The algicidal mechanism of prodigiosin from Hahella sp. KA22 against Microcystis aeruginosa

Changes of vertical distribution of Microcystis colonies driven by short-term rainfall: Disappearance and reformation of surface bloom

Global climate warming and human activities have increased the magnitude and frequency of Microcystis surface blooms, posing significant threats to freshwater ecosystems and human health over recent decades. Heavy rainfall events have been reported to cause the disappearance of these blooms. Although some studies have employed turbulence models to analyze the movement characteristics of Microcystis colonies, the impact of rainfall is complex, comprehensive investigations on their vertical migration induced by short-term rainfall are still necessary. Utilizing monitoring data from eutrophic ponds and controlled simulation experiments, this study examines the short-term impacts of rainfall on the vertical distribution of Microcystis in the water column. Our findings indicate that rainfall contributes to the disappearance of Microcystis blooms by reducing the quantity of small to medium-sized colonies (0-100 μm) at the surface, subsequently decreasing the overall Microcystis biomass. As rainfall intensity increases, larger colonies migrate deeper into the water column. At a rainfall threshold of 666 mm, the difference in the median volume diameter (DV50) of Microcystis colonies between the surface and bottom reaches a minimal value of 3.09%. Post-rainfall, these colonies rapidly ascend, aggregate into larger formations, and re-establish surface blooms. The greater the rainfall, the smaller the resultant Microcystis biomass, albeit with larger aggregated colony sizes. When rainfall exceeds 222 mm, the recovery rate of surface Microcystis biomass remains below 100%, decreasing to 19.48% at 666 mm of rainfall, while the median volume diameter (DV50) of the colonies increases to 139.07% of its pre-rainfall level. Furthermore, compared to pre-rainfall conditions, the photosynthetic activity of the surface Microcystis colonies was enhanced and the secretion of EPS was increased under heavy rainfall conditions. Our results identify a critical response time of 30 min for Microcystis colonies to rainfall, after which the response ceases to intensify. These insights are crucial for predicting post-rain Microcystis bloom dynamics and aiding management authorities in timely interventions.

Alleviating effects of microplastics together with tetracycline hydrochloride on the physiological stress of Closterium sp

Microplastics have significant influence on both freshwater cyanobacteria and marine microalgae, especially under co-exposure with other pollutants such as heavy metals, antibiotics, and pharmaceuticals. In the present study, combined effects of microplastics (polyethylene terephthalate (PET) or polybutylene terephthalate (PBT)) and tetracycline hydrochloride (TCH) on the microalgae Closterium sp. were studied to evaluate their acute toxicity, and the cell density, total chlorophyll concentration, photosynthetic activity, antioxidant system, and subcellular structure of Closterium sp. under different treatments were used to explain the physiological stress mechanism of the combined effects. The results indicate that both the single and combined treatments have inhibition effects on the cell growth and photosynthetic activity, with inhibition efficiencies (in terms of cell density) of 5.0%, 9.2%, 66.7%, 55.1%, and 59.8% for PET (100 mg L-1), PBT (100 mg L-1), TCH (10 mg L-1), PET/TCH (PET 100 mg L-1 and TCH 10 mg L-1), and PBT/TCH (PBT 100 mg L-1 and TCH 10 mg L-1), respectively, and relative electron-transport rates (rETRs) of 7.3%, 12.7%, 66.8%, 54.0%, and 59.9%, respectively, for each treatment compared with the control on the 7th day. Moreover, both PET and PBT have positive effects in alleviating TCH toxicity toward Closterium sp., and at the same time, the malondialdehyde level (MDA), superoxide dismutase (SOD) activity, and catalase (CAT) activity induced by the combined treatments were much higher than those from the single microplastic treatments but lower than those from TCH treatment after 7 days. It was demonstrated that TCH causes a much more serious oxidative stress than PET/TCH and PBT/TCH, and the lower oxidative stress of the PET/TCH and PBT/TCH groups could be attributed to the adsorption of TCH to PET or PBT. This work improves the understanding of the combined toxicity effects of microplastics and TCH on Closterium sp.

Succession of particle-attached and free-living bacterial communities in response to microalgal dynamics induced by the biological cyanocide paucibactin A

Microalgae, including cyanobacteria and eukaryotic algae, are hotspots of primary production and play a critical role in global carbon cycling. However, these species often form blooms that poses a threat to aquatic ecosystems. Although the use of bacteria-derived cyanocides is regarded as an environmentally friendly method for controlling cyanobacterial blooms, only a few studies have examined their potential impact on ecosystems. This study is the first to explore the response of particle-attached (PA) and free-living (FL) bacteria to the dynamics of microalgal communities induced by the biological cyanocide paucibactin A. The microalgal community dynamics were divided into two distinct phases [phase I (days 0-2) and phase II (days 3-7)]. In phase I, paucibactin A caused a sudden decrease in the cyanobacterial concentration. Phase II was characterized by increased growth of eukaryotic microalgae (Scenedesmus, Pediastrum, Selenastrum, and Coelastrum). The stability of the bacterial community and the contribution of stochastic processes to community assembly were more pronounced in phase II than in phase I. The microalgal dynamics triggered by paucibactin A coincided with the succession of the PA and FL bacterial communities. The lysis of cyanobacteria in phase I favored the growth of microbial organic matter degraders in both the PA (e.g., Aeromonas and Rheinheimera) and FL (e.g., Vogesella) bacterial communities. In phase II, Lacibacter, Phycisphaeraceae, and Hydrogenophaga in the PA bacterial community and Lacibacter, Peredibacter, and Prosthecobacter in the FL bacterial community showed increased relative abundances. Overall, the FL bacterial community exhibited greater sensitivity to the two sequential processes compared with the PA bacterial community. These results highlight the need for studies evaluating the impact of biological cyanocides on aquatic ecosystems when used to control natural cyanobacterial blooms.

