Nanoscale elemental and morphological imaging of nitrogen-fixing cyanobacteria

Nitrogen-fixing cyanobacteria bind atmospheric nitrogen and carbon dioxide using sunlight. This experimental study focused on a laboratory-based model system, Anabaena sp., in nitrogen-depleted culture. When combined nitrogen is scarce, the filamentous prokaryotes reconcile photosynthesis and nitrogen fixation by cellular differentiation into heterocysts. To better understand the influence of micronutrients on cellular function, 2D and 3D synchrotron X-ray fluorescence mappings were acquired from whole biological cells in their frozen-hydrated state at the Bionanoprobe, Advanced Photon Source. To study elemental homeostasis within these chain-like organisms, biologically relevant elements were mapped using X-ray fluorescence spectroscopy and energy-dispersive X-ray microanalysis. Higher levels of cytosolic K+, Ca2+, and Fe2+ were measured in the heterocyst than in adjacent vegetative cells, supporting the notion of elevated micronutrient demand. P-rich clusters, identified as polyphosphate bodies involved in nutrient storage, metal detoxification, and osmotic regulation, were consistently co-localized with K+ and occasionally sequestered Mg2+, Ca2+, Fe2+, and Mn2+ ions. Machine-learning-based k-mean clustering revealed that P/K clusters were associated with either Fe or Ca, with Fe and Ca clusters also occurring individually. In accordance with XRF nanotomography, distinct P/K-containing clusters close to the cellular envelope were surrounded by larger Ca-rich clusters. The transition metal Fe, which is a part of nitrogenase enzyme, was detected as irregularly shaped clusters. The elemental composition and cellular morphology of diazotrophic Anabaena sp. was visualized by multimodal imaging using atomic force microscopy, scanning electron microscopy, and fluorescence microscopy. This paper discusses the first experimental results obtained with a combined in-line optical and X-ray fluorescence microscope at the Bionanoprobe.

Calcium‐dependent antimicrobials: Nature‐inspired materials and designs

Bacterial infection remains a major complication answering for the failures of various implantable medical devices. Tremendous extraordinary advances have been published in the design and synthesis of antimicrobial materials addressing this issue; however, the clinical translation has largely been blocked due to the challenge of balancing the efficacy and safety of these materials. Here, calcium's biochemical features, natural roles in pathogens and the immune systems, and advanced uses in infection medications are illuminated, showing calcium is a promising target for developing implantable devices with less infection tendency. The paper gives a historical overview of biomedical uses of calcium and summarizes calcium's merits in coordination, hydration, ionization, and stereochemistry for acting as a structural former or trigger in biological systems. It focuses on the involvement of calcium in pathogens' integrity, motility, and metabolism maintenance, outlining the potential antimicrobial targets for calcium. It addresses calcium's uses in the immune systems that the authors can learn from for antimicrobial synthesis. Additionally, the advances in calcium's uses in infection medications are highlighted to sketch the future directions for developing implantable antimicrobial materials. In conclusion, calcium is at the nexus of antimicrobial defense, and future works on taking advantage of calcium in antimicrobial developments are promising in clinical translation.

A TonB-Like Protein, SjdR, Is Involved in the Structural Definition of the Intercellular Septa in the Heterocyst-Forming Cyanobacterium Anabaena

