Dominance and Growth Factors of Pseudanabaena sp. in Drinking Water Source Reservoirs, Southern China

Molecular mechanisms underlying sodium percarbonate treatment suppress the recovery and growth of Pseudanabaena sp. in early spring

Increasing frequency and intensity of cyanobacterial blooms in water sources is a growing global issue. Algicides are usually implemented in summer or autumn when blooms break out, however, the blooms will form again when algicide's concentration declines to a certain extent. Preventing the recovery and growth of cyanobacteria in early spring may be conducive to abatement of the blooms in summer or autumn. In this study solid sodium percarbonate (SPC) was used as an algicide to suppress recovery and growth of Pseudanabaena sp., a common odour-producing cyanobacterium, in early spring (12 °C). Results showed that 3.0 and 6.0 mg/L SPC were able to kill most of the algal cells after 12 h treatment at 12 °C, and the residual cells gradually died during the re-cultivation period at 25 °C. As a control, although SPC also caused most of algal cells to lyse at 25 °C, regrowth of cells was found during the period of re-cultivation at 25 °C. Transcriptomic analysis revealed that the dysregulated genes were strongly associated with translation and photosynthesis after SPC treatment. All differentially expressed unigenes related to translation and photosynthesis were down-regulated after SPC oxidation at 12 °C, whereas key genes associated with translation and photosynthesis were upregulated after SPC treatment at 25 °C.

Evaluating a river's ecological health: A multidimensional approach

Evaluating the health of river surface water is essential, as rivers support significant biological resources and serve as vital drinking water sources. While the Water Quality Index (WQI) is commonly employed to evaluate surface water quality, it fails to consider biodiversity and does not fully capture the ecological health of rivers. Here we show a comprehensive assessment of the ecological health of surface water in the lower Yangtze River (LYR), integrating chemical and biological metrics. According to traditional WQI metrics, the LYR's surface water generally meets China's Class II standards. However, it also contains 43 high-risk emerging contaminants; nitrobenzenes are found at the highest concentrations, representing 25-90% of total detections, while polycyclic aromatic hydrocarbons present the most substantial environmental risks, accounting for 81-93% of the total risk quotient. Notably, the plankton-based index of biological integrity (P-IBI) rates the ecological health of the majority of LYR water samples (59.7%) as 'fair', with significantly better health observed in autumn compared to other seasons (p < 0.01). Our findings suggest that including emerging contaminants and P-IBI as additional metrics can enhance the traditional WQI analysis in evaluating surface water's ecological health. These results highlight the need for a multidimensional assessment approach and call for improvements to LYR's ecological health, focusing on emerging contaminants and biodiversity rather than solely on reducing conventional indicators.

Effect of Culture Temperature on 2-Methylisoborneol Production and Gene Expression in Two Strains of Pseudanabaena sp

The presence of the odorant 2-methylisoborneol (2-MIB) in drinking water sources is undesirable. Although 2-MIB production is known to be influenced by temperature, its regulation at the gene level and its relationship with Chlorophyll-a (Chl-a) at different temperatures remain unclear. This study investigates the impact of temperature on 2-MIB production and related gene expression in Pseudanabaena strains PD34 and PD35 isolated from Lake Paldang, South Korea. The strains were cultured at three temperatures (15, 25, and 30 °C) to examine cell growth, 2-MIB production, and mic gene expression levels. 2-MIB production per cell increased with higher temperatures, whereas mic gene expression levels were higher at lower temperatures, indicating a complex regulatory mechanism involving post-transcriptional and enzyme kinetics factors. Additionally, the relationship between Chl-a and 2-MIB involved in metabolic competition was analyzed, suggesting that high temperatures appear to favor 2-MIB synthesis more than Chl-a synthesis. The distinct difference in the total amount of the two products and the proportion of 2-MIB between the two strains partially explains the variations in 2-MIB production. These findings highlight the significant effect of temperature on 2-MIB biosynthesis in Pseudanabaena and provide a valuable background for gene data-based approaches to manage issues regarding 2-MIB in aquatic environments.