Anticyanobacterial effect of p-coumaric acid on Limnothrix sp. determined by proteomic and metabolomic analysis

Regulating photosynthetic machinery is a powerful but challenging strategy for selectively inhibiting bloom-forming cyanobacteria, in which photosynthesis mainly occurs in thylakoids. P-coumaric acid (p-CA) has several biological properties, including free radical scavenging and antibacterial effects, and studies have shown that it can damage bacterial cell membranes, reduce chlorophyll a in cyanobacteria, and effectively inhibit algal growth at concentrations exceeding 0.127 g/L. Allelochemicals typically inhibit cyanobacteria by inhibiting photosynthesis; however, research on inhibiting harmful algae using phenolic acids has focused mainly on their inhibitory and toxic effects and metabolite levels, and the molecular mechanism by which p-CA inhibits photosynthesis remains unclear. Thus, we examined the effect of p-CA on the photosynthesis of Limnothrix sp. in detail. We found that p-CA inhibits algal growth and damages photosynthesis-related proteins in Limnothrix sp., reduces carotenoid and allophycocyanin levels, and diminishes the actual quantum yield of Photosystem II (PSII). Moreover, p-CA significantly altered algal cell membrane protein systems, and PSII loss resulting from p-CA exposure promoted reactive oxygen species production. It significantly altered algae cell membrane protein systems. Finally, p-CA was found to be environmentally nontoxic; 80 % of 48-h-old Daphnia magna larvae survived when exposed to 0.15 g/L p-CA. These findings provide insight into the mechanism of cyanobacterial inhibition by p-CA, providing a more practical approach to controlling harmful algal blooms.

Zinc oxide nanoparticles alleviate cadmium toxicity and promote tolerance by modulating programmed cell death in alfalfa (Medicago sativa L.)

Cadmium (Cd) can induce programmed cell death (PCD) and zinc oxide nanoparticles (ZnO NPs) effectively alleviate Cd stress. However, the mechanisms of ZnO NPs-mediated Cd detoxification in alfalfa (Medicago sativa L.) are limited. The pot experiment was conducted with Cd soil (19.2 mg kg-1) and foliar ZnO NPs (100 mg L-1) on alfalfa. The results showed that Cd reduced shoot height and biomass, and accumulated reactive oxygen species (ROS), resulting in oxidative stress and further PCD (plasmolysis, cytosolic and nuclear condensation, subcellular organelle swelling, and cell death). ZnO NPs positively regulated the antioxidant system, cell membrane stability, ultrastructure, osmotic homeostasis, and reduced PCD, indicating a multi-level coordination for the increased Cd tolerance. ZnO NPs up-regulated the activity and expression of antioxidant enzymes and regulated PCD-related genes to scavenge ROS and mitigate PCD caused by Cd. The genes related to ZnO NPs-mediated Cd detoxification were significantly enriched in cell death and porphyrin and chlorophyll metabolism. Overall, it elucidates the molecular basis of ZnO NPs-mediated Cd-tolerance by promoting redox and osmotic homeostasis, maintaining cellular ultrastructure, reducing Cd content, and attenuating Cd-induced PCD. it provides a promising application of ZnO NPs to mitigate Cd phytotoxicity and the related cellular and biochemical mechanisms. ENVIRONMENTAL IMPLICATION: Cd, one of the most toxic heavy metals, has caused serious environmental pollution. ZnO NPs can effectively alleviate Cd stress on plants and the environment. This study revealed that foliar-applied ZnO NPs alleviate Cd toxicity by mitigating the oxidative damage and regulating Cd-induced PCD via morphological, physiological, and transcriptomic levels. The findings elucidated the molecular basis of ZnO NPs-mediated Cd tolerance by promoting osmotic and redox homeostasis, reducing Cd content and lipid peroxidation, attenuating Cd-induced PCD features, and altering PCD-related genes in alfalfa. The study laid a theoretical foundation for the safe production of alfalfa under Cd pollution.

Decylprodigiosin: a new member of the prodigiosin family isolated from a seaweed-associated Streptomyces

Bioprospecting actinobacterial secondary metabolism from untapped marine sources may lead to the discovery of biotechnologically-relevant compounds. While studying the diversity and bioactive potential of Actinomycetota associated with Codium tomentosum, a green seaweed collected in the northern Portuguese cost, strain CT-F61, identified as Streptomyces violaceoruber, was isolated. Its extracts displayed a strong anticancer activity on breast carcinoma T-47D and colorectal carcinoma HCT116 cells, being effective as well against a panel of human and fish pathogenic bacteria. Following a bioactivity-guided isolation pipeline, a new analogue of the red-pigmented family of the antibiotics prodigiosins, decylprodigiosin (1), was identified and chemically characterized. Despite this family of natural products being well-known for a long time, we report a new analogue and the first evidence for prodigiosins being produced by a seaweed-associated actinomycete.

Inhibitory effect and mechanism of algicidal bacteria on Chaetomorpha valida

Chaetomorpha valida, filamentous green tide algae, poses a significant threat to both aquatic ecosystems and aquaculture. Vibrio alginolyticus Y20 is a new algicidal bacterium with an algicidal effect on C. valida. This study aimed to investigate the physiological and molecular responses of C. valida to exposure to V. alginolyticus Y20. The inhibitory effect of V. alginolyticus Y20 on C. valida was content dependent, with the lowest inhibitory content being 3 × 105 CFU mL-1. The microscopic results revealed that C. valida experienced severe morphological damage under the influence of V. alginolyticus Y20, with a dispersion of intracellular pigments. V. alginolyticus Y20 caused the decrease in chlorophyll-a content and Fv/Fm in C. valida. At the molecular level, V. alginolyticus Y20 downregulated the expression of genes related to photosynthetic pigment synthesis, light capture, and electron transport. Furthermore, V. alginolyticus Y20 induced oxidative damage to algal cells. The production of reactive oxygen species significantly increased after 11 days of exposure. Malondialdehyde content significantly increased, and the cell membranes were severely damaged by lipid peroxidation. The content of superoxide dismutase and peroxidase also markedly increased, whereas catalase content decreased significantly. Additionally, peroxisomes were inhibited due to the downregulation of PEX expression, leading to irreversible oxidative damage to algal cells. Our findings provided a new theoretical basis for exploring the interaction between algicidal bacteria and green tide algae at the molecular level.

