Inhibition of harmful algae Phaeocystis globosa and Prorocentrum donghaiense by extracts of coastal invasive plant Spartina alterniflora

Identification of algicidal monoterpenoids from four chemotypes of Cinnamomum camphora and their algicidal mechanisms on Microcystis aeruginosa

Cyanobacterial blooms cause serious environmental issues, and plant secondary metabolites are considered as new algaecide for controlling them. Cinnamomum camphora produces a wide spectrum of terpenoids and has 4 main chemotypes, including linalool, camphor, eucalyptol and borneol chemotype. To develop the new cyanobacterial algaecide by using suitable chemotype of Cinnamomum camphora and the main terpenoids, we analyzed the terpenoid composition in the 4 chemotype extracts, evaluated the algicidal effects of the extracts and their typical monoterpenoids on Microcystis aeruginosa, and investigated the algicidal mechanism of the stronger algicidal agents. Among the 4 chemotypes, eucalyptol and borneol chemotype extracts exhibited stronger algicidal effects. In the 4 chemotype extracts, monoterpenoids were the main compounds, of which linalool, camphor, eucalyptol and borneol were the typical components. Among the 4 typical monoterpenoids, eucalyptol and borneol showed stronger algicidal effects, which killed 78.8% and 100% M. aeruginosa cells, respectively, at 1.2 mM after 48 h. In 1.2 mM eucalyptol and borneol treatments, the reactive oxygen species levels markedly increased, and the caspase-3-like activity also raised. With prolonging the treatment time, M. aeruginosa cells gradually shrank and wrinkled, and the cell TUNEL fluorescence intensity and DNA degradation gradually enhanced, indicating that the lethal mechanism is causing apoptosis-like programmed cell death (PCD). Therefore, eucalyptol and borneol chemotype extracts and their typical monoterpenoids have the potential for developing as algaecides to control cyanobacteria through triggering apoptosis-like PCD.

Down-regulation of iron/zinc ion transport and toxin synthesis in Microcystis aeruginosa exposed to 5,4'-dihydroxyflavone

Flavonoids, common natural polyphenolic compounds from plants, have been proposed as highly effective and safe algicides. However, the molecular mechanism of flavonoids inhibiting Microcystis aeruginosa remains unclear. This study aims in exploring the global transcriptional changes and molecular docking in cyanobacterial cells in response to flavonoids. Transcriptomic analysis revealed that 5,4'-dihydroxyflavone (DHF) primarily affected the genes transcription of iron and zinc ion transport, resulting in the blockage of transport for iron (II), iron (III) and zinc (II), which eventually led to a decrease in intracellular iron and zinc content. 5,4'-DHF can also interfere with iron and zinc transport by binding to metal ion transport-related proteins, leading to eliminated biological activities in M. aeruginosa. Meanwhile, 5,4'-DHF inhibit microcystin synthesis and reduce the content of intercellular toxin by inhibiting the transcription of mcyC and binding with McyC protein, implying that 5,4'-DHF have potential to reduce the risk of microcystins in the environment. Moreover, iron starvation and down-regulation of photosynthesis-related genes transcription led to the inhibition of electron transport in photosynthetic system. These results provide more information for the inhibitory mechanism of flavonoids, and the inhibition of flavonoids on metal ion transmembrane transport provides a new perspective for the development of allelochemical algicides.

Algal community structure prediction by machine learning

The algal community structure is vital for aquatic management. However, the complicated environmental and biological processes make modeling challenging. To cope with this difficulty, we investigated using random forests (RF) to predict phytoplankton community shifting based on multi-source environmental factors (including physicochemical, hydrological, and meteorological variables). The RF models robustly predicted the algal communities composed by 13 major classes (Bray-Curtis dissimilarity = 9.2 ± 7.0%, validation NRMSE mostly <10%), with accurate simulations to the total biomass (validation R² > 0.74) in Norway's largest lake, Lake Mjosa. The importance analysis showed that the hydro-meteorological variables (Standardized MSE and Node Purity mostly >0.5) were the most influential factors in regulating the phytoplankton. Furthermore, an in-depth ecological interpretation uncovered the interactive stress-response effect on the algal community learned by the RF models. The interpretation results disclosed that the environmental drivers (i.e., temperature, lake inflow, and nutrients) can jointly pose strong influence on the algal community shifts. This study highlighted the power of machine learning in predicting complex algal community structures and provided insights into the model interpretability.

