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

Broad spectrum and species specificity of plant allelochemicals 1,2-benzenediol and 3-indoleacrylic acid against marine and freshwater harmful algae

Allelochemicals derived from plants have shown great potential in mitigating harmful algal blooms (HABs), although different algal species can respond differently to these chemicals. Therefore, we first investigated the allelopathic effects of two newly identified plant-derived allelochemicals, 1,2-benzenediol (1,2-BD) and 3-indoleacrylic acid (3-IDC), on six algal species. Then we further evaluated the allelopathic responses of two bloom-forming species, Microcystis aeruginosa FACHB-905 and Heterosigma akashiwo to 1,2-BD. Results showed that 1,2-BD had a broader antialgal spectrum than 3-IDC. Allelopathic response analysis indicated that 1,2-BD consistently and stably inhibit the growth of M. aeruginosa FACHB-905, with inhibitory mechanism being disruption of photosynthetic activity, overwhelming of the antioxidant system and activation of programmed cell death (PCD). H. akashiwo displayed resistance to 1,2-BD during exposure, and the growth inhibition was mainly attributed to PCD. Therefore, the species-specific allelopathic responses provide new insights for controlling HABs using 1,2-BD and 3-IDC.

The growth inhibitory effects and non-targeted metabolomic profiling of Microcystis aeruginosa treated by Scenedesmus sp

The health of the aquatic ecosystem has recently been severely affected by cyanobacterial blooms brought on by eutrophication. Therefore, it is critical to develop efficient and secure methods to control dangerous cyanobacteria, such as Microcystis aeruginosa. In this research, we tested the inhibition of M. aeruginosa growth by a Scenedesmus sp. strain isolated from a culture pond. Scenedesmus sp. culture filtrate that had been lyophilized was added to M. aeruginosa, and cultivation for seven days, the cell density, chlorophyll a (Chl-a) concentration, maximum quantum yield of photosystem II (Fv/Fm), the activities of superoxide dismutase (SOD), catalase (CAT), and the concentration of malondialdehyde (MDA) and glutathione (GSH) were measured. Moreover, non-targeted metabolomics was carried out to provide light on the inhibitory mechanism in order to better understand the metabolic response. According to the results, M. aeruginosa is effectively inhibited by the lyophilized Scenedesmus sp. culture filtrate at a rate of 51.2%. Additionally, the lyophilized Scenedesmus sp. clearly inhibit the photosystem and damages the antioxidant defense system of M. aeruginosa cells, resulting in oxidative damage, which worsens membrane lipid peroxidation, according to changes in Chl-a, Fv/Fm, SOD, CAT enzyme activities and MDA, GSH. Metabolomics analysis revealed that the secondary metabolites of Scenedesmus sp. significantly interfere with the metabolism of M. aeruginosa involved in amino acid synthesis, membrane creation and oxidative stress, which is coherent with the morphology and physiology outcomes. These results demonstrate that the secondary metabolites of Scenedesmus sp. exert algal inhibition effect by breaked the membrane structure, destroyed the photosynthetic system of microalgae, inhibited amino acid synthesis, reduced antioxidant capacity, and eventually caused algal cell lysis and death. Our research provides a reliable basis for the biological control of cyanobacterial blooms on the one hand, and on other hand supply application of non-targeted metabolome on the study of microalgae allelochemicals.

Aquatic plant allelochemicals inhibit the growth of microalgae and cyanobacteria in aquatic environments

Excess nitrogen and phosphorus nutrients in the aquatic environment result in the growth of algal cells and water eutrophication, which adversely affect the aquatic environment and human health. Therefore, discovering a safe and efficient algae suppression method is necessary to ensure the ecological safety of water. Recently, the allelopathic effects of aquatic plants on algae have attracted extensive attention from researchers. This review demonstrates the current research hotspot of allelopathic algal inhibition in aquatic plants and lists the common aquatic plant species and allelochemicals. In addition, the inhibition mechanism of allelochemicals from aquatic plants on algae is systematically discussed. Moreover, the key factors affecting the inhibition of allelopathy in algae, such as pH, temperature, algal cell density, and concentration of allelochemicals, are summarized. The present utilization modes of allelochemicals on algae are also presented. Finally, the problems existing in the study of allelopathic algal inhibition of aquatic plants are highlighted, and suggestions for further research are proposed.

Natural Algaecide Sphingosines Identified in Hybrid Straw Decomposition Driven by White-Rot Fungi

Harmful algal blooms (HABs), which are promoted by eutrophication and intensified by global warming, occur worldwide. Allelochemicals, which are natural chemicals derived from plants or microbes, are emerging weapons to eliminate these blooms. However, the cost and technical challenges have limited the discovery of novel antialgal allelochemicals. Herein, the decomposition of agricultural straws is manipulated by white-rot fungi and achieved elevated antialgal efficiency. The transcriptomic analysis reveals that nutrient limitation activated fungal decomposition. By using a comparative nontarget metabolomics approach, a new type of allelochemical sphingosines (including sphinganine, phytosphingosine, sphingosine, and N-acetylsphingosine) is identified. These novel natural algaecides exhibit superior antialgal capability, with as high as an order of magnitude lower effective concentration on blooming species than other prevalent allelochemicals. The co-expression relationship between transcriptomic and metabolomic results indicate that sphinganine is strongly correlated with the differentially expressed lignocellulose degradation unigenes. The algal growth suppression is triggered by the activation of programmed cell death, malfunction of algal photosystem and antioxidant system, the disruption on CO2 assimilation and light absorption. The sphingosines reported here are a new category of allelochemicals in addition to the well-known antialgal natural chemicals, which are potential species-specific agents for HABs control identified by multi-omics methodology.