Spatio-temporal dynamics of phytoplankton in a diversion reservoir and the major influencing factors: taxonomic versus functional groups classification

Identifying factors affecting phytoplankton dynamics is crucial to the management of aquatic ecosystems. A lot of scholars have conducted intensive studies on phytoplankton in lake or reservoirs, but not many studies have been conducted on diversion reservoirs. To explore the seasonal and spatial variation of phytoplankton communities and their relationship with environmental factors in the context of water diversion, a case study was carried out at XiKeng (XK) reservoir in South China. In this study, month-by-month water samples and phytoplankton were collected from this reservoir from December, 2021, to July, 2022. The results showed that the phytoplankton community was characterized by significant spatial and temporal variations. There were significant differences in phytoplankton abundance and structure in the reservoirs in terms of time. The abundance of phytoplankton cells and the proportion of Cyanobacteria in the reservoir showed a trend of increasing from autumn to spring and then decreasing from spring to summer, while the functional group evolved from S1 in autumn to SN in spring and summer. The abundance of phytoplankton was influenced by the dynamic water division and the characteristics of the reservoir itself, resulting in a spatial distribution characteristic of AIII > AII > AI. Water temperature (WT) and nutrients were the key factors driving the changes in phytoplankton abundance and community structure in the reservoir. These findings will deepen our understanding of the spatial and temporal dynamics of phytoplankton community structure in diversion reservoirs and provide a basis for freshwater water ecological management strategies.

MIB-derived odor management based upon hydraulic regulation in small drinking water reservoirs: Principle and application

The musty odorant (2-methylisoborneol, MIB) is prevalent in source water reservoirs and has become one of the major challenges for drinking water quality. This study proposes an approach to control the growth of MIB-producing cyanobacteria in a small reservoir based on hydraulic regulation, according to the results of long-term field investigations, laboratory culture experiments, model construction, and field application. Field investigations found that longer hydraulic retention time (HRT) is a factor that triggers MIB episodes. The culture study revealed that the maximum cell density, growth rate of MIB-producing Planktothricoides raciborskii, and MIB concentration are determined by the HRT (R2= 0.94, p-value < 0.001) and can be minimized by decreasing the HRT to less than 10 d. On this basis, an HRT regulation model was constructed and validated by field investigation, and critical HRT values were evaluated for 14 cyanobacteria genera. By decreasing the HRT to 5.4 ± 0.8 d, which is lower than the critical value of 7.5 ∼ 15.0 d, an MIB episode was successfully terminated in ZXD Reservoir in 2021. The results suggest that the proposed principle can provide a scientific basis for HRT regulation, which has been proved to be effective and feasible. This approach avoids negative impacts on water quality, does not require extra investment in engineering infrastructure, and in some cases may be applied readily by changing existing operational procedures. Therefore, HRT-based regulation is a promising strategy targeting MIB control and possibly for other cyanobacterial-derived water quality problems in small reservoirs.

A novel cyanolytic bacterium, Pseudomonas fluorescens BG-E as a potential biological control agent for freshwater bloom-forming cyanobacteria Pseudanabaena spp

