Discerning biodegradation and adsorption of microcystin-LR in a shallow semi-enclosed bay and bacterial community shifts in response to associated process

Elucidating microbial mechanisms of microcystin-LR degradation in Lake Erie beach sand through metabolomics and metatranscriptomics

As one of five Laurentian Great Lakes, Lake Erie ranks among the top freshwater drinking sources and ecosystems globally. Historical and current agriculture mismanagement and climate change sustains the environmental landscape for late summer cyanobacterial harmful algal blooms, and consequently, cyanotoxins such as microcystin (MC). Microcystin microbial degradation is a promising mitigation strategy, however the mechanisms controlling the breakdown of MCs in Lake Erie are not well understood. Pelee Island, Ontario, Canada is located in the western basin of Lake Erie and the bacterial community in the sand has demonstrated the capacity of metabolizing the toxin. Through a multi-omic approach, the metabolic, functional and taxonomical signatures of the Pelee Island microbial community during MC-LR degradation was investigated over a 48-hour period to comprehensively study the degradation mechanism. Cleavage of bonds surrounding nitrogen atoms and the upregulation of nitrogen deamination (dadA, alanine dehydrogenase, leucine dehydrogenase) and assimilation genes (glnA, gltB) suggests a targeted isolation of nitrogen by the microbial community for energy production. Methylotrophic pathways RuMP and H4MPT control assimilation and dissimilation of carbon, respectively and differential abundance of Methylophilales indicates an interconnected role through electron exchange of denitrification and methylotrophic pathways. The detected metabolites did not resolve a clear breakdown pathway, but rather the diversity of products in combination with taxonomic and functional results supports that a variety of strategies are applied, such as epoxidation, hydroxylation, and aromatic degradation. Annual repeated exposure to the toxin may have allowed the community to adaptatively establish a novel pathway through functional plasticity and horizontal gene transfer. The culmination of these results reveals the complexity of the Pelee Island sand community and supports a dynamic and cooperative metabolism between microbial species to achieve MC degradation.

Environmental risk of microplastics after field aging: Reduced rice yield without mitigating yield-scale ammonia volatilization from paddy soils

Microplastics (MPs, <5 mm) are enriched in paddy ecosystems as emerging environmental pollutants. Biochar (BC) is a controversial recalcitrant carbon product that poses potential environmental risks. The presence of these two exogenous organic substances has been demonstrated to have impacts on soil nitrogen cycling and crop production. However, the after-effects of MPs and BC on soil ammonia (NH₃) volatilization and rice yield after field aging remain unexplored. In this study, two common MPs, including polyethylene (PE) and polyacrylonitrile (PAN), and BC were selected for rice growing season observations to study the impacts on soil NH₃ volatilization and rice yield after field aging. The results showed that the reduction of cumulative soil NH₃ losses by MPs was around 45% after one-year field aging, which was within the range of 40-57% in the previous rice season. Abatement of NH₃ volatilization by MPs mainly occurred in basal fertilization and was related to floodwater pH. Besides, the reduction rate of NH₃ volatilization by BC and MPs + BC was enhanced after field aging (63% and 50-57%) compared to that in the previous rice season (5% and 11-19%), with the abatement process occurring in the first supplementary fertilization. There was a significant positive correlation between cumulative NH₃ volatilization and soil urease activity. Notably, field aging removed the positive effect of MPs and MPs + BC in reducing yield-scale NH₃ losses in the previous rice season (∼62%). Furthermore, despite BC affecting rice yield insignificantly after field aging, the presence of MPs led to a significant 17-19% reduction in rice yield. Our findings reveal that differences in the after-effects of BC and MPs in field aging emerge, where the negative impacts of MPs on soil NH₃ abatement and crop yield are progressively becoming apparent and should be taken into serious consideration.

