The Alpha 7 Nicotinic Acetylcholine Receptor Does Not Affect Neonatal Brain Injury.

Inflammation plays a central role in the development of neonatal brain injury. The alpha 7 nicotinic acetylcholine receptor (α7nAChR) can modulate inflammation and has shown promising results as a treatment target in rodent models of adult brain injury. However, little is known about the role of the α7nAChR in neonatal brain injury. Hypoxic-ischemic (HI) brain injury was induced in male and female C57BL/6 mice, α7nAChR knock-out (KO) mice and their littermate controls on postnatal day (PND) 9-10. C57BL/6 pups received i.p. injections of α7nAChR agonist PHA 568487 (8 mg/kg) or saline once daily, with the first dose given directly after HI. Caspase-3 activity and cytokine mRNA expression in the brain was analyzed 24 h after HI. Motor function was assessed 24 and 48 h after HI, and immunohistochemistry was used to assess tissue loss at 24 h and 7 days after HI and microglial activation 7 days after HI. Activation of α7nAChR with the agonist PHA 568487 significantly decreased CCL2/MCP-1, CCL5/RANTES and IL-6 gene expression in the injured brain hemisphere 24 h after HI compared with saline controls in male, but not female, pups. However, α7nAChR activation did not alter caspase-3 activity and TNFα, IL-1ß and CD68 mRNA expression. Furthermore, agonist treatment did not affect motor function (24 or 48 h), neuronal tissue loss (24 h or 7 days) or microglia activation (7 days) after HI in either sex. Knock-out of α7nAChR did not influence neuronal tissue loss 7 days after HI. In conclusion, targeting the α7nAChR in neonatal brain injury shows some effect on dampening acute inflammatory responses in male pups. However, this does not lead to an effect on overall injury outcome.

Polymorphisms in alpha 7 nicotinic acetylcholine receptor gene, CHRNA7, and its partially duplicated gene, CHRFAM7A, associate with increased inflammatory response in human peripheral mononuclear cells.

The vagus nerve can, via the alpha 7 nicotinic acetylcholine receptor (α7nAChR), regulate inflammation. The gene coding for the α7nAChR, CHRNA7, can be partially duplicated, that is, CHRFAM7A, which is reported to impair the anti-inflammatory effect mediated via the α7nAChR. Several single nucleotide polymorphisms (SNPs) have been described in both CHRNA7 and CHRFAM7A, however, the functional role of these SNPs for immune responses remains to be investigated. In the current study, we set out to investigate whether genetic variants of CHRNA7 and CHRFAM7A can influence immune responses. By investigating data available from the Swedish SciLifeLab SCAPIS Wellness Profiling (S3WP) study, in combination with droplet digital PCR and freshly isolated PBMCs from the S3WP participants, challenged with lipopolysaccharide (LPS), we show that CHRNA7 and CHRFAM7A are expressed in human PBMCs, with approximately four times higher expression of CHRFAM7A compared with CHRNA7. One SNP in CHRFAM7A, rs34007223, is positively associated with hsCRP in healthy individuals. Furthermore, gene ontology (GO)-terms analysis of plasma proteins associated with gene expression of CHRNA7 and CHRFAM7A demonstrated an involvement for these genes in immune responses. This was further supported by in vitro data showing that several SNPs in both CHRNA7 and CHRFAM7A are significantly associated with cytokine response. In conclusion, genetic variants of CHRNA7 and CHRFAM7A alters cytokine responses. Furthermore, given that CHRFAM7A SNP rs34007223 is associated with inflammatory marker hsCRP in healthy individuals suggests that CHRFAM7A may have a more pronounced role in regulating inflammatory processes in humans than previously been recognized.

The selective alpha7 nicotinic acetylcholine receptor agonist AR-R17779 does not affect ischemia-reperfusion brain injury in mice.