Efficient removal of Phaeocystis globosa from seawater with the persulfate activation by arbutin-modified cellulose nanocrystals

Harmful algal blooms (HABs) from seawater have a severe threat to human health, aquaculture, and coastal nuclear power safety. Thus, it is highly desirable to explore environmentally friendly, efficient, and economic methods for controlling HABs. Herein, the arbutin-modified cellulose nanocrystals (AT-CNC) activated persulfate (PS), as a novel heterogeneous Fenton-like process, was proposed to remove Phaeocystis globosa (P. globosa) from seawater. The AT-CNC was synthesized via the surface modification of AT on CNC. The effects of AT dosage, CNC dosage, and PS dosage on the removal performance of P. globosa were investigated. With the addition of 530 mg/L AT-CNC (6 wt% AT/CNC of AT loading) and 120 mg/L PS, the removal percentage of chlorophyll a (R_c), optical density at 680 nm (R_o) and turbidity (R_t) reached 97.7%, 91.9% and 85.2% at 24 h. According to electron paramagnetic resonance (EPR) spectra and radical quenching tests, the predominant free radicals inactivating P. globosa were hydroxyl radicals (^•OH). Additionally, the flocculation of the inactivated algae cells by AT-CNC was also critical for removing P. globosa. Moreover, a positive environmental impact was achieved in the AT-CNC-PS system due to the reduction of nitrogen, phosphorus and organic carbon contents. Based on the excellent removal performance for P. globosa, we believe that the AT-CNC activated persulfate is a promising option for HABs control.

Inhibitory effects of Ipomoea cairica extracts on the harmful algae Phaeocystis globosa

Ipomoea cairica (L.) Sweet is an invasive plant that cause serious invasion and damage in South China. Phaeocystis globosa is a common harmful algal bloom species on the southeast coast of China. Both species cause great environmental disturbances and serious economic damage to the localregion. This study explored the potential inhibitory effects of I. cairica leaf extracts on P. globosa. The results showed that solitary cells growth was inhibited at extract concentrations higher than 0.25 % (v/v). Although the colony diameter did not change, and the colony number increased rapidly in the first 36 h, we found that cells in the colonies had been damaged using scanning electron microscope and SYTOX-Green staining at 48 h. In addition, the rapid light-response curve of cells treated with extracts decreased, along with down-regulation of photosynthesis-related genes (psbA, psbD, and rbcL), suggesting damage to the photosynthetic system. Finally, the activities of antioxidant enzymes including superoxide dismutase, peroxidase, and catalase increased with increasing treatment time, indicating that cells activate antioxidant enzyme defense systems to alleviate the production of reactive oxygen species (ROS). Increased ROS levels disrupt cell membranes, alter cellular ultrastructures, and ultimately lead to cell death. This study not only achieved the reuse of invasive plant resources, but also demonstrated that I. cairica leaf extract has potential value as an algaecide.

Programmed cell death process in freshwater Microcystis aeruginosa and marine Phaeocystis globosa induced by a plant derived allelochemical

