Microcystis genotypes in a tropical freshwater lake: Discovery of novel MIB-producing Microcystis with potentially unique synthesis pathway.

Harmful algal blooms (HABs) are a threat to freshwater systems over the world due to the production of hepatotoxins like microcystin (MC), and nuisance taste and odour (T&O) compounds like 2-methylisoborneol (MIB). While MCs are known to cause detrimental effects to both water quality and human health, MIB is only reported to cause aesthetical problems. In this study, we investigated a tropical, urban lake that was experiencing persistent MC and MIB events. Although it was dominated by Microcystis blooms, analysis revealed that the toxigenic Microcystis were not the only species driving the MC concentrations. Additionally, there was also a lack of causative species for the MIB events. Through isolation, we have identified three toxigenic Microcystis found to produce four different variants of MCs, and two novel non-toxigenic Microcystis that were capable of producing MIB. The ability to produce MIB had never been previously reported for this species. Compared to other major producers such as Planktothricoides sp. and Streptomyces sp., the MIB synthase genes of our Microcystis sp. strains were partial, illustrating the possibility of unique synthesis pathways. The Microcystis sp. strains were found to produce about 2.77-5.22 fg MIB cell-1, with a majority of the contents (70-80 %) existing in the extracellular phase. Correlation analysis of field study indicated that phosphorus limitation may have an indirect effect on non-toxigenic Microcystis abundance and proportion by influencing the toxigenic genotype, suggesting that current measures to control HABs may favour the proliferation of the non-toxigenic Microcystis. The potential for Microcystis sp. to produce MIB through unique synthesis pathway, coupled with the potential dominance of non-toxigenic genotypes in Microcystis blooms, signals the possibility that non-toxigenic Microcystis should be monitored as well.

Corrected and Republished from: "Isolation and Characterization of the First Freshwater Cyanophage Infecting Pseudanabaena".

Cyanobacteria are the major primary producers in both freshwater and marine environments. However, the majority of freshwater cyanophages remain unknown due to the limited number of cyanophage isolates. In this study, we present a novel lytic freshwater cyanophage, PA-SR01, which was isolated from the Singapore Serangoon Reservoir. To our knowledge, this is the first isolate of a cyanophage that has been found to infect the cyanobacterium Pseudanabaena. PA-SR01 has a narrow host range, a short latent period, and is chloroform sensitive. PA-SR01 is a member of Siphoviridae with a long noncontractile tail. It is a double-stranded DNA virus with a 137,012-bp genome. Functional annotation for the predicted open reading frames (ORFs) of the PA-SR01 genome identified genes with putative functions related to DNA metabolism, structural proteins, lysis, host-derived metabolic genes, and DNA packaging. Out of 166 predicted ORFs, only 17 ORFs have homology with genes with known function. Phylogenetic analysis of the major capsid protein and terminase large subunit further suggests that phage PA-SR01 is evolutionary distinct from known cyanophages. Metagenomics sequence recruitment onto the PA-SR01 genome indicates that PA-SR01 represents a new evolutionary lineage of phage which shares considerable genetic similarities with phage sequences in aquatic environments and could play key ecological roles. IMPORTANCE This study presents the isolation of the very first freshwater cyanophage, PA-SR01, that infects Pseudanabaena, and fills an important knowledge gap on freshwater cyanophages as well as cyanophages infecting Pseudanabaena.

Coexistence of Synechococcus and Microcystis Blooms in a Tropical Urban Reservoir and Their Links with Microbiomes.