Inflammation plays a central role in stroke-induced brain injury. The alpha7 nicotinic acetylcholine receptor (α7nAChR) can modulate immune responses in both the periphery and the brain. The aims of the present study were to investigate α7nAChR expression in different brain regions and evaluate the potential effect of the selective α7nAChR agonist AR-R17779 on ischemia-reperfusion brain injury in mice. Droplet digital PCR (ddPCR) was used to evaluate the absolute expression of the gene encoding α7nAChR (Chrna7) in hippocampus, striatum, thalamus and cortex in adult, naïve mice. Mice subjected to transient middle cerebral artery occlusion (tMCAO) or sham surgery were treated with α7nAChR agonist AR-R17779 (12 mg/kg) or saline once daily for 5 days. Infarct size and microglial activation 7 days after tMCAO were analyzed using immunohistochemistry. Chrna7 expression was found in all analyzed brain regions in naïve mice with the highest expression in cortex and hippocampus. At sacrifice, white blood cell count was significantly decreased in AR-R17779 treated mice compared with saline controls in the sham groups, although, no effect was seen in the tMCAO groups. Brain injury and microglial activation were evident 7 days after tMCAO. However, no difference was found between mice treated with saline or AR-R17779. In conclusion, α7nAChR expression varies in different brain regions and, despite a decrease in white blood cells in sham mice receiving AR-R17779, this compound does not affect stroke-induced brain injury.

Spleen proteomics data from high fat diet fed mice.

The composition of the diet affects many processes in the body, including body weight and endocrine system. We have previously shown that dietary fat also affects the immune system. Mice fed high fat diet rich in polyunsaturated fatty acids survive S. aureus infection to a much greater extent than mice fed high fat diet rich in saturated fatty acids. Here we present data regarding the dietary effects on protein expression in spleen from mice fed three different diets, I) low fat/chow diet (LFD, n = 4), II) high fat diet rich in saturated fatty acids (HFD-S, n = 4) and III) high fat diet rich in polyunsaturated fatty acids (HFD-P, n = 4). We performed mass spectrophotometry based quantitative proteomics analysis of isolated spleen by implementing the isobaric tags for relative and absolute quantification (iTRAQ) approach. Mass spectrometry data were analyzed using Proteome Discoverer 2.4 software using the search engine mascot against Mus musculus in SwissProt. 924 proteins are identified in all sets (n = 4) for different dietary effects taken for statistical analysis using Qlucore Omics Explorer software. Only 20 proteins were found to be differentially expressed with a cut-off value of false discovery rate < 0.1 (q-value) when comparing HFD-S and HFD-P but no differentially expressed proteins were found when LFD was compared with HFD-P or HFD-S. The identified proteins and statistical analysis comparing HFD-S and HFD-P diets are available as a supplementary file S1. We identified a subset of proteins that showed an inverse expression pattern between two high fat diets. These differentially expressed proteins were further classified by gene ontology for their role in biological processes and molecular functions. Mass spectrometry raw data are also available via ProteomeXchange with identifier PXD020365.

Introduction of a green algal squalene synthase enhances squalene accumulation in a strain of Synechocystis sp. PCC 6803.

Squalene is a triterpene which is produced as a precursor for a wide range of terpenoid compounds in many organisms. It has commercial use in food and cosmetics but could also be used as a feedstock for production of chemicals and fuels, if generated sustainably on a large scale. We have engineered a cyanobacterium, Synechocystis sp. PCC 6803, for production of squalene from CO2. In this organism, squalene is produced via the methylerythritol-phosphate (MEP) pathway for terpenoid biosynthesis, and consumed by the enzyme squalene hopene cyclase (Shc) for generation of hopanoids. The gene encoding Shc in Synechocystis was inactivated (Δshc) by insertion of a gene encoding a squalene synthase from the green alga Botryococcus braunii, under control of an inducible promoter. We could demonstrate elevated squalene generation in cells where the algal enzyme was induced. Heterologous overexpression of genes upstream in the MEP pathway further enhanced the production of squalene, to a level three times higher than the Δshc background strain. During growth in flat panel bioreactors, a squalene titer of 5.1 â€‹mg/L of culture was reached.