Microcystin drives the composition of small-sized bacterioplankton communities from a coastal lagoon

Cyanobacterial blooms affect biotic interactions in aquatic ecosystems, including those involving heterotrophic bacteria. Ultra-small microbial communities are found in both surface water and groundwater and include diverse heterotrophic bacteria. Although the taxonomic composition of these communities has been described in some environments, the involvement of these small cells in the fate of environmentally relevant molecules has not been investigated. Here, we aimed to test if small-sized microbial fractions from a polluted urban lagoon were able to degrade the cyanotoxin microcystin (MC). We obtained cells after filtration through 0.45 as well as 0.22 μm membranes and characterized the morphology and taxonomic composition of bacteria before and after incubation with and without microcystin-LR (MC-LR). Communities from different size fractions (< 0.22 and < 0.45 μm) were able to remove the dissolved MC-LR. The originally small-sized cells grew during incubation, as shown by transmission electron microscopy, and changed in both cell size and morphology. The analysis of 16S rDNA sequences revealed that communities originated from < 0.22 and < 0.45 μm fractions diverged in taxonomic composition although they shared certain bacterial taxa. The presence of MC-LR shifted the structure of < 0.45 μm communities in comparison to those maintained without toxin. Actinobacteria was initially dominant and after incubation with MC-LR Proteobacteria predominated. There was a clear enhancement of taxa already known to degrade MC-LR such as Methylophilaceae. Small-sized bacteria constitute a diverse and underestimated fraction of microbial communities, which participate in the dynamics of MC-LR in natural environments.

Efficient Microcystis removal and sulfonamide-resistance gene propagation mitigation by constructed wetlands and functional genes analysis

Increasingly prevalent Microcystis blooms and the propagation of the associated resistance genes represent global environmental problems. Constructed wetlands (CWs) are a cost-effective technology used for wastewater treatment. In this study, the herb Alisma orientale and three industrial byproducts, namely, blast furnace slag, biochar, and sawdust, were selected to construct mini-CW units. Their potential to remediate toxic Microcystis and their influences on the behaviors of antibiotic-resistant genes (ARGs, sul1, sul2, and intl1) were analyzed. Approximately 98.46% of Microcystis cells were removed by the sawdust-based CW in just 2 d, wherein <0.37 μg/L residual microcystin (MC)-LR was detected, with a removal efficiency of >96.47%, which is potentially caused by the higher relative abundance of MC-degrading gene mlrA on the substrate. Lower target ARG accumulations in the sawdust-based CW may be attributed to the lower intl1 relative abundance and microbial function mobile element content, which could influence horizontal gene transfer. In three sequential batches for the treatment of eutrophic lake water, six sawdust-based CW units were assembled into CW microcosms. The efficiency of removal of Microcystis and MC-LR by planted CW microcosms ranged between 92.00% and 95.88% and between 86.48% and 94.82%, respectively; this was significantly (P < 0.05) higher than that by unplanted ones. Less accumulation of target ARGs was also observed in planted CWs. Planting considerably improved nitrogen removal, possibly owing to the enrichment of genes involved in the KEGG nitrogen metabolism pathway in the substrate through metagenomic analysis.

Multiple pathways for the anaerobic biodegradation of microcystin-LR in the enriched microbial communities from Lake Taihu

Anaerobic biodegradation is a non-negligible elimination approach for microcystin (MC) pollution and exhibits important bioremediation potential for environmental problems. However, the specific anaerobic MC-degrading mechanism remains unclear and few functional bacteria have been found. In this study, three microbial communities of sludges from different locations in Lake Taihu were collected and further enriched by microcystin-LR (MC-LR) under anaerobic conditions. MC-LR (1 mg/L) could be completely degraded by these enriched microbial communities under anaerobic conditions, but their degradation rates were significantly different. In addition, two different ring-opening sites of MC-LR in Ala-Leu and Arg-Adda were observed, and three new anaerobic degradation products were first identified, including two hexapeptides (MeAsp-Arg-Adda-Glu-Mdha-Ala and Adda-Glu-Mdha-Ala-Leu-MeAsp) and one end-product pentapeptide (Glu-Mdha-Ala-Leu-MeAsp). Based on the chemical structures and temporal trends of all detected degradation products, two novel anaerobic biodegradation pathways of MC-LR were proposed. Moreover, the MC-degrading genes mlrABC were not detected among all microbial communities, which suggested that some new MC-degrading mechanisms might exist under anaerobic conditions. Finally, through the comparison of microbial community structure, Gemmatimonas and Smithella were deduced as possible anaerobic MC-degrading bacteria. These findings strongly indicate that anaerobic biodegradation is an important method of self-repair in the natural environment and provides a potential removal strategy for MC pollution.