Cyanobacteria are photosynthetic organisms with a Gram-negative envelope structure. Certain filamentous species such as Anabaena sp. strain PCC 7120 can fix dinitrogen upon depletion of combined nitrogen. Because the nitrogen-fixing enzyme, nitrogenase, is oxygen sensitive, photosynthesis and nitrogen fixation are spatially separated in Anabaena. Nitrogen fixation takes place in specialized cells called heterocysts, which differentiate from vegetative cells. During heterocyst differentiation, a microoxic environment is created by dismantling photosystem II and restructuring the cell wall. Moreover, solute exchange between the different cell types is regulated to limit oxygen influx into the heterocyst. The septal zone containing nanopores for solute exchange is constricted between heterocysts and vegetative cells, and cyanophycin plugs are located at the heterocyst poles. We identified a protein previously annotated as TonB1 that is largely conserved among cyanobacteria. A mutant of the encoding gene formed heterocysts but was impaired in diazotrophic growth. Mutant heterocysts appeared elongated and exhibited abnormal morphological features, including a reduced cyanophycin plug, an enhanced septum size, and a restricted nanopore zone in the septum. In spite of this, the intercellular transfer velocity of the fluorescent marker calcein was increased in the mutant compared to the wild type. Thus, the protein is required for proper formation of septal structures, expanding our emerging understanding of Anabaena peptidoglycan plasticity and intercellular solute exchange, and is therefore renamed SjdR (septal junction disk regulator). Notably, calcium supplementation compensated for the impaired diazotrophic growth and alterations in septal peptidoglycan in the sjdR mutant, emphasizing the importance of calcium for cell wall structure. IMPORTANCE Multicellularity in bacteria confers an improved adaptive capacity to environmental conditions and stresses. This includes an enhanced capability of resource utilization through a distribution of biochemical processes between constituent cells. This specialization results in a mutual dependency of different cell types, as is the case for nitrogen-fixing heterocysts and photosynthetically active vegetative cells in Anabaena. In this cyanobacterium, intercellular solute exchange is facilitated through nanopores in the peptidoglycan between adjacent cells. To ensure functionality of the specialized cells, septal size as well as the position, size, and frequency of nanopores in the septum need to be tightly established. The novel septal junction disk regulator SjdR characterized here is conserved in the cyanobacterial phylum. It influences septal size and septal nanopore distribution. Consequently, its absence severely affects the intercellular communication and the strains' growth capacity under nitrogen depletion. Thus, SjdR is involved in septal structure remodeling in cyanobacteria.

The small Ca2+-binding protein CSE links Ca2+ signalling with nitrogen metabolism and filament integrity in Anabaena sp. PCC 7120

Background

Filamentous cyanobacteria represent model organisms for investigating multicellularity. For many species, nitrogen-fixing heterocysts are formed from photosynthetic vegetative cells under nitrogen limitation. Intracellular Ca²⁺ has been implicated in the highly regulated process of heterocyst differentiation but its role remains unclear. Ca²⁺ is known to operate more broadly in metabolic signalling in cyanobacteria, although the signalling mechanisms are virtually unknown. A Ca²⁺-binding protein called the Ca²⁺ Sensor EF-hand (CSE) is found almost exclusively in filamentous cyanobacteria. Expression of asr1131 encoding the CSE protein in Anabaena sp. PCC 7120 was strongly induced by low CO₂ conditions, and rapidly downregulated during nitrogen step-down. A previous study suggests a role for CSE and Ca²⁺ in regulation of photosynthetic activity in response to changes in carbon and nitrogen availability.

Results

In the current study, a mutant Anabaena sp. PCC 7120 strain lacking asr1131 (Δcse) was highly prone to filament fragmentation, leading to a striking phenotype of very short filaments and poor growth under nitrogen-depleted conditions. Transcriptomics analysis under nitrogen-replete conditions revealed that genes involved in heterocyst differentiation and function were downregulated in Δcse, while heterocyst inhibitors were upregulated, compared to the wild-type.

Conclusions

These results indicate that CSE is required for filament integrity and for proper differentiation and function of heterocysts upon changes in the cellular carbon/nitrogen balance. A role for CSE in transmitting Ca²⁺ signals during the first response to changes in metabolic homeostasis is discussed.

Evidence of high Ca uptake by cyanobacteria forming intracellular CaCO3 and impact on their growth

