Unsaturation of fatty acids in membrane lipids enhances tolerance of the cyanobacterium Synechocystis PCC6803 to low-temperature photoinhibition

Mechanisms of temperature acclimatisation in the psychrotolerant green alga Coccomyxa subellipsoidea C-169 (Trebouxiophyceae)

Despite the interest in different temperature acclimatisations of higher plants, few studies have considered the mechanisms that allow psychrotolerant microalgae to live in a cold environment. Although the analysis of the genomes of some algae revealed the presence of specific genes that encode enzymes that can be involved in the response to stress, this area has not been explored deeply. This work aims to clarify the acclimatisation mechanisms that enable the psychrotolerant green alga Coccomyxa subellipsoidea C-169 to grow in a broad temperature spectrum. The contents of various biochemical compounds in cells, the lipid composition of the biological membranes of entire cells, and the thylakoid fraction as well as the electron transport rate and PSII efficiency were investigated. The results demonstrate an acclimatisation mechanism that is specific for C. subellipsoidea and that allows the maintenance of appropriate membrane fluidity, for example, in thylakoid membranes. It is achieved almost exclusively by changes within the unsaturated fatty acid pool, like changes from C18:2 into C18:3 and C16:2 into C16:3 or vice versa. This ensures, for example, an effective transport rate through PSII and in consequence a maximum quantum yield of it in cells growing at different temperatures. Furthermore, reactions characteristic for both psychrotolerant and mesophilic microalgae, involving the accumulation of lipids and soluble sugars in cells at temperatures other than optimal, were observed. These findings add substantially to our understanding of the acclimatisation of psychrotolerant organisms to a wide range of temperatures and prove that this process could be accomplished in a species-specific manner.

Cyanobacterial dominance and succession: Factors, mechanisms, predictions, and managements

Eutrophication of natural water bodies worldwide has led to cyanobacteria becoming the dominant species in phytoplankton communities, causing serious harm environmentally and economically. Cyanobacterial succession makes effective treatment of cyanobacterial blooms a challenge. Although there are many studies about cyanobacterial dominance and succession, it is still lack of relevant review summarizing the advances on this topic. To control cyanobacterial blooms and manage water quality effectively, we conducted a critical review and drew the following conclusions: (1) cyanobacterial dominance and succession occur from spring to summer, with changes of multiple environmental factors dominated by temperature and nutrients conditions; (2) the cyanobacterial dominance and succession are inherently attributed to the distinctive traits of cyanobacteria including colony formation, gas vesicles, toxin release, and nitrogen fixation; (3) given the current meta-omics explorations on mechanisms of cyanobacterial succession, how to combine the extensive data to draw general conclusions is a challenge in the future; (4) the dominant niche of high temperature-adapted cyanobacteria genera will be further reinforced with global warming and elevated carbon dioxide in the future; (5) considering the causes and future developments of cyanobacterial blooms, the management strategies for controlling cyanobacterial blooms include reducing external nutrient input and removing internal nutrient in sediment, artificial mixing waters to decrease buoyancy of cyanobacteria, and biological control using allelopathy of aquatic plants and/or enhancing zooplankton feeding.

Reading and surviving the harsh conditions in desert biological soil crust: The cyanobacterial viewpoint

Biological soil crusts (BSCs) are found in drylands, cover ∼12% of the Earth's surface in arid and semi-arid lands and their destruction is considered an important promoter of desertification. These crusts are formed by the adhesion of soil particles to polysaccharides excreted mostly by filamentous cyanobacteria, which are the pioneers and main primary producers in BSCs. Desert BSCs survive in one of the harshest environments on Earth, and are exposed to daily fluctuations of extreme conditions. The cyanobacteria inhabiting these habitats must precisely read the changing conditions and predict, for example, the forthcoming desiccation. Moreover, they evolved a comprehensive regulation of multiple adaptation strategies to enhance their stress tolerance. Here we focus on what distinguishes cyanobacteria able to revive after dehydration from those that cannot. While important progress has been made in our understanding of physiological, biochemical and omics aspects, clarification of the sensing, signal transduction and responses enabling desiccation tolerance are just emerging. We plot the trajectory of current research and open questions ranging from general strategies and regulatory adaptations in the hydration/desiccation cycle, to recent advances in our understanding of photosynthetic adaptation. The acquired knowledge provides new insights to mitigate desertification and improve plant productivity under drought conditions.

Molecular Cloning and Characterization of FAD6 Gene from Chia (Salvia hispanica L.)

Plastidial Δ12 fatty acid desaturase (FAD6) is a key enzyme for linoleic acid (LA) and α-linolenic acid (ALA) biosynthesis. Chia (Salvia hispanica L.) is a revived omega-3 plant source that is richest in ALA level. In this study, based on the RACE method, one full-length cDNA sequence encoding FAD6, named ShFAD6, was isolated from chia. There exist three alternative transcription start sites and five alternative poly(A) tailing sites in ShFAD6. The 5'UTR of ShFAD6 contains a purine-stretch of 44 bp. ShFAD6 has an ORF of 1335 bp encoding a 444 aa protein of 51.33 kDa. ShFAD6 contains a conserved Delta12-FADS-like domain together with three strong trans-membrane helices and three histidine motifs. There also exists a chloroplast transmit peptide in ShFAD6 N-terminal. Phylogenetic analyses validated its identity of dicot FAD6 protein and suggested some critical evolutionary features of plant FAD6 genes. Heterologous yeast expression confirmed the catalytic activity of ShFAD6. The qRT-PCR assay showed that ShFAD6 is mainly expressed in leaves, stems, flowers, buds and early-stage seeds, and also responded to various stresses and hormone treatments. Under Sclerotinia infection, qRT-PCR and fluorescence imaging illustrated the possible correlation of ShFAD6 expression and photosynthesis. This study provides insight for further function study of ShFAD6 in oil quality improvement in staple oilseed crops as well as stress response and adaptation in plants.

The effects of different temperatures on the storage characteristics of lotus (Nelumbo nucifera G.) root

Lotus root (Nelumbo nucifera G.) is a high economic value crop in the world. In this study, the storage characteristics (color, sensory, texture, and fatty acids) of lotus root ("Elian No.5″) were evaluated at different harvest periods (September 2018, October 2018, November 2018, December 2018, and January 2019). Moreover, the storage characteristics were evaluated after the short- term and long-term storage of lotus root at 4 °C and 20 °C. The hardness of lotus root significantly decreased at both temperatures (4 °C and 20 °C) during the first 3 days of storage. In contrast, the decrease in hardness delayed at 4 °C (beyond 3 days of storage). Further, genes related to hardness at different storage temperatures were identified using the RNA-seq and qRT-PCR. The results of this study provide a reference for lotus root storage and a basis for the molecular breeding of longterm-storable lotus root.