Bacteria play a crucial role in driving ecological processes in aquatic ecosystems. Studies have shown that bacteria-cyanobacteria interactions contributed significantly to phytoplankton dynamics. However, information on the contribution of bacterial communities to blooms remains scarce. Here, we tracked changes in the bacterial community during the development of a cyanobacterial bloom in an equatorial estuarine reservoir. Two forms of blooms were observed simultaneously corresponding to the lotic and lentic characteristics of the sampling sites where significant spatial variabilities in physicochemical water quality, cyanobacterial biomass, secondary metabolites, and cyanobacterial/bacterial compositions were detected. Microcystis dominated the upstream sites during peak periods and were succeeded by Synechococcus when the bloom subsided. For the main body of the reservoir, a mixed bloom featuring coccoid and filamentous cyanobacteria (Microcystis, Synechococcus, Planktothricoides, Nodosilinea, Raphidiopsis, and Prochlorothrix) was observed. Concentrations of the picocyanobacteria Synechococcus remained high throughout the study, and their positive correlations with cylindrospermopsin and anatoxin-a suggested that they could produce cyanotoxins, which pose more damaging impacts than previously supposed. Succession of different cyanobacteria (Synechococcus and Microcystis) following changes in nutrient composition and ionic strength was demonstrated. The microbiomes associated with blooms were unique to the dominant cyanobacteria. Generic and specialized bloom biomarkers for the Microcystis and downstream mixed blooms were also identified. Microscillaceae, Chthoniobacteraceae, and Roseomonas were the major heterotrophic bacteria associated with Microcystis bloom, whereas Phycisphaeraceae and Methylacidiphilaceae were the most prominent groups for the Synechococcus bloom. Collectively, bacterial community can be greatly deviated by the geological condition, monsoon season, cyanobacterial density, and dominant cyanobacteria.

Rapid detection methods and modelling simulations provide new insights into cyanobacteria detection and bloom management in a tropical reservoir.

The increasing occurrence of cyanobacteria blooms is of global concern, and is often associated with environmental and socio-economic problems, such as degenerated ecosystems and aquaculture impairment. The diazotrophic cyanobacterium Raphidiopsis raciborskii (R. raciborskii) grows rapidly in the tropics, and produces the toxin, cylindrospermopsin (CYN), which has harmful effects on aquatic organisms. Thus, to protect water quality and ecosystem, it is essential to have rapid and reliable methods for cyanobacteria and R. raciborskii detection and prediction so that early warning can be provided for management. Molecular assays, such as PCR, real-time quantitative PCR (qPCR), two-step PCR assays are accurate and widely used, but still require several hours from sample preparation to data analysis. In this study, insulated isothermal PCR (iiPCR) assays in conjunction with fast DNA extraction method, were developed and verified as a rapid detection assay in detecting cyanobacteria and R. raciborskii within 50 min, and also with high detection accuracy (98.8%) and the overall high agreement level (98.8%, k = 97.5%)) comparing to conventional qPCR assay. However, the limitation of the iiPCR assay is that it only generates qualitative results. Therefore, the quantified iiPCR assay, named as A-iiPCR, by coupling iiPCR device with fluorescence signal catching and interpretation instrument (Andor spectrometer with Solis spectroscopy software) was developed and verified with in situ environmental samples. The fluorescence intensity decreased accordingly with the drop of DNA concentration until reaching 1.32 ng/µL. Also, Delft 3D modelling was established to simulate R. raciborskii change in predicting spatial and temporal variabilities for reservoir management, as the simulated R. raciborskii concentration was the highest at sampling site 1, as well as temporally highest in April and October, posing as the most high-risk location and time periods for R. raciborskii bloom-forming requiring corresponding governance measures.

Effects of Light and Temperature on the Metabolic Profiling of Two Habitat-Dependent Bloom-Forming Cyanobacteria.

Rapid proliferation of cyanobacteria in both benthic and suspended (planktonic) habitats is a major threat to environmental safety, as they produce nuisance compounds such as cytotoxins and off-flavors, which degrade the safety and quality of water supplies. Temperature and light irradiance are two of the key factors in regulating the occurrence of algal blooms and production of major off-flavors. However, the role of these factors in regulating the growth and metabolism is poorly explored for both benthic and planktonic cyanobacteria. To fill this gap, we studied the effects of light and temperature on the growth and metabolic profiling of both benthic (Hapalosiphon sp. MRB220) and planktonic (Planktothricoides sp. SR001) environmental species collected from a freshwater reservoir in Singapore. Moreover, this study is the first report on the metabolic profiling of cyanobacteria belonging to two different habitats in response to altered environmental conditions. The highest growth rate of both species was observed at the highest light intensity (100 µmol photons/m²/s) and at a temperature of 33 °C. Systematic metabolite profiling analysis suggested that temperature had a more profound effect on metabolome of the Hapalosiphon, whereas light had a greater effect in the case of Planktothricoides. Interestingly, Planktothricoides sp. SR001 showed a specialized adaptation mechanism via biosynthesis of arginine, and metabolism of cysteine and methionine to survive and withstand higher temperatures of 38 °C and higher. Hence, the mode of strategies for coping with different light and temperature conditions was correlated with the growth and alteration in metabolic activities for physiological and ecological adaptations in both species. In addition, we putatively identified a number of unique metabolites with a broad range of antimicrobial activities in both species in response to both light and temperature. These metabolites could play a role in the dominant behavior of these species in suppressing competition during bloom formation. Overall, this study elucidated novel insights into the effects of environmental factors on the growth, metabolism, and adaptation strategies of cyanobacteria from two different habitats, and could be useful in controlling their harmful effects on human health and environmental concerns.