The majority of bacterial antagonists identified to date are active against Microcystis. Therefore, this study aimed to isolate and characterize novel cyanolytic bacterial strains antagonistic against bloom-forming filamentous cyanobacteria. The bacterial strain BG-E isolated from the Bandagiriya Wewa in Sri Lanka was identified as Pseudomonas fluorescens (MZ007859) based on the 16S rRNA gene sequencing. BG-E showed 82% and 73% cyanolytic activity (CA) against Pseudanabaena sp. LW2 (MW288948) and Pseudanabaena lonchoides LW1 (MW288940), respectively, after 10 days of inoculation. The light microscopic images affirmed the complete disintegration in the filamentous structures of the tested Pseudanabaena species. The bacterial cell density of 15% v/v showed the CA with 95% and 89% cell lysis, respectively, in P. lonchoides and Pseudanabaena sp. LW2. Moreover, the results showed that >50% CA could be achieved by 0.100 and 1.00 (OD₇₃₀ ) cell densities for these same species. The highest CA of the cell-free supernatant of BG-E against P. lonchoides and bacterial culture against Pseudanabaena sp. LW2 illustrated the species-specific mode of action of BG-E. Although BG-E efficiently lysed the tested cyanobacterial species, the results of the MC-biodegradation assay confirmed its inability to degrade MC-LR cyanotoxin. Further, the BG-E strain lacks the mlrABCD gene cluster which is known to be responsible for the enzymatic degradation of MCs. The overall findings highlighted the applicability of P. fluorescens BG-E as a biological controlling agent to terminate blooms of freshwater filamentous cyanobacteria genus Pseudanabaena. The incorporation of cyanotoxin-degrading heterotrophic bacteria is recommended as a means of controlling toxic Pseudanabaena blooms.

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.

Biophotoelectrochemical process co-driven by dead microalgae and live bacteria

Anaerobic reduction processes in natural waters can be promoted by dead microalgae that have been attributed to nutrient substances provided by the decomposition of dead microalgae for other microorganisms. However, previous reports have not considered that dead microalgae may also serve as photosensitizers to drive microbial reduction processes. Here we demonstrate a photoelectric synergistic linkage between dead microalgae and bacteria capable of extracellular electron transfer (EET). Illumination of dead Raphidocelis subcapitata resulted in two-fold increase in the rate of anaerobic bioreduction by pure Geobacter sulfurreducens, suggesting that photoelectrons generated from the illuminated dead microalgae were transferred to the EET-capable microorganisms. Similar phenomena were observed in NO3- reduction driven by irradiated dead Chlorella vulgaris and living Shewanella oneidensis, and Cr(VI) reduction driven by irradiated dead Raphidocelis subcapitata and living Bacillus subtilis. Enhancement of bioreduction was also seen when the killed microalgae were illuminated in mixed-culture lake water, suggesting that EET-capable bacteria were naturally present and this phenomenon is common in post-bloom systems. The intracellular ferredoxin-NADP+-reductase is inactivated in the dead microalgae, allowing the production and extracellular transfer of photoelectrons. The use of mutant strains confirmed that the electron transport pathway requires multiheme cytochromes. Taken together, these results suggest a heretofore overlooked biophotoelectrochemical process jointly mediated by illumination of dead microalgae and live EET-capable bacteria in natural ecosystems, which may add an important component in the energetics of bioreduction phenomena particularly in microalgae-enriched environments.

Driving forces for the growth of MIB-producing Planktothricoides raciborskii in a low-latitude reservoir

In comparison with the middle- and high-latitude regions, the low-latitude regions are less associated with the occurrence of 2-methylisoborneol (MIB) episodes, since most of the previously identified MIB producers favor moderate/low light/temperature conditions. Here, we report a serious MIB outbreak over the period from Jul. 2018 to Jun. 2019 in a low-latitude reservoir with a mean annual water temperature of 25.6 °C. The MIB episode lasted for a long period, from Jul. 2018 to Jan. 2019, and Planktothricoides raciborskii was confirmed to be the main MIB producer. The growth characteristics of P. raciborskii were explored through both laboratory culturing and on-site verification experiments. The results indicated that this strain was not nutrient-sensitive at TN > 800 μg L^-1 and TP > 10 μg L^-1, but favored moderate light intensity (54 μmol photon m^-2·s^-1) and high temperature (30 °C). The two bloom-forming genera, Limnothrix and Aphanizomenon, favoring lower temperature and similar or relatively higher light intensity, showed much greater proliferation, about 13 folds (Limnothrix) and 58 folds (Aphanizomenon), from Dec. to Jun.; by contrast, the high water temperature (29.9 ± 2.8 °C) and light intensity (189.1 ± 87.6 μmol photon m^-2·s^-1) from Jul. to Nov. were not favorable to Limnothrix or Aphanizomenon, which might have created an opportunity for the growth of MIB-producing P. raciborskii. In addition, we also found that high temperature could promote the release of MIB from P. raciborskii cells, therefore exerting increased pressure on drinking water treatment processes.

Save reservoirs of humid subtropical cities from eutrophication threat

Reservoir water is the most important freshwater resource for many cities, especially in densely populated humid subtropical areas. Economic growth, population increase, and urbanization have been putting reservoir water of Shenzhen (China), a humid subtropical city, under severe threat of eutrophication and water supply shortage. In this study, we focused on an upstream reservoir of Shenzhen and established a 3-dimensional hydrodynamic-ecological model to investigate the water dynamics and nutrient budget. Tributaries to the reservoir were identified as the greatest contributors to nitrogen and phosphorus loads. Zones with weak flows and high nutrient concentration have high risks of causing blooms. Several mitigation measures were proposed, including improving flow by adding additional water exit locations in the reservoir, reducing nutrients in tributaries, and enhancing algal predation, and were evaluated with the established model. The strategies combining hydrodynamic improvement and phosphorus reduction were suggested to decision makers and government managers for short-term management. However, for future water safety, excessive nitrogen is a potential danger. This study provides a modeling framework that can be applied to anthropogenic-influenced reservoirs elsewhere.

2-Methylisoborneol (2-MIB) Excretion by Pseudanabaena&nbsp;yagii under Low Temperature

Outbreaks of 2-methylisoborneol (2-MIB) contamination in drinking water sources cause inconvenient odor issues in the water distribution system. In this study, microscopy-based isolation with physiological and molecular phylogenetic characterization were performed to investigate and characterize the 2-MIB odor producers that caused an odor problem in the freshwater system of the North Han River in the autumn of 2018. A benthic cyanobacterium was isolated from 2-MIB odor-issue freshwater samples and was found to be phylogenetically affiliated with Pseudanabaena yagii (99.66% sequence similarity), which was recorded in South Korea for the first time. The 2-MIB synthesis gene sequences from the odor-issue freshwater samples showed 100% similarity with those in the P. yagii strains. Protein sequences of 2-MIB synthase observed in the genome of the isolated strain showed structural and functional characteristics similar to those observed in other Pseudanabaena species. The 2-MIB production rate increased slowly during mat formation on the vessel wall; however, it rapidly increased after the temperature dropped. The 2-MIB gene was continuously expressed regardless of the temperature changes. These results suggest that the 2-MIB odor issue in the North Han River might be caused by the release of 2-MIB from the mat-forming P. yagii species in a low-temperature freshwater environment.

Whole-genome characterization and comparative genomics of a novel freshwater cyanobacteria species: Pseudanabaena punensis

Cyanobacteria are emerging as a potential source of novel, beneficial bioactive compounds. However, some cyanobacteria species can harm water quality and public health through the production of toxins. Therefore, surveying the occurrence and generating genomic resources of cyanobacteria producing harmful compounds could help develop the control methods necessary to manage their growth and limit the release contaminants into the water bodies. Here, we describe a novel strain, Pseudanabaena punensis isolated from the open ends of pipelines supplying freshwater. This isolate was characterized morphologically, biochemically and by whole-genome sequence analysis. We also provide genomic information for P. punensis to help understand and highlight the features unique to this isolate. Morphological and genetic (analysis using 16S rRNA and rbcL genes) data were used to assign this novel strain to phylogenetic and taxonomic groups. The isolate was identified as a filamentous and non-heterocystous cyanobacteria. Based on morphological and 16S rRNA phylogeny, this isolate shares characteristics with the Pseudanabaenaceae family, but remains distinct from well-characterized species suggesting its polyphyletic assemblage. The whole-genome sequence analysis suggests greater genomic and phenotypic plasticity. Genome-wide sequence and comparative genomic analyses, comparing against several closely related species, revealed diverse and important genes associated with synthesizing bioactive compounds, multi-drug resistance pathway, heavy metal resistance, and virulence factors. This isolate also produces several important fatty acids with potential industrial applications. The observations described in this study emphasize both industrial applications and risks associated with the freshwater contamination, and therefore genomic resources provided in this study offer an opportunity for further investigations.

