV resistance has been reported in several eukaryotic organisms, from algae to higher organisms, although the underlying mechanisms remain unknown. Here, we demonstrate that the unicellular cyanobacterium Synechococcus elongatus PCC 7942 exhibits a circadian rhythm in resistance to UV-C and UV-B light, which is higher during subjective dawn and lower during subjective dusk. Nullification of the clock gene cluster kaiABC or the DNA-photolyase phr abolished rhythmicity with constitutively lower resistance to UV-C light, and amino acid substitutions of KaiC altered the period lengths of the UV-C resistance rhythm. In order to elucidate the molecular mechanism underlying the circadian regulation of UV-C resistance, transposon insertion mutants that alter UV-C resistance were isolated. Mutations to the master circadian output mediator genes sasA and rpaA and the glycogen degradation enzyme gene glgP abolished circadian rhythms of UV-C resistance with constitutively high UV-C resistance. Combining these results with further experiments using ATP synthesis inhibitor and strains with modified metabolic pathways, we showed that UV-C resistance is weakened by directing more metabolic flux from the glycogen degradation to catabolic pathway such as oxidative pentose phosphate pathway and glycolysis. We suggest glycogen-related metabolism in the dark affects circadian control in UV sensitivity, while the light masks this effect through the photolyase function.

Single-Organelle Quantification Reveals Stoichiometric and Structural Variability of Carboxysomes Dependent on the Environment.

The carboxysome is a complex, proteinaceous organelle that plays essential roles in carbon assimilation in cyanobacteria and chemoautotrophs. It comprises hundreds of protein homologs that self-assemble in space to form an icosahedral structure. Despite its significance in enhancing CO2 fixation and potentials in bioengineering applications, the formation of carboxysomes and their structural composition, stoichiometry, and adaptation to cope with environmental changes remain unclear. Here we use live-cell single-molecule fluorescence microscopy, coupled with confocal and electron microscopy, to decipher the absolute protein stoichiometry and organizational variability of single ß-carboxysomes in the model cyanobacterium Synechococcus elongatus PCC7942. We determine the physiological abundance of individual building blocks within the icosahedral carboxysome. We further find that the protein stoichiometry, diameter, localization, and mobility patterns of carboxysomes in cells depend sensitively on the microenvironmental levels of CO2 and light intensity during cell growth, revealing cellular strategies of dynamic regulation. These findings, also applicable to other bacterial microcompartments and macromolecular self-assembling systems, advance our knowledge of the principles that mediate carboxysome formation and structural modulation. It will empower rational design and construction of entire functional metabolic factories in heterologous organisms, for example crop plants, to boost photosynthesis and agricultural productivity.

Interplay between differentially expressed enzymes contributes to light color acclimation in marine Synechococcus.

Marine Synechococcus, a globally important group of cyanobacteria, thrives in various light niches in part due to its varied photosynthetic light-harvesting pigments. Many Synechococcus strains use a process known as chromatic acclimation to optimize the ratio of two chromophores, green-light-absorbing phycoerythrobilin (PEB) and blue-light-absorbing phycourobilin (PUB), within their light-harvesting complexes. A full mechanistic understanding of how Synechococcus cells tune their PEB to PUB ratio during chromatic acclimation has not yet been obtained. Here, we show that interplay between two enzymes named MpeY and MpeZ controls differential PEB and PUB covalent attachment to the same cysteine residue. MpeY attaches PEB to the light-harvesting protein MpeA in green light, while MpeZ attaches PUB to MpeA in blue light. We demonstrate that the ratio of mpeY to mpeZ mRNA determines if PEB or PUB is attached. Additionally, strains encoding only MpeY or MpeZ do not acclimate. Examination of strains of Synechococcus isolated from across the globe indicates that the interplay between MpeY and MpeZ uncovered here is a critical feature of chromatic acclimation for marine Synechococcus worldwide.

Glycogen Metabolism Supports Photosynthesis Start through the Oxidative Pentose Phosphate Pathway in Cyanobacteria.