Application of a Mechanistic Model for the Prediction of Microcystin Production by Microcystis in Lab Cultures and Tropical Lake.

Microcystin is an algal toxin that is commonly found in eutrophic freshwaters throughout the world. Many studies have been conducted to elucidate the factors affecting its production, but few studies have attempted mechanistic models of its production to aid water managers in predicting its occurrence. Here, a mechanistic model was developed based on microcystin production by Microcystis spp. under laboratory culture and ambient field conditions. The model was built on STELLA, a dynamic modelling software, and is based on constitutive cell quota that varies with nitrogen, phosphorus, and temperature. In addition to these factors, varying the decay rate of microcystin according to its proportion in the intracellular and extracellular phase was important for the model's performance. With all these effects, the model predicted most of the observations with a model efficiency that was >0.72 and >0.45 for the lab and field conditions respectively. However, some large discrepancies were observed. These may have arisen from the non-constitutive microcystin production that appear to have a precondition of nitrogen abundance. Another reason for the large root mean square error is that cell quota is affected by factors differently between strains.

Multi-class secondary metabolites in cyanobacterial blooms from a tropical water body: Distribution patterns and real-time prediction.

Cyanobacterial blooms that produce toxins occur in freshwaters worldwide and yet, the occurrence and distribution patterns of many cyanobacterial secondary metabolites particularly in tropical regions are still not fully understood. Moreover, predictive models for these metabolites by using easily accessible water quality indicators are rarely discussed. In this study, we investigated the co-occurrence and spatiotemporal trends of 18 well-known and less-studied cyanobacterial metabolites (including [D-Asp3] microcystin-LR (DM-LR), [D-Asp3] microcystin-RR (DM-RR), microcystin-HilR (MC-HilR), microcystin-HtyR (MC-HtyR), microcystin-LA (MC-LA), microcystin-LF (MC-LF), microcystin-LR (MC-LR), microcystin-LW (MC-LW), microcystin-LY (MC-LY), microcystin-RR (MC-RR) and microcystin-WR (MC-WR), Anatoxin-a (ATX-a), homoanatoxin-a (HATX-a), cylindrospermospin (CYN), nodularin (NOD), anabaenopeptin A (AptA) and anabaenopeptin B (AptB)) in a tropical freshwater lake often plagued with blooms. Random forest (RF) models were developed to predict MCs and CYN and assess the relative importance of 22 potential predictors that determined their concentrations. The results showed that 11 MCs, CYN, ATX-a, HATX-a, AptA and AptB were found at least once in the studied water body, with MC-RR and CYN being the most frequently occurring, intracellularly and extracellularly. AptA and AptB were detected for the first time in tropical freshwaters at low concentrations. The metabolite profiles were highly variable at both temporal and spatial scales, in line with spatially different phytoplankton assemblages. Notably, MCs decreased with the increase of CYN, possibly revealing interspecific competition of cyanobacteria. The rapid RF prediction models for MCs and CYN were successfully developed using 4 identified drivers (i.e., chlorophyll-a, total carbon, rainfall and ammonium for MCs prediction; and chloride, total carbon, rainfall and nitrate for CYN prediction). The established models can help to better understand the potential relationships between cyanotoxins and environmental variables as well as provide useful information for making policy decisions.