RcaE-Dependent Regulation of Carboxysome Structural Proteins Has a Central Role in Environmental Determination of Carboxysome Morphology and Abundance in Fremyella diplosiphon.

Carboxysomes are central to the carbon dioxide-concentrating mechanism (CCM) and carbon fixation in cyanobacteria. Although the structure is well understood, roles of environmental cues in the synthesis, positioning, and functional tuning of carboxysomes have not been systematically studied. Fremyella diplosiphon is a model cyanobacterium for assessing impacts of environmental light cues on photosynthetic pigmentation and tuning of photosynthetic efficiency during complementary chromatic acclimation (CCA), which is controlled by the photoreceptor RcaE. Given the central role of carboxysomes in photosynthesis, we investigated roles of light-dependent RcaE signaling in carboxysome structure and function. A ΔrcaE mutant exhibits altered carboxysome size and number, ccm gene expression, and carboxysome protein accumulation relative to the wild-type (WT) strain. Several Ccm proteins, including carboxysome shell proteins and core-nucleating factors, overaccumulate in ΔrcaE cells relative to WT cells. Additionally, levels of carboxysome cargo RuBisCO in the ΔrcaE mutant are lower than or unchanged from those in the WT strain. This shift in the ratios of carboxysome shell and nucleating components to the carboxysome cargo appears to drive carboxysome morphology and abundance dynamics. Carboxysomes are also occasionally mislocalized spatially to the periphery of spherical mutants within thylakoid membranes, suggesting that carboxysome positioning is impacted by cell shape. The RcaE photoreceptor links perception of external light cues to regulating carboxysome structure and function and, thus, to the cellular capacity for carbon fixation. IMPORTANCE Carboxysomes are proteinaceous subcellular compartments, or bacterial organelles, found in cyanobacteria that consist of a protein shell surrounding a core primarily composed of the enzyme ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO) that is central to the carbon dioxide-concentrating mechanism (CCM) and carbon fixation. Whereas significant insights have been gained regarding the structure and synthesis of carboxysomes, limited attention has been given to how their size, abundance, and protein composition are regulated to ensure optimal carbon fixation in dynamic environments. Given the centrality of carboxysomes in photosynthesis, we provide an analysis of the role of a photoreceptor, RcaE, which functions in matching photosynthetic pigmentation to the external environment during complementary chromatic acclimation and thereby optimizing photosynthetic efficiency, in regulating carboxysome dynamics. Our data highlight a role for RcaE in perceiving external light cues and regulating carboxysome structure and function and, thus, in the cellular capacity for carbon fixation and organismal fitness.

Sll1783, a monooxygenase associated with polysaccharide processing in the unicellular cyanobacterium Synechocystis PCC 6803.

Cyanobacteria play a pivotal role as the primary producer in many aquatic ecosystems. The knowledge on the interacting processes of cyanobacteria with its environment - abiotic and biotic factors - is still very limited. Many potential exocytoplasmic proteins in the model unicellular cyanobacterium Synechocystis PCC 6803 have unknown functions and their study is essential to improve our understanding of this photosynthetic organism and its potential for biotechnology use. Here we characterize a deletion mutant of Synechocystis PCC 6803, Δsll1783, a strain that showed a remarkably high light resistance which is related with its lower thylakoid membrane formation. Our results suggests Sll1783 to be involved in a mechanism of polysaccharide degradation and uptake and we hypothesize it might function as a sensor for cell density in cyanobacterial cultures.

Terpenoids and their biosynthesis in cyanobacteria.