Harmful algal blooms (HAB) are a serious problem worldwide. Allelochemicals from natural plants were recently thought to be promising anti-algaecide in controlling harmful algae. However, the programmed cell death (PCD) process of algae under allelopathic pressure induced by 5,4'-dihydroxyflavone (5,4'-DHF) was poorly understood. In this study, two common and worldwide distributed microalgae, Microcystis aeruginosa and Phaeocystis globosa were selected as target algae, and the PCD processes induced by 5,4'-DHF were cross-compared between the two species. Both algae species were inhibited significantly by 5,4'-DHF with the relative sensitivity of 0.11. To uncover the PCD progress systematically, signals for PCD triggering, antioxidant enzyme activity, photosynthetic ability variation, caspase-like activities and typical indicators were investigated. In both species, typical indicators of PCD - phosphatidylserine externalization and chromatin condensation - were detected. The intracellular reactive oxygen species (ROS), nitric oxide (NO) and H₂O₂ were the potential signal molecules to stimulate PCD, and caspase-like activities were activated with an elevation of cytochrome c indicating the initiation of PCD in both species. However, P. globosa responded to 5,4'-DHF immediately after 3 h with the elevation of ROS and not in M. aeruginosa. Antioxidant enzyme activities of superoxide dismutase (SOD) and catalase (CAT) in M. aeruginosa and P. globosa also showed different patterns on day 3. Specifically, SOD activity in M. aeruginosa increased significantly while it decreased significantly in P. globosa, CAT activity in M. aeruginosa decreased significantly while it increased significantly in P. globosa (p < 0.05). Malondialdehyde (MDA) content in P. globosa increased significantly (p < 0.001) while it showed no variation in M. aeruginosa. Overall, this study is one of the earliest studies to explore the inhibition and action mechanism of plant derived flavonoids on harmful algae from the perspective of PCD, and provide new insights into the antialgal mechanism of allelochemicals.

Flavonoids as biopesticides - Systematic assessment of sources, structures, activities and environmental fate

Biopesticides obtained from renewable resources and associated with biodegradability have the potential to address resource limitations and environmental pollution, often caused by many conventional pesticides, due to the facility of natural products to run in natural nutrient cycles. Flavonoids are considered benign substitutes for pesticides, however, little comprehensive information of their pesticidal activities and critical evaluation of their associated advantages is available. Therefore, this systematic review assessed sources, structures, activities and the environmental fate of flavonoids on a basis of 201 selected publications. We identified 281 different flavonoids that were investigated for their pesticidal activity as either a pure compound or a flavonoid-containing extract, with quercetin, kaempferol, apigenin, luteolin and their glycosides as the most studied compounds. Agricultural or food waste, a potential sustainable source for flavonoids, represent 10.6% of the plant sources of flavonoids within these studies, showing the currently underutilization of these preferable feedstocks. Analysis of pesticidal activities and target organisms revealed a broad target spectrum for the class of flavonoids, including fungi, insects, plants, bacteria, algae, nematodes, molluscs and barnacles. Little information is available on the environmental fate and biodegradation of flavonoids, and a connection to studies investigating pesticidal activities is largely missing. Emerging from these findings is the need for comprehensive understanding of flavonoids pesticidal activities with emphasis on structural features that influence activity and target specificity to avoid risks for non-target organisms. Only if the target spectrum and environmental fate of a potential biopesticide are known it can serve as a benign substitute. Then, flavonoids can be integrated in a valorization process of agricultural and food waste shifting the extract-produce-consume linear chain to a more circular economy.

3-Indoleacrylic acid from canola straw as a promising antialgal agent - Inhibition effect and mechanism on bloom-forming Prorocentrum donghaiense

Harmful algal blooms (HABs) have induced severe damage worldwide. A novel high-efficient antialgal natural chemical, 3-indoleacrylic acid (3-IDC) with a 5-day half-maximal inhibitory concentration (IC_{50, 5d}), was discovered from canola straw, and its algal inhibition mechanism was investigated. Adverse effects were observed on the growth of P. donghaiense with 3-IDC addition, following an increase in reactive oxygen species (ROS) production. 3-IDC also hindered the photosynthetic mechanism of P. donghaiense cells. Transcriptional results showed 3-IDC inhibiting the functions of all the nutrient assimilating genes, down-regulated ribulose-1,5-bisphosphate carboxylase/oxygenase II, and cytochrome f genes. The expression of heat shock protein (HSP) 70 and 90 and rhodopsin genes were also suppressed. The binding affinity of investigated receptors was observed. The conformational changes induced by the spatial microstructural alteration through 3-IDC may further contribute to the perturbation of those enzyme catalytic activities. The present results provide new insights on controlling HABs using 3-IDC.