Multi-Soil-Layering Technology: A New Approach to Remove Microcystis aeruginosa and Microcystins from Water

Eutrophication of surface waters caused by toxic cyanobacteria such as Microcystis aeruginosa leads to the release of secondary metabolites called Microcystins (MCs), which are heptapeptides with adverse effects on soil microbiota, plants, animals, and human health. Therefore, to avoid succumbing to the negative effects of these cyanotoxins, various remediation approaches have been considered. These techniques involve expensive physico-chemical processes because of the specialized equipment and facilities required. Thus, implementing eco-technologies capable of handling this problem has become necessary. Indeed, multi-soil-layering (MSL) technology can essentially meet this requirement. This system requires little space, needs simple maintenance, and has energy-free operation and high durability (20 years). The performance of the system is such that it can remove 1.16 to 4.47 log10 units of fecal contamination from the water, 98% of suspended solids (SS), 92% of biological oxygen demand (BOD), 98% of chemical oxygen demand (COD), 92% of total nitrogen (TN), and 100% of total phosphorus (TP). The only reported use of the system to remove cyanotoxins has shown a 99% removal rate of MC-LR. However, the mechanisms involved in removing this toxin from the water are not fully understood. This paper proposes reviewing the principal methods employed in conventional water treatment and other technologies to eliminate MCs from the water. We also describe the principles of operation of MSL systems and compare the performance of this technology with others, highlighting some advantages of this technology in removing MCs. Overall, the combination of multiple processes (physico-chemical and biological) makes MSL technology a good choice of cyanobacterial contamination treatment system that is applicable in real-life conditions, especially in rural areas.

Prevalence and persistence of microcystin in shoreline lake sediments and porewater, and associated potential for human health risk

Midlatitude waterbodies are experiencing increased cyanobacteria blooms that necessitate health advisories to protect waterbody users. Although surface waters may contain cyanotoxins such as microcystin (MC), at concentrations that pose potential public health risks, little is known about MC contamination of shoreline sediments. Based on growing evidence that lake and reservoir sediments can accumulate MCs, we hypothesized that shoreline sediments (i.e., recreational beaches) may accumulate MCs and thereby pose a potential health risk to recreational users even if people stay out of contaminated water. We sampled nearshore surface water, shoreline sediment, and porewater from seven Washington State, USA, lakes/reservoirs recreational beaches to determine MC presence/absence during or immediately following cyanobacteria blooms. We found MCs in shoreline sediments at all waterbodies using ELISA and LC-MS/MS. MC concentrations in shoreline sediments and porewaters persisted for 20 days following dissipation of cyanobacteria blooms when MC concentrations were near analytical reporting limits in corresponding surface waters. A human health risk assessment based on potential MC exposure through incidental ingestion of porewaters and sediments found, even when very high MC concentrations occur in surface waters (i.e., >11,000 μg/L), estimated ingestion doses are below MC World Health Organization tolerable daily intake and U.S. Environmental Protection Agency's risk reference dose. While our findings suggest MCs in Washington State recreational beaches in 2018 did not present a significant human health risk, future blooms with higher MC concentrations could pose human health risks via the shoreline sediment/porewater exposure pathway.

Investigating the microbial dynamics of microcystin-LR degradation in Lake Erie sand