Cyanobacteria experience drastic changes in their carbon metabolism under daily light/dark cycles. During the day, the Calvin-Benson cycle fixes CO2 and diverts excess carbon into glycogen storage. At night, glycogen is degraded to support cellular respiration. The dark/light transition represents a universal environmental stress for cyanobacteria and other photosynthetic lifeforms. Recent studies revealed the essential genetic background necessary for the fitness of cyanobacteria during diurnal growth. However, the metabolic processes underlying the dark/light transition are not well understood. In this study, we observed that glycogen metabolism supports photosynthesis in the cyanobacterium Synechococcus elongatus PCC 7942 when photosynthesis reactions start upon light exposure. Compared with the wild type, the glycogen mutant ∆glgC showed a reduced photosynthetic efficiency and a slower P700+ rereduction rate when photosynthesis starts. Proteomic analyses indicated that glycogen is degraded through the oxidative pentose phosphate (OPP) pathway during the dark/light transition. We confirmed that the OPP pathway is essential for the initiation of photosynthesis and further showed that glycogen degradation through the OPP pathway contributes to the activation of key Calvin-Benson cycle enzymes by modulating NADPH levels. This strategy stimulates photosynthesis in cyanobacteria following dark respiration and stabilizes the Calvin-Benson cycle under fluctuating environmental conditions, thereby offering evolutionary advantages for photosynthetic organisms using the Calvin-Benson cycle for carbon fixation.

The structural basis of far-red light absorbance by allophycocyanins.

Phycobilisomes (PBS), the major light-harvesting antenna in cyanobacteria, are supramolecular complexes of colorless linkers and heterodimeric, pigment-binding phycobiliproteins. Phycocyanin and phycoerythrin commonly comprise peripheral rods, and a multi-cylindrical core is principally assembled from allophycocyanin (AP). Each AP subunit binds one phycocyanobilin (PCB) chromophore, a linear tetrapyrrole that predominantly absorbs in the orange-red region of the visible spectrum (600-700 nm). AP facilitates excitation energy transfer from PBS peripheral rods or from directly absorbed red light to accessory chlorophylls in the photosystems. Paralogous forms of AP that bind PCB and are capable of absorbing far-red light (FRL; 700-800 nm) have recently been identified in organisms performing two types of photoacclimation: FRL photoacclimation (FaRLiP) and low-light photoacclimation (LoLiP). The FRL-absorbing AP (FRL-AP) from the thermophilic LoLiP strain Synechococcus sp. A1463 was chosen as a platform for site-specific mutagenesis to probe the structural differences between APs that absorb in the visible region and FRL-APs and to identify residues essential for the FRL absorbance phenotype. Conversely, red light-absorbing allophycocyanin-B (AP-B; ~ 670 nm) from the same organism was used as a platform for creating a FRL-AP. We demonstrate that the protein environment immediately surrounding pyrrole ring A of PCB on the alpha subunit is mostly responsible for the FRL absorbance of FRL-APs. We also show that interactions between PCBs bound to alpha and beta subunits of adjacent protomers in trimeric AP complexes are responsible for a large bathochromic shift of about ~ 20 nm and notable sharpening of the long-wavelength absorbance band.

Cell size control driven by the circadian clock and environment in cyanobacteria.

How cells maintain their size has been extensively studied under constant conditions. In the wild, however, cells rarely experience constant environments. Here, we examine how the 24-h circadian clock and environmental cycles modulate cell size control and division timings in the cyanobacterium Synechococcus elongatus using single-cell time-lapse microscopy. Under constant light, wild-type cells follow an apparent sizer-like principle. Closer inspection reveals that the clock generates two subpopulations, with cells born in the subjective day following different division rules from cells born in subjective night. A stochastic model explains how this behavior emerges from the interaction of cell size control with the clock. We demonstrate that the clock continuously modulates the probability of cell division throughout day and night, rather than solely applying an on-off gate to division, as previously proposed. Iterating between modeling and experiments, we go on to identify an effective coupling of the division rate to time of day through the combined effects of the environment and the clock on cell division. Under naturally graded light-dark cycles, this coupling narrows the time window of cell divisions and shifts divisions away from when light levels are low and cell growth is reduced. Our analysis allows us to disentangle, and predict the effects of, the complex interactions between the environment, clock, and cell size control.

Far-red light allophycocyanin subunits play a role in chlorophyll d accumulation in far-red light.