Co-occurrence of chromophytic phytoplankton and the Vibrio community during Phaeocystis globosa blooms in the Beibu Gulf

Accumulating research evidence has revealed that harmful algal blooms (HABs) can substantially affect the community structures of phytoplankton and heterotrophic bacteria in marine ecosystems. However, little is known about their species-specific interactions between phytoplankton and heterotrophic bacteria during the HABs period and about their interaction shifts in response to blooms. From this perspective, we investigated the co-occurrence of chromophytic phytoplankton and Vibrio during Phaeocystis globosa blooms in the Beibu Gulf. The results showed that Vibrio communities were distinct during the blooms, and P. globosa blooms resulted in a decline in phytoplankton alpha diversity, revealing that the blooms could affect their community compositions. The regression lines between the Shannon indices and Bray-Curtis distances of phytoplankton and Vibrio showed positive correlations with each other (p < 0.001), suggesting that they may have intrageneric symbiotic interactions overall. In addition, network analysis further demonstrated that relationships between phytoplankton and Vibrio were dominated by positive correlations, and more interaction modules were observed during the blooms, revealing that the blooms intensified synergistic association and mutual symbiotic interactions between them. Environmental factors (SiO₃^2-, NH₄⁺, NO₃^- and TN,) and P. globosa density more deeply affected network interactions between phytoplankton and Vibrio during the periods of P. globosa blooms than those before the blooms and after the blooms. This study provided new insight to elucidate community structure and interaction relationships between phytoplankton and Vibrio in response to P. globosa blooms and their ecological effects in marine ecosystems.

Spatiotemporal BME characterization and mapping of sea surface chlorophyll in Chesapeake Bay (USA) using auxiliary sea surface temperature data

Improving the spatiotemporal coverage of remote sensing (RS) products, such as sea surface chlorophyll concentration (SSCC), can offer a better understanding of the spatiotemporal SSCC distribution for ocean management purposes. In the first part of this work, 834 in-situ SSCC measurements of the SeaBASS-NASA (National Aeronautics and Space Administration) during 2002-2016 served as the empirical dataset. A moving window with ±3 days and ±0.5° centered at each of the in-situ SSCC measurements established a search neighborhood for Moderate Resolution Imaging Spectroradiometer Level 2 (MODIS L2) SSCC and MODIS L2 sea surface temperature (SST) data, and the matched SSCC and SST data were used for building a linear SSCC-SST relationship. The unmatched SST was introduced to the linear model for generating soft SSCC data with uniform distributions. The inherent spatiotemporal dependency of the SSCC distribution was then represented by the Bayesian maximum entropy (BME) method, which incorporated the soft SSCC data as auxiliary variable for SSCC estimation and mapping purposes. The results showed that a 75.3% accuracy improvement of remote SSCC retrieval in terms of R² can be achieved by BME-based method compared to the original MODIS L2 product. Subsequently, the BME-based method was applied to obtain daily SSCC dataset in Chesapeake Bay (USA) during the period 2010-2019. It was found that the SSCC distribution exhibited a decreasing spatial trend from the upper bay to the outer bay, whereas decreasing and increasing temporal trends were detected during the periods 2011-2014 and 2016-2019, respectively. The generalized Cauchy process was used to quantitatively describe the autocorrelation SSCC function in the Chesapeake Bay. The results showed that the outer bay exhibited the strongest long-range dependence among the four sub-regions, whereas the middle bay exhibited the weakest long-range dependence. Finally, one-point and two-point stochastic site indicators (SSIs) were employed to explore the spatiotemporal SSCC characteristics in Chesapeake Bay. The one-point SSI results showed that nearly 100% of the upper, middle and the lower bay areas experienced a high SSCC level (>5 mg/m³) during the entire study period. The area with SSCC >5 mg/m³ in the outer bay increased a lot during the winter season, but the area with SSCC >10 or 20 mg/m³ decreased significantly in the upper, middle and lower bay. Simultaneously, the SSCC dispersion in these areas was rather small during the winter season. On the other hand, the two-point SSI results showed that although the SSCC levels differ among the four sub-regions, but the SSCC connectivity structures between pairs of points also displayed some similarities in terms of their spatiotemporal dependency. In conclusion, the proposed BME-based method was shown to be a promising remote SSCC mapping technique that exhibited a powerful ability to improve both accuracy and coverage of RS products. The SSIs can be also used to explore the spatiotemporal characteristics of a variety of natural attributes in waters.