Biological and Chemical Approaches for Controlling Harmful Microcystis Blooms

The proliferation of harmful cyanobacterial blooms dominated by Microcystis aeruginosa has become an increasingly serious problem in freshwater ecosystems due to climate change and eutrophication. Microcystis-blooms in freshwater generate compounds with unpleasant odors, reduce the levels of dissolved O2, and excrete microcystins into aquatic ecosystems, potentially harming various organisms, including humans. Various chemical and biological approaches have thus been developed to mitigate the impact of the blooms, though issues such as secondary pollution and high economic costs have not been adequately addressed. Red clays and H2O2 are conventional treatment methods that have been employed worldwide for the mitigation of the blooms, while novel approaches, such as the use of plant or microbial metabolites and antagonistic bacteria, have also recently been proposed. Many of these methods rely on the generation of reactive oxygen species, the inhibition of photosynthesis, and/or the disruption of cellular membranes as their mechanisms of action, which may also negatively impact other freshwater microbiota. Nevertheless, the underlying molecular mechanisms of anticyanobacterial chemicals and antagonistic bacteria remain unclear. This review thus discusses both conventional and innovative approaches for the management of M. aeruginosa in freshwater bodies.

Antagonistic actions of Paucibacter aquatile B51 and its lasso peptide paucinodin toward cyanobacterial bloom-forming Microcystis aeruginosa PCC7806

Superior antagonistic activity against axenic Microcystis aeruginosa PCC7806 was observed with Paucibacter sp. B51 isolated from cyanobacterial bloom samples among 43 tested freshwater bacterial species. Complete genome sequencing, analyzing average nucleotide identity and digital DNA-DNA hybridization, designated the B51 strain as Paucibacter aquatile. Electron and fluorescence microscopic image analyses revealed the presence of the B51 strain in the vicinity of M. aeruginosa cells, which might provoke direct inhibition of the photosynthetic activity of the PCC7806 cells, leading to perturbation of cellular metabolisms and consequent cell death. Our speculation was supported by the findings that growth failure of the PCC7806 cells led to low pH conditions with fewer chlorophylls and down-regulation of photosystem genes (e.g., psbD and psaB) during their 48-h co-culture condition. Interestingly, the concentrated ethyl acetate extracts obtained from B51-grown supernatant exhibited a growth-inhibitory effect on PCC7806. The physical separation of both strains by a filter system led to no inhibitory activity of the B51 cells, suggesting that contact-mediated anti-cyanobacterial compounds might also be responsible for hampering the growth of the PCC7806 cells. Bioinformatic tools identified 12 gene clusters that possibly produce secondary metabolites, including a class II lasso peptide in the B51 genome. Further chemical analysis demonstrated anti-cyanobacterial activity from fractionated samples having a rubrivinodin-like lasso peptide, named paucinodin. Taken together, both contact-mediated inhibition of photosynthesis and the lasso peptide secretion of the B51 strain are responsible for the anti-cyanobacterial activity of P. aquatile B51.

Genomic Insights into Paucibacter aquatile DH15, a Cyanobactericidal Bacterium, and Comparative Genomics of the Genus Paucibacter

Microcystis blooms threaten ecosystem function and cause substantial economic losses. Microorganism-based methods, mainly using cyanobactericidal bacteria, are considered one of the most ecologically sound methods to control Microcystis blooms. This study focused on gaining genomic insights into Paucibacter aquatile DH15 that exhibited excellent cyanobactericidal effects against Microcystis. Additionally, a pan-genome analysis of the genus Paucibacter was conducted to enhance our understanding of the ecophysiological significance of this genus. Based on phylogenomic analyses, strain DH15 was classified as a member of the species Paucibacter aquatile. The genome analysis supported that strain DH15 can effectively destroy Microcystis, possibly due to the specific genes involved in the flagellar synthesis, cell wall degradation, and the production of cyanobactericidal compounds. The pan-genome analysis revealed the diversity and adaptability of the genus Paucibacter, highlighting its potential to absorb external genetic elements. Paucibacter species were anticipated to play a vital role in the ecosystem by potentially providing essential nutrients, such as vitamins B7, B12, and heme, to auxotrophic microbial groups. Overall, our findings contribute to understanding the molecular mechanisms underlying the action of cyanobactericidal bacteria against Microcystis and shed light on the ecological significance of the genus Paucibacter.

Dynamic response of bacterial communities to Microcystis blooms: A three-year study

Although nutrient availability is widely recognized as the driving force behind Microcystis blooms, identifying the microorganisms that play a pivotal role in their formation is a challenging task. Our understanding of the contribution of bacterial communities to the development of Microcystis blooms remains incomplete, despite the fact that the relationship between Microcystis and bacterial communities has been extensively investigated. Most studies have focused on their interaction for a single year rather than for multiple years. To determine key bacteria crucial for the formation of Microcystis blooms, we collected samples from three sites in the Daechung Reservoir (Chuso, Hoenam, and Janggye) over three years (2017, 2019, and 2020). Our results indicated that Microcystis bloom-associated bacterial communities were more conserved across stations than across years. Bacterial communities could be separated into modules corresponding to the different phases of Microcystis blooms. Dolichospermum and Aphanizomenon belonged to the same module, whereas the module of Microcystis was distinct. The microbial recurrent association network (MRAN) showed that amplicon sequence variants (ASVs) directly linked to Microcystis belonged to Pseudanabaena, Microscillaceae, Sutterellaceae, Flavobacterium, Candidatus Aquiluna, Bryobacter, and DSSD61. These ASVs were also identified as key indicators of the bloom stage, indicating that they were fundamental biological elements in the development of Microcystis blooms. Overall, our study highlights that, although bacterial communities change annually, they continue to share core ASVs that may be crucial for the formation and maintenance of Microcystis blooms.