Some terrestrial cyanobacteria acclimate to and utilize far-red light (FRL; λ = 700-800 nm) for oxygenic photosynthesis, a process known as far-red light photoacclimation (FaRLiP). A conserved, 20-gene FaRLiP cluster encodes core subunits of Photosystem I (PSI) and Photosystem II (PSII), five phycobiliprotein subunits of FRL-bicylindrical cores, and enzymes for synthesis of chlorophyll (Chl) f and possibly Chl d. Deletion mutants for each of the five apc genes of the FaRLiP cluster were constructed in Synechococcus sp. PCC 7335, and all had similar phenotypes. When the mutants were grown in white (WL) or red (RL) light, the cells closely resembled the wild-type (WT) strain grown under the same conditions. However, the WT and mutant strains were very different when grown under FRL. Mutants grown in FRL were unable to assemble FRL-bicylindrical cores, were essentially devoid of FRL-specific phycobiliproteins, but retained RL-type phycobilisomes and WL-PSII. The transcript levels for genes of the FaRLiP cluster in the mutants were similar to those in WT. Surprisingly, the Chl d contents of the mutant strains were greatly reduced (~ 60-99%) compared to WT and so were the levels of FRL-PSII. We infer that Chl d may be essential for the assembly of FRL-PSII, which does not accumulate to normal levels in the mutants. We further infer that the cysteine-rich subunits of FRL allophycocyanin may either directly participate in the synthesis of Chl d or that FRL bicylindrical cores stabilize FRL-PSII to prevent loss of Chl d.

Predicting the metabolic capabilities of Synechococcus elongatus PCC 7942 adapted to different light regimes.

There is great interest in engineering photoautotrophic metabolism to generate bioproducts of societal importance. Despite the success in employing genome-scale modeling coupled with flux balance analysis to engineer heterotrophic metabolism, the lack of proper constraints necessary to generate biologically realistic predictions has hindered broad application of this methodology to phototrophic metabolism. Here we describe a methodology for constraining genome-scale models of photoautotrophy in the cyanobacteria Synechococcus elongatus PCC 7942. Experimental photophysiology parameters coupled to genome-scale flux balance analysis resulted in accurate predictions of growth rates and metabolic reaction fluxes at low and high light conditions. Additionally, by constraining photon uptake fluxes, we characterized the metabolic cost of excess excitation energy. The predicted energy fluxes were consistent with known light-adapted phenotypes in cyanobacteria. Finally, we leveraged the modeling framework to characterize existing photoautotrophic and photomixtotrophic engineering strategies for 2,3-butanediol production in S. elongatus. This methodology, applicable to genome-scale modeling of all phototrophic microorganisms, can facilitate the use of flux balance analysis in the engineering of light-driven metabolism.

The endogenous redox rhythm is controlled by a central circadian oscillator in cyanobacterium Synechococcus elongatus PCC7942.

The intracellular redox and the circadian clock in photosynthetic organisms are two major regulators globally affecting various biological functions. Both of the global control systems have evolved as systems to adapt to regularly or irregularly changing light environments. Here, we report that the two global regulators mutually interact in cyanobacterium Synechococcus elongatus PCC7942, a model photosynthetic organism whose clock molecular mechanism is well known. Electrochemical assay using a transmembrane electron mediator revealed that intracellular redox of S. elongatus PCC7942 cell exhibited circadian rhythms under constant light conditions. The redox rhythm disappeared when transcription/translation of clock genes is defunctionalized, indicating that the transcription/translation controlled by a core KaiABC oscillator generates the circadian redox rhythm. Importantly, the amplitude of the redox rhythm at a constant light condition was large enough to affect the KaiABC oscillator. The findings indicated that the intracellular redox state is actively controlled to change in a 24-h cycle under constant light conditions by the circadian clock system.

Energy transfer from chlorophyll f to the trapping center in naturally occurring and engineered Photosystem I complexes.