Synthesis of ZnO Nanoparticles Loaded on Biochar Derived from Spartina alterniflora with Superior Photocatalytic Degradation Performance

Spartina alterniflora is an invasive plant from coastal wetlands, and its use in applications has garnered much interest. In this study, a composite photocatalyst (ZnO@BC) was synthesized by preparing zinc oxide (ZnO) nanoparticles with S. alterniflora extracts, S. alterniflora, and one-step carbonization, which was characterized using scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Raman, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy (UV-vis DRS), photoluminescence (PL) and N₂ adsorption-desorption isotherm. The degradation capacity and mechanism of malachite green (MG) using ZnO@BC were analyzed under visible irradiation, and the degradation products of malachite green were detected by LC-MS. The results show that ZnO@BC has a larger surface area (83.2 m²/g) and various reactive groups, which enhance its photocatalytic efficiency, with the presence of oxygen vacancy further improving the photocatalytic activity. The total removal rate of malachite green (400 mg/L) using ZnO@BC is up to 98.38%. From the LC-MS analysis, it could be concluded that malachite green is degraded by demethylation, deamination, conjugate structure and benzene ring structure destruction. This study provides a novel idea for the high-value utilization of S. alterniflora.

Antioxidant, Mineralogenic and Osteogenic Activities of Spartina alterniflora and Salicornia fragilis Extracts Rich in Polyphenols

Osteoporosis is an aging-related disease and a worldwide health issue. Current therapeutics have failed to reduce the prevalence of osteoporosis in the human population, thus the discovery of compounds with bone anabolic properties that could be the basis of next generation drugs is a priority. Marine plants contain a wide range of bioactive compounds and the presence of osteoactive phytochemicals was investigated in two halophytes collected in Brittany (France): the invasive Spartina alterniflora and the native Salicornia fragilis. Two semi-purified fractions, prepared through liquid-liquid extraction, were assessed for phenolic and flavonoid contents, and for the presence of antioxidant, mineralogenic and osteogenic bioactivities. Ethyl acetate fraction (EAF) was rich in phenolic compounds and exhibited the highest antioxidant activity. While S. fragilis EAF only triggered a weak proliferative effect in vitro, S. alterniflora EAF potently induced extracellular matrix mineralization (7-fold at 250 μg/mL). A strong osteogenic effect was also observed in vivo using zebrafish operculum assay (2.5-fold at 10 μg/mL in 9-dpf larvae). Results indicate that polyphenol rich EAF of S. alterniflora has both antioxidant and bone anabolic activities. As an invasive species, this marine plant may represent a sustainable source of molecules for therapeutic applications in bone disorders.

Hemolytic Activity in Relation to the Photosynthetic System in Chattonella marina and Chattonella ovata

Chattonella species, C. marina and C. ovata, are harmful raphidophycean flagellates known to have hemolytic effects on many marine organisms and resulting in massive ecological damage worldwide. However, knowledge of the toxigenic mechanism of these ichthyotoxic flagellates is still limited. Light was reported to be responsible for the hemolytic activity (HA) of Chattonella species. Therefore, the response of photoprotective, photosynthetic accessory pigments, the photosystem II (PSII) electron transport chain, as well as HA were investigated in non-axenic C. marina and C. ovata cultures under variable environmental conditions (light, iron and addition of photosynthetic inhibitors). HA and hydrogen peroxide (H2O2) were quantified using erythrocytes and pHPA assay. Results confirmed that% HA of Chattonella was initiated by light, but was not always elicited during cell division. Exponential growth of C. marina and C. ovata under the light over 100 µmol m-2 s-1 or iron-sufficient conditions elicited high hemolytic activity. Inhibitors of PSII reduced the HA of C. marina, but had no effect on C. ovata. The toxicological response indicated that HA in Chattonella was not associated with the photoprotective system, i.e., xanthophyll cycle and regulation of reactive oxygen species, nor the PSII electron transport chain, but most likely occurred during energy transport through the light-harvesting antenna pigments. A positive, highly significant relationship between HA and chlorophyll (chl) biosynthesis pigments, especially chl c2 and chl a, in both species, indicated that hemolytic toxin may be generated during electron/energy transfer through the chl c2 biosynthesis pathway.