Green synthesis of ZnFe2O4 nanoparticles using plant extracts and their applications: A review

Magnetic nanoparticles, particularly ZnFe₂O₄ are of enormous significance in biomedical and water treatment fields. However, chemical synthesis of ZnFe₂O₄ nanoparticles endures some major limitations, e.g., the use of toxic substances, unsafe procedure, and cost-ineffectiveness. Biological methods are more preferable approaches since they take advantages of biomolecules available in plant extract serving as reducing, capping, and stabilizing agents. Herein, we review plant-mediated synthesis and properties of ZnFe₂O₄ nanoparticles for multiple applications in catalytic and adsorption performance, biomedical, catalyst, and others. Effect of several factors such as Zn²⁺/Fe³⁺/extract ratio, and calcination temperature on morphology, surface chemistry, particle size, magnetism and bandgap energy of obtained ZnFe₂O₄ nanoparticles was discussed. The photocatalytic activity and adsorption for removal of toxic dyes, antibiotics, and pesticides were also evaluated. Main results of antibacterial, antifungal and anticancer activities for biomedical applications were summarized and compared. Several limitations and prospects of green ZnFe₂O₄ as an alternative to traditional luminescent powders have been proposed.

Effects of Ginkgo biloba extract on growth, photosynthesis, and photosynthesis-related gene expression in Microcystis flos-aquae

The inhibitory effect of plants on algae offers a new and promising alternative method for controlling harmful algal blooms. Previous studies showed that anti-algal effects might be obvious from extracts of fallen leaves from terrestrial plants, which had great potential for cyanobacterial control in field tests. To investigate the anti-algal activities and main algicidal mechanisms of Ginkgo biloba fallen leaves extracts (GBE) on Microcystis flos-aquae, the cell density, photosynthetic fluorescence, and gene expression under different concentrations of GBE treatments were tested. GBE (3.00 g L^-1) showed a strong inhibitory effect against M. flos-aquae with an IC₅₀ (96h) of 0.79 g L^-1. All the inhibition rates of maximal quantum yield (F_v/F_m), effective quantum yield (F_q'/F_m'), and maximal relative electron transfer rate (rETR_max) were more than 70% at 96 h at 3.00 g L^-1 and more than 90% at 6.00 g L^-1. Further results of gene expression of the core proteins of PSII (psbD), limiting enzyme in carbon assimilation (rbcL), and phycobilisome degradation protein (nblA) were downregulated after exposure. These findings emphasized that photosynthetic damage is one of the main toxic mechanisms of GBE on M. flos-aquae. When exposed to 12.00 g L^-1 GBE, no significant influence on the death rate of zebrafish or photosynthetic activity of the three submerged plants was found. Therefore, appropriate use of GBE could control the expansion of M. flos-aquae colonies without potential risks to the ecological safety of aquatic environments, which means that GBE could actually be used to regulate cyanobacterial blooms in natural waters.

Treatment of the red tide dinoflagellate Karenia brevis and brevetoxins using USEPA-registered algaecides

The effectiveness of USEPA-registered algaecides for managing algae in lakes and reservoirs has been extensively evaluated in laboratory studies, mesocosm studies and in situ treatment. However, the use of these algaecides in marine environments for the management of dinoflagellates and associated toxins remains largely unknown. Karenia brevis is a toxic dinoflagellate that causes red tides in the Gulf of Mexico. In this study, we investigated the efficacy of six USEPA-registered algaecides, three copper-based and three peroxide-based, on treating toxic K. brevis with a natural bloom density (1.79 × 10⁷ cells/L). Our results indicate that the application of as low as 0.31-0.34 mg Cu/L led to a significant decrease of K. brevis cells within 24 h after treatment, while peroxide-based algaecides required a relatively higher concentration for the effective removal of K. brevis cells (4.89-7.08 mg H₂O₂/L), but still lower than maximum label rate. Total brevetoxin levels 72 h after treatment revealed that 1.00 mg Cu/L for Algimycin® PWF, 6.48 mg H₂O₂/L for PAK® 27 and 7.08 mg H₂O₂/L for Oximycin® P5 had the greatest impact on decreasing toxin levels. The correlation analysis showed that brevetoxin reduction rate was significantly positively related with the peroxide-based algaecide exposure concentration, which is caused by the oxidation of hydroxyl radicals produced by hydrogen peroxide. The degradation dynamics of the three peroxide-based algaecides revealed that salinity, microorganisms and organic matter (≥ 0.2 μm) impact the stability of hydrogen peroxide, and Oximycin® P5 showed the highest stability among tested peroxide-based algaecides with a degradation rate of 0.467 mg/d in natural seawater. Hence, our laboratory work provided new insights into potential emergency treatment methods for immediate mitigation of K. brevis and brevetoxins. More work on the fate and persistence of algaecide active ingredients and phycotoxins, effects of site characteristics, and pilot studies on marine non-targets are still needed before safe application of this method for HABs in marine systems.