Environmental factors associated with cyanobacterial assemblages in a mesotrophic subtropical plateau lake: A focus on bloom toxicity

Harmful algal blooms caused by cyanobacteria have been increasing in frequency worldwide. However, the main environmental drivers of this change are often difficult to identify because of the effects of the interaction between eutrophication and climate change. Recently, filamentous N₂-fixing cyanobacteria and non-diazotrophic Microcystis have been observed to be co-existing and undergoing succession in some eutrophic lakes. However, the succession patterns of dominant cyanobacteria and the factors driving this in mesotrophic lakes are not well understood. We hypothesized that the changes in cyanobacterial assemblages in mesotrophic lakes could result in a relatively high risks of toxic blooms, and that these changes are associated with the global climatic changes. We tested these hypotheses using data from the subtropical mesotrophic Lake Erhai. We found that the high spatiotemporal variability in the cyanobacterial community, and the increase in biomass were driven primarily by the growth of bloom-forming cyanobacterial taxa. Species-specific biomasses were related to a different environmental stressor; increases in dissolved organic carbon (DOC) concentrations were statistically associated with an increase of Microcystis biomass, whereas increases in surface water temperature favored higher biomass of Pseudanabaena at low transparency and high concentration of phosphorus. In addition, low nitrogen- to- phosphorus ratios were identified as potential determinants of the abundance of N₂-fixing Dolichospermum. Furthermore, changes in the concentration of DOC, total nitrogen, pH and water transparency levels were found to affect the composition of Microcystis morphotypes and genotypes mostly. This study highlights that the toxic to non-toxic Microcystis ratio might increase with the water darkening and browning (which occurs in many subtropical plateau lakes). Lake management strategies, therefore, need to consider the toxicity of cyanobacterial assemblages in mesotrophic lakes over the intensity of the cyanobacterial blooms.

Contrasting patterns of 2-methylisoborneol (MIB) vs. geosmin across depth in a drinking water reservoir are mediated by cyanobacteria and actinobacteria

Taste and odor episodes caused by off-flavor secondary metabolites, such as 2-methylisoborneol (MIB) and geosmin, pose one of the greatest challenges for drinking water utilities around the world. The prevalence of these compounds is predicted to increase in the future as a function of nutrient enrichment and elevated temperatures of surface drinking water sources. We conducted a manipulative field experiment in a drinking water reservoir to elucidate patterns for two taste and odor compounds, MIB and geosmin, as well as two taxa known to produce these compounds, phytoplankton (more specifically, cyanobacteria) and actinobacteria, across different depths in response to nutrient enrichment with two common dissolved nitrogen forms, organic urea or inorganic nitrate. In general, we found that MIB levels increased by greater than 250% with nutrient enrichment mediated by increased phytoplankton biomass. However, the effect of the fertilization treatments on MIB decreased with depth with a 35% reduction at 7 m versus 1.5 m. In contrast, geosmin levels reached a maximum at the lowest measured depth (7 m), were unaffected by the fertilization treatments, and followed a similar pattern to the abundance of actinobacteria. Thus, our data suggest that the positive response of phytoplankton (e.g., cyanobacteria, such as Oscillatoria species) to the fertilization treatments is likely responsible for increased MIB, while geosmin concentrations may be a function of actinobacteria-mediated decomposition in the hypolimnion in our study system.

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.