Untargeted Lipidomics Analysis of the Cyanobacterium Synechocystis sp. PCC 6803: Lipid Composition Variation in Response to Alternative Cultivation Setups and to Gene Deletion

Cyanobacteria play an important role in several ecological environments, and they are widely accepted to be the ancestors of chloroplasts in modern plants and green algae. Cyanobacteria have become attractive models for metabolic engineering, with the goal of exploring them as microbial cell factories. However, the study of cyanobacterial lipids’ composition and variation, and the assessment of the lipids’ functional and structural roles have been largely overlooked. Here, we aimed at expanding the cyanobacterial lipidomic analytical pipeline by using an untargeted lipidomics approach. Thus, the lipid composition variation of the model cyanobacterium Synechocystis sp. PCC 6803 was investigated in response to both alternative cultivation setups and gene deletion. This approach allowed for detecting differences in total lipid content, alterations in fatty-acid unsaturation level, and adjustments of specific lipid species among the identified lipid classes. The employed method also revealed that the cultivation setup tested in this work induced a deeper alteration of the cyanobacterial cell lipidome than the deletion of a gene that results in a dramatic increase in the release of lipid-rich outer membrane vesicles. This study further highlights how growth conditions must be carefully selected when cyanobacteria are to be engineered and/or scaled-up for lipid or fatty acids production.

Plastidic Δ6 Fatty-Acid Desaturases with Distinctive Substrate Specificity Regulate the Pool of C18-PUFAs in the Ancestral Picoalga Ostreococcus tauri

Eukaryotic Δ6-desaturases are microsomal enzymes that balance the synthesis of ω-3 and ω-6 C18-polyunsaturated fatty acids (C18-PUFAs) according to their specificity. In several microalgae, including Ostreococcus tauri, plastidic C18-PUFAs are strictly regulated by environmental cues suggesting an autonomous control of Δ6-desaturation of plastidic PUFAs. Here, we identified two putative front-end Δ6/Δ8-desaturases from O tauri that, together with putative homologs, cluster apart from other characterized Δ6-desaturases. Both were plastid-located and unambiguously displayed a Δ6-desaturation activity when overexpressed in the heterologous hosts Nicotiana benthamiana and Synechocystis sp. PCC6803, as in the native host. Detailed lipid analyses of overexpressing lines unveiled distinctive ω-class specificities, and most interestingly pointed to the importance of the lipid head-group and the nonsubstrate acyl-chain for the desaturase efficiency. One desaturase displayed a broad specificity for plastidic lipids and a preference for ω-3 substrates, while the other was more selective for ω-6 substrates and for lipid classes including phosphatidylglycerol as well as the peculiar 16:4-galactolipid species occurring in the native host. Overexpression of both Δ6-desaturases in O tauri prevented the regulation of C18-PUFA under phosphate deprivation and triggered glycerolipid fatty-acid remodeling, without causing any obvious alteration in growth or photosynthesis. Tracking fatty-acid modifications in eukaryotic hosts further suggested the export of plastidic lipids to extraplastidic compartments.

The Trait Repertoire Enabling Cyanobacteria to Bloom Assessed through Comparative Genomic Complexity and Metatranscriptomics

Water bloom development due to eutrophication constitutes a case of niche specialization among planktonic cyanobacteria, but the genomic repertoire allowing bloom formation in only some species has not been fully characterized. We posited that the habitat relevance of a trait begets its underlying genomic complexity, so that traits within the repertoire would be differentially more complex in species successfully thriving in that habitat than in close species that cannot. To test this for the case of bloom-forming cyanobacteria, we curated 17 potentially relevant query metabolic pathways and five core pathways selected according to existing ecophysiological literature. The available 113 genomes were split into those of blooming (45) or nonblooming (68) strains, and an index of genomic complexity for each strain's version of each pathway was derived. We show that strain versions of all query pathways were significantly more complex in bloomers, with complexity in fact correlating positively with strain blooming incidence in 14 of those pathways. Five core pathways, relevant everywhere, showed no differential complexity or correlations. Gas vesicle, toxin and fatty acid synthesis, amino acid uptake, and C, N, and S acquisition systems were most strikingly relevant in the blooming repertoire. Further, we validated our findings using metagenomic gene expression analyses of blooming and nonblooming cyanobacteria in natural settings, where pathways in the repertoire were differentially overexpressed according to their relative complexity in bloomers, but not in nonbloomers. We expect that this approach may find applications to other habitats and organismal groups.IMPORTANCE We pragmatically delineate the trait repertoire that enables organismal niche specialization. We based our approach on the tenet, derived from evolutionary and complex-system considerations, that genomic units that can significantly contribute to fitness in a certain habitat will be comparatively more complex in organisms specialized to that habitat than their genomic homologs found in organisms from other habitats. We tested this in cyanobacteria forming harmful water blooms, for which decades-long efforts in ecological physiology and genomics exist. Our results essentially confirm that genomics and ecology can be linked through comparative complexity analyses, providing a tool that should be of general applicability for any group of organisms and any habitat, and enabling the posing of grounded hypotheses regarding the ecogenomic basis for diversification.

Effects of gelatin-based antifreeze peptides on cell viability and oxidant stress of Streptococcus thermophilus during cold stage

Cold stage adversely affects cell proliferation and cell viability of probiotics such as Streptococcus thermophilus in food industry, new type of cryoprotectants continues to be needed. Gelatin-based antifreeze peptide becomes a popular topic because of its cryoprotective effects on cold-stressed probiotics. In this study the effects of tilapia scales antifreeze peptides (TSAPP) on cell viability and oxidant stress of S. thermophilus during cold stage were investigated. The results showed that the percentage of viable cells was increased 10.85 folds compared with control groups. Addition of TSAPP activated the activities of ATPases, relieved the hyperpolarization of cell membrane potential and regulated the intracellular Ca2+ concentration. Furthermore, TSAPP significantly inhibited reactive oxygen species level and malonaldehyde content in cells. Under cryopreservation with TSAPP, cells of S. thermophilus maintained higher activities of antioxidant enzymes including catalase, peroxidase and total antioxidant capacity. These findings indicate that TSAPP likely offered its cellular protection by maintaining membrane integrity and alleviation of oxidative stress.