Cyanobacterial blooms and the associated hepatotoxins produced (e.g., microcystins, MCs) create a significant human health risk in freshwater lakes around the world, including Lake Erie. Though various physical and chemical treatment options are utilized, these are costly and their effectiveness decreases when other organics are present. Laboratory studies have identified a remediation option based on a mlr gene operon that can systematically degrade this toxin; however, studies on Lake Erie have been unable to amplify mlr genes from MC-degrading bacteria. These results suggest that either existing primers may be inefficient for broad identification of the mlr genes or that MC degradation genes and/or pathways may vary among bacterial taxa. To investigate the dynamics of the Lake Erie microbial community involved in the degradation of microcystin-LR (MC-LR), a flow-through column experiment using collected beach sand was conducted over a period of six weeks. Increasing concentrations of lake water spiked with MC-LR were continuously delivered to both biotic and abiotic (sterilized) sand columns, with influent and effluent MC-LR concentrations measured by LC-MS/MS. Despite the toxin concentrations far exceeding natural conditions during a bloom event (maximum dosage = 15.4 μg/L), MC-LR was completely removed within 21 h of contact time in the biotic columns. Stimulation of community taxa during the degradation process included Burkholderiaceae, Illumatobacteraceae, Pseudomonadaceae, Rhodocyclaceae and Nitrosomonadaceae. The overall results suggest several critical species may be required for the most complete and effective degradation of MC-LR.

Functional and structural analyses for MlrC enzyme of Novosphingobium sp. THN1 in microcystin-biodegradation: Involving optimized heterologous expression, bioinformatics and site-directed mutagenesis

Enzymatic function of MlrC from Novosphingobium sp. THN1 (i.e., THN1-MlrC) towards linearized microcystins (MCs) and structural/physic-chemical properties of MlrC enzyme deserved urgent research, and heterologous expression (HE) optimization for MlrC is yet to be solved. This study achieved HE of THN1-MlrC by rapid-efficiently constructing HE system, and revealed that THN1-MlrC can degrade linearized MC-LR and linearized MC-RR to produce Adda, providing direct evidence for catalytic function of THN-MlrC and its ecological implication in MC-detoxification. Consequently, to maximize THN1-MlrC expression for production and application, induction conditions for HE of THN1-MlrC was optimized as at 0.1 mM of isopropyl-β-_D-thiogalactoside and 30 °C for 8 h, which could be widely applicable for heterologous production of other MlrC homologs. Using bioinformatics and site-mutation experiment, THN1-MlrC was evaluated as a cytoplasm-locating hydrophilic protein with theoretical isoelectric point of 5.57, and contained six verified active sites Glu³⁹, His¹³³, Asp¹⁶⁷, His¹⁶⁹, His¹⁹¹ and Asp³³² in two domains of its 3D structure, among which Glu³⁹, His¹³³ and Asp³³² were newly-discovered ones here. The Glu³⁹, His¹³³, Asp¹⁶⁷, His¹⁶⁹ and His¹⁹¹ gathered more closely in 3D structure than in amino acid sequence, while they and Asp³³² surrounded protein center to constitute a potential active pocket for mediating linearized MCs degradation. Due to MlrC sequence homology and conservative active sites, structural/physic-chemical characteristics of THN1-MlrC presented in this study provided a helpful reference for other MlrC homologs of diverse bacteria. This study shed novel insights for functional-structural relationships of THN1-MlrC during MC-biodegradation, and was crucial for research and practical applications in MC-decontamination.

Growth and Cellular Responses of Toxigenic Microcystis to Chloramphenicol-Stress at Various Environmentally-Relevant Nitrogen Levels

This study explored nitrogen (N)-dependent interaction between Microcystis and chloramphenicol (CAP) along 20 day-test. Results showed that 5 mg/L N largely alleviated inhibitory effects of CAP on Microcystis growth, while 50 and 0.5 mg/L N exacerbated growth-inhibition by CAP especially in early (before day 8) and mid-late stage, respectively. At each N level, CAP-induced antioxidant defense and cell damage extents were negatively correlated to growth state in each stage, and CAP-biodegradation coincided with Microcystis growth and glutathione synthesis dynamics, implying that antioxidant defense, cell damage and CAP-removal closely linked to N-dependent Microcystis growth under CAP-stress. Microcystin (MC)-production and -release under CAP-stress were also N-dependent. Although Microcystis growth was greatly-inhibited by prolonged CAP-stress at 0.5 mg/L N, delayed CAP-loss and high MC-release at 0.5 mg/L N should be emphasized during Microcystis-dominated cyanobacterial blooms (MCBs) and CAP co-occurrence. This study had great implication in risk assessment for MCBs-CAP co-occurrence in different waters.