ROS-dependent cell death of Heterosigma akashiwo induced by algicidal bacterium Hahella sp. KA22

Marine algicidal bacteria and their metabolites are considered to be one of the most effective strategies to mitigate the harmful algal blooms (HABs). The bacterium Hahella sp. KA22 has previously been confirmed to have strong algicidal activity against the HABs causing microalgae, Heterosigma akashiwo. In this study, the molecular mechanism of microalgae cell death was detected. The results showed that the cell growth rate and photosynthetic efficiency were inhibited with addition of algicidal strain KA22, while the accumulation of reactive oxygen species (ROS) and oxidative damage in H. akashiwo cells increased. A total of 2056 unigenes were recognized to be differentially expressed in transcriptome sequences. In particular, the transcriptional levels of light-harvesting pigments and structural proteins in the oxygen-evolving-complex were continuously down-regulated, corresponding to the significant reduction of photosynthetic efficiency and the accumulation of ROS. Furthermore, glutamate dehydrogenase was significantly up-regulated in abundance. Meanwhile, calcium-dependent protein kinases were also detected with significant changes. Collectively, algicidal stress caused the suppressed electron transfer in chloroplast and impaired detoxification of intracellular oxidants by glutathione, which may subsequently result in multiple cell regulation and metabolic responses and ultimately lead to the ROS-dependent cell death of H. akashiwo.

Effects of perfluorooctanoic acid on Microcystis aeruginosa: Stress and self-adaptation mechanisms

The persistent organic pollutant perfluorooctanoic acid (PFOA) is ubiquitous in aquatic environments. However, little is known about its toxicity to microalgae or the mechanisms by which they may self-adapt to it. We found that growth of the bloom-forming cyanobacterium Microcystis aeruginosa was initially inhibited, with inhibition attenuated after 12 d of PFOA exposure. Growth inhibition gradually decreased and stabilized over time. With increasing PFOA concentration, reactive oxygen species levels and superoxide dismutase and photosystem II activity significantly increased, while respiration, NDH-1 activity, and total carbohydrate content significantly decreased. Self-adaptation mechanisms included antioxidant pathways, energy transfer and distribution of photosystems, and repair of the PSI and NDH complexes. The patterns of change in these parameters were consistent with those of the expression levels of genes in their associated metabolic pathways. Our data suggest that PSII overcompensation might be a strategy by which M. aeruginosa contends with oxidative stress induced by PFOA. Multiple downstream photosynthesis-related proteins were upregulated as a function of PFOA exposure time. These findings may help elucidate physiological, genetic stress and self-adaptive responses of microalgae to PFOA exposure.

The effects of extracellular algicidal compounds of Bacillus sp. B1 on Heterosigma akashiwo: a metabolomics approach

Heterosigma akashiwo (H. akashiwo), a harmful algal species, has been a global environmental problem. Extracellular algicidal compounds (EACs) extracted from Bacillus sp. B1 exhibited algicidal effects against H. akashiwo. However, little is known about the algicidal mechanism and metabolic process. In this study, metabolomics and physiological analyses were combined to investigate the cellular responses of H. akashiwo when treated with EACs. The results indicated that EACs at 10% (v_EACs/v_sample) showed more than 90% inhibition of H. akashiwo. EAC treatment resulted in excessive reactive oxygen species (ROS) production in algal cells, causing stress responses such as inhibition of photosynthetic pigment synthesis, reduction of sugar synthesis, imbalance of osmotic pressure in the cell membrane, disruption of cell size and morphology, and eventual cell death. The results reveal the underlying mechanism of the algicidal process and provide new insights into algae-bacteria interactions and the application of metabolomics to algal research.

Response of cytotoxin production ability to gene expression and cell molecular structure of Microcystis aeruginosa FACHB-905

To investigate the inhibitory mechanism of artemisinin sustained-release microspheres (ASMs) on Microcystis aeruginosa (M. aeruginosa) from the molecular level, prx, psbA, fabZ, and mcyD were studied, and the cell death mode were also explored. The results showed that expression of prx was slightly up-regulated, while the expression of psbA, fabZ, and mcyD was significantly reduced. It can infer that oxidant damage and photic damage are the main mechanisms for the algicidal effect of ASMs on M. aeruginosa. It can be seen from the changes in cell morphology and structure that microspheres stress triggers apoptosis-like cell death, and the cell membrane is intact effectively preventing the leakage of microcystin-LR (MC-LR). Moreover, the down-regulation of mcyD gene also played major role in less extracellular MC-LR than intracellular MC-LR. It was concluded that the ASMs will not cause secondary ecological hazards while killing algae cells and have good application prospects.

Effect of three commercial algaecides on cyanobacteria and microcystin-LR: implications for drinking water treatment using activated carbon

Toxic cyanobacterial blooms in aquatic ecosystems are associated to both public health and environmental concerns worldwide. Depending on the treatment technologies used, the removal capacity of cyanotoxins by drinking water treatment plants (DWTPs) is not sufficient to reach safe levels in drinking water. Likewise, controlling these blooms with algaecide may impair the efficiency of DWTPs due to the possible lysis of cyanobacterial cells and consequent release of cyanotoxins. We investigated the effects of three commercial algaecides (cationic polymer, copper sulfate, and hydrogen peroxide) on the growth parameters of the cyanobacterium Microcystis aeruginosa and the release of microcystin-LR (MC-LR). The potential interference of each algaecide on the MC-LR removal by adsorption on activated carbon (AC) was also tested through adsorption isotherms and kinetics experiments. Most algaecides significantly decreased the cell density and biovolume of M. aeruginosa, as well as increased the release of MC-LR. Interestingly, the presence of the algaecides in binary mixtures with MC-LR affected the adsorption of the cyanotoxin. Relevant adsorption parameters (e.g., maximum adsorption capacity, adsorption intensity, and affinity between MC-LR and AC) were altered when the algaecides were present, especially in the case of the cationic polymer. Also, the algaecides influenced the kinetics (e.g., by shifting the initial adsorption and the desorption constant), which may directly affect the design and operation of DWTPs. Our study indicated that algaecides can significantly impact the fate and the removal of MC-LR in DWTPs when the adsorption process is employed, with important implications for the management and performance of such facilities.