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.

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.

Physiological, biochemical and transcriptional responses of cyanobacteria to environmentally relevant concentrations of a typical antibiotic-roxithromycin

The frequent occurrence of antibiotics in source waters may affect the formation of harmful algal blooms (HABs) dominated by the cyanobacterium Microcystis aeruginosa. However, it remains poorly understood whether dissolved algal organic matters (AOM) can be altered by the introduction of antibiotics in source waters. To resolve these discrepancies, this study investigated the physiological, biochemical, and transcriptional responses of a toxigenic strain of M. aeruginosa to the commonly-detected antibiotic roxithromycin (ROX) at environmentally relevant concentrations ranging from 30 to 8000 ng L^-1. The growth and microcystin (MC) production of M. aeruginosa was significantly stimulated by 300 and 1000 ng L^-1 ROX, whereas inhibited by 5000 and 8000 ng L^-1 ROX. This may be owing to the regulation of genes related to photosynthesis and MCs. Although the membrane of cyanobacterial cells remained intact, the release of MCs was increased significantly with the growing ROX dosages, which may cause additional challenges in drinking water treatment. The amounts of AOM were enhanced by 300 and 1000 ng L^-1 ROX, while decreased by 5000 and 8000 ng L^-1 ROX. It may be attributed to the changes of cyanobacterial cell growth and the gene expression related to carbon fixation, carbohydrate metabolism and nitrogen metabolism. To further understand the regulation of related genes in M. aeruginosa exposed to ROX, trend analysis of differentially expressed genes was performed. The results indicated that the regulation of metabolism-related genes (e.g., lipopolysaccharide biosynthesis) may be also responsible for the changes of cyanobacterial cell densities. Generally, low levels of ROX (300 and 1000 ng L^-1) could stimulated the cyanobacterial growth, MC synthesis and AOM production, which may promote the formation of HABs and reduce the source water quality. Although higher levels of ROX (5000 and 8000 ng L^-1) inhibited the formation of HABs, the threat of increasing extracellular MCs should be considered.

Green synthesis of ZrO2 nanoparticles and nanocomposites for biomedical and environmental applications: a review

Pollution and diseases such as the coronavirus pandemic (COVID-19) are major issues that may be solved partly by nanotechnology. Here we review the synthesis of ZrO₂ nanoparticles and their nanocomposites using compounds from bacteria, fungi, microalgae, and plants. For instance, bacteria, microalgae, and fungi secret bioactive metabolites such as fucoidans, digestive enzymes, and proteins, while plant tissues are rich in reducing sugars, polyphenols, flavonoids, saponins, and amino acids. These compounds allow reducing, capping, chelating, and stabilizing during the transformation of Zr⁴⁺ into ZrO₂ nanoparticles. Green ZrO₂ nanoparticles display unique properties such as a nanoscale size of 5-50 nm, diverse morphologies, e.g. nanospheres, nanorods and nanochains, and wide bandgap energy of 3.7-5.5 eV. Their high stability and biocompatibility are suitable biomedical and environmental applications, such as pathogen and cancer inactivation, and pollutant removal. Emerging applications of green ZrO₂-based nanocomposites include water treatment, catalytic reduction, nanoelectronic devices, and anti-biofilms.

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.

Allelopathic effects and potential allelochemical of Sargassum fusiforme on red tide microalgae Heterosigma akashiwo

In this study, we aim to explore the allelochemicals of marine brown algae Sargassum fusiforme and the mechanism of allelopathy effect on Heterosigma akashiwo. Six potential allelopathic substances, palmitic acid, arachidonic acid, α-linolenic acid (ALA), fucosterol, 24-hydroperoxy-24-vinylcholesterol, and saringosterowere, were isolated and identified from S. fusiforme by column chromatography, NMR, TOF-MS and GC-MS analyses. The ALA and 24-hydroperoxy-24-vinylcholesterol showed remarkable inhibitory activities on H. akashiwo growth. Besides, the allelopathic mechanism between the ALA and H. akashiwo was preliminarily investigated. The results indicated that the activities of SOD and POD of H. akashiwo gradually decreased under high ALA concentration. The reduction of photosynthetic pigments and chlorophyll fluorescence parameters, as well as apparent electron transport rate in H. akashiwo cells, aggravated gradually with increasing the concentration of the ALA. In summary, this study revealed the responses of morphology and physiology of H. akashiwo when exposed by ALA, while revealing the potential of macroalgae in red tide control.