Certain cyanobacteria can thrive in environments enriched in far-red light (700-800 nm) due to an acclimation process known as far-red light photoacclimation (FaRLiP). During FaRLiP, about 8% of the Chl a molecules in the photosystems are replaced by Chl f and a very small amount of Chl d. We investigated the spectroscopic properties of Photosystem I (PSI) complexes isolated from wild-type (WT) Synechococcus sp. PCC 7335 and a chlF mutant strain (lacking Chl f synthase) grown in white and far-red light (WL-PSI and FRL-PSI, respectively). WT-FRL-PSI complexes contain Chl f and Chl a but not Chl d. The light-minus dark difference spectrum of the trapping center at high spectral resolution indicates that the special pair in WT-FRL-PSI consists of Chl a molecules with maximum bleaching at 703-704 nm. The action spectrum for photobleaching of the special pair showed that Chl f molecules absorbing at wavelengths up to 800 nm efficiently transfer energy to the trapping center in FRL-PSI complexes to produce a charge-separated state. This is ~ 50 nm further into the near IR than WL-PSI; Chl f has a quantum yield equivalent to that of Chl a in the antenna, i.e., ~ 1.0. PSI complexes from Synechococcus 7002 carrying 3.8 Chl f molecules could promote photobleaching of the special pair by energy transfer at wavelengths longer than WT PSI complexes. Results from these latter studies are directly relevant to the issue of whether introduction of Chl f synthase into plants could expand the wavelength range available for oxygenic photosynthesis in crop plants.

Characterization of chlorophyll f synthase heterologously produced in Synechococcus sp. PCC 7002.

In diverse terrestrial cyanobacteria, Far-Red Light Photoacclimation (FaRLiP) promotes extensive remodeling of the photosynthetic apparatus, including photosystems (PS)I and PSII and the cores of phycobilisomes, and is accompanied by the concomitant biosynthesis of chlorophyll (Chl) d and Chl f. Chl f synthase, encoded by chlF, is a highly divergent paralog of psbA; heterologous expression of chlF from Chlorogloeopsis fritscii PCC 9212 led to the light-dependent production of Chl f in Synechococcus sp. PCC 7002 (Ho et al., Science 353, aaf9178 (2016)). In the studies reported here, expression of the chlF gene from Fischerella thermalis PCC 7521 in the heterologous system led to enhanced synthesis of Chl f. N-terminally [His]10-tagged ChlF7521 was purified and identified by immunoblotting and tryptic-peptide mass fingerprinting. As predicted from its sequence similarity to PsbA, ChlF bound Chl a and pheophytin a at a ratio of ~ 3-4:1, bound ß-carotene and zeaxanthin, and was inhibited in vivo by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Cross-linking studies and the absence of copurifying proteins indicated that ChlF forms homodimers. Flash photolysis of ChlF produced a Chl a triplet that decayed with a lifetime (1/e) of ~ 817 µs and that could be attributed to intersystem crossing by EPR spectroscopy at 90 K. When the chlF7521 gene was expressed in a strain in which the psbD1 and psbD2 genes had been deleted, significantly more Chl f was produced, and Chl f levels could be further enhanced by specific growth-light conditions. Chl f synthesized in Synechococcus sp. PCC 7002 was inserted into trimeric PSI complexes.

Elemental Stoichiometry and Photophysiology Regulation of Synechococcus sp. PCC7002 Under Increasing Severity of Chronic Iron Limitation.

Iron (Fe) is an essential cofactor for many metabolic enzymes of photoautotrophs. Although Fe limits phytoplankton productivity in broad areas of the ocean, phytoplankton have adapted their metabolism and growth to survive in these conditions. Using the euryhaline cyanobacterium Synechococcus sp. PCC7002, we investigated the physiological responses to long-term acclimation to four levels of Fe availability representative of the contemporary ocean (36.7, 3.83, 0.47 and 0.047 pM Fe'). With increasing severity of Fe limitation, Synechococcus sp. cells gradually decreased their volume and growth while increasing their energy allocation into organic carbon and nitrogen cellular pools. Furthermore, the total cellular content of pigments decreased. Additionally, with increasing severity of Fe limitation, intertwined responses of PSII functional cross-section (σPSII), re-oxidation time of the plastoquinone primary acceptor QA (τ) and non-photochemical quenching revealed a shift in the photophysiological response between mild to strong Fe limitation compared with severe limitation. Under mild and strong Fe limitation, there was a decrease in linear electron transport accompanied by progressive loss of state transitions. Under severe Fe limitation, state transitions seemed to be largely supplanted by alternative electron pathways. In addition, mechanisms to dissipate energy excess and minimize oxidative stress associated with high irradiances increased with increasing severity of Fe limitation. Overall, our results establish the sequence of physiological strategies adopted by the cells under increasing severity of chronic Fe limitation, within a range of Fe concentrations relevant to modern ocean biogeochemistry.