Transcriptomic analysis provides insights into the algicidal mechanism of cocamidopropyl betaine against the red tide microalgae Skeletonema costatum

This study investigated the effect of the surfactant cocamidopropyl betaine (CAB) on the growth of red tide microalgae Skeletonema costatum. It was found that CAB caused cell lysis in a time- and dose-dependent manner and significantly inhibited the growth of S. costatum. Additionally, the transcriptomic approach was coupled with physiological analysis to elucidate the inhibitory mechanism of CAB on S. costatum. Among the 30726 genes identified, 17720 and 20583 genes were differentially expressed after treatment for 3 h and 6 h, respectively, which revealed that CAB redirected metabolic pathways, of which the expressions of genes related to the proteasome, ABC transporters, and amino acid-related metabolism were significantly upregulated, while genes involved in photosynthesis, biofilm and cell wall synthesis, mitogen-activated protein kinase (MAPK) cascades and antioxidant system were downregulated. The results above corresponded to the decreasing antioxidant enzymes activities, protein and photosynthetic pigments contents, as well as the increasing malondialdehyde (MDA) content. Our study presented herein shed light on the algicidal mechanism of CAB at the transcriptome level and was useful to red tide control, and marine environmental protection.

Paraquat induces different programmed cell death patterns in Microcystis aeruginosa and Chlorella luteoviridis

Although programmed cell death (PCD) has been reported in phytoplankton, knowledge of the characterization of the PCD pathway and cascade process in different phytoplankton species is still limited. In this study, PCD progression in cyanobacterium Microcystis aeruginosa and green algae Chlorella luteoviridis by paraquat-induced oxidative stress was monitored. The results showed that paraquat-induced PCD in the two species belonged to the caspase-dependent pathway. Dose- and time-dependent PCD characteristics in the two strains under paraquat included the increase in caspase-like activity, DNA fragmentation, and chromatin condensation. However, the signaling pathway and cascade events of PCD in M. aeruginosa and C. luteoviridis differed. In M. aeruginosa, the free Ca²⁺ concentration was rapidly increased at 8 h, followed by a significant elevation of the reactive oxygen species (ROS) level at 24 h, and eventual cell death. In C. luteoviridis, the mitochondrial apoptosis pathway, revealed by the depolarization of the mitochondrial membrane potential at 1 h and increase in the ROS level and caspase-like activity at 8 h, might contribute to cell death. In addition, the dynamics of ROS levels and metacaspase activity were synchronized, suggesting that paraquat-triggered PCD was ROS-mediated in both M. aeruginosa and C. luteoviridis. These results provide insights into PCD patterns in prokaryotic cyanobacteria and eukaryotic green algae under similar stress.

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.

Exogenous melatonin promotes the growth of alfalfa (Medicago sativa L.) under NaCl stress through multiple pathways

Salinity is one of the most common factors affecting alfalfa (Medicago sativa L.), and NaCl is one of the main factors of salinity stress which can cause heavy losses in agricultural production in the world. The application of exogenous melatonin (MT) plays a major role in numerous plants against various stress environments. The effects of exogenous MT on the NaCl tolerance of alfalfa treated with the control, 100 µmol L^-1 MT, 150 mmol L^-1 NaCl, or 150 mmol L^-1 NaCl+ 100 µmol L^-1 MT were investigated. The results showed that MT increased growth parameters, inhibited chlorophyll degradation and promoted photosynthetic gas exchange parameters (photosynthetic rate, conductance to H₂O, and transpiration rate) and stomatal opening under NaCl stress. Osmotic regulation substances such as soluble sugar, proline and glycine betaine were the highest in the NaCl treatment and the second in the NaCl+MT treatment. Nitrogen, phosphorus, potassium, calcium and magnesium were reduced and sodium was increased by NaCl, whereas these levels were reversed by the NaCl+MT treatment. MT inhibited cell membrane imperfection, lipid peroxidation and reactive oxygen species (ROS) accumulation caused by NaCl stress. MT up-regulated the gene expression and activity of antioxidant enzymes and increased the content of antioxidant non-enzyme substances to scavenge excessive ROS in NaCl-treated plants. In addition, all indicators interacted with each other to a certain extent and could be grouped according to the relative values. All variables were divided into PC 1 (89.2 %) and PC 2 (4 %). They were clustered into two categories with opposite effects, and most of them were significant variables. Hence, these findings reveal that exogenous MT alleviates the inhibitory effects of NaCl stress on photosynthesis, stomata opening, osmotic adjustment, ion balance and redox homeostasis, enhancing tolerance and growth of alfalfa. Furthermore, it suggests that MT could be implemented to improve the NaCl tolerance of alfalfa.

To Die or Not to Die—Regulated Cell Death and Survival in Cyanobacteria

Regulated cell death (RCD) is central to the development, integrity, and functionality of multicellular organisms. In the last decade, evidence has accumulated that RCD is a universal phenomenon in all life domains. Cyanobacteria are of specific interest due to their importance in aquatic and terrestrial habitats and their role as primary producers in global nutrient cycling. Current knowledge on cyanobacterial RCD is based mainly on biochemical and morphological observations, often by methods directly transferred from vertebrate research and with limited understanding of the molecular genetic basis. However, the metabolism of different cyanobacteria groups relies on photosynthesis and nitrogen fixation, whereas mitochondria are the central executioner of cell death in vertebrates. Moreover, cyanobacteria chosen as biological models in RCD studies are mainly colonial or filamentous multicellular organisms. On the other hand, unicellular cyanobacteria have regulated programs of cellular survival (RCS) such as chlorosis and post-chlorosis resuscitation. The co-existence of different genetically regulated programs in cyanobacterial populations may have been a top engine in life diversification. Development of cyanobacteria-specific methods for identification and characterization of RCD and wider use of single-cell analysis combined with intelligent image-based cell sorting and metagenomics would shed more light on the underlying molecular mechanisms and help us to address the complex colonial interactions during these events. In this review, we focus on the functional implications of RCD in cyanobacterial communities.