Correlation between specific groups of heterotrophic bacteria and microcystin biodegradation in freshwater bodies of central Europe

Microcystins produced by several toxic cyanobacterial strains constitute an important problem for public health. Bacterial degradation of these hepatotoxins may play an important role in natural ecosystems, however the nature of the process is very poorly understood. The aim of our study was to investigate the possible interactions between cyanotoxin producers and degraders. Samples collected from 24 water bodies in western Poland were analysed to determine the chemo-physical parameters, phytoplankton content, bacterial community structure and microcystin-biodegradation potency. A redundancy analysis identified a positive correlation between the capacity of a community to degrade microcystin LR (MC-LR) and temperature, pH, chlorophyll a concentration and the abundance of MC-producers. The relative abundance of classes F38, TM7-3 and the order WCHB1-81c (Actinobacteria) was significantly higher in the lakes with MC-biodegradation potency. Some specific bacterial genera belonging to Acidobacteria, Chloroflexi, Gemmatimonadetes, Firmicutes and TM7 were closely correlated with the occurrence of Microcystis spp. Furthermore, the MC biodegradation process was connected with the same bacterial groups. Thus, our approach allowed us to provide a broader picture of some specific relations between microcystin producers and potential microcystin degraders. A more comprehensive analysis of the existing correlations may be helpful in our understanding of natural mechanisms of MC elimination using bacteria such as MC-degraders.

Functional role of bloom-forming cyanobacterium Planktothrix in ecologically shaping aquatic environments

Diverse metabolic behaviors endow microorganisms with various ecological functions, and metabolic activities of microbial species may affect the environmental conditions of their habitats. In this study, genome-guided analysis of Planktothrix spp. first divided these strains into six distinct groups, and comparisons of Planktothrix genomes revealed the inter- and intra-species variation. Prediction of central metabolism showed the functional diversity with regard to uptake of carbon, nitrogen, and sulfur sources. As the carbon-fixing microorganisms, Planktothrix isolates played a critical role in transforming the atmospheric carbon into organic carbon-the waterbodies' pool of available carbon. Diazotrophic lifestyle in certain Planktothrix strains may provide valuable avenues for supporting the equilibrium community. Furthermore, genome mining supported the exploration of biosynthetic gene clusters dedicated to cyanobacterial natural products, mainly including non-ribosomal peptide, polyketide, cyanobactin, and microviridin. Notably, some Planktothrix strains had the potential to non-ribosomally synthesize the microcystin (MC), a potent cyclic heptapeptide toxin, and MC-mediated cycling might strengthen the association between MC-producing and MC-degrading microorganisms. In short, genome-wide study of Planktothrix strains advances our current understanding of their metabolic potential and especially ecological roles in shaping natural environments.

Insight Into the Molecular Mechanisms for Microcystin Biodegradation in Lake Erie and Lake Taihu

Microcystins are potent hepatotoxins that are frequently detected in fresh water lakes plagued by toxic cyanobacteria. Microbial biodegradation has been referred to as the most important avenue for removal of microcystin from aquatic environments. The biochemical pathway most commonly associated with the degradation of microcystin is encoded by the mlrABCD (mlr) cassette. The ecological significance of this pathway remains unclear as no studies have examined the expression of these genes in natural environments. Six metatranscriptomes were generated from microcystin-producing Microcystis blooms and analyzed to assess the activity of this pathway in environmental samples. Seventy-eight samples were collected from Lake Erie, United States/Canada and Lake Tai (Taihu), China, and screened for the presence of mlr gene transcripts. Read mapping to the mlr cassette indicated transcripts for these genes were absent, with only 77 of the collective 3.7 billion reads mapping to any part of the mlr cassette. Analysis of the assembled metatranscriptomes supported this, with only distantly related sequences identified as mlrABC-like. These observations were made despite the presence of microcystin and over 500,000 reads mapping to the mcy cassette for microcystin production. Glutathione S-transferases and alkaline proteases have been previously hypothesized to be alternative pathways for microcystin biodegradation, and expression of these genes was detected across space and time in both lakes. While the activity of these alternative pathways needs to be experimentally confirmed, they may be individually or collectively more important than mlr genes in the natural environment. Importantly, the lack of mlr expression could indicate microcystin biodegradation was not occurring in the analyzed samples. This study raises interesting questions about the ubiquity, specificity and locality of microcystin biodegradation, and highlights the need for the characterization of relevant mechanisms in natural communities to understand the fate of microcystin in the environment and risk to public health.