Several species of cyanobacteria biomineralizing intracellular amorphous calcium carbonates (ACC) were recently discovered. However, the mechanisms involved in this biomineralization process and the determinants discriminating species forming intracellular ACC from those not forming intracellular ACC remain unknown. Recently, it was hypothesized that the intensity of Ca uptake (i.e., how much Ca was scavenged from the extracellular solution) might be a major parameter controlling the capability of a cyanobacterium to form intracellular ACC. Here, we tested this hypothesis by systematically measuring the Ca uptake by a set of 52 cyanobacterial strains cultured in the same growth medium. The results evidenced a dichotomy among cyanobacteria regarding Ca sequestration capabilities, with all strains forming intracellular ACC incorporating significantly more calcium than strains not forming ACC. Moreover, Ca provided at a concentration of 50 μM in BG-11 was shown to be limiting for the growth of some of the strains forming intracellular ACC, suggesting an overlooked quantitative role of Ca for these strains. All cyanobacteria forming intracellular ACC contained at least one gene coding for a mechanosensitive channel, which might be involved in Ca influx, as well as at least one gene coding for a Ca²⁺ /H⁺ exchanger and membrane proteins of the UPF0016 family, which might be involved in active Ca transport either from the cytosol to the extracellular solution or the cytosol toward an intracellular compartment. Overall, massive Ca sequestration may have an indirect role by allowing the formation of intracellular ACC. The latter may be beneficial to the growth of the cells as a storage of inorganic C and/or a buffer of intracellular pH. Moreover, high Ca scavenging by cyanobacteria biomineralizing intracellular ACC, a trait shared with endolithic cyanobacteria, suggests that these cyanobacteria should be considered as potentially significant geochemical reservoirs of Ca.

Physiological mechanisms of aluminum (Al) toxicity tolerance in nitrogen-fixing aquatic macrophyte Azolla microphylla Kaulf: phytoremediation, metabolic rearrangements, and antioxidative enzyme responses

To investigate the extent of aluminum toxicity tolerance of eco-friendly, fast-growing, fresh water, pteridophytic Azolla-Anabaena symbiotic association in terms of altered physiological signals; Azolla microphylla Kaulf was exposed to 0 (control), 100, 250, 500, and 750 μM AlCl₃, at pH 4.5 for 6 days. The adversity of Al was increased in a dose-dependent manner and the highest was recorded at 750 μM AlCl₃. Despite the significant loss in membrane integrity (80% electrolyte leakage) due to an enhanced generation of H₂O₂, A. microphylla reflected only 50% growth inhibition (fresh and dry weight) at 500 μM AlCl₃ (LD₅₀). However, the average root length of Azolla was drastically reduced at high concentration due to their direct contact with aluminum-containing growth medium. Contrary to this, the whole association maintained moderate chlorophyll, carbohydrate content, photosynthetic efficiency, nitrogen-fixing ability, and nitrogen content at high Al concentration. Probably, growth protection was pertained through significant detoxification of H₂O₂ by employing an efficient antioxidative defense system including antioxidative enzymes (SOD, APX, and CAT) and non-enzymatic antioxidant carotenoids. An enhanced level of phenolics and flavonoids in the root exudates possibly maintained a non-toxic level of aluminum inside the cell (195.8 μg Al/g FW) which makes A. microphylla a suitable pteridophytic plant to not only remove toxic Al from the contaminated sites but also to improve nitrogen status of those regions. Graphical abstract ᅟ.

Characterization of extracellular polysaccharide/protein contents during the adsorption of Cd(II) by Synechocystis sp. PCC6803

Cyanobacteria have been proven to be cheaper and more effective for the removal of metallic elements in aqueous solutions. In this study, the living cyanobacteria Synechocystis sp. PCC6803 was used to adsorb Cd(II) and its extracellular polymeric substances (EPS) were investigated in the adsorption process. The initial stage of adsorption of Cd(II) was a rapid process, and then increase slowly accompanied with the increases of biomass. The final adsorption percentage could achieve 86% when the Cd(II) concentration was 0.5 mg/L. It proved that Synechocystis sp. PCC6803 has a good adsorption capacity for heavy metal ions. EPS was extracted to investigate the secretion of which was dynamic and the maximum extracellular polysaccharides and proteins were 134.2 and 100.9 mg/g, respectively. Furthermore, the real-time PCR (RT-PCR) results of genes (slr0977 and exoD) involved in EPS synthesis and secretion indicated that the EPS production was firstly increased and then decreased slightly. Transmission electron microscope (TEM) observation revealed that heavy metal ions were absorbed into EPS layer. Fourier transform infrared spectrum (FT-IR) analysis showed that EPS was rich in functional groups which could combine with heavy metal ions, such as -OH and -NH groups. All the results obtained show that the secretion of EPS by cyanobacteria was one of the ways to resist heavy metal stress. And it shows a trend of rising first and then decreasing, the change regulation of which was consistent with adsorptive behavior.

Characterization of Pustular Mats and Related Rivularia-Rich Laminations in Oncoids From the Laguna Negra Lake (Argentina)

Stromatolites are organo-sedimentary structures that represent some of the oldest records of the early biosphere on Earth. Cyanobacteria are considered as a main component of the microbial mats that are supposed to produce stromatolite-like structures. Understanding the role of cyanobacteria and associated microorganisms on the mineralization processes is critical to better understand what can be preserved in the laminated structure of stromatolites. Laguna Negra (Catamarca, Argentina), a high-altitude hypersaline lake where stromatolites are currently formed, is considered as an analog environment of early Earth. This study aimed at characterizing carbonate precipitation within microbial mats and associated oncoids in Laguna Negra. In particular, we focused on carbonated black pustular mats. By combining Confocal Laser Scanning Microscopy, Scanning Electron Microscopy, Laser Microdissection and Whole Genome Amplification, Cloning and Sanger sequencing, and Focused Ion Beam milling for Transmission Electron Microscopy, we showed that carbonate precipitation did not directly initiate on the sheaths of cyanobacterial Rivularia, which dominate in the mat. It occurred via organo-mineralization processes within a large EPS matrix excreted by the diverse microbial consortium associated with Rivularia where diatoms and anoxygenic phototrophic bacteria were particularly abundant. By structuring a large microbial consortium, Rivularia should then favor the formation of organic-rich laminations of carbonates that can be preserved in stromatolites. By using Fourier Transform Infrared spectroscopy and Synchrotron-based deep UV fluorescence imaging, we compared laminations rich in structures resembling Rivularia to putatively chemically-precipitated laminations in oncoids associated with the mats. We showed that they presented a different mineralogy jointly with a higher content in organic remnants, hence providing some criteria of biogenicity to be searched for in the fossil record.

Cyanobacterial formation of intracellular Ca-carbonates in undersaturated solutions

Cyanobacteria have long been thought to induce the formation of Ca-carbonates as secondary by-products of their metabolic activity, by shifting the chemical composition of their extracellular environment to conditions favoring mineral precipitation. Some cyanobacterial species forming Ca-carbonates intracellularly were recently discovered. However, the environmental conditions under which this intracellular biomineralization process can occur and the impact of cyanobacterial species forming Ca-carbonates intracellularly on extracellular carbonatogenesis are not known. Here, we show that these cyanobacteria can form Ca-carbonates intracellularly while growing in extracellular solutions undersaturated with respect to all Ca-carbonate phases, that is, conditions thermodynamically unfavorable to mineral precipitation. This shows that intracellular Ca-carbonate biomineralization is an active process; that is, it costs energy provided by the cells. The cost of energy may be due to the active accumulation of Ca intracellularly. Moreover, unlike cyanobacterial strains that have been usually considered before by studies on Ca-carbonate biomineralization, cyanobacteria forming intracellular carbonates may slow down or hamper extracellular carbonatogenesis, by decreasing the saturation index of their extracellular solution following the buffering of the concentration of extracellular calcium to low levels.

Calcium mediates the cellular response of Chlamydomonas reinhardtii to the emerging aquatic pollutant Triclosan

The present study was aimed at investigating the role of intracellular free calcium, [Ca²⁺]_c, in the early cellular response of the green alga Chlamydomonas reinhardtii to the emergent pollutant Triclosan (13.8μM; 24h of exposure). There is a growing concern about the persistence and toxicity of this antimicrobial in aquatic environments, where non-target organisms such as C. reinhardtii, a primary producer of ecological relevance, might be severely impacted. A mechanistic study was undertaken which combined flow cytometry protocols, physiological as well as gene expression analysis. As an early response, Triclosan strongly altered [Ca²⁺]_c homeostasis which could be prevented by prechelation with the intracellular calcium chelator BAPTA-AM. Triclosan induced ROS overproduction which ultimately leads to oxidative stress with loss of membrane integrity, membrane depolarization, photosynthesis inhibition and mitochondrial membrane depolarization; within this context, Triclosan also induced an increase in caspase 3/7 activity and altered the expression of metacaspase genes which are indicative of apoptosis. All these adverse outcomes were dependent on [Ca²⁺]_c. Interestingly, an interconnection between [Ca²⁺]_c alterations and increased ROS formation by Triclosan was found. Taken altogether these results shed light on the mechanisms behind Triclosan toxicity in the green alga Chlamydomonas reinhardtii and demonstrate the role of [Ca²⁺]_c in mediating the observed toxicity.

Exopolysaccharide production in Anabaena sp. PCC 7120 under different CaCl2 regimes

Influence of various levels of CaCl₂ (0, 1, 10 and 100 mM) on exopolysaccharide production has been investigated in the cyanobacterium Anabaena 7120. At the concentration of 1 mM CaCl₂, growth was found to be stimulatory while 100 mM was sub lethal for the cyanobacterial cells. Estimation of EPS content revealed that EPS production depends on the concentration of calcium ions in the immediate environment with maximum being at10 mM CaCl₂. A possible involvement of alr2882 gene in the process of EPS production was also revealed through qRT-PCR. Further, FTIR-spectra marked the presence of aliphatic alkyl-group, primary amine-group, and polysaccharides along with shift in major absorption peaks suggesting that calcium levels in the external environment regulate the composition of EPS produced by Anabaena 7120. Thus, both quantity and composition of EPS is affected under different calcium chloride concentrations presenting possibilities of EPS with novel unexplored features that may offer biotechnological applications.

Calcium impacts carbon and nitrogen balance in the filamentous cyanobacterium Anabaena sp. PCC 7120

Calcium is integral to the perception, communication and adjustment of cellular responses to environmental changes. However, the role of Ca(2+) in fine-tuning cellular responses of wild-type cyanobacteria under favourable growth conditions has not been examined. In this study, extracellular Ca(2+) has been altered, and changes in the whole transcriptome of Anabaena sp. PCC 7120 have been evaluated under conditions replete of carbon and combined nitrogen. Ca(2+) induced differential expression of many genes driving primary cellular metabolism, with transcriptional regulation of carbon- and nitrogen-related processes responding with opposing trends. However, physiological effects of these transcriptional responses on biomass accumulation, biomass composition, and photosynthetic activity over the 24h period following Ca(2+) adjustment were found to be minor. It is well known that intracellular carbon:nitrogen balance is integral to optimal cell growth and that Ca(2+) plays an important role in the response of heterocystous cyanobacteria to combined-nitrogen deprivation. This work adds to the current knowledge by demonstrating a signalling role of Ca(2+) for making sensitive transcriptional adjustments required for optimal growth under non-limiting conditions.

Unraveling of cross talk between Ca(2+) and ROS regulating enzymes in Anabaena 7120 and ntcA mutant

In order to understand a cross talk between Ca(2+) and ROS regulating enzymes and the possible involvement of ntcA gene, Anabaena sp. PCC 7120 and its derivative ntcA mutant grown in varied levels of calcium chloride (0, 1, 10, and 100 mM) have been investigated. Scanning Electron Microscopy showed abnormal structure formation at high calcium concentration (100 mM) both in wild type and mutant. Fv /Fm values suggested that 100 mM calcium concentration was detrimental for photosynthetic apparatus. SOD, catalase, APX, GR, and peroxidase activity were found to be maximum for 100 mM and minimum for 1 mM of exogenously supplied calcium salt. NADPH contents were higher for wild type than mutant. RAPD-PCR and SDS-PAGE analysis revealed a difference in DNA as well as proteome pattern with changes in calcium chloride regime. Prominent bands of approximately 70, 33, 21, and 14 kDa expressed in the wild type served as the marker polypeptide bands under calcium supplementation. Results suggest that higher levels of calcium ion disturb the cellular homeostasis generating ROS, thereby inducing enhanced levels of antioxidative enzymes. Further, data also suggests possible involvement of ntcA gene in cross talk between calcium ion and ROS regulating enzymes.