Monitoring and control methods of harmful algal blooms in Chinese freshwater system: a review

Harmful algal blooms (HABs) are a worldwide problem with substantial adverse effects on the aquatic environment as well as human health, which have prompted researchers to study measures to stem and control them. Meanwhile, it is key to research and develop monitoring methods to establish early warning HABs. However, both the current monitoring methods and control methods have some shortcomings, making the field application limited. Thus, we need to improve current approaches for monitoring and controlling HABs efficiently. Based on the freshwater system features in China, we review various monitoring and control methods of HABs, summarize and discuss the problems with these methods, and propose the future development direction of monitoring and control HABs. Finally, we envision that it can combine physical, chemical, and biological methods to inhibit HAB expansion in the future, complementing each other with advantages. Further, we promise to establish a long-term strategy of controlling HABs with various algicidal bacteria co-cultivate for field applications in China. Efforts in studying algicidal bacteria must be increased to better control HABs and mitigate the risks of aquatic ecosystems and human health in China.

In silico insight of cell-death-related proteins in photosynthetic cyanobacteria

Cyanobacteria are a large group of ubiquitously found photosynthetic prokaryotes that are constantly exposed to different kinds of stressors of varying intensities and seem to overcome these in a precise and regulated manner. However, a high dose and duration of given stress induce cell death in a few select cyanobacteria, mainly to protect other cells (altruism). Despite the recent findings for the presence of biochemical and molecular hallmarks of cell death in cyanobacteria, it is yet a sketchily understood phenomenon. Regulation of metacaspase-like genes during Programmed Cell Death suggests it to be a genetically controlled mechanism like other eukaryotes. In addition to providing a comprehensive understanding of the current status of cell death in cyanobacteria, this review has used in silico analyses to directly compare the existence of some important molecular players operating in the intrinsic and extrinsic apoptotic pathways. Phylogenetic trees for all sequences indicate a cluster with a common ancestry and also a divergence from sequences of eukaryotic origin. To the best of our knowledge, such a comparison (except for orthocaspases) has not been attempted earlier and hopes to encourage workers in the field to investigate this altruistic phenomenon in detail.

Transcriptome Analysis Reveals the Algicidal Mechanism of Brevibacillus laterosporus against Microcystis aeruginosa through Multiple Metabolic Pathways

It is widely accepted that eutrophication has played an important role in the formation of harmful cyanobacterial blooms in recent decades, which impacts water quality and ecological environment and causes huge economic losses. Algicidal bacteria have a promising application prospect in controlling cyanobacterial blooms in aquaculture water. Here, the process of the algicidal bacterium Brevibacillus laterosporus strain Bl-zj acting on Microcystis aeruginosa was explored using transcriptome analysis to elucidate the algicidal mechanism. The results of the co-culture of bacterium and alga showed a strong alga-lysing effect of B. laterosporus against M. aeruginosa with an extreme morphology deformation of the algal cells. A total of 2744 differentially expressed genes of B. laterosporus were identified, which were mainly involved in the metabolism of amino acid, carbohydrate, and lipid. In the co-cultured group, the expression of genes mainly enriched in valine, leucine and isoleucine degradation, and fatty acid degradation were significantly increased. However, the expression of the genes related to ribosome were mainly inhibited. Transcriptome analysis showed that B. laterosporus obtained ATP and energy by the degradation of valine, leucine, isoleucine, and fatty acids, and destroyed algal cells by efflux pump transporters, secretion of hydrolytic enzymes, antibiotics, proteases, and other secondary metabolites, resulting in algal death and achieving the algicidal effect.

The Effect of Algicidal and Denitrifying Bacteria on the Vertical Distribution of Cyanobacteria and Nutrients

Algicidal bacteria combined with the ability of aerobic denitrification is considered to be a promising way to control harmful cyanobacterial bloom and remove nitrogen. However, the effect of these bacteria on the vertical distribution of colonial cyanobacteria and nutrients remained unknown. In this study, two algicidal and denitrifying bacteria were respectively co-cultured with the colonial Microcystis aeruginosa to construct the microcosm systems, and then the cyanobacteria number, the ratio of bacterial to cyanobacterial abundance, the content of dissolved nitrogen, phosphorus and organic carbon in different water layers were investigated. The results showed that the distribution difference of Microcystis among the vertical water layers was further enlarged due to the short-term influence of algicidal bacteria Brevundimonas diminuta and Pseudomonas stutzeri. The number of Microcystis in the lower layer was further reduced by the inhibitory effect of the algicidal bacteria. However, there was a dramatic increase in the number of Microcystis in the upper layer, even when the ratio of algicidal bacteria to cyanobacteria increased significantly. B. diminuta and P. stutzeri both greatly promoted the removal of dissolved total nitrogen in the upper and middle layers of cyanobacteria blooming water, but they also boosted the release of dissolved phosphorus in all layers. These results enable us to better understand the possible limitations of algicidal bacteria in their application to control cyanobacteria blooms.

Identifying Algicides of Enterobacter hormaechei F2 for Control of the Harmful Alga Microcystis aeruginosa

Eutrophication has become an increasingly serious environmental issue and has contributed towards an explosion in harmful algal blooms (HABs) affecting local development. HABs can cause serious threats to ecosystems and human health. A newly isolated algicidal strain, Enterobacter hormaechei F2, showed high algicidal activity against the typical HAB species Microcystis aeruginosa. Potential algicides were detected through liquid chromatograph–mass spectrometer analysis, revealing that prodigiosin is an algicide and PQS is a quorum sensing molecule. RNA-seq was used to understand the algicidal mechanisms and the related pathways. We concluded that the metabolism of prodigiosin and PQS are active at the transcriptional level. The findings indicate that E. hormaechei F2 can be used as a potential biological agent to control harmful algal blooms to prevent the deterioration of the ecological and economic value of water bodies.