Cyanobacterial bloom mitigation by sanguinarine and its effects on aquatic microbial community structure

Sanguinarine has strong inhibitory effects against the cyanobacterium Microcystis aeruginosa. However, previous studies were mainly limited to laboratory tests. The efficacy of sanguinarine for mitigation of cyanobacterial blooms under field conditions, and its effects on aquatic microbial community structure remain unknown. To elucidate these issues, we carried out in situ cyanobacterial bloom mitigation tests. Our results showed that sanguinarine decreased population densities of the harmful cyanobacteria Microcystis and Anabaena. The inhibitory effects of sanguinarine on these cyanobacteria lasted 17 days, after which the harmful cyanobacteria recovered and again became the dominant species. Concentrations of microcystins in the sanguinarine treatments were lower than those of the untreated control except during the early stage of the field test. The results of community DNA pyrosequencing showed that sanguinarine decreased the relative abundance of the prokaryotic microorganisms Cyanobacteria, Actinobacteria, Planctomycetes and eukaryotic microorganisms of Cryptophyta, but increased the abundance of the prokaryotic phylum Proteobacteria and eukaryotic microorganisms within Ciliophora and Choanozoa. The shifting of prokaryotic microbial community in water column was directly related to the toxicity of sanguinarine, whereas eukaryotic microbial community structure was influenced by factors other than direct toxicity. Harmful cyanobacteria mitigation efficacy and microbial ecological effects of sanguinarine presented in this study will inform the broad application of sanguinarine in cyanobacteria mitigation.

A comparative study of the effects of microbial agents and anaerobic sludge on microalgal biotransformation into organic fertilizer

The effects of Lactobacillus bulgaricus, Rhodopseudomonas palustris, Issatchenkia orientalis and anaerobic sludge on anaerobic digestion of microalgae to organic fertilizer were studied. High-throughput sequencing was used to analyze characteristics of microbial community structure during anaerobic digestion of microalgae using different inocula. Lactobacillales and Saccharomycetales were more likely to become dominant bacteria and eukaryotes. The relative abundance of Lactobacillales was 98.15%, 88.61% and 81.73% of total bacteria at the beginning, middle and end of the experiment, respectively. Meanwhile, the relative abundance of Saccharomycetales was 90.91%, 98.41% and 98.8% of eukaryotes at the beginning, middle and end of the experiment, respectively. At the end of digestion, the microcystin content in the reactor inoculated with Issatchenkia orientalis decreased to 0.71 μg/kg, which met drinking water standards. Rhodopseudomonas palustris did not become a dominant microorganism and had the most negative impact on the atmosphere. Volatile organic compounds were 11.92 mg/kg while the odor concentration reached 97,724 ou/m³. The organic matter content in reactors inoculated with specific groups of microbial agents, which was higher than the standard required for bio-organic fertilizer, occupying over 96% dry weight. In addition, the effective microorganism counts of Issatchenkia orientalis and Lactobacillus bulgaricus in fermentation products reached 1.8E+09 colony-forming units (cfu)/g and 1.6E+09 cfu/g, respectively, which are suitable values for microbial fertilizer.

Source Community and Assembly Processes Affect the Efficiency of Microbial Microcystin Degradation on Drinking Water Filtration Membranes

Microbial biofilms in gravity-driven membrane (GDM) filtration systems can efficiently degrade the cyanotoxin microcystin (MC), but it is unclear if this function depends on the presence of MC-producing cyanobacteria in the source water habitat. We assessed the removal of MC from added Microcystis aeruginosa biomass in GDMs fed with water from a lake with regular blooms of toxic cyanobacteria (ExpL) or from a stream without such background (ExpS). While initial MC removal was exclusively due to abiotic processes, significantly higher biological MC removal was observed in ExpL. By contrast, there was no difference in MC degradation capacity between lake and stream bacteria in separately conducted liquid enrichments on pure MC. Co-occurrence network analysis revealed a pronounced modularity of the biofilm communities, with a clear hierarchic distinction according to feed water origin and treatment type. Genotypes in the network modules associated with ExpS had significantly more links to each other, indicating that these biofilms had assembled from a more coherent source community. In turn, signals for stochastic community assembly were stronger in ExpL biofilms. We propose that the less "tightly knit" ExpL biofilm assemblages allowed for the better establishment of facultatively MC degrading bacteria, and thus for higher overall functional efficiency.