Dynamics and energetics of cyanobacterial photosystem I:ferredoxin complexes in different redox states.

Fast turnover of ferredoxin/Fd reduction by photosystem-I/PSI requires that it dissociates rapidly after it has been reduced by PSI:Fd intracomplex electron transfer. The rate constants of Fd dissociation from PSI have been determined by flash-absorption spectroscopy with different combinations of cyanobacterial PSIs and Fds, and different redox states of Fd and of the terminal PSI acceptor (FAFB). Newly obtained values were derived firstly from the fact that the dissociation constant between PSI and redox-inactive gallium-substituted Fd increases upon (FAFB) reduction and secondly from the characterization and elucidation of a kinetic phase following intracomplex Fd reduction to binding of oxidized Fd to PSI, a process which is rate-limited by the foregoing dissociation of reduced Fd from PSI. By reference to the complex with oxidized partners, dissociation rate constants were found to increase moderately with (FAFB) single reduction and by about one order of magnitude after electron transfer from (FAFB)- to Fd, therefore favoring turnover of Fd reduction by PSI. With Thermosynechococcus elongatus partners, values of 270, 730 and >10000s-1 were thus determined for (FAFB)Fdoxidized, (FAFB)-Fdoxidized and (FAFB)Fdreduced, respectively. Moreover, assuming a conservative upper limit for the association rate constant between reduced Fd and PSI, a significant negative shift of the Fd midpoint potential upon binding to PSI has been calculated (< -60mV for Thermosynechococcus elongatus). From the present state of knowledge, the question is still open whether this redox shift is compatible with a large (>10) equilibrium constant for intracomplex reduction of Fd from (FAFB)-.

Connectivity among Photosystem II centers in phytoplankters: Patterns and responses.

Fast Repetition and Relaxation chlorophyll fluorescence induction is used to estimate the effective absorption cross section of PSII (σPSII), to analyze phytoplankton acclimation and electron transport. The fitting coefficient ρ measures excitation transfer from closed PSII to remaining open PSII upon illumination, which could theoretically generate a progressive increase in σPSII for the remaining open PSII. To investigate how ρ responds to illumination we grew marine phytoplankters with diverse antenna structures (Prochlorococcus, Synechococcus, Ostreococcus and Thalassiosira pseudonana) under limiting or saturating growth light. Initial ρ varied with growth light in Synechococcus and Thalassiosira. With increasing actinic illumination PSII closed progressively and ρ decreased for all four taxa, in a pattern explicable as an exponential decay of ρ with increasing distance between remaining open PSII reaction centers. This light-dependent down-regulation of ρ allows the four phytoplankters to limit the effect of increasing light upon σPSII. The four structurally distinct taxa showed, however, distinct rates of response of ρ to PSII closure, likely reflecting differences in the spacing or orientation among their PSII centers. Following saturating illumination recovery of ρ in darkness coincided directly with PSII re-opening in Prochlorococcus. Even after PSII had re-opened in Synechococcus a transition to State II slowed dark recovery of ρ. In Ostreococcus sustained NPQ slowed dark recovery of ρ. In Thalassiosira dark recovery of ρ was slowed, possibly by a light-induced change in PSII spacing. These patterns of ρ versus PSII closure are thus a convenient probe of comparative PSII spacings.

Light dilution via wavelength management for efficient high-density photobioreactors.

The spectral distribution of light influences microalgae productivity; however, development of photobioreactors has proceeded largely without regard to spectral optimization. Here, we use monochromatic light to quantify the joint influence of path length, culture density, light intensity, and wavelength on productivity and efficiency in Synechococcus elongatus. The productivity of green light was ∼4× that of red at the highest levels of culture density, depth, and light intensity. This performance is attributed to the combination of increased dilution and penetration of this weakly absorbed wavelength over a larger volume fraction of the reactor. In contrast, red light outperformed other wavelengths in low-density cultures with low light intensities. Low-density cultures also adapted more rapidly to reduce absorption of longer wavelengths, allowing for prolonged cultivation. Taken together, these results indicate that, particularly for artificially lit photobioreactors, wavelength needs to be included as a critical operational parameter to maintain optimal performance. Biotechnol. Bioeng. 2017;114: 1160-1169. © 2017 Wiley Periodicals, Inc.