Algicide capacity of Paucibacter aquatile DH15 on Microcystis aeruginosa by attachment and non-attachment effects

The excessive proliferation of Microcystis aeruginosa can lead to ecological damage, economic losses, and threaten animal and human health. For controlling Microcystis blooms, microorganism-based methods have attracted much attention from researchers because of their eco-friendliness and species-specificity. Herein, we first found that a Paucibacter strain exhibits algicidal activity against M. aeruginosa and microcystin degradation capability. The algicidal activity of DH15 (2.1 × 10⁴ CFU/ml) against M. aeruginosa (2 × 10⁶ cells/ml) was 94.9% within 36 h of exposure. DH15 also degraded microcystin (1.6 mg/L) up to 62.5% after 72 h. We demonstrated that the algicidal activity of DH15 against M. aeruginosa can be mediated by physical attachment and indirect attack: (1) Both washed cells and cell-free supernatant could kill M. aeruginosa efficiently; (2) Treatment with DH15 cell-free supernatants caused oxidative stress, altered the fatty acid profile, and damaged photosynthetic system, carbohydrate, and protein metabolism in M. aeruginosa. The combination of direct and indirect attacks supported that strain DH15 exerts high algicidal activity against M. aeruginosa. The expression of most key genes responsible for photosynthesis, antioxidant activity, microcystin synthesis, and other metabolic pathways in M. aeruginosa was downregulated. Strain DH15, with its microcystin degradation capacity, can overcome the trade-off between controlling Microcystis blooms and increasing microcystin concentration. Our findings suggest that strain DH15 possesses great potential to control outbreaks of Microcystis blooms.

Recent Advances in the Research on the Anticyanobacterial Effects and Biodegradation Mechanisms of Microcystis aeruginosa with Microorganisms

Harmful algal blooms (HABs) have attracted great attention around the world due to the numerous negative effects such as algal organic matters and cyanobacterial toxins in drinking water treatments. As an economic and environmentally friendly technology, microorganisms have been widely used for pollution control and remediation, especially in the inhibition/biodegradation of the toxic cyanobacterium Microcystis aeruginosa in eutrophic water; moreover, some certain anticyanobacterial microorganisms can degrade microcystins at the same time. Therefore, this review aims to provide information regarding the current status of M. aeruginosa inhibition/biodegradation microorganisms and the acute toxicities of anticyanobacterial substances secreted by microorganisms. Based on the available literature, the anticyanobacterial modes and mechanisms, as well as the in situ application of anticyanobacterial microorganisms are elucidated in this review. This review aims to enhance understanding the anticyanobacterial microorganisms and provides a rational approach towards the future applications.

A Novel Algicidal Bacterium and Its Effects against the Toxic Dinoflagellate Karenia mikimotoi (Dinophyceae)

The toxic dinoflagellate Karenia mikimotoi is a harmful algal bloom-forming species in coastal areas around the world. It produces ichthyotoxins and hemolytic toxins, with deleterious effects on marine ecosystems. In this study, the bacterium Pseudoalteromonas sp. FDHY-MZ2, with high algicidal efficiency against K. mikimotoi, was isolated from a bloom event. Based on the results, it completely lysed K. mikimotoi cells within 24 h 0.5% (vol/vol), with the algicidal activity of the supernatant of the bacterium culture. Algal cell wall fragmentation occurred, leading to cell death. There was a marked decline in various photochemical traits. When treated with the supernatant, cellulase, pheophorbide a oxygenase (PAO) and cyclin B genes were significantly increased, suggesting induced cell wall deterioration, chloroplast degradation and cell cycle regulation of K. mikimotoi cells. In addition, the expression levels of reactive oxygen species (ROS) scavenging gene was significantly inhibited, indicating that the ROS removal system was damaged. The bacterial culture was dried to obtain the spray-dried powder, which showed algicidal activity rates of 92.2 and 100% against a laboratory K. mikimotoi culture and a field microcosm of Karlodinium sp. bloom within 24 h with the addition of 0.04% mass fraction powder. Our results demonstrate that FDHY-MZ2 is a suitable strain for K. mikimotoi and Karlodinium sp. blooms management. In addition, this study provides a new strategy for the anthropogenic control of harmful algal bloom-forming species in situ. IMPORTANCE K. mikimotoi is a noxious algal bloom-forming species that cause damaging of the aquaculture industry and great financial losses. Bacterium with algicidal activity is an ideal agency to inhibit the growth of harmful algae. In this approach application, the bacterium with high algicidal activity is required and the final management material is ideal for easy-to-use. The algicidal characteristics are also needed to understand the effects of the bacterium for managing strategy exploration. In this study, we isolated a novel algicidal bacterium with extremely high lysis efficiency for K. mikimotoi. The algicidal characteristics of the bacterium as well as the chemical and molecular response of K. mikimotoi with the strain challenge were examined. Finally, the algicidal powder was explored for application. The results demonstrate that FDHY-MZ2 is suitable for K. mikimotoi and Karlodinium sp. blooms controlling, and this study provides a new strategy for algicidal bacterium application.

Cercosporin-bioinspired photoinactivation of harmful cyanobacteria under natural sunlight via bifunctional mechanisms

Harmful cyanobacterial blooms (HCBs), mainly caused by eutrophication, have deleterious impacts on water resources and pose a great threat to human health and natural ecosystems. Thus, an environmentally-friendly method to inhibit HCBs is urgently needed. Learning from nature, herein, natural product cercosporin, produced by the fungi Cercospora to damage plant cells under natural sunlight, was developed as a powerful photosensitive algicidal reagent to inhibit HCBs. Microcystis aeruginosa could be severely inactivated by 20 μM cercosporin in 36 h with 95% inhibition ratio under 23 W compact fluorescent light irradiation. Further mechanism investigation showed that algal cell walls and membranes along with the antioxidant and photosynthetic systems were damaged via two mechanisms, those being, reactive oxygen species generation and cell adsorption. More importantly, the practical applicability of cercosporin was demonstrated by its effectiveness in a 2 L-scale photoinactivation experiment using cyanobacterial blooms from Taihu Lake, China under natural sunlight with a lower dosage of cercosporin (7.5 μM). This study established the bifunctional mechanisms by which cercosporin inactivates HCBs, opening design possibilities for the development of novel photosensitive algicidal reagents to control HCBs.