Phosphorus Influences the Interaction Between Toxigenic Microcystis and Chloramphenicol

Microcystis growth and physiological responses to chloramphenicol (CAP)-stress were explored at different phosphorus (P) concentrations during 20-day exposure. Under CAP-stress, Microcystis exhibited (i) stronger total protein synthesis and antioxidant defenses at 5 mg/L P than 0.05-0.5 mg/L P in early test period (before day 8), and (ii) greater CAP-removal via biodegradation at 5 mg/L P in mid-late period. Due to above mechanisms, 5 mg/L P largely alleviated the inhibitory effect of CAP on Microcystis growth until test end, thus minimizing CAP toxicity to Microcystis, compared with 0.05-0.5 mg/L P. Moreover, microcystin-production and -release by Microcystis under CAP-stress were also P-dependent. These results suggested that under CAP-stress, although Microcystis growth was more inhibited at 0.05-0.5 mg/L P, higher microcystin-release and CAP residual at 0.05-0.5 mg/L P than at 5 mg/L P still caused eco-risks, which had important implication for risk assessment during Microcystis-dominated blooms and CAP pollution co-occurrence in different waters.

Enhanced removal of Microcystis bloom and microcystin-LR using microcosm constructed wetlands with bioaugmentation of degrading bacteria

The prevalence of cyanobacterial bloom (Cyano-bloom) and hepatotoxic microcystin (MC) pollution caused by eutrophication poses serious problems to aquatic ecosystems and public health. However, conventional water treatment technologies are inefficient for removing cyanotoxins. In this study, the performance of microcosm constructed wetlands (CWs) in the removal of Cyano-bloom, microcystin-LR (MC-LR), and nutrients was investigated following repeated loading of pollutants. The effects of plant and bioaugmentation of selected MC-LR degrading bacteria on removal efficiency, degrading gene mlrA abundance, and bacterial community structure were examined. More than 90% of the MC-LR and chlorophyll-a was eliminated by CWs after 3 d of hydraulic retention time (HRT) without a lag phase. No significant differences between planted and unplanted CWs were found in the MC-LR and Cyano-bloom removal and mlrA gene abundance. Nevertheless, the plants improved nutrient removal to reduce eutrophication. Bioaugmentation markedly enhanced the degradation of MC-LR from 16.7 μg L^-1 to below the threshold value within 12 h, which could help shorten the HRT of CWs by increasing functional MC-LR degrading bacteria. In the soil of CWs, the following six bacterial genera with MC-LR-degrading potential were found: Sphingopyxis, Methylotenera, Pseudomonas, Methylosinus, Novosphingobium, and Sphingomonas. Among them, the first three also significantly proliferated in CWs with bioaugmentation during MC-LR degradation, indicating their high adaptability and MC-LR removal contribution. These results suggested that CWs could provide suitable conditions for MC-LR degrading microorganism proliferation, and CWs with bioaugmentation could be effective and practical measures for the remediation of eutrophication and MC pollution.