Terpenoids, or isoprenoids, are a family of compounds with great structural diversity which are essential for all living organisms. In cyanobacteria, they are synthesized from the methylerythritol-phosphate (MEP) pathway, using glyceraldehyde 3-phosphate and pyruvate produced by photosynthesis as substrates. The products of the MEP pathway are the isomeric five-carbon compounds isopentenyl diphosphate and dimethylallyl diphosphate, which in turn form the basic building blocks for formation of all terpenoids. Many terpenoid compounds have useful properties and are of interest in the fields of pharmaceuticals and nutrition, and even potentially as future biofuels. The MEP pathway, its function and regulation, and the subsequent formation of terpenoids have not been fully elucidated in cyanobacteria, despite its relevance for biotechnological applications. In this review, we summarize the present knowledge about cyanobacterial terpenoid biosynthesis, both regarding the native metabolism and regarding metabolic engineering of cyanobacteria for heterologous production of non-native terpenoids.

Responses to iron limitation are impacted by light quality and regulated by RcaE in the chromatically acclimating cyanobacterium Fremyella diplosiphon.

Photosynthetic organisms adapt to environmental fluctuations of light and nutrient availability. Iron is critical for photosynthetic organismal growth, as many cellular processes depend upon iron cofactors. Whereas low iron levels can have deleterious effects, excess iron can lead to damage, as iron is a reactive metal that can result in the production of damaging radicals. Therefore, organisms regulate cellular iron levels to maintain optimal iron homeostasis. In particular, iron is an essential factor for the function of photosystems associated with photosynthetic light-harvesting complexes. Photosynthetic organisms, including cyanobacteria, generally respond to iron deficiency by reduced growth, degradation of non-essential proteins and in some cases alterations of cellular morphology. In response to fluctuations in ambient light quality, the cyanobacterium Fremyella diplosiphon undergoes complementary chromatic adaptation (CCA). During CCA, phycobiliprotein composition of light-harvesting antennae is altered in response to green light (GL) and red light (RL) for efficient utilization of light energy for photosynthesis. We observed light-regulated responses to iron limitation in F. diplosiphon. RL-grown cells exhibited significant reductions in growth and pigment levels, and alterations in iron-associated proteins, which impact the accumulation of reactive oxygen species under iron-limiting conditions, whereas GL-grown cells exhibited partial resistance to iron limitation. We investigated the roles of known CCA regulators RcaE, RcaF and RcaC in this light-dependent iron-acclimation response. Through comparative analyses of wild-type and CCA mutant strains, we determined that photoreceptor RcaE has a central role in light-induced oxidative stress associated with iron limitation, and impacts light-regulated iron-acclimation responses, physiologically and morphologically.

Production of squalene in Synechocystis sp. PCC 6803.

In recent years, there has been an increased interest in the research and development of sustainable alternatives to fossil fuels. Using photosynthetic microorganisms to produce such alternatives is advantageous, since they can achieve direct conversion of carbon dioxide from the atmosphere into the desired product, using sunlight as the energy source. Squalene is a naturally occurring 30-carbon isoprenoid, which has commercial use in cosmetics and in vaccines. If it could be produced sustainably on a large scale, it could also be used instead of petroleum as a raw material for fuels and as feedstock for the chemical industry. The unicellular cyanobacterium Synechocystis PCC 6803 possesses a gene, slr2089, predicted to encode squalene hopene cyclase (Shc), an enzyme converting squalene into hopene, the substrate for forming hopanoids. Through inactivation of slr2089 (shc), we explored the possibility to produce squalene using cyanobacteria. The inactivation led to accumulation of squalene, to a level over 70 times higher than in wild type cells, reaching 0.67 mg OD750(-1) L(-1). We did not observe any significant growth deficiency in the Δshc strain compared to the wild type Synechocystis, even at high light conditions, suggesting that the observed squalene accumulation was not detrimental to growth, and that formation of hopene by Shc is not crucial for growth under normal conditions, nor for high-light stress tolerance. Effects of different light intensities and growth stages on squalene accumulation in the Δshc strain were investigated. We also identified a gene, sll0513, as a putative squalene synthase in Synechocystis, and verified its function by inactivation. In this work, we show that it is possible to use the cyanobacterium Synechocystis to generate squalene, a hydrocarbon of commercial interest and a potential biofuel. We also report the first identification of a squalene hopene cyclase, and the second identification of squalene synthase, in cyanobacteria.

Light Quantity Affects the Regulation of Cell Shape in Fremyella diplosiphon.

In some cyanobacteria, the color or prevalent wavelengths of ambient light can impact the protein or pigment composition of the light-harvesting complexes. In some cases, light color or quality impacts cellular morphology. The significance of changes in pigmentation is associated strongly with optimizing light absorption for photosynthesis, whereas the significance of changes in light quality-dependent cellular morphology is less well understood. In natural aquatic environments, light quality and intensity change simultaneously at varying depths of the water column. Thus, we hypothesize that changes in morphology that also have been attributed to differences in the prevalent wavelengths of available light may largely be associated with changes in light intensity. Fremyella diplosiphon shows highly reproducible light-dependent changes in pigmentation and morphology. Under red light (RL), F. diplosiphon cells are blue-green in color, due to the accumulation of high levels of phycocyanin, a RL-absorbing pigment in the light-harvesting complexes or phycobilisomes (PBSs), and the shape of cells are short and rounded. Conversely, under green light (GL), F. diplosiphon cells are red in color due to accumulation of GL-absorbing phycoerythrin in PBSs, and are longer and brick-shaped. GL is enriched at lower depths in the water column, where overall levels of light also are reduced, i.e., to 10% or less of the intensity found at the water surface. We hypothesize that longer cells under low light intensities at increasing depths in the water column, which are generally also enriched in green wavelengths, are associated with greater levels of total photosynthetic pigments in the thylakoid membranes. To test this hypothesis, we grew F. diplosiphon under increasing intensities of GL and observed whether the length of cells diminished due to reduced pressure to maintain larger cells and the associated increased photosynthetic membrane capacity under high light intensity, independent of whether it is light of green wavelengths.

Interspecific resource competition-combined effects of radiation and nutrient limitation on two diazotrophic filamentous cyanobacteria.

The cyanobacterial blooms in the Baltic Sea are dominated by diazotrophic cyanobacteria, the potentially toxic species Aphanizomenon sp. and the toxic species Nodularia spumigena. The seasonal succession with peaks of Aphanizomenon sp., followed by peaks of N. spumigena, has been explained by the species-specific niches of the two species. In a three-factorial outdoor experiment, we tested if nutrient and radiation conditions may impact physiological and biochemical responses of N. spumigena and Aphanizomenon sp. in the presence or absence of the other species. The two nutrient treatments were f/2 medium without NO (3) (-) (-N) and f/2 medium without PO (4) (3-) (-P), and the two ambient radiation treatments were photosynthetic active radiation >395 nm (PAR) and PAR + UV-A + UV-B >295 nm. The study showed that Aphanizomenon sp. was not negatively affected by the presence of N. spumigena and that N. spumigena was better adapted to both N and P limitation in interaction with ultraviolet radiation (UVR, 280-400 nm). In the Baltic Sea, these physical conditions are likely to prevail in the surface water during summer. Interestingly, the specific growth rate of N. spumigena was stimulated by the presence of Aphanizomenon sp. We suggest that the seasonal succession, with peaks of Aphanizomenon sp. followed by peaks of N. spumigena, is a result from species-specific preferences of environmental conditions and/or stimulation by Aphanizomenon sp. rather than an allelopathic effect of N. spumigena. The results from our study, together with a predicted stronger stratification due to effects of climate change in the Baltic Sea with increased temperature and increased precipitation and increased UV-B due to ozone losses, reflect a scenario with a continuing future dominance of the toxic N. spumigena.

Convergence and divergence of the photoregulation of pigmentation and cellular morphology in Fremyella diplosiphon.

Photosynthetic pigment accumulation and cellular and filament morphology are regulated reversibly by green light (GL) and red light (RL) in the cyanobacterium Fremyella diplosiphon during complementary chromatic adaptation (CCA). The photoreceptor RcaE (regulator of chromatic adaptation), which appears to function as a light-responsive sensor kinase, controls both of these responses. Recent findings indicate that downstream of RcaE, the signaling pathways leading to light-dependent changes in morphology or pigment synthesis and/or accumulation branch, and utilize distinct molecular components. We recently reported that the regulation of the accumulation of the GL-absorbing photosynthetic accessory protein phycoerythrin (PE) and photoregulation of cellular morphology are largely independent, as many mutants with severe PE accumulation defects do not have major disruptions in the regulation of cellular morphology. Furthermore, morphology can be disrupted under GL without impacting GL-dependent PE accumulation. Most recently, however, we determined that the disruption of the cpeR gene, which encodes a protein that is known to function as an activator of PE synthesis under GL, results in disruption of cellular morphology under GL and RL. Thus, apart from RcaE, CpeR is only the second known regulator to impact morphology under both light conditions in F. diplosiphon.

Regulation of phycoerythrin synthesis and cellular morphology in Fremyella diplosiphon green mutants.

Light-dependent modification of photosynthetic pigmentation and cellular growth responses is commonly associated with increased fitness in photosynthetic organisms, including cyanobacteria. Prior analyses of pigmentation mutants in the freshwater cyanobacterium Fremyelladiplosiphon has resulted in the observation that RcaE is a photosensor responsible for regulating organismal responses to changes in red light (RL) and green light (GL). RcaE regulates both pigmentation and cellular morphology, yet previous investigations and the analysis of additional pigmentation mutants here show that the signaling pathways regulating pigmentation and morphology appear to branch downstream of RcaE. We provide evidence that a ΔcpeR mutant has altered regulation of cellular morphology in addition to a known disruption in phycoerythrin synthesis. This marks the first description of the association of a regulator with the control of cellular morphology under both RL and GL in F.diplosiphon, apart from RcaE. In addition to providing a link between CpeR and the photoregulation of morphology in F.diplosiphon, the isolation of a ΔcpeR::IS66 mutant in the UTEX 481 strain represents both the first isolation of an IS66-based gene disruption and verification of the existence of an IS66-related element in F. diplosiphon.

Characterization of green mutants in Fremyella diplosiphon provides insight into the impact of phycoerythrin deficiency and linker function on complementary chromatic adaptation.

Functions of phycobiliprotein (PBP) linkers are less well studied than other PBP polypeptides that are structural components or required for the synthesis of the light-harvesting phycobilisome (PBS) complexes. Linkers serve both structural and functional roles in PBSs. Here, we report the isolation of a phycoerythrin (PE) rod-linker mutant and a novel PE-deficient mutant in Fremyella diplosiphon. We describe their phenotypic characterization, including light-dependent photosynthetic pigment accumulation and photoregulation of cellular morphology. PE-linker protein CpeE and a novel protein impact PE accumulation, and thus PBS function, primarily under green light conditions.

A novel role for a TonB-family protein and photoregulation of iron acclimation in Fremyella diplosiphon.

Photosynthetic organisms display adaptations to changes in light and nutrient availability. Iron, which is required for the function of photosynthetic photosystems and other important biochemical processes, is an essential mineral that consequently impacts not only overall photosynthetic efficiency, but also the physiology of organisms in general. Our recent study represents the first functional characterization of a cyanobacterial TonB protein. TonB proteins classically are membrane proteins that support the transport of iron and vitamin B12 into cells. TonB proteins thus generally serve a critical role in organismal iron acclimation. We recently identified FdTonB, a TonB-family protein, in the filamentous freshwater cyanobacterium Fremyella diplosiphon. FdTonB contains conserved TonB residues and domains, as well as novel protein domains. Our recent study, however, supports a novel function for this protein in the photoregulation of morphology, rather than iron acclimation, in F. diplosiphon. Our detailed investigations into the responses of SF33 wild-type and ΔtonB mutant strains did not support a role for FdTonB in organismal responses to iron limitation. However, close examination of our recent results did highlight a novel interaction between light and iron acclimation in F. diplosiphon.

FdTonB is involved in the photoregulation of cellular morphology during complementary chromatic adaptation in Fremyella diplosiphon.

We have characterized a Fremyella diplosiphon TonB protein (FdTonB) and investigated its function during complementary chromatic adaptation. Sequence similarity analysis of FdTonB (571 aa) led to identification of several conserved domains characteristic of TonB proteins, including an N-terminal transmembrane domain, a central proline-rich spacer and a C-terminal TonB-related domain (TBRD). We identified a novel glycine-rich domain containing (Gly-X)(n) repeats. To assess FdTonB function, we constructed a DeltatonB mutant through homologous recombination based upon truncation of the central proline-rich spacer, glycine-rich domain and TBRD. Our DeltatonB mutant exhibited an aberrant cellular morphology under green light, with expanded cell width compared to the parental wild-type (WT) strain. The cellular morphology of the DeltatonB mutant recovered upon WT tonB expression. Interestingly, tonB expression was found to be independent of RcaE. As DeltatonB and WT strains respond in the same way when grown under iron-replete versus iron-limited conditions, our results suggest that FdTonB is not involved in the classic TonB function of mediating cellular adaptation to iron limitation, but exhibits a novel function related to the photoregulation of cellular morphology in F. diplosiphon.

Photosynthetic response of Nodularia spumigena to UV and photosynthetically active radiation depends on nutrient (N and P) availability.

Biomass of N. spumigena is distributed within the dynamic photic zone that changes in both light quantity and quality. This study was designed to determine whether nutrient status can mitigate the negative impacts of experimental radiation treatments on the photosynthetic performance of N. spumigena. Cyanobacterial suspensions were exposed to radiation consisting of photosynthetically active radiation (PAR=400-700 nm), PAR+UV-A (=PA, 320-700 nm), and PAR+UV-A+UV-B (=PAB, 280-700 nm) under different nutrient media either replete with external dissolved nitrate (N) and orthophosphate (P; designated as +N/+P), replete with P only (-N/+P), or replete with N only (+N/-P). Under low PAR (75 micromol photons m(-2) s(-1)), nutrient status had no significant effect on the photosynthetic performance of N. spumigena in terms of rETRmax, alpha, and E(k). Nodularia spumigena was able to acclimate to high PAR (300 micromol photons m(-2) s(-1)), with a corresponding increase in rETRmax and E(k). The photosynthetic performance of N. spumigena cultured with supplemental nitrogen was more susceptible to experimental PAR irradiance. Under UVR, P-enrichment in the absence of additional external N (-N/+P) induced lower photoinhibition of photosynthesis compared with +N/-P cultures. However, the induction of NPQ may have provided PSII protection under P-deplete and PAR+UVR conditions. Because N. spumigena are able to fix nitrogen, access to available P can render them less susceptible to photoinhibition, effectively promoting blooms. Under a P-deficient condition, N. spumigena were more susceptible to radiation but were capable of photosynthetic recovery immediately after removal of radiation stress. In the presence of an internal P pool in the Baltic Sea, which may be seasonally available to the diazotrophic cyanobacteria, summer blooms of the resilient N. spumigena will persist.

Isolate-specific effects of ultraviolet radiation on photosynthesis, growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium Microcoleus chthonoplastes.

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12-15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 mumol photon m(-2) s(-1)) = P; PAR + UV-A (8 W m(-2)) = PA; PAR + UV-A + UV-B (0.4 W m(-2)) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F (v)/F (m)) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F (v)/F (m) during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F (v)/F (m )sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.