Linear Six-Carbon Sugar Alcohols Induce Lysis of Microcystis aeruginosa NIES-298 Cells

Cyanobacterial blooms are a global concern due to their adverse effects on water quality and human health. Therefore, we examined the effects of various compounds on Microcystis aeruginosa growth. We found that Microcystis aeruginosa NIES-298 cells were lysed rapidly by linear six-carbon sugar alcohols including mannitol, galactitol, iditol, fucitol, and sorbitol, but not by other sugar alcohols. Microscopic observations revealed that mannitol treatment induced crumpled inner membrane, an increase in periplasmic space, uneven cell surface with outer membrane vesicles, disruption of membrane structures, release of intracellular matter including chlorophylls, and eventual cell lysis in strain NIES-298, which differed from the previously proposed cell death modes. Mannitol metabolism, antioxidant-mediated protection of mannitol-induced cell lysis by, and caspase-3 induction in strain NIES-298 were not observed, suggesting that mannitol may not cause organic matter accumulation, oxidative stress, and programmed cell death in M. aeruginosa. No significant transcriptional expression was induced in strain NIES-298 by mannitol treatment, indicating that cell lysis is not induced through transcriptional responses. Mannitol-induced cell lysis may be specific to strain NIES-298 and target a specific component of strain NIES-298. This study will provide a basis for controlling M. aeruginosa growth specifically by non-toxic substances.

Algicidal Properties of Microbial Fermentation Products on Inhibiting the Growth of Harmful Dinoflagellate Species

The fermentation processes of algicidal bacteria offer an eco-friendly and promising approach for controlling harmful algae blooms (HABs). The strain Ba3, previously isolated and identified as Bacillus sp., displays robust algicidal activity against HABs dinoflagellate in particular. Microbial fermentation products have also been found to provide metabolites with multiple bioactivities, which has been shown to reduce harmful algae species’ vegetative cells and thus reduce red tide outbreaks. In this study, the microbial fermentation of algicidal bacterium Ba3 was analyzed for its potential ability of algicidal compounds. A treatment time increased the algicidal efficiency of the fermentation products against Prorocentrum donghaiense (91%) and Alexandrium tamarense (82%). Among the treatment groups, the changing trend for the 2% treatment group was faster than that for the other treatments, showing that the inhibition rate could reach 99.1% in two days. Active components were separated by organic solvent extraction and macroporous resin, and the molecular weight of the active components was analyzed by LC-MS. The result shows that the microbial fermentation products offer a potential, not practical use for controlling the outbreaks of dinoflagellate blooms. As a result of its potential application for inhibiting HABs, these findings provide an encouraging basis for promoting large-scale fermentation production and the controlling the outbreaks of red tide.

A Novel Algicidal Bacterium, Microbulbifer sp. YX04, Triggered Oxidative Damage and Autophagic Cell Death in Phaeocystis globosa, Which Causes Harmful Algal Blooms

Phaeocystis globosa causes severe marine pollution by forming harmful algal blooms and releasing hemolytic toxins and is therefore harmful to marine ecosystems and aquaculture industries. In this study, Microbulbifer sp. YX04 exerted high algicidal activity against P. globosa by producing and secreting metabolites. The algicidal activity of the YX04 supernatant was stable after exposure to different temperatures (-80 to 100°C) and pH values (4 to 12) for 2 h, suggesting that algicidal substances could temporarily be stored under these temperature and pH value conditions. To explore the algicidal process and mechanism, morphological and structural changes, oxidative stress, photosynthesis, autophagic flux, and global gene expression were investigated. Biochemical analyses showed that the YX04 supernatant induced reactive oxygen species (ROS) overproduction, which caused lipid peroxidation and malondialdehyde (MDA) accumulation in P. globosa. Transmission electron microscopy (TEM) observation and the significant decrease in both maximum photochemical quantum yield (Fv/Fm) and relative electron transfer rate (rETR) indicated damage to thylakoid membranes and destruction of photosynthetic system function. Immunofluorescence, immunoblot, and TEM analyses indicated that cellular damage caused autophagosome formation and triggered large-scale autophagic flux in P. globosa. Transcriptome analysis revealed many P. globosa genes that were differentially expressed in response to YX04 stress, most of which were involved in photosynthesis, respiration, cytoskeleton, microtubule, and autophagosome formation and fusion processes, which may trigger autophagic cell death. In addition to P. globosa, the YX04 supernatant showed high algicidal activity against Thalassiosira pseudonana, Thalassiosira weissflogii, Skeletonema costatum, Heterosigma akashiwo, and Prorocentrum donghaiense. This study highlights multiple mechanisms underlying YX04 supernatant toxicity toward P. globosa and its potential for controlling the occurrence of harmful algal blooms. IMPORTANCEP. globosa is one of the most notorious harmful algal bloom (HAB)-causing species, which can secrete hemolytic toxins, frequently cause serious ecological pollution, and pose a health hazard to animals and humans. Hence, screening for bacteria with high algicidal activity against P. globosa and studies on the algicidal characteristics and mechanism will contribute to providing an ecofriendly microorganism-controlling agent for preventing the occurrence of algal blooms and reducing the harm of algal blooms to the environment. Our study first reported the algicidal characteristic and mechanism of Microbulbifer sp. YX04 against P. globosa and demonstrated that P. globosa shows different response mechanisms, including movement ability, antioxidative systems, photosynthetic systems, gene expression, and cell death mode, to adapt to the adverse environment when algicidal compounds are present.

Different Algicidal Modes of the Two Bacteria Aeromonas bestiarum HYD0802-MK36 and Pseudomonas syringae KACC10292T against Harmful Cyanobacteria Microcystis aeruginosa

Blooms of harmful cyanobacteria Microcystis aeruginosa lead to an adverse effect on freshwater ecosystems, and thus extensive studies on the control of this cyanobacteria’s blooms have been conducted. Throughout this study, we have found that the two bacteria Aeromonas bestiarum HYD0802-MK36 and Pseudomonas syringae KACC10292^T are capable of killing M. aeruginosa. Interestingly, these two bacteria showed different algicidal modes. Based on an algicidal range test using 15 algal species (target and non-target species), HYD0802-MK36 specifically attacked only target cyanobacteria M. aeruginosa, whereas the algicidal activity of KