Responses to oxidative and heavy metal stresses in cyanobacteria: recent advances

Identification of a New Target slr0946 of the Response Regulator Sll0649 Involving Cadmium Tolerance in Synechocystis sp. PCC 6803

Survival of photosynthetic cyanobacteria is challenged by environmental contaminations like heavy metals. Among them, deciphering the regulatory mechanisms for cadmium (Cd) in cyanobacteria would facilitate the construction of Cd-resistant strains. In this study, the DNA-Affinity-Purified-chromatin immunoprecipitation assay was employed to identify the direct targets of Sll0649, which was a Cd²⁺-related response regulator identified in our previous work in model cyanobacteria Synechocystis sp. PCC 6803. As a result, the promoter region of slr0946 encoding the arsenate reductase was enriched fourfolds by quantitative real time PCR analysis. Further, deletion of slr0946 led to a sensitive phenotype to Cd²⁺ stress compared with the wild type (WT) and the sensitive phenotype of Δslr0946 could be rescued by complementation assay via introducing slr0946 back into Δslr0946. Finally, individually overexpression of slr0946 as well as two Cd²⁺-related genes identified priviously (i.e., sll1598 and slr0798) in WT could significantly improve the tolerance of Synechocystis sp. PCC 6803 to Cd²⁺. This study provided a better understanding of the tolerance mechanism to Cd²⁺ in cyanobacteria and also feasible strategies for tolerance modifications to heavy metals in the future.

Acclimation to High CO2 Requires the ω Subunit of the RNA Polymerase in Synechocystis

Inactivation of the nonessential ω-subunit of the RNA polymerase core in the ΔrpoZ strain of the model cyanobacterium Synechocystis sp. PCC 6803 leads to a unique high-CO₂-sensitive phenotype. Supplementing air in the growth chamber with 30 mL L^-1 (3%) CO₂ accelerated the growth rate of the control strain (CS) 4-fold, whereas ΔrpoZ did not grow faster than under ambient air. The slow growth of ΔrpoZ during the first days in high CO₂ was due to the inability of the mutant cells to adjust photosynthesis to high CO₂ The light-saturated photosynthetic activity of ΔrpoZ in high CO₂ was only half of that measured in CS, Rubisco content was one-third lower, and cells of ΔrpoZ were not able to increase light-harvesting phycobilisome antenna like CS upon high-CO₂ treatment. In addition, altered structural and functional organization of photosystem I and photosystem II were detected in the ΔrpoZ strain compared with CS when cells were grown in high CO₂ but not in ambient air. Moreover, respiration of ΔrpoZ did not acclimate to high CO₂ Unlike the photosynthetic complexes, the RNA polymerase complex and ribosomes were produced in high CO₂ similarly as in CS Our results indicate that the deletion of the ω-subunit specifically affects photosynthesis and respiration, but transcription and translation remain active. Thus, the specific effect of the ω-subunit on photosynthesis but not on all household processes suggests that the ω-subunit might have a regulatory function in cyanobacteria.

Comparative Genomics of DNA Recombination and Repair in Cyanobacteria: Biotechnological Implications

Cyanobacteria are fascinating photosynthetic prokaryotes that are regarded as the ancestors of the plant chloroplast; the purveyors of oxygen and biomass for the food chain; and promising cell factories for an environmentally friendly production of chemicals. In colonizing most waters and soils of our planet, cyanobacteria are inevitably challenged by environmental stresses that generate DNA damages. Furthermore, many strains engineered for biotechnological purposes can use DNA recombination to stop synthesizing the biotechnological product. Hence, it is important to study DNA recombination and repair in cyanobacteria for both basic and applied research. This review reports what is known in a few widely studied model cyanobacteria and what can be inferred by mining the sequenced genomes of morphologically and physiologically diverse strains. We show that cyanobacteria possess many E. coli-like DNA recombination and repair genes, and possibly other genes not yet identified. E. coli-homolog genes are unevenly distributed in cyanobacteria, in agreement with their wide genome diversity. Many genes are extremely well conserved in cyanobacteria (mutMS, radA, recA, recFO, recG, recN, ruvABC, ssb, and uvrABCD), even in small genomes, suggesting that they encode the core DNA repair process. In addition to these core genes, the marine Prochlorococcus and Synechococcus strains harbor recBCD (DNA recombination), umuCD (mutational DNA replication), as well as the key SOS genes lexA (regulation of the SOS system) and sulA (postponing of cell division until completion of DNA reparation). Hence, these strains could possess an E. coli-type SOS system. In contrast, several cyanobacteria endowed with larger genomes lack typical SOS genes. For examples, the two studied Gloeobacter strains lack alkB, lexA, and sulA; and Synechococcus PCC7942 has neither lexA nor recCD. Furthermore, the Synechocystis PCC6803 lexA product does not regulate DNA repair genes. Collectively, these findings indicate that not all cyanobacteria have an E. coli-type SOS system. Also interestingly, several cyanobacteria possess multiple copies of E. coli-like DNA repair genes, such as Acaryochloris marina MBIC11017 (2 alkB, 3 ogt, 7 recA, 3 recD, 2 ssb, 3 umuC, 4 umuD, and 8 xerC), Cyanothece ATCC51142 (2 lexA and 4 ruvC), and Nostoc PCC7120 (2 ssb and 3 xerC).

Fully automated two-step assay for detection of metallothionein through magnetic isolation using functionalized γ-Fe2O3 particles

Metallothioneins (MTs) are involved in heavy metal detoxification in a wide range of living organisms. Currently, it is well known that MTs play substantial role in many pathophysiological processes, including carcinogenesis, and they can serve as diagnostic biomarkers. In order to increase the applicability of MT in cancer diagnostics, an easy-to-use and rapid method for its detection is required. Hence, the aim of this study was to develop a fully automated and high-throughput assay for the estimation of MT levels. Here, we report the optimal conditions for the isolation of MTs from rabbit liver and their characterization using MALDI-TOF MS. In addition, we described a two-step assay, which started with an isolation of the protein using functionalized paramagnetic particles and finished with their electrochemical analysis. The designed easy-to-use, cost-effective, error-free and fully automated procedure for the isolation of MT coupled with a simple analytical detection method can provide a prototype for the construction of a diagnostic instrument, which would be appropriate for the monitoring of carcinogenesis or MT-related chemoresistance of tumors.

Extracellular Proteins: Novel Key Components of Metal Resistance in Cyanobacteria?

Metals are essential for all living organisms and required for fundamental biochemical processes. However, when in excess, metals can turn into highly-toxic agents able to disrupt cell membranes, alter enzymatic activities, and damage DNA. Metal concentrations are therefore tightly controlled inside cells, particularly in cyanobacteria. Cyanobacteria are ecologically relevant prokaryotes that perform oxygenic photosynthesis and can be found in many different marine and freshwater ecosystems, including environments contaminated with heavy metals. As their photosynthetic machinery imposes high demands for metals, homeostasis of these micronutrients has been widely studied in cyanobacteria. So far, most studies have focused on how cells are capable of controlling their internal metal pools, with a strong bias toward the analysis of intracellular processes. Ultrastructure, modulation of physiology, dynamic changes in transcription and protein levels have been studied, but what takes place in the extracellular environment when cells are exposed to an unbalanced metal availability remains largely unknown. The interest in studying the subset of proteins present in the extracellular space has only recently begun and the identification and functional analysis of the cyanobacterial exoproteomes are just emerging. Remarkably, metal-related proteins such as the copper-chaperone CopM or the iron-binding protein FutA2 have already been identified outside the cell. With this perspective, we aim to raise the awareness that metal-resistance mechanisms are not yet fully known and hope to motivate future studies assessing the role of extracellular proteins on bacterial metal homeostasis, with a special focus on cyanobacteria.

Sensitivity of Four Cyanobacterial Isolates from Tropical Freshwaters to Environmentally Realistic Concentrations of Cr(6+), Cd(2+) and Zn(2.)

Sensitivity of four tropical cyanobacteria viz. Coelosphaerium sp., Synechococcus sp., Oscillatoria sp. and Chroococcus sp. to environmentally relevant concentrations of Cr(6+), Cd(2+) and Zn(2+)was assessed based on fluorescence change as a proxy for growth reduction. At 24 h exposure, the growth reduction inthe cyanobacteria followed the order: Zn(2+) < Cr(6+) ≤ Cd(2+). Of the four cyanobacteria, Synechococcus was the most sensitive for Cr(6+), where as Chroococcus was the most sensitive for Cd(2+)and Zn(2+). Sensitivity was gradually decreased by 96 h implying the acquisition of tolerance by cyanobacteria to heavy metal ions with prolonged exposure.

Effects of exogenous calcium and spermidine on cadmium stress moderation and metal accumulation in Boehmeria nivea (L.) Gaudich

Cadmium (Cd) is a detrimental metal in the environment and it is easily taken up by plants, thus entering the food chain and posing a severe threat to human health. Phytoremediation being low cost, highly stable, and environmentally friendly has been considered as a promising green technology for Cd remediation. The addition of exogenous substances to the culture media has been recognized as an efficient strategy to improve plant phytoremediation capability. Pot trials were conducted to investigate the combined effects of exogenous calcium (Ca) and spermidine (Spd) on Cd-induced toxicity in Boehmeria nivea (L.) Gaudich. (ramie). Results showed that the application of 5-mM exogenous Ca significantly alleviated Cd toxicity in ramie by reducing Cd accumulation, depressing H2O2 and malondialdehyde contents, increasing plants dry weights and chlorophyll concentrations, as well as altering the activities of total superoxide dismutase and guaiacol peroxidase. Furthermore, as a non-Cd hyperaccumulator plant, ramie hyperaccumulated Cd and suffered more severe toxic effects of Cd by the treatment of 1 mM Ca/Cd. The aggravated Cd toxicity could be compensated by the addition of exogenous Spd via the promotion of plant growth and the reduction of the oxidative stress. Overall, the combination effects of 1 mM Ca and Spd appeared to be more superior compared to other treatments in the plants under Cd stress with a higher Cd accumulation ability and the evaluated Cd stress tolerance.

Effects of exogenous calcium and spermidine on cadmium stress moderation and metal accumulation in Boehmeria nivea (L.) Gaudich

Cadmium (Cd) is a detrimental metal in the environment and it is easily taken up by plants, thus entering the food chain and posing a severe threat to human health. Phytoremediation being low cost, highly stable, and environmentally friendly has been considered as a promising green technology for Cd remediation. The addition of exogenous substances to the culture media has been recognized as an efficient strategy to improve plant phytoremediation capability. Pot trials were conducted to investigate the combined effects of exogenous calcium (Ca) and spermidine (Spd) on Cd-induced toxicity in Boehmeria nivea (L.) Gaudich. (ramie). Results showed that the application of 5-mM exogenous Ca significantly alleviated Cd toxicity in ramie by reducing Cd accumulation, depressing H2O2 and malondialdehyde contents, increasing plants dry weights and chlorophyll concentrations, as well as altering the activities of total superoxide dismutase and guaiacol peroxidase. Furthermore, as a non-Cd hyperaccumulator plant, ramie hyperaccumulated Cd and suffered more severe toxic effects of Cd by the treatment of 1 mM Ca/Cd. The aggravated Cd toxicity could be compensated by the addition of exogenous Spd via the promotion of plant growth and the reduction of the oxidative stress. Overall, the combination effects of 1 mM Ca and Spd appeared to be more superior compared to other treatments in the plants under Cd stress with a higher Cd accumulation ability and the evaluated Cd stress tolerance.

Involvement of Potassium Transport Systems in the Response of Synechocystis PCC 6803 Cyanobacteria to External pH Change, High-Intensity Light Stress and Heavy Metal Stress

The unicellular photosynthetic cyanobacterium, able to survive in varying environments, is the only prokaryote that directly converts solar energy and CO2 into organic material and is thus relevant for primary production in many ecosystems. To maintain the intracellular and intrathylakoid ion homeostasis upon different environmental challenges, the concentration of potassium as a major intracellular cation has to be optimized by various K(+)uptake-mediated transport systems. We reveal here the specific and concerted physiological function of three K(+)transporters of the plasma and thylakoid membranes, namely of SynK (K(+)channel), KtrB (Ktr/Trk/HKT) and KdpA (Kdp) in Synechocystis sp. strain PCC 6803, under specific stress conditions. The behavior of the wild type, single, double and triple mutants was compared, revealing that only Synk contributes to heavy metal-induced stress, while only Ktr/Kdp is involved in osmotic and salt stress adaptation. With regards to pH shifts in the external medium, the Kdp/Ktr uptake systems play an important role in the adaptation to acidic pH. Ktr, by affecting the CO2 concentration mechanism via its action on the bicarbonate transporter SbtA, might also be responsible for the observed effects concerning high-light stress and calcification. In the case of illumination with high-intensity light, a synergistic action of Kdr/Ktp and SynK is required in order to avoid oxidative stress and ensure cell viability. In summary, this study dissects, using growth tests, measurement of photosynthetic activity and analysis of ultrastructure, the physiological role of three K(+)transporters in adaptation of the cyanobacteria to various environmental changes.

Regulation of Plant Mineral Nutrition: Transport, Sensing and Signaling

Limitation in crop yield productivity significantly contributes to the pressing problem of food security and malnutrition worldwide. [...].

Effect of Metals, Metalloids and Metallic Nanoparticles on Microalgae Growth and Industrial Product Biosynthesis: A Review

Microalgae are a source of numerous compounds that can be used in many branches of industry. Synthesis of such compounds in microalgal cells can be amplified under stress conditions. Exposure to various metals can be one of methods applied to induce cell stress and synthesis of target products in microalgae cultures. In this review, the potential of producing diverse biocompounds (pigments, lipids, exopolymers, peptides, phytohormones, arsenoorganics, nanoparticles) from microalgae cultures upon exposure to various metals, is evaluated. Additionally, different methods to alter microalgae response towards metals and metal stress are described. Finally, possibilities to sustain high growth rates and productivity of microalgal cultures in the presence of metals are discussed.