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Cyanobacteria are vital photosynthetic prokaryotes, but some form harmful algal blooms (cyanoHABs) that disrupt ecosystems and produce toxins. The mechanisms by which these blooms form have yet to be fully understood, particularly the role of extracellular components. Here, we present a 2.4 Å cryo-EM structure of a pilus, termed the cyanobacterial tubular (CT) pilus, found in the cyanoHAB-forming Microcystis aeruginosa . The pilin exhibits a unique protein fold, forming a tubular pilus structure with tight, double-layer anti-parallel β-sheet interactions. We show that CT pili are essential for buoyancy by facilitating the formation of micro-colonies, which increases drag force and prevents sinking. The CT pilus surface is heavily glycosylated with ten monosaccharide modifications per pilin. Furthermore, CT pili can enrich microcystin, potentially enhancing cellular resilience, and co-localize with iron-enriched extracellular matrix components. Thus, we propose that this pilus plays an important role in the proliferation of cyanoHABs. This just discovered pilus family appears to be widely distributed across several cyanobacterial orders. Our structural and functional characterization of CT pili provide insights into cyanobacterial cell morphology, physiology, and toxin interactions, and identify potential targets for disrupting bloom formation. Ricca et al discover a new family of tubular pili in Microcystis aeruginosa, a harmful algal bloom-forming cyanobacterium. These pili are crucial for buoyancy by forming cell micro-colonies, which increases drag and prevents sinking. The pili also enrich microcystin and co-localize with iron-enriched extracellular matrix components, suggesting a vital role in bloom proliferation.

An isolated strain Streptomyces sp. HJC-D1 was applied to inhibit the growth of cyanobacterium Microcystis aeruginosa FACHB-905. The effect of Streptomyces sp. HJC-D1 culture broth on the cell integrity and physiological characteristics of M. aeruginosa FACHB-905 was investigated using the flow cytometry (FCM), enzyme activity and transmission electron microscopy (TEM) methods. Results showed that the growth of M. aeruginosa FACHB-905 was significantly inhibited, and the percentage of live cells depended on the culture broth concentration and exposure time. The activities of antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) increased with exposure concentration and exposure time, and the significant increase of reactive oxygen species (ROS) led to the disruption of the subcellular structure of M. aeruginosa FACHB-905, and caused the increase of malondialdehyde (MDA). Furthermore, TEM observation suggested the presence of three stages (cell breakage, organelle release and cell death) for the cyanobactericidal process of Streptomyces sp. HJC-D1. Therefore, Streptomyces sp. HJC-D1 not only affected antioxidant enzyme activities and ROS level, but also destroyed the subcellular structure of M. aeruginosa FACHB-905, demonstrating excellent cyanobactericidal properties.

Microcystis aeruginosa, as one of the major players in algal bloom, produces microcystins, which are strongly hepatotoxic, endangering human health and damaging the ecological environment. Biological control of the overgrowth of Microcystis with cyanophage has been proposed to be a promising solution for algal bloom. In this study, a novel strain of Microcystis cyanophage, MinS1, was isolated. MinS1 contains an icosahedral head approximately 54 nm in diameter and a 260 nm-long non-contractile tail. The phage genome consists of a linear, double-stranded 49,966 bp DNA molecule, which shares very low homology with known phages in the NCBI database (only 1% of the genome showed weak homology with known phages when analyzed by megablast). The phage contains 75 ORFs, of which 23 ORFs were predicted to code for proteins of known function, 39 ORFs were predicted to code for proteins of unknown function, and 13 ORFs showed no similarity to any protein sequences. Transmission electron microscopy and phylogenetic analysis showed that MinS1 belongs to the family Siphoviridae. Various experiments confirmed that the phage could infect several different orders of cyanobacteria, including Chroococcales, Nostocales, Oscillatoriales, Hormogonales, and Synechococcales, indicating that it has a very broad host range. In addition, MinS1 has no known antibiotic tolerance genes, virulence genes, and tRNAs, and it is tolerant to temperature, pH, UV, and salinity, suggesting that MinS1 has good potential for application as a biological control agent against cyanobacterial blooms. This study expands the diversity and knowledge of cyanophages, and it provides useful information for the development of novel prevention and control measures against cyanobacterial blooms.