Biodiversity, phylogeny and toxin production profile of cyanobacterial strains isolated from lake Latyan in Iran.

Monitoring toxigenic cyanobacteria in freshwaters is of great importance due to the adverse health impacts on humans and aquatic organisms. Here we studied cyanobacterial occurrence and biodiversity in a drinking water reservoir in Tehran province, Iran. In total, nine different species representing three orders of Synechococcales, Oscillatoriales and Nostocales were isolated and classified into six families and seven genera ranging from 92.3% to 99.0% similarities in their partial 16S rDNA with GenBank sequences. The cultures were analyzed for cyanotoxins production by the Artemia salina bioassay, ultrahigh performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) and also screened for the presence of marker genes involved in toxins production. Ethyl acetate extracts of three strains showed more than 50% mortality on A. salina larvae after 24 h at a concentration of 500 µg/ml. Production of at least one of the cyanotoxins, microcystin (MC), cylindrospermopsin (CYN) and anatoxin-a (ATX-a), was detected in 6 of the strains. Seven MC variants with a total concentration of 130.6 ng/mg of biomass dry weight were detected for the strain Phormidium sp. UTMC6001 and molecular screening of the mcyE gene also confirmed the presence of this biomarker in its genome. Our study also revealed the production of CYN in a novel picocyanobacterial strain Cyanobium sp. UTMC6007 at 1.0 ng/mg of biomass dry weight. Considering the limited information on freshwater toxic cyanobacteria taxonomy in the Middle East, these findings will expand our knowledge and consequently aid in development of new water management policies in future.

Novel cyanotoxin-producing Synechococcus in tropical lakes.

Picocyanobacteria are small cyanobacteria, being about 0.8-1.5 µm in size. They are present in freshwater environments all over the world and are known to cause harmful algal blooms, although their effects are not well understood. Algal blooms are important to manage because they threaten freshwater resources, with potentially severe effects on ecological and human health. There is also increased urgency due to urbanization and climate change trends which are expected to exacerbate these bloom dynamics. These changes are expected to especially favour picocyanobacteria groups, emphasizing the need for better characterization of their effects in the environment. In this study, we report the discovery that Synechococcus sp. could produce cylindrospermopsin (CYN) and anatoxin-a (ATX). This ability had never been previously reported for this species. Their toxin genes were also partial compared to other major producers such as Raphidiopsis sp. and Anabaena sp., demonstrating potentially unique synthesis pathways that provides insight into the various mechanisms of genetic variation that drives toxin synthesis. The Synechococcus sp. strains were found to produce about 9.0 × 10-5-6.8 × 10-4 fg CYN cell-1 and 4.7 × 10-4-1.5 × 10-2 fg ATX cell-1. The potential for Synechococcus sp. to be toxic highlights a global concern due to its widespread distribution, and through environmental trends that increasingly favour its productivity within freshwater systems around the world.

Isolation and Characterization of the First Freshwater Cyanophage Infecting Pseudanabaena.

Cyanobacteria are the major primary producers in both freshwater and marine environments. However, the majority of freshwater cyanophages remain unknown due to the limited number of cyanophage isolates. In this study, we present a novel lytic freshwater cyanophage, PA-SR01, which was isolated from the Singapore Serangoon Reservoir. To our knowledge, this is the first isolate of a cyanophage that has been found to infect the cyanobacterium Pseudanabaena PA-SR01 has a narrow host range, a short latent period, and is chloroform sensitive. Distinct from the majority of cyanophage isolates, PA-SR01 has a tailless morphology. It is a double-stranded DNA virus with a 137,012-bp genome. Functional annotation for the predicted open reading frames (ORFs) of the PA-SR01 genome identified genes with putative functions related to DNA metabolism, structural proteins, lysis, host-derived metabolic genes, and DNA packaging. Out of 166 predicted ORFs, only 17 ORFs have homology with genes with known function. Phylogenetic analysis of the major capsid protein and terminase large subunit further suggests that phage PA-SR01 is evolutionary distinct from known cyanophages. Metagenomics sequence recruitment onto the PA-SR01 genome indicates that PA-SR01 represents a new evolutionary lineage of phage which shares considerable genetic similarities with phage sequences in aquatic environments and could play key ecological roles.IMPORTANCE This study presents the isolation of the very first freshwater cyanophage, PA-SR01, that infects Pseudanabaena, and fills an important knowledge gap on freshwater cyanophages as well as cyanophages infecting Pseudanabaena.