Proteomic De-Regulation in Cyanobacteria in Response to Abiotic Stresses

Cyanobacteria are oxygenic photoautotrophs, exhibiting a cosmopolitan distribution in almost all possible environments and are significantly responsible for half of the global net primary productivity. They are well adapted to the diverse environments including harsh conditions by evolving a range of fascinating repertoires of unique biomolecules and secondary metabolites to support their growth and survival. These phototrophs are proved as excellent models for unraveling the mysteries of basic biochemical and physiological processes taking place in higher plants. Several known species of cyanobacteria have tremendous biotechnological applications in diverse fields such as biofuels, biopolymers, secondary metabolites and much more. Due to their potential biotechnological and commercial applications in various fields, there is an imperative need to engineer robust cyanobacteria in such a way that they can tolerate and acclimatize to ever-changing environmental conditions. Adaptations to stress are mainly governed by a precise gene regulation pathways resulting in the expression of novel protein/enzymes and metabolites. Despite the demand, till date few proteins/enzymes have been identified which play a potential role in improving tolerance against abiotic stresses. Therefore, it is utmost important to study environmental stress responses related to post-genomic investigations, including proteomic changes employing advanced proteomics, synthetic and structural biology workflows. In this respect, the study of stress proteomics offers exclusive advantages to scientists working on these aspects. Advancements on these fields could be helpful in dissecting, characterization and manipulation of physiological and metabolic systems of cyanobacteria to understand the stress induced proteomic responses. Till date, it remains ambiguous how cyanobacteria perceive changes in the ambient environment that lead to the stress-induced proteins thus metabolic deregulation. This review briefly describes the current major findings in the fields of proteome research on the cyanobacteria under various abiotic stresses. These findings may improve and advance the information on the role of different class of proteins associated with the mechanism(s) of stress mitigation in cyanobacteria under harsh environmental conditions.

Strategies related to light quality and temperature to improve lutein production of marine microalga Chlamydomonas sp

The marine microalga Chlamydomonas sp. JSC4 was examined for its potential as a lutein producer. Environmental conditions, including light quality, temperature and light wavelength mixing ratio, were individually altered to enhance the cell growth rate and lutein production in strain JSC4. Results showed that optimal cell growth was obtained under white light and a temperature of 35 °C, while the optimal lutein content was obtained under blue light and a lower temperature of 20-25 °C. The best lutein production occurred when using a mixing ratio of 3:1 (white light: blue light). Strategies related to light quality and temperature (namely, temperature-gradient and two-stage strategies) were then used to further improve lutein production. Among them, the two-stage strategy proved to be effective markedly improving lutein content from 2.52 to 4.24 mg/g and resulting in the highest lutein productivity of 3.25 mg/L/day.

Molecular breeding of water lily: engineering cold stress tolerance into tropical water lily

Water lilies (order Nymphaeales) are rich in both economic and cultural values. They grow into aquatic herbs, and are divided into two ecological types: tropical and hardy. Although tropical water lilies have more ornamental and medicinal values compared to the hardy water lily, the study and utilization of tropical water lilies in both landscaping and pharmaceutical use is greatly hindered due to their limited planting area. Tropical water lilies cannot survive the winter in areas beyond 24.3°N latitude. Here, the transgenic pipeline through the pollen-tube pathway was generated for water lily for the first time. To improve cold stress tolerance of tropical water lilies, a gene encoding choline oxidase (CodA) driven by a cold stress-inducible promoter was transformed into a tropical water lily through the pollen-tube transformation. Six independent transgenic lines were tested for survival rate during two winter seasons from 2015 to 2017 in Hangzhou (30.3°N latitude). PCR and southern blot detection revealed that the CodA gene had been integrated into the genome. Reverse transcription PCR showed that CodA gene was induced after cold stress treatment, and further quantitative real-time PCR revealed different expressions among six 4 lines and line 3 had the highest expression. Multiple physiological experiments showed that after cold stress treatment, both the conductivity and malondialdehyde (MDA) levels from transgenic plants were significantly lower than those of non-transgenic plants, whereas the content of betaine and the activity of superoxide dismutase, catalase, and peroxidase were higher than those from non-transgenic plants. These results suggest that expression of exogenous CodA gene significantly improved the cold stress tolerance of tropical water lilies through a wide range of physiological alterations. Our results currently expanded a six-latitude cultivating area of the tropical water lilies. These results not only illuminate the bright future for water lily breeding but will also facilitate the functional genomic studies.

Comparative Analysis of Membrane Lipids in Psychrophilic and Mesophilic Freshwater Dinoflagellates

Here we report the lipid profiles of ten dinoflagellate species originating from different freshwater habitats and grown at 4, 13, or 20°C akin to their natural occurrence. Lipids were determined by High Performance Liquid Chromatography-ElectroSpray Ionization-Mass Spectrometry in positive and negative ion modes. Besides the well-studied monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) lipids, our study revealed the presence of intact molecular lipid species of trigalactosyldiacylglycerols, betaine diacylglyceryl-carboxyhydroxymethylcholine, sulfolipid sulfoquinovosyldiacylglycerols (SQDG) and phospholipids, in particular phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol. In multivariate ordination, the freshwater dinoflagellates studied could be distinguished into two groups based on their lipid profiles. Peridinium aciculiferum, Borghiella dodgei, B. tenuissima and Tovellia coronata belonged to group 1 while Ceratium cornutum, Gymnodinium palustre, Jadwigia applanata, P. cinctum, P. willei, and P. gatunense belonged to group 2. Indicator species analysis evidenced that group 1 was characterized by 36:9 MGDG and 36:9 DGDG and group 2 by 38:9 and 38:10 MGDG, 38:9 and 38:10 DGDG and 34:1 SQDG. We suggest that the grouping of dinoflagellates indicated their range of temperature tolerance. Furthermore, non-thylakoid lipids were linked to dinoflagellate phylogeny based on the large ribosomal sub-unit (28S LSU) rather than their temperature tolerance. Thus certain lipids better reflected habitat adaptation while other lipids better reflected genetic diversity.

Chlorella mirabilis as a Potential Species for Biomass Production in Low-Temperature Environment

Successful adaptation/acclimatization to low temperatures in micro-algae is usually connected with production of specific biotechnologically important compounds. In this study, we evaluated the growth characteristics in a micro-scale mass cultivation of the Antarctic soil green alga Chlorella mirabilis under different nitrogen and carbon sources followed by analyses of fatty acid contents. The micro-scale mass cultivation was performed in stable (in-door) and variable (out-door) conditions during winter and/or early spring in the Czech Republic. In the in-door cultivation, the treatments for nitrogen and carbon sources determination included pure Z medium (control, Z), Z medium + 5% glycerol (ZG), Z medium + 5% glycerol + 50 μM KNO₃ (ZGN), Z medium + 5% glycerol + 200 μM NH₄Cl (ZGA), Z medium + 5% glycerol + 1 mM Na₂CO₃ (ZNC), Z medium + 5% glycerol + 1 mM Na₂CO₃ + 200 μM NH₄Cl (ZGCA) and Z medium + 5% glycerol + 1 mM Na₂CO₃ + 50 μM KNO₃ (ZGCN) and were performed at 15°C with an irradiance of 75 μmol m⁻² s⁻¹. During the out-door experiments, the night-day temperature ranged from −6.6 to 17.5°C (daily average 3.1 ± 5.3°C) and irradiance ranged from 0 to 2,300 μmol m⁻² s⁻¹ (daily average 1,500 ± 1,090 μmol m⁻² s⁻¹). Only the Z, ZG, ZGN, and ZGC treatments were used in the out-door cultivation. In the in-door mass cultivation, all nitrogen and carbon sources additions increased the growth rate with the exception of ZGA. When individual sources were considered, only the effect of 5% glycerol addition was significant. On the other hand, the growth rate decreased in the ZG and ZGN treatments in the out-door experiment, probably due to carbon limitation. Fatty acid composition showed increased production of linoleic acid in the glycerol treatments. The studied strain of C. mirabilis is proposed to be a promising source of linoleic acid in low-temperature-mass cultivation biotechnology. This strain is a perspective model organism for biotechnology in low-temperature conditions.