In this study, two cyanobacterial strains (morphologically identified as Microcystis novacekii BA005 and Nostoc paludosum BA033) were exposed to different Mn concentrations: 7.0, 10.5, 15.7, 23.6 and 35.4 mg L−1 for BA005; and 15.0, 22.5, 33.7, 50.6, and 76.0 mg L−1 for BA033. Manganese toxicity was assessed by growth rate inhibition (EC50), chlorophyll a content, quantification of Mn accumulation in biomass and monitoring morphological and ultrastructural effects. The Mn EC50 values were 16 mg L−1 for BA005 and 39 mg L−1 for BA033, respectively. Reduction of chlorophyll a contents and ultrastructural changes were observed in cells exposed to Mn concentrations greater than 23.6 and 33.7 mg L−1 for BA005 and BA033. Damage to intrathylakoid spaces, increased amounts of polyphosphate granules and an increased number of carboxysomes were observed in both strains. In the context of the potential application of these strains in bioremediation approaches, BA005 was able to remove Mn almost completely from aqueous medium after 96 h exposure to an initial concentration of 10.5 mg L−1, and BA033 was capable of removing 38% when exposed to initial Mn concentration of 22.5 mg L−1. Our data shed light on how these cyanobacterial strains respond to Mn stress, as well as supporting their utility as organisms for monitoring Mn toxicity in industrial wastes and potential bioremediation application.

Identification of extracellular conductive pilus-like structures (PLS) i.e. microbial nanowires has spurred great interest among scientists due to their potential applications in the fields of biogeochemistry, bioelectronics, bioremediation etc. Using conductive atomic force microscopy, we identified microbial nanowires in Microcystis aeruginosa PCC 7806 which is an aerobic, photosynthetic microorganism. We also confirmed the earlier finding that Synechocystis sp. PCC 6803 produces microbial nanowires. In contrast to the use of highly instrumented continuous flow reactors for Synechocystis reported earlier, we identified simple and optimum culture conditions which allow increased production of nanowires in both test cyanobacteria. Production of these nanowires in Synechocystis and Microcystis were found to be sensitive to the availability of carbon source and light intensity. These structures seem to be proteinaceous in nature and their diameter was found to be 4.5–7 and 8.5–11 nm in Synechocystis and M. aeruginosa , respectively. Characterization of Synechocystis nanowires by transmission electron microscopy and biochemical techniques confirmed that they are type IV pili (TFP) while nanowires in M. aeruginosa were found to be similar to an unnamed protein (GenBank : CAO90693.1). Modelling studies of the Synechocystis TFP subunit i.e. PilA1 indicated that strategically placed aromatic amino acids may be involved in electron transfer through these nanowires. This study identifies PLS from Microcystis which can act as nanowires and supports the earlier hypothesis that microbial nanowires are widespread in nature and play diverse roles.

Oleamide, a fatty acid derivative, shows inhibitory effect against the bloom-forming cyanobacterium Microcystis aeruginosa . The EC 50 of oleamide on the growth of M . aeruginosa NIES-843 was 8.60 ± 1.20 mg/L. In order to elucidate the possible mechanism of toxicity of oleamide against M . aeruginosa , chlorophyll fluorescence transient, cellular ultrastructure, fatty acids composition and the transcription of the mcyB gene involved in microcystins synthesis were studied. The results of chlorophyll fluorescence transient showed that oleamide could destruct the electron accepting side of the photosystem II of M . aeruginosa NIES-843. Cellular ultrastructure examination indicated that the destruction of fatty acid constituents, the distortion of thylakoid membrane and the loss of integrity of cell membrane were associated with oleamide treatment and concentration. The damage of cellular membrane increased the release of microcystins from intact cells into the medium. Results presented in this study provide new information on the possible mechanisms involved and potential utilization of oleamide as an algicide in cyanobacterial bloom control.

The cyanobacterium Microcystis aeruginosa is a mesophilic freshwater organism, which cannot tolerate sulphide. However, it was possible to isolate a sulphideresistant (S r ) mutant strain that was able to survive in a normally lethal medium sulphide. In order to evaluate the cost of the mutation conferring sulphide resistance in the S r strain of M. aeruginosa, the morphology and the photosynthetic performance were compared to that found in the wild-type, sulphide-sensitive (S s ) strain. An increase in size and a disrupted morphology was observed in S r cells in comparison to the S s counterpart. Phycoerythrin and phycocyanin levels were higher in the S r than in the S s cells, whereas a higher carotenoid content, per unit volume, was found in the S s strain. The irradiance-saturated photosynthetic oxygen-production rate (GPRmax) and the photosynthetic efficiency (measured both by oxygen production and fluorescence,αGPR and αETR) were lower in the S r strain than in the wild-type. These results appear to be the result of package effect. On the other hand, the S r strain showed higher quantum yield of nonphotochemical quenching, especially those regulated mechanisms (estimated throughout q N and Y(NPQ)) and a significantly lower slope in the maximum quantum yield of lightadapted samples ( F v ' / F m ' ) compared to the S s strain. These findings point to a change in the regulation of the quenching of the transition states (q T ) in the S r strain which may be generated by a change in the distribution of thylakoidal membranes, which somehow could protect metalloenzymes of the electron transport chain from the lethal effect of sulphide.