Occurrence and distribution of viruses and picoplankton in tropical freshwater bodies determined by flow cytometry.

This study aimed to examine the drivers in shaping the occurrence and distribution of total viruses and picoplankton in tropical freshwater ecosystems. Flow cytometry was used to quantify the concentrations of total viruses, picoheterotrophs, picophytoplankton, and picocyanobacteria. Three land use patterns (urban-, agriculture- and parkland-dominated) were evaluated using ArcGIS. Significant correlations were observed between water-borne microbial targets and water quality parameters (0.175 ≤ |r| ≤ 0.441), nutrients (0.250 ≤ r ≤ 0.570) and land use factors (0.200 ≤ |r| ≤ 0.460). In particular, the concentrations of total viruses and picoheterotrophic cells were higher in catchments whereas the abundances of picophytoplankton and picocyanobacteria were higher in reservoirs. Total viruses and picoplankton had higher concentrations in urban- and agriculture-dominated areas, probably due to anthropogenic inputs and agricultural inputs, respectively. Although surface water is a complex matrix influenced by niche-based (i.e., physicochemical properties, nutrients, land use impact etc.) and neutral-based factors (i.e., ecological drift, dispersal and species), land use patterns could help to elucidate the occurrence and distribution of the total microbial community at the macroscopic level. Meanwhile, inter-correlations among viruses, picoplankton and picoheterotrophs (0.715 ≤ r ≤ 0.990) also substantiates their mutual interactions in influencing the microbial community. Furthermore, the relationships between total microbial cells and bacterial and viral indicators were also investigated. Concentrations of total viruses, picoplankton and picoheterotrophs were positively correlated with bacterial indicators (0.427 ≤ r ≤ 0.590) and viral indicators (0.201 ≤ r ≤ 0.563). These results indicated that faecal and viral contamination could contribute to the numbers of total viruses and bacteria.

The Characteristics and Dynamics of Cyanobacteria-Heterotrophic Bacteria Between Two Estuarine Reservoirs - Tropical Versus Sub-Tropical Regions.

In this study, Illumina MiSeq sequencing technique was employed to explore the characteristics and dynamics of cyanobacteria-heterotrophic bacteria between two estuarine reservoirs in sub-tropical (reservoir A in Shanghai) and tropical (reservoir B in Singapore) regions. The results indicated that significant differences in bacterial community composition were found between two estuarine reservoirs, which influenced by varied environmental variables. The environmental heterogeneity in reservoir A was much higher, which indicated that the composition of bacterial community in reservoir A was more complex. In contrast, reservoir B provided a suitable and temperate water environment conditions for bacterial growth, which resulted in higher community diversity and less co-exclusion correlations. The molecular ecological network indicated that the presence of dominant bacterial community in each of the reservoir were significant different. These differences mainly reflected the responses of bacterial community to the variations of environmental variables. Although Synechococcus was the dominant cyanobacterial species in both reservoirs, it exhibited co-occurrence patterns with different heterotrophic bacteria between reservoirs. In addition, the cyanobacteria-heterotrophic bacteria interaction exhibited highly dynamic variations, which was affected by nutrition and survive space. Also, the co-occurrence of Microcystis and Pseudanabaena found in reservoir B implied that the non-N-fixing Microcystis accompanied with N-fixing Pseudanabeana occurrence in freshwater lakes, so as to better meet the demand for nitrogen source.

Biotransformation of Sulfluramid (N-ethyl perfluorooctane sulfonamide) and dynamics of associated rhizospheric microbial community in microcosms of wetland plants.