A DFT study of the interaction between [Cd(H2O)3]2+ and monodentate O-, N-, and S-donor ligands: bond interaction analysis

A series of B3LYP/6-311+G(d,p) calculations of the affinity of monodentate ligands for [Cd(H₂O)₃]²⁺ are performed. Three types of ligands containing O (phosphine oxide, lactam, amide, carboxylic acid, ester, ketone, aldehyde, ether, halohydrin, enol, furan), N (thiocyanate, amine, ammonia, azide), and S (thioester, thioketone, thiol, thiophene, disulfide) interacting atoms are investigated. The results show that phosphine oxide has the largest affinity for the cadmium cation due to the polarization of the P=O bond. As the P atom has a large atomic radius, the O atom can polarize the electronic cloud enhancing its amount of electronic charge and favoring the interaction with Cd²⁺. The affinity order found is phosphine oxide > thioester > lactam > amide > carboxylic acid > ester > thioketone > ketone > thiocyanate > amine > ammonia > aldehyde > ether > thiol > thiophene > enol > halohydrin > disulfide > azide > furan ligands. These results were also corroborated by the functional M06-2X. The electronic effects (resonance and induction) of neighboring groups of the interacting atom modulate the strength of metal-ligand binding. For almost all the O-donor ligands the electrostatic component has the same magnitude as the covalent term, while for the N- and S-donor ligands the covalent term is predominant. The polarization term accounts for twice the exchange term as part of the covalent component. The dispersion term varies less than 2 kcal mol^-1 for the complexes analyzed. The Pauli repulsion term is correlated with the metal ligand distance, increasing in the compounds with decreased metal-ligand bond length. The charge between the interacting atoms is also strongly correlated with both the interacting strength and the electrostatic interaction component. The natural bond orbital analysis highlights correlations of the bond order, and S and P contributions of the interacting metal-ligand orbital with the coordination strength. Graphical abstract The affinity of 20 monodentate ligands with different functional groups for the [Cd(H₂O)₃]²⁺ cation is calculated based on the interaction enthalpy and Gibbs free energy for the substitution of one water molecule from the fully hydrated cation. The affinity is correlated with geometric, electronic, and energetic parameters of the ligands and the complexes as well as with energy decomposition and natural bond order analyses results.

Heterologous expression of mlrA gene originated from Novosphingobium sp. THN1 to degrade microcystin-RR and identify the first step involved in degradation pathway

Information on the catalytic role of mlrA gene-encoded enzyme (MlrA) in microcystin-RR (MC-RR) biodegradation was limited. This study succeeded in expressing mlrA homolog of Novosphingobium sp. THN1 in heterologous host for the first time, by constructing a recombinant bacterium. Mass spectrometric analysis showed that the recombinant MlrA hydrolyzed MC-RR into linear intermediate product by cleaving the peptide bond between Adda and arginine residue, greatly detoxifying MC-RR. This finding clearly manifested that the MlrA homolog of THN1 strain possesses its original catalytic function, and ring-opening constituted the first step in MC-RR biodegradation pathway of THN1 strain. Moreover, MC-RR degradation by intact recombinant cells and cell-free crude enzyme (CE) from recombinant was compared. Results exhibited that intact recombinant was able to degrade 20 μg mL^-1 MC-RR more quickly than CE, with the maximum rate of 9.22 μg mL^-1 h^-1 in the first 8 h. Thus, this study provided new insights on the catalytic activity and roles of MlrA originated from THN1 strain in MC-RR biodegradation process, which lay a foundation for efficiently removing and detoxifying MC-RR, and exploring downstream steps in MC-RR biodegradation pathway of THN1 strain.

Current research scenario for microcystins biodegradation - A review on fundamental knowledge, application prospects and challenges

Microcystins (MCs) are common cyanotoxins produced by harmful cyanobacterial blooms (HCBs) and severely threaten human and ecosystems health. Biodegradation is an efficient and sustainable biological strategy for MCs removal. Many novel findings in fundamental knowledge and application potential of MC-biodegradation have been documented. Little effort has devoted to summarize and comment recent research progress on MC-biodegradation, and discuss the research problems and gaps. This review deals with current research scenario in aerobic and anaerobic biodegradation for MCs. Diverse organisms capable of degrading MCs are encapsulated. Enzymatic mechanisms and influence factors regulating aerobic and anaerobic MC-biodegradation are summarized and discussed, which are essential for assessing and reducing MC-risks during HCBs episodes. Also, we propose some ideas to solve the challenges and bottleneck problems in practical application of MC-biodegradation, and discuss research gaps and promising research methods which deserve special attention. This review may provide new insights on future direction of MC-biodegradation research, in order to further broaden its application prospects for bioremediation.