Genetic decrease in fatty acid unsaturation of phosphatidylglycerol increased photoinhibition of photosystem I at low temperature in tobacco leaves

Leaves of transgenic tobacco plants with decreased levels of fatty acid unsaturation in phosphatidylglycerol (PG) exhibited a slightly lower level of the steady state oxidation of the photosystem I (PSI) reaction center P700 (P700(+)) than wild-type plants. The PSI photochemistry of wild-type plants was only marginally affected by high light treatments. Surprisingly, all plants of transgenic lines exhibited much higher susceptibility to photoinhibition of PSI than wild-type plants. This was accompanied by a 2.5-fold faster re-reduction rate of P700(+) in the dark, indicating a higher capacity for cyclic electron flow around PSI in high light treated transgenic leaves. This was associated with a much higher intersystem electron pool size suggesting over-reduction of the PQ pool in tobacco transgenic lines with altered PG unsaturation compared to wild-type plants. The physiological role of PG unsaturation in PSI down-regulation and modulation of the capacity of PSI-dependent cyclic electron flows and distribution of excitation light energy in tobacco plants under photoinhibitory conditions at low temperatures is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Regioisomer separation and identification of triacylglycerols containing vaccenic and oleic acids, and α- and γ-linolenic acids, in thermophilic cyanobacteria Mastigocladus laminosus and Tolypothrix sp

Reversed phase liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (RP-HPLC/APCI-MS) was used for direct analysis of triacylglycerols (TAGs) from different strains of the cyanobacteria Mastigocladus laminosus, Tolypothrix cf. tenuis and Tolypothrix distorta. This technique enabled us to identify and quantify the specific molecular species of TAGs directly from lipid extracts of the cyanobacteria. The regioisomeric series of TAGs having α-linolenic and γ-linolenic and also oleic and cis-vaccenic acids were separated by RP-HPLC and identified by APCI-MS. M. laminosus produced only a few molecular species of TAGs, including both isomers of octadecenoic (oleic and vaccenic) acid, while T. distorta contained tens of molecular species of TAGs having FAs with up to four double bonds (stearidonic acid and including also its positional isomer, i.e. 3,6,9,12-octadecatetraenoic acid) and both positional isomers (α and γ) of linolenic acids. Individual strains of both cyanobacteria exhibited different contents of polyunsaturated fatty acids (Tolypothrix sp.) and different distribution of positional isomers of monoenoic fatty acids in TAGs (M. laminosus).

Physiological effects of free fatty acid production in genetically engineered Synechococcus elongatus PCC 7942

The direct conversion of carbon dioxide into biofuels by photosynthetic microorganisms is a promising alternative energy solution. In this study, a model cyanobacterium, Synechococcus elongatus PCC 7942, is engineered to produce free fatty acids (FFA), potential biodiesel precursors, via gene knockout of the FFA-recycling acyl-ACP synthetase and expression of a thioesterase for release of the FFA. Similar to previous efforts, the engineered strains produce and excrete FFA, but the yields are too low for large-scale production. While other efforts have applied additional metabolic engineering strategies in an attempt to boost FFA production, we focus on characterizing the engineered strains to identify the physiological effects that limit cell growth and FFA synthesis. The strains engineered for FFA-production show reduced photosynthetic yields, chlorophyll-a degradation, and changes in the cellular localization of the light-harvesting pigments, phycocyanin and allophycocyanin. Possible causes of these physiological effects are also identified. The addition of exogenous linolenic acid, a polyunsaturated FFA, to cultures of S. elongatus 7942 yielded a physiological response similar to that observed in the FFA-producing strains with only one notable difference. In addition, the lipid constituents of the cell and thylakoid membranes in the FFA-producing strains show changes in both the relative amounts of lipid components and the degree of saturation of the fatty acid side chains. These changes in lipid composition may affect membrane integrity and structure, the binding and diffusion of phycobilisomes, and the activity of membrane-bound enzymes including those involved in photosynthesis. Thus, the toxicity of unsaturated FFA and changes in membrane composition may be responsible for the physiological effects observed in FFA-producing S. elongatus 7942. These issues must be addressed to enable the high yields of FFA synthesis necessary for large-scale biofuel production.

Role of Slr1045 in environmental stress tolerance and lipid transport in the cyanobacterium Synechocystis sp. PCC6803

ATP-binding cassette (ABC) transporter proteins mediate energy-dependent transport of substrates across cell membranes. Numerous ABC transporter-related genes have been found in the Synechocystis sp. PCC6803 genome by genome sequence analysis including H(+), iron, phosphate, polysaccharide, and CO(2) transport-related genes. The substrates of many other ABC transporters are still unknown. To identify ABC transporters involved in acid tolerance, deletion mutants of ABC transporter genes with unknown substrates were screened for acid stress sensitivities in low pH medium. It was found that cells expressing the deletion mutant of slr1045 were more sensitive to acid stress than the wild-type cells. Moreover, slr1045 expression in the wild-type cells was increased under acid stress. These results indicate that slr1045 is an essential gene for survival under acid stress. The mutant displayed high osmotic stress resistance and high/low temperature stress sensitivity. Considering the temperature-sensitive phenotype and homology to the organic solvent-resistant ABC system, we subsequently compared the lipid profiles of slr1045 mutant and wild-type cells by thin-layer chromatography. In acid stress conditions, the phosphatidylglycerol (PG) content in the slr1045 mutant cells was approximately 40% of that in the wild-type cells. Moreover, the addition of PG to the medium compensated for the growth deficiency of the slr1045 mutant cells under acid stress conditions. These data suggest that slr1045 plays a role in the stabilization of cell membranes in challenging environmental conditions. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Effects of temperature shifts on growth rate and lipid characteristics of Synechocystis sp. PCC6803 in a bench-top photobioreactor

Synechocystis sp. PCC6803 exhibited a high degree of variation in biomass and lipid production rates in response to temperature changes in a photobioreactor. Compared with an optimal temperature of 30-33°C, a higher temperature of 44°C and lower temperatures of 22°C and 18°C severely inhibited the specific growth rate (up to a 66% decrease), biomass production rate (up to a 71% decrease), nutrient utilization rates (up to a 77% decrease), and lipid production rate (up to a 80% decrease). Temperature stress triggered changes in the relative percentage of individual fatty acids (mainly for C16:0 and C18:3), and degree of unsaturation significantly changed: 0.87 at 30°C, 0.62 at 44°C, and 1.29 at 18°C. Although PCC6803 survived temperature stress and maintained its predominate position in the culture, it could not fully recover from long-term temperature stress. Thus, avoiding prolonged exposure to extreme temperature is crucial for using PCC6803 as feedstock for biofuel production.

LHCII organization and thylakoid lipids affect the sensitivity of the photosynthetic apparatus to high-light treatment

Pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photosynthetic oxygen evolution were used to investigate the role of the different amount and organization of light-harvesting complexes of photosystem II (LHCII) in four pea species on the susceptibility of the photosynthetic apparatus to high-light treatment. In this work we analyzed the thylakoid membrane lipid composition of the studied pea plants. A relationship between the structural organization of LHCII proteins, the amount of the main lipid classes and the sensitivity of the photosynthetic apparatus to high-light treatment was found. The results reveal that the photosynthetic apparatus, enriched in oligomeric forms of LHCII concomitant with decreased amount of anionic lipids and increased content of the monogalactosyldiacylglycerol (MGDG), is less sensitive to high light. Our data also suggest that the degree of LHCII oligomerization, as well as the lipid composition do not influence the degree of recovery of the PSII photochemistry after excess light exposure.

Glutathione protects Lactobacillus sanfranciscensis against freeze-thawing, freeze-drying, and cold treatment

Lactobacillus sanfranciscensis DSM20451 cells containing glutathione (GSH) displayed significantly higher resistance against cold stress induced by freeze-drying, freeze-thawing, and 4 degrees C cold treatment than those without GSH. Cells containing GSH were capable of maintaining their membrane structure intact when exposed to freeze-thawing. In addition, cells containing GSH showed a higher proportion of unsaturated fatty acids in cell membranes upon long-term cold treatment. Subsequent studies revealed that the protective role of GSH against cryodamage of the cell membrane is partly due to preventing peroxidation of membrane fatty acids and protecting Na(+),K(+)-ATPase. Intracellular accumulation of GSH enhanced the survival and the biotechnological performance of L. sanfranciscensis, suggesting that the robustness of starters for sourdough fermentation can be improved by selecting GSH-accumulating strains. Moreover, the results of this study may represent a further example of mechanisms for stress responses in lactic acid bacteria.

Overexpression of sweet pepper glycerol-3-phosphate acyltransferase gene enhanced thermotolerance of photosynthetic apparatus in transgenic tobacco

In order to investigate the relationship between the lipid composition in thylakoid membrane and thermostability of photosynthetic apparatus, tobacco transformed with sweet pepper sense glycerol-3-phosphate acyltransferase (GPAT) gene were used to analyze the lipid composition in thylakoid membrane, the net photosynthetic rate and chlorophyll fluorescence parameters under high temperature stress. The results showed that the saturated extent of monogalactosyldiacylglycerol (MGDG), sulfoquinovosyldiacylglycerol, digalactosyldiacylglycerol and phosphatidylglycerol in thylakoid membrane of transgenic tobacco T(1) lines increased generally. Particularly, the saturated extent in MGDG increased obviously by 16.2% and 12.0% in T(1)-2 and T(1)-1, respectively. With stress temperature elevating, the maximum efficiency of photosystem II (PSII) photochemistry (Fv/Fm), actual photochemical efficiency of PSII in the light (Phi(PSII)) and net photosynthetic rate (Pn) of the two lines and wild type tobacco plants decreased gradually, but those parameters decreased much less in transgenic plants. Even though the recovery process appeared differently in the donor and acceptor side of PSII in transgenic tobacco compared with wild-type plants, the entire capability of PSII recovered faster in transgenic tobacco, which was shown in the parameters of PI, Fv/Fm and Phi(PSII), as a result, the recovery of Pn was accelerated. Conclusively, we proposed that the increase in saturated extent of thylakoid membrane lipids in transgenic plants enhanced the stability of photosynthetic apparatus under high temperature stress.

Desaturase genes in a psychrotolerant Nostoc sp. are constitutively expressed at low temperature

Antarctic psychrotolerant, Nostoc sp. (SO-36), when grown at 25 degrees C and then shifted to 10 degrees C, showed an increase in the tri-unsaturated fatty acid [C(18:3(9,12,15))] at the expense of mono- [C(18:1(9))] and di-unsaturated [C(18:2(9,12))] fatty acids. These results indicate that the activities of the enzymes DesA and DesB are up-regulated, when cultures were grown at 10 degrees C or shifted to 10 degrees C from 25 degrees C. However, RT-PCR studies indicated a constitutive expression of desA, desB, desC, and desC2 genes when cultures grown at 25 degrees C were shifted to 10 degrees C. This constitutive expression of des genes is in contrast to that observed in mesophilic cyanobacteria, in which desA and desB are transcriptionally up-regulated in response to lowering of growth temperature.

Identification of a gene, ccr-1 (sll1242), required for chill-light tolerance and growth at 15 °C in Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803 exposed to chill (5 degrees C)-light (100 mumol photons m(-2) s(-1)) stress loses its ability to reinitiate growth. From a random insertion mutant library of Synechocystis sp. PCC 6803, a sll1242 mutant showing increased sensitivity to chill plus light was isolated. Mutant reconstruction and complementation with the wild-type gene confirmed the role of sll1242 in maintaining chill-light tolerance. At 15 degrees C, the autotrophic and mixotrophic growth of the mutant were both inhibited, paralleled by decreased photosynthetic activity. The expression of sll1242 was upregulated in Synechocystis sp. PCC 6803 after transfer from 30 to 15 degrees C at a photosynthetic photon flux density of 30 mumol photons m(-2) s(-1). sll1242, named ccr (cyanobacterial cold resistance gene)-1, may be required for cold acclimation of cyanobacteria in light.

Mechanisms of temperature adaptation in poikilotherms

For good function, membrane lipids have to be arranged appropriately and be in the correct physical state. In poikilotherms, exposure to cold stress or heat shock can alter membrane properties such that, unless they are corrected quickly, damage and, possibly, death can result. Low temperature stress is countered by modifying membrane lipids such that their average transition temperature is lowered. There are various ways in which this can be achieved but an increase in fatty acid unsaturation is the most common. For heat shock, various changes in lipids have been noted and some defensive strategies involving heat shock proteins noted. In this short review, we will describe recent results where adaptive lipid changes, as a result of temperature stress, have been found. Mechanisms for bringing about such alterations are discussed, together with the contrasting data for different organisms.

Proteomic analysis of the heat shock response in Synechocystis PCC6803 and a thermally tolerant knockout strain lacking the histidine kinase 34 gene

Proteomic analysis of the heat shock response of wild type and a mutant of the histidine kinase 34 gene (Deltahik34), which shows increased thermal tolerance, has been performed in the cyanobacterium Synechocystis sp. PCC6803. In vivo radioactive labelling demonstrates that major proteomic changes occur within 1 h of heat shock. 2-D DIGE and MS have been used to quantify changes in specific proteins following heat shock in the wild type and the mutant. Over 100 spots, corresponding to 65 different proteins alter following heat shock. Changes occur not only in the classical heat shock proteins but also in the protein biosynthetic machinery, amino acid biosynthetic enzymes, components of the light and dark acts of photosynthesis and energy metabolism. The Deltahik34 cells have elevated levels of heat shock proteins under both non-heat shock and heat shock conditions, in comparison to the wild type, consistent with Hik34, or a down stream component, being a negative regulator of heat shock-responsive genes.

Differential responses of three acyl-lipid desaturases to immediate temperature reduction occurring in two lipid membranes of Spirulina platensis strain C1

In cyanobacteria, the elevation of unsaturated fatty acid levels in membrane lipids has been shown to play a major role in the response to temperature change. The cyanobacterium Spirulina platensis strain C1 contains three desaturases--delta9, delta12 and delta6 desaturases--which are encoded by desC, desA and desD, respectively. In light of the above, a study was conducted of the regulation of desaturase gene expression in response to temperature change in S. platensis strain C1. The two lipid membranes, thylakoid and plasma, were separated, while the expressions of the desaturase genes to the downward shift of growth temperature were studied in the translation level by Western blot analysis. The results revealed that the expression of delta9 desaturase is independent of temperature. In the case of delta12 desaturase, two forms of the enzyme were found, at 45 and 40 kDa. In terms of correlation with the results in the transcription level, it is more likely that the 45 kDa-delta12 desaturase and the 40 kDa-delta12 desaturase are translated from 1.7 kb and 1.5 kb mRNA, respectively. Taken together, the results indicate that the expression of the 40 kDa-delta12 desaturase is temperature independent, whereas, the 45 kDa protein form demonstrates a response to the immediate temperature reduction. Furthermore, the activity of delta6 desaturase in the two lipid membranes is possibly regulated by temperature reduction. However, alteration in the level of gamma-linolenic acid, the product synthesized by delta6 desaturase, was observed in the plasma membrane prior to the thylakoid membrane.

Temperature and Light Modulate the trans-Δ3-Hexadecenoic Acid Content of Phosphatidylglycerol: Light-harvesting Complex II Organization and Non-photochemical Quenching

The interaction of light and temperature in the modulation of the trans-delta3-hexadecenoic acid (trans-16:1) content of phosphatidylglycerol (PG) in winter rye (Secale cereale L.) was assessed and related to the organization of light-harvesting complex II (LHCII). Increasing the growth irradiance from 50 to 800 micromol m(-2) s(-1) at 20 degrees C resulted in a 1.8-fold increase in the trans-16:1 content in PG which favoured a greater preponderance of oligomeric LHCII, measured in vitro as the ratio of oligomer : monomer. Similar irradiance-dependent increases were observed during growth at 5 degrees C; however, 1.4-fold lower trans-16:1 contents and lower LHCII oligomer : monomer ratios were observed compared with growth at 20 degrees C and the same irradiance. These trends were also observed under natural field conditions. Thus, the accumulation of trans-16:1, as well as the organization of LHCII are modulated by both growth irradiance and growth temperature in an independent but additive manner. We also examined how changes in the supramolecular organization of LHCII affected the capacity for non-photochemical quenching (q(N)) and photoprotection via antenna quenching (q(O)). While q(O) was positively correlated with q(N), there was no correlation with either LHCII organization or xanthophyll cycle activity under the steady-state growth conditions examined.

Irreversible photoinhibition of photosystem II is caused by exposure of Synechocystis cells to strong light for a prolonged period

Irreversible photoinhibition of photosystem II (PSII) occurred when Synechocystis sp. PCC 6803 cells were exposed to very strong light for a prolonged period. When wild-type cells were illuminated at 20 degrees C for 2 h with light at an intensity of 2,500 micromol photons m(-2) s(-1), the oxygen-evolving activity of PSII was almost entirely and irreversibly lost, whereas the photochemical reaction center in PSII was inactivated only reversibly. The extent of irreversible photoinhibition was enhanced at lower temperatures and by the genetically engineered rigidification of membrane lipids. Western and Northern blotting demonstrated that, after cells had undergone irreversible photoinhibition, the precursor to D1 protein in PSII was synthesized but not processed properly. These observations may suggest that exposure of Synechocystis cells to strong light results in the irreversible photoinhibition of the oxygen-evolving activity of PSII via impairment of the processing of pre-D1 and that this effect of strong light is enhanced by the rigidification of membrane lipids.

Photoinhibition and recovery in a herbicide-resistant mutant from Glycine max (L.) Merr. cell cultures deficient in fatty acid unsaturation

Photoinhibition and recovery were studied in two photosynthetic cell suspensions from soybean (Glycine max L. Merr): the wild type (WT) and the herbicide-resistant D1 mutant STR7. This mutant also showed an increase in saturated fatty acids from thylakoid lipids. STR7 was more sensitive to photoinhibition under culture conditions. In vivo photoinhibition experiments in the presence of chloramphenicol, in vitro studies in isolated thylakoid membranes, and immunoblot analysis indicated that the process of light-induced degradation of the D1 protein was not involved in the response of STR7 to light. At growth temperature (24 degrees C), the recovery rate of photoinhibited photosystem II (PSII) was slower in STR7 relative to WT. Photoinhibition and recovery were differentially affected by temperature in both cell lines. The rates of photoinhibition were faster in STR7 at any temperature below 27 degrees C. The rates of PSII recovery from STR7 were more severely affected than those of WT at temperatures lower than 24 degrees C. The photoinhibition and recovery rates of WT at 17 degrees C mimicked those of STR7 at 24 degrees C. In organelle translation studies indicated that synthesis and elongation of D1 were substantially similar in both cell lines. However, sucrose gradient fractionation of chloroplast membranes demonstrated that D1 and also other PSII proteins such as D2, OEE33, and LCHII had a reduced capability to incorporate into PSII to yield a mature assembled complex in STR7. This effect may become the rate-limiting step during the recovery of photoinhibited PSII and may explain the increased sensitivity to high light found in STR7. Our data may hint at a possible role of fatty acids from membrane lipids in the assembly and dynamics of PSII.

Dissecting a cyanobacterial proteolytic system: efficiency in inducing degradation of the D1 protein of photosystem II in cyanobacteria and plants

A chromatography fraction, prepared from isolated thylakoids of a fatty acid desaturation mutant (Fad6/desA Colon, two colons Km(r)) of the cyanobacterium Synechocystis 6803, could induce an initial cleavage of the D1 protein in Photosystem II (PSII) particles of Synechocystis 6803 mutant and Synechococcus 7002 wild type as well as in supercomplexes of PSII-light harvesting complex II of spinach. Proteolysis was demonstrated both in darkness and in light as a reduction in the amount of full-length D1 protein or as a production of C-terminal initial degradation fragments. In the Synechocystis mutant, the main degradation fragment was a 10-kDa C-terminal one, indicating an initial cleavage occurring in the cytoplasmic DE-loop of the D1 protein. A protein component of 70-90 kDa isolated from the chromatographic fraction was found to be involved in the production of this 10-kDa fragment. In spinach, only traces of the corresponding fragment were detected, whereas a 24-kDa C-terminal fragment accumulated, indicating an initial cleavage in the lumenal AB-loop of the D1 protein. Also in Synechocystis the 24-kDa fragment was detected as a faint band. An antibody raised against the Arabidopsis DegP2 protease recognized a 35-kDa band in the proteolytically active chromatographic fraction, suggesting the existence of a lumenal protease that may be the homologue DegP of Synechocystis. The identity of the other protease cleaving the D1 protein in the DE-loop exposed on the stromal (cytoplasmic) side of the membrane is discussed.

Requirement of phosphatidylglycerol for maintenance of photosynthetic machinery

Phosphatidylglycerol (PG) is a ubiquitous component of thylakoid membranes. Experiments with the pgsA mutant of the cyanobacterium Synechocystis sp. PCC6803 defective in biosynthesis of PG have demonstrated an indispensable role of PG in photosynthesis. In the present study, we have investigated the light susceptibility of the pgsA mutant with regard to the maintenance of the photosynthetic machinery. Growth of the mutant cells without PG increased the light susceptibility of the cells and resulted in severe photoinhibition of photosynthesis upon a high-light treatment, whereas the growth in the presence of PG was protected against photoinhibition. Photoinhibition induced by PG deprivation was mainly caused by an impairment of the restoration process. The primary target of the light-induced damage in thylakoid membranes, the D1 protein of photosystem (PS) II was, however, synthesized and degraded with similar rates irrespective of whether the mutant cells were incubated with PG or not. Intriguingly, it was found that instead of the synthesis of the D1 protein, the dimerization of the PSII core monomers was impaired in the PG-deprived mutant cells. Addition of PG to photoinhibited cells restored the dimerization capacity of PSII core monomers. These results suggest that PG plays an important role in the maintenance of the photosynthetic machinery through the dimerization and reactivation of the PSII core complex.

Photoinhibition in mutants of Arabidopsis deficient in thylakoid unsaturation

Thylakoid lipid composition in higher plants is characterized by a high level of fatty acid unsaturation. We have screened four mutants of Arabidopsis that have reduced levels of fatty acid unsaturation. Three of the mutant lines tested, fad5, fad6, and the fad3-2 fad7-2 fad8 triple mutant, were more susceptible to photoinhibition than wild-type Arabidopsis, whereas one mutant, fab1, was indistinguishable from wild type. The fad3-2 fad7-2 fad8 triple mutant, which contains no trienoic fatty acids in its thylakoid membranes, was most susceptible to photoinhibition. Detailed investigation of photoinhibition in the triple mutant revealed that the rate of photoinactivation of PSII was the same in wild-type and mutant plants. However, the recovery of photoinactivated PSII was slower in fad3-2 fad7-2 fad8, relative to wild type, at all temperatures below 27 degrees C. These results indicate that trienoic fatty acids of thylakoid membrane lipids are required for low-temperature recovery from photoinhibition in Arabidopsis.

The role of glycine betaine in the protection of plants from stress: clues from transgenic plants

The acclimation of a plant to a constantly changing environment involves the accumulation of certain organic compounds of low molecular mass, known collectively as compatible solutes, in the cytoplasm. The evidence from numerous investigations of the physiology, genetics, biophysics and biochemistry of plants strongly suggests that glycine betaine (GB), an amphoteric quaternary amine, plays an important role as a compatible solute in plants under various types of environmental stress, such as high levels of salts and low temperature. Plant species vary in their capacity to synthesize GB and some plants, such as spinach and barley, accumulate relatively high levels of GB in their chloroplasts while others, such as Arabidopsis and tobacco, do not synthesize this compound. Genetic engineering has allowed the introduction into GB-deficient species of biosynthetic pathways to GB from both micro-organisms and higher plants; this approach has facilitated investigations of the importance of GB in stress protection. In this review, we summarize recent progress in the genetic manipulation of the synthesis of GB, with special emphasis on the relationship between the protective effects of GB in vivo and those documented in vitro.

Synergistic effect of high-light and low temperature on cell growth of the Delta12 fatty acid desaturase mutant in Synechococcus sp. PCC 7002

The effect of polyunsaturated fatty acids on photosynthesis and the growth of the marine cyanobacterium Synechococcus sp. PCC 7002 was examined using wild-type and Delta12 fatty acid desaturase mutant strains. Under a light intensity of 250 mumol m(-2) s(-1), wild-type cells could grow exponentially in a temperature range of 20-38 degrees C, but growth was non-exponential below 20 degrees C and ceased at 12 degrees C. The Delta12 desaturase mutant cells lacking polyunsaturated fatty acids had the same growth rate as wild-type cells in a temperature range of 25-38 degrees C but grew slowly at 22 degrees C, and no cell growth took place below 18 degrees C. Under a very high-light intensity of 2.5 mmol m(-2) s(-1), wild-type cells could grow exponentially in a temperature range of 30-38 degrees C, although the high-light grown cells became chlorotic because of nitrogen limitation. The temperature sensitive phenotype in the Delta12 desaturase mutant was enhanced in cells grown under high-light illumination; the mutant cells could grow at 38 degrees C, but were killed at 30 degrees C. The decrease of oxygen evolution and nitrate consumption by whole cells as a function of temperature was similar in both wild type and the Delta12 desaturase mutant. No differences were observed in either light-induced damage of oxygen evolution or recovery from this damage. No inactivation of oxygen evolution took place at 22 degrees C under the normal light intensity of 250 mumol m(-2) s(-1). These results suggest that growth of the Delta12 desaturase mutant at low temperature is not directly limited by the inactivation of photosynthesis, and raise new questions about the functions of polyunsaturated membrane lipids on low temperature acclimation in cyanobacteria.

Bioenergetic responses of Synechocystis 6803 fatty acid desaturase mutants at low temperatures

Fatty acid composition of the membrane lipids in the mesophilic cyanobacterium Synechocystis sp. PCC 6803 was altered in earlier work by targeted mutagenesis of genes for fatty acid desaturases. In this work, cells of several mutant strains, depleted in the unsaturated fatty acids in membrane lipids, were grown at 34 degrees C. Spheroplasts (permeabilized cells) were prepared by lysozyme digestion of the cell wall followed by gentle osmotic shock. The bioenergetic parameters ATP formation, electron transport, and H+ uptake were measured at various temperatures. All three bioenergetic parameters for spheroplasts from wild-type cells (which had abundant polyunsaturated fatty acids) were active down to the lowest temperatures used (1 degrees - 2 degrees C). In two strains, which lacked the capacity to desaturate fatty acids at the A 12 position and at the A 12 and A6 positions (designated as desA- and desA-/desD-, respectively), the spheroplasts lost the capacity to form ATP (measured as phenazine methosulfate cyclic phosphorylation) at about 5 degrees C but retained electron transport (water oxidation-dependent ferricyanide reduction) and H+ uptake linked to phenazine methosulfate cyclic electron transport. It appears that the absence of the unsaturation of fatty acids in the A 12 and A6 positions blocks the ability of the photosynthetic membranes to couple a bioenergetically competent proton-motive force to the ATP formation mechanism at temperatures below 5 degrees C. It remains to be determined whether the loss of ATP formation in the mutant strains is the failure of available protons to properly flow into the CF0CF1-ATP synthase or a failure in the CF1 part of the complex in coupling the dissipative H+ flow to the enzyme mechanism of the synthase.

Photosystem II fluorescence quenching in the cyanobacterium Synechocystis PCC 6803: involvement of two different mechanisms

The structural changes associated to non-photochemical quenching in cyanobacteria is still a matter of discussion. The role of phycobilisome and/or photosystem mobility in this mechanism is a point of interest to be elucidated. Changes in photosystem II fluorescence induced by different quality of illumination (state transitions) or by strong light were characterized at different temperatures in wild-type and mutant cells, that lacked polyunsaturated fatty acids, of the cyanobacterium Synechocystis PCC 6803. The amplitude and the rate of state transitions decreased by lowering temperature in both strains. Our results support the hypothesis that a movement of membrane complexes and/or changes in the oligomerization state of these complexes are involved in the mechanism of state transitions. The quenching induced by strong blue light which was not associated to D1 damage and photoinhibition, did not depend on temperature or on the membrane state. Thus, the mechanism involved in the formation of this type of quenching seems to be unrelated to the movement of membrane complexes. Our results strongly support the idea that the mechanism involved in the fluorescence quenching induced by light 2 is different from that involved in strong blue light induced quenching.

Cold shock response of Bacillus subtilis: isoleucine-dependent switch in the fatty acid branching pattern for membrane adaptation to low temperatures

Bacillus subtilis has developed sophisticated mechanisms to withstand fluctuations in temperature. Membrane fatty acids are the major determinants for a sufficiently fluid membrane state to ensure the membrane's function at all temperatures. The fatty acid profile of B. subtilis is characterized by a high content of branched fatty acids irrespective of the growth medium. Here, we report on the importance of isoleucine for B. subtilis to survive cold shock from 37 to 15 degrees C. Cold shock experiments with strain JH642 revealed a cold-protective function for all intermediates of anteiso-branched fatty acid biosynthesis. Metabolites related to iso-branched or straight-chain fatty acid biosynthesis were not protective. Fatty acid profiles of different B. subtilis wild-type strains proved the altered branching pattern by an increase in the anteiso-branched fatty acid content and a concomitant decrease of iso-branched species during cold shock. There were no significant changes in the fatty acid saturation or acyl chain length. The cold-sensitive phenotype of isoleucine-deficient strains in the absence of isoleucine correlated with their inability to synthesize more anteiso-branched fatty acids, as shown by the fatty acid profile. The switch to a fatty acid profile dominated by anteiso-C(15:0) and C(17:0) at low temperatures and the cold-sensitive phenotype of isoleucine-deficient strains in the absence of isoleucine focused our attention on the critical role of anteiso-branched fatty acids in the growth of B. subtilis in the cold.

Iron superoxide dismutase protects against chilling damage in the cyanobacterium synechococcus species PCC7942

A strain of Synechococcus sp. PCC7942 lacking functional Fe superoxide dismutase (SOD), designated sodB-, was characterized by its growth rate, photosynthetic pigments, inhibition of photosynthetic electron transport activity, and total SOD activity at 0 degrees C, 10 degrees C, 17 degrees C, and 27 degrees C in moderate light. At 27 degrees C, the sodB- and wild-type strains had similar growth rates, chlorophyll and carotenoid contents, and cyclic photosynthetic electron transport activity. The sodB- strain was more sensitive to chilling stress at 17 degrees C than the wild type, indicating a role for FeSOD in protection against photooxidative damage during moderate chilling in light. However, both the wild-type and sodB- strains exhibited similar chilling damage at 0 degrees C and 10 degrees C, indicating that the FeSOD does not provide protection against severe chilling stress in light. Total SOD activity was lower in the sodB- strain than in the wild type at 17 degrees C and 27 degrees C. Total SOD activity decreased with decreasing temperature in both strains but more so in the wild type. Total SOD activity was equal in the two strains when assayed at 0 degrees C.

Nitrate transport and not photoinhibition limits growth of the freshwater Cyanobacterium synechococcus species PCC 6301 at low temperature

The effect of low temperature on cell growth, photosynthesis, photoinhibition, and nitrate assimilation was examined in the cyanobacterium Synechococcus sp. PCC 6301 to determine the factor that limits growth. Synechococcus sp. PCC 6301 grew exponentially between 20 degreesC and 38 degreesC, the growth rate decreased with decreasing temperature, and growth ceased at 15 degreesC. The rate of photosynthetic oxygen evolution decreased more slowly with temperature than the growth rate, and more than 20% of the activity at 38 degreesC remained at 15 degreesC. Oxygen evolution was rapidly inactivated at high light intensity (3 mE m-2 s-1) at 15 degreesC. Little or no loss of oxygen evolution was observed under the normal light intensity (250 microE m-2 s-1) for growth at 15 degreesC. The decrease in the rate of nitrate consumption by cells as a function of temperature was similar to the decrease in the growth rate. Cells could not actively take up nitrate or nitrite at 15 degreesC, although nitrate reductase and nitrite reductase were still active. These data demonstrate that growth at low temperature is not limited by a decrease in the rate of photosynthetic electron transport or by photoinhibition, but that inactivation of the nitrate/nitrite transporter limits growth at low temperature.