Photoinhibition in marine picocyanobacteria.

Marine Synechococcus and Prochlorococcus cyanobacteria have different antenna compositions although they are genetically near to each other, and different strains thrive in very different illumination conditions. We measured growth and photoinhibition of PSII in two low-light and one high-light Prochlorococcus strains and in one Synechococcus strain. All strains were found to be able to shortly utilize moderate or even high light, but the low-light strains bleached rapidly in moderate light. Measurements of photoinhibition in the presence of the antibiotic lincomycin showed that a low-light Prochlorococcus strain was more sensitive than a high-light strain and both were more sensitive than the marine Synechococcus. The action spectrum of photoinhibition showed an increase from blue to ultraviolet wavelengths in all strains, suggesting contribution of manganese absorption to photoinhibition, but blue light caused less photoinhibition in marine cyanobacteria than expected on the basis of earlier results from plants and cyanobacteria. The visible-light part of the action spectrum resembled the absorption spectrum of the organism, suggesting that photosynthetic antenna pigments, especially divinyl chlorophylls, have a more important role as photoreceptors of visible-light photoinhibition in marine cyanobacteria than in other photoautotrophs.

Different physiological responses of cyanobacteria to ultraviolet-B radiation under iron-replete and iron-deficient conditions: Implications for underestimating the negative effects of UV-B radiation.

Iron deficiency has been considered one of the main limiting factors of phytoplankton productivity in some aquatic systems including oceans and lakes. Concomitantly, solar ultraviolet-B radiation has been shown to have both deleterious and positive impacts on phytoplankton productivity. However, how iron-deficient cyanobacteria respond to UV-B radiation has been largely overlooked in aquatic systems. In this study, physiological responses of four cyanobacterial strains (Microcystis and Synechococcus), which are widely distributed in freshwater or marine systems, were investigated under different UV-B irradiances and iron conditions. The growth, photosynthetic pigment composition, photosynthetic activity, and nonphotochemical quenching of the different cyanobacterial strains were drastically altered by enhanced UV-B radiation under iron-deficient conditions, but were less affected under iron-replete conditions. Intracellular reactive oxygen species (ROS) and iron content increased and decreased, respectively, with increased UV-B radiation under iron-deficient conditions for both Microcystis aeruginosa FACHB 912 and Synechococcus sp. WH8102. On the contrary, intracellular ROS and iron content of these two strains remained constant and increased, respectively, with increased UV-B radiation under iron-replete conditions. These results indicate that iron-deficient cyanobacteria are more susceptible to enhanced UV-B radiation. Therefore, UV-B radiation probably plays an important role in influencing primary productivity in iron-deficient aquatic systems, suggesting that its effects on the phytoplankton productivity may be underestimated in iron-deficient regions around the world.

Integrated transcriptomic and proteomic analysis of the global response of Synechococcus to high light stress.

Sufficient light is essential for the growth and physiological functions of photosynthetic organisms, but prolonged exposure to high light (HL) stress can cause cellular damage and ultimately result in the death of these organisms. Synechococcus sp. PCC 7002 (hereafter Synechococcus 7002) is a unicellular cyanobacterium with exceptional tolerance to HL intensities. However, the molecular mechanisms involved in HL response by Synechococcus 7002 are not well understood. Here, an integrated RNA sequencing transcriptomic and quantitative proteomic analysis was performed to investigate the cellular response to HL in Synechococcus 7002. A total of 526 transcripts and 233 proteins were identified to be differentially regulated under HL stress. Data analysis revealed major changes in mRNAs and proteins involved in the photosynthesis pathways, resistance to light-induced damage, DNA replication and repair, and energy metabolism. A set of differentially expressed mRNAs and proteins were validated by quantitative RT-PCR and Western blot, respectively. Twelve genes differentially regulated under HL stress were selected for knockout generation and growth analysis of these mutants led to the identification of key genes involved in the response of HL in Synechococcus 7002. Taken altogether, this study established a model for global response mechanisms to HL in Synechococcus 7002 and may be valuable for further studies addressing HL resistance in photosynthetic organisms.