Although the use of Sulfluramid (N-ethyl perfluorooctane sulfonamide (N-EtFOSA)) has been restricted by the Stockholm Convention, it is still frequently detected in the environmental matrices and in use in some countries. Employing constructed wetlands as treatment systems requires understanding of the biodegradation process in the rhizosphere and the effect of contaminants on the microbes of wetlands. This study aimed to investigate the interactions between the microbial community and N-EtFOSA under aerobic and anaerobic conditions. Aerobic biotransformation of N-EtFOSA occurred with a half-life of 0.51 day and yielded 85.1 mol% PFOS of after 91 days. Kinetic modelling revealed that cleavage of the SN was the rate-limiting degradation step. Biotransformation was not observed under anaerobic and anoxic conditions, suggesting that N-EtFOSA is recalcitrant to biodegradation without dissolved oxygen. Under aerobic condition, the presence of N-EtFOSA and its biotransformation products decreased the microbial richness and diversity and exerted selective pressure on the microbial community. Enrichment of Methylocaldum was significant (49%) in the presence of N-EtFOSA compared to unexposed conditions (11%), suggesting that Methylocaldum is relatively tolerant to N-EtFOSA and potentially degrading N-EtFOSA. Under anaerobic conditions, the microbial richness and diversity were not significantly altered by the presence of N-EtFOSA. Only Methanomethylovorans increased significantly in the spiked microcosm (30% vs. 20%). These findings provide knowledge for comprehending the contribution of N-EtFOSA to other PFASs in various environmental conditions, information about microbial community changes in response to PFASs and robust microbial species which can degrade N-EtFOSA in the environment.

Variations of Bacterial Community Composition and Functions in an Estuary Reservoir during Spring and Summer Alternation.

In this study, we focused on the dynamics of bacterial community composition in a large reservoir in the Yangtze estuary during spring and summer seasons, especially the variations of functional mechanisms of microbial community during the seasonal alternation between spring and summer. Both 16S rRNA gene sequencing and shotgun metagenomic sequencing technology were used for these purposes. The results indicated that obvious variations of bacterial community structures were found at different sites. Particle-associated bacterial taxa exhibited higher abundance at the inlet site, which was closer to the Yangtze River with a high level of turbidity. In other sites, Synechococcus, as the most dominant cyanobacterial species, revealed high abundance driven by increased temperature. Moreover, some heterotrophic bacterial taxa revealed high abundance following the increased Synechococcus in summer, which indicated potential correlations about carbon source utilization between these microorganisms. In addition, the shotgun metagenomic data indicated during the period of seasonal alternation between spring and summer, the carbohydrate transport and metabolism, energy production and conversion, translation/ribosomal biogenesis, and cell wall/membrane/envelope biogenesis were significantly enhanced at the exit site. However, the course of cell cycle control/division was more active at the internal site.

Geospatial distribution of viromes in tropical freshwater ecosystems.

This study seeks to understand the general distribution of virome abundance and diversity in tropical freshwater ecosystems in Singapore and the geospatial distribution of the virome under different landuse patterns. Correlations between diversity, environmental parameters and land use patterns were analyzed and significant correlations were highlighted. Overall, the majority (65.5%) of the annotated virome belonged to bacteriophages. The percentage of Caudovirales was higher in reservoirs whereas the percentages of Dicistroviridae, Microviridae and Circoviridae were higher in tributaries. Reservoirs showed a higher Shannon-index virome diversity compared to upstream tributaries. Land use (urbanized, agriculture and parkland areas) influenced the characteristics of the virome distribution pattern. Dicistroviridae and Microviridae were enriched in urbanized tributaries while Mimiviridae, Phycodnaviridae, Siphoviridae and Podoviridae were enriched in parkland reservoirs. Several sequences closely related to the emerging zoonotic virus, cyclovirus, and the human-related virus (human picobirnavirus), were also detected. In addition, the relative abundance of PMMoV (pepper mild mottle virus) sequences was significantly correlated with RT-qPCR measurements (0.588 < r < 0.879, p < 0.05). This study shows that spatial factors (e.g., reservoirs/tributaries, land use) are the main drivers of the viral community structure in tropical freshwater ecosystems.

Seasonal variation in the bacterial community composition of a large estuarine reservoir and response to cyanobacterial proliferation.

This study employed high-throughput sequencing (HTS) to understand the variations in microbial community in the largest estuary reservoir located at the Yangtze River for a year. Correlations between the heterotrophic bacterial and cyanobacterial communities in the estuarine ecosystem were also investigated. Significant spatial and temporal changes were observed in the microbial community composition at all sites. These differences were mainly reflected on the variations of bacterial relative abundance. The modularity analysis on the network indicate that bacterial community response to the variations of environmental factors in the form of co-occurrence/exclusion patterns. In warm season, Synechococcus spp. being the dominant Cyanobacteria taxa exhibited high relative abundance in the reservoir. Water temperature was the critical driver for the proliferation of Synechococcus. Moreover, heterotrophic bacteria belonging to Actinobacteria, Proteobacteria (α-, ß-, and γ-Proteobacteria), Bacteroidetes and Chlorobi, exhibited positive correlations with Synechococcus. The co-occurrence of these bacterial OTUs suggests that specific taxa may benefit from the proliferation of Synechococcus. In cold season, bacterial OTUs belonging to Actinobacteria and Bacteroidetes shown co-occurrence pattern with salt ions (including K+, Na+, Mg2+, Ca2+, Cl- and SO42-) inside the reservoir. In conclusion, further research is required to investigate the ecological functions of these taxa in estuarine ecosystems.

Genomics insights into production of 2-methylisoborneol and a putative cyanobactin by Planktothricoides sp. SR001.

Planktothricoides is a free-living filamentous cyanobacterium belonging to the order Oscillatoriales and the family Phormidiaceae, capable of forming bloom in fresh and brackish waters. A unicyanobacterial non-axenic culture dominated by Planktothricoides sp. SR001 was obtained from a freshwater reservoir in Singapore. The draft genome presented here is the first tropical freshwater Planktothricoides sp. ever sequenced. The genome of 7.0Mbp contains 5,776 genes predicted using the JGI IMG pipeline. The whole genome sequence allows identification of genes encoding for nitrogen-fixation, accessory photosynthetic pigments and biosynthesis of an off-flavor compound, 2-methylisoborneol, which has been experimentally verified here based on metabolite detection. In addition, strain SR001 genome contains an operon putatively involved in the production of a linear tripeptide cyanobactin related to viridisamide A and aeruginosamide, with the later known to possess anti-microbial or cytotoxic effect.

Relationship of Microbiota and Cyanobacterial Secondary Metabolites in Planktothricoides-Dominated Bloom.

The identification of phytoplankton species and microbial biodiversity is necessary to assess water ecosystem health and the quality of water resources. We investigated the short-term (2 days) vertical and diel variations in bacterial community structure and microbially derived secondary metabolites during a cyanobacterial bloom that emerged in a highly urbanized tropical reservoir. The waterbody was largely dominated by the cyanobacteria Planktothricoides spp., together with the Synechococcus, Pseudanabaena, Prochlorothrix, and Limnothrix. Spatial differences (i.e., water depth) rather than temporal differences (i.e., day versus night) better-explained the short-term variability in water quality parameters and bacterial community composition. Difference in bacterial structure suggested a resource-driven distribution pattern for the community. We found that the freshwater bacterial community associated with cyanobacterial blooms is largely conserved at the phylum level, with Proteobacteria (ß-proteobateria), Bacteroidetes, and Actinobacteria as the main taxa despite the cyanobacterial species present and geographical (Asia, Europe, Australia, and North America) or climatic distinctions. Through multivariate statistical analyses of the bacterial community, environmental parameters, and secondary metabolite concentrations, we observed positive relationships between the occurrences of cyanobacterial groups and off-flavor compounds (2-methyisoborneol and ß-ionone), suggesting a cyanobacterial origin. This study demonstrates the potential of 16S rRNA gene amplicon sequencing as a supporting tool in algal bloom monitoring or water-resource management.

Draft Genome Sequence of a Tropical Freshwater Cyanobacterium, Limnothrix sp. Strain P13C2.

A nonaxenic unialgal culture of Limnothrix sp. strain P13C2 was obtained through multiple subculturing of an inoculum obtained from a tropical freshwater lake. Here, we report the genome of P13C2 of 4.6 Mbp, extracted from the metagenome of this coculture.