In vitro allelopathic effects of compounds from Cerbera manghas L. on three Dinophyta species responsible for harmful common red tides

Allelopathy is regarded as an economic and eco-friendly approach for the control of harmful algal blooms (HABs) because allelochemicals degrade easily and cause less pollution than traditional algicides. We first surveyed the inhibitory effect of the traditional medicinal plant Cerbera manghas L. on the notorious dinoflagellates Alexandrium tamarense, Scrippsiella trochoidea, and Karenia mikimotoi. Then, we identified and quantified the potential algicidal compounds by UPLC-MS and determined their activity. The aqueous extract inhibited algae with EC_{50-120 h} at 0.986, 1.567 and 1.827 g L^-1 for A. tamarense, S. trochoidea, and K. mikimotoi, respectively. Three potential allelochemicals were quantified in the stock solution: quinic acid (QA) (28.81 mg L^-1), protocatechuic acid (PA) (53.91 mg L^-1), and phloridzin (PD) (26.17 mg L^-1). Our results illustrated that 1) QA did not have an inhibitory effect, 2) PA had medium toxicity to algae (EC_50-120h: 0.22, 0.28, and 0.35 mM for A. tamarense, S. trochoidea, and K. mikimotoi), and 3) PD had low toxicity (EC_50-120h > 0.66 mM). These findings suggested that PA might be the main allelopathic compound in the aqueous extract of the studied algae. In addition, PA could have a negative effect on the photosynthesis of S. trochoidea by impeding the reduction of quinone electrons and destroying electron transfer in PSII. In summary, this was the first study to quantify allelochemicals in C. manghas fruit. Moreover, C. manghas and protocatechuic have the potential to be algicides to control and mitigate the HABs caused by dinoflagellates.

Remediation of cadmium-contaminated coastal saline-alkaline soil by Spartina alterniflora derived biochar

Following oil extraction in the wetland of the Yellow River Delta, heavy metal contamination of coastal saline-alkaline soil, especially with cadmium (Cd), has become a serious environmental problem in some regions. Biochar application has been proposed to remedy Cd-contaminated soil, but the remediation effect is related to preparation conditions of biochar (e.g., pyrolysis temperature and raw material) and soil properties. The invasive plant, Spartina alterniflora, produces a high amount of biomass, making it suitable for biochar production in coastal China. We investigated the effect of S. alterniflora-derived biochar (SDB) pyrolyzed at four temperatures (350, 450, 550, and 650 °C) crossed with three addition ratios (1, 5, and 10%) and control on Cd contamination of coastal saline-alkaline soil. Pyrolysis temperature affected pH, surface area, and functional groups of SDB. SDB markedly improved soil pH and soil organic matter, but the degree of improvement was affected by pyrolysis temperature and addition ratio. SDB significantly altered available Cd content in soil, but reduced it only at low pyrolysis temperatures (350 and 450 °C). Available Cd content had a positive correlation with soil pH (R² = 0.298, P < 0.01), but was not related to salinity and soil organic matter content. Thus, SDB pyrolyzed at 350 °C with 5% addition was optimal for passivating Cd in coastal saline-alkaline soil, since available Cd content in soil decreased mostly (by 26.9%). These findings act as a reference for the development of an application strategy for SDB to ameliorate Cd-contaminated coastal saline-alkaline soil.

Harmful Cyanobacterial Blooms (HCBs): innovative green bioremediation process based on anti-cyanobacteria bioactive natural products

Over the last decades, Harmful Cyanobacterial Blooms (HCBs) represent one of the most conspicuous hazards to human health in freshwater ecosystems, due to the uses of the water for drinking, recreation and aquaculture. Cyanobacteria are one of the main biological components in freshwater ecosystems and they may proliferate in nutrients rich ecosystems causing severe impacts at different levels. Therefore, several methods have been applied to control cyanobacterial proliferation, including physical, chemical and biological strategies. However, the application of those methods is generally not very efficient. Research on an eco-friendly alternative leading to the isolation of new bioactive compounds with strong impacts against harmful cyanobacteria is a need in the field of water environment protection. Thus, this paper aims to give an overview of harmful cyanobacterial blooms and reviews the state of the art of studying the activities of biological compounds obtained from plants, seaweeds and microorganisms in the cyanobacterial bloom control.

New insights into the harmful algae inhibition by Spartina alterniflora: Cellular physiology and metabolism of extracellular secretion

This study investigated the response of three toxic harmful algal blooms (HABs) species, Heterosigma akashiwo, Chattonella marina, and Alexandrium tamarense to coastal invasive plant, Spartina alterniflora. In this study, the growth of three HABs species were suppressed significantly by S. alterniflora extracts, showing a dose-response relationship. The raphidophyte H. akashiwo and C. marina exhibited higher inhibitory response with EC_50,7d decreased by 14% and 75% as compared to the dinoflagellate A. tamarense. C. marina was the most sensitive among the three species. S. alterniflora extracts disrupted algal cellular integrity and photosynthesis. Furthermore, the extracellular organic matters were detected by fluorescence excitation-emission matrix. Algal metabolites, protein-like substances (tyrosine-like peak and tryptophan-like peak) decreased as time prolonged and the humic-like substances (UVA marine humic-like peak) increased when algal cells were exposed to S. alterniflora extracts. These results provide new insights to the inhibition mechanism of S. alterniflora extracts on HABs species.

The allelopathic effects of aqueous extracts from Spartina alterniflora on controlling the Microcystis aeruginosa blooms

The Microcystis aeruginosa (M. aeruginosa) blooms and Spartina alterniflora (S. alterniflora) invasion have caused serious damage to local ecological environment. This study validated the possibility of transforming the abandoned S. alterniflora into a biological resource to inhibit M. aeruginosa blooms through allelopathy. The results showed that the inhibitory effect became stronger with the increasing S. alterniflora concentration by decreasing chlorophyll a and weakening photosynthesis when S. alterniflora aqueous extract concentration was over 0.05 g/mL. The results of GC-MS showed that Cyclohexane, Heptane, 2-Cyclohexen-1-one, Hexadecanoic acid, 2,4-Di-tert-butylphenol and Hydrocinnamic acid may be the main allelochemicals. In addition, the S. alterniflora aqueous extract had little effect on the relative abundance and diversity of microbial communities in the culture system. This study provided a novel idea of controlling the M. aeruginosa blooms using the rapidly expanding S. alterniflora.

Soil microorganisms interacting with residue-derived allelochemicals effects on seed germination

Despite the knowledge regarding allelopathy, known as a major ecological mechanism for biological weed control, had increased greatly, the role of soil microorganisms in that field remained controversial. The study sought to evaluate the interference potential of soil microorganisms, residues-derived allelochemicals and their interaction on seed germination and understand the variation of microbial community in allelopathic activities. Three different rice residues-derived fractions from variety PI312777 (extracts, straw fraction and fresh residue) were applied to sterile and live soils to disentangle the interference potential of soil microorganisms, residues-derived allelochemicals and their interaction concerned allelopathic activities. The results demonstrated that microbe-only and residues-only exerted onefold promotion and inhibition effects on lettuce (Lactuca sativa Linn.) seed germination, respectively, whereas, microbe-by-residues interaction showed an inhibition at the beginning, and a feeble promotion later. The 20 most dominant genera of microbes were classified into three clusters, with 13 genera in one cluster, only 1 in the second cluster and 6 in the third one. The genera in the first cluster commonly exerted negative effects on phenol content, while showed positive correlation with seed germination. Interestingly, Bacillus, clustered in the second cluster, had an opposite effect alone. The third cluster genera somehow had a weak correlation with both germination as well as the release of the allelochemicals. Overall, we incorporated molecular methodology for tracking bacterial impacts during incubation with allelochemicals, and demonstrated the mutable role of soil microbes in allelopathy. It may be potentially important for stimulating the beneficial roles of microbes for environmentally friendly weed management.