This study aimed to investigate the performance of chitosan-modified nano-sized montmorillonite (CTS/NMMT) during the flocculation of Microcystis aeruginosa (MA). The release of intracellular microcystins (MCs) caused by the damage of intact MA cells during the flocculation and floc storage processes was also comprehensively evaluated through scanning electron microscopy (SEM) and measurement of K + and Mg 2+ release. With the application of the Box–Behnken experimental design combined with response surface methodology, the quadratic statistical model was established to predict and optimize the interactive effects of content of CTS/NMMT, weight ratio of NMMT to CTS, and agitation time on the removal efficiency of MA cells. A maximum removal of 94.7 % MA cells was observed with content of CTS/NMMT 300–320 mg L −1 , weight ratio of NMMT to CTS 14–16, and agitation time 16–50 min. During the flocculation process, CTS/NMMT aggregated MA cells as flocs and served as a protection shield for cells. The extracellular and intracellular microcystin–leucine–arginine (MC-LR) decreased remarkably and the yield of intracellular MC-LR showed a decreasing trend during the flocculation. The cell integrity was slightly damaged by the mechanical actions rather than by the flocculant. During the floc storage process, cell lysis and membrane damage were remarkably aggravated. The noticeable increase of K + and Mg 2+ release indicated that CTS/NMMT damaged the integrity of most MA cells in the flocs and liberated the intracellular MC-LR. Meanwhile, NMMT and CTS polymers assisted the adsorptive removal of extracellular MC-LR released to water. The flocs should be timely treated within 12 h to prevent the leakage of MCs.

Cyanobacteria are a diverse group of Gram-negative bacteria that produce an array of secondary compounds with selective bioactivity against vertebrates, invertebrates, fungi, bacteria and cell lines. Recently the main methods of controlling cyanobacteria are using chemicals, medicinal plants and microorganism but fewer involved the safety research in hydrophytic ecosystems. In search of an environmentally safe compound, 53 chemicals were screened against the developed heavy cyanobacteria bloom Microcystis aeruginosa using coexistence culture system assay. The results of the coexistence assay showed that 9 chemicals inhibited M. aeruginosa effectively at 20 mg L⁻¹ after 7 days of exposure. Among them dimethomorph, propineb, and paraquat were identified that they are safe for Chlorella vulgaris, Scenedesmus obliquus, Carassius auratus (Goldfish) and Bacillus subtilis within half maximal effective concentration (EC₅₀) values 5.2, 4.2 and 0.06 mg L⁻¹ after 7 days, respectively. Paraquat as the positive control observed to be more efficient than the other compounds with the inhibitory rate (IR) of 92 % at 0.5 mg L⁻¹. For the potential inhibition mechanism, the chemicals could destroy the cell ultrastructure in different speed. The safety assay proved dimethomorph, propineb and paraquat as harmless formulations or products having potential value in M. aeruginosa controlling, with the advantage of its cell morphology degrading ability.

In cyanobacteria the protein on the outside of the gas vesicle, GvpC, is characterised by the presence of a 33 amino acid residue repeat (33 RR), which in some genera is highly conserved. The number of 33 RRs correlates with the diameter of the gas vesicle and inversely with its strength. Gas vesicles isolated from Microcystis aeruginosa strain PCC 7806 were found to be wider and have a lower critical collapse pressure than those from Microcystis sp. strain BC 8401. The entire gas-vesicle gene cluster of the latter strain was sequenced and compared with the published sequence of the former: the sequences of nine of the ten gvp genes differed by only 1–5% between the two strains; the only substantial difference was in gvpC which in strain BC 8401 lacked a 99-nucleotide section encoding a 33 RR. This observation further narrows the correlation of gas vesicle width to the number of 33 RRs and suggests how Microcystis strains might be used in experimental manipulation of gas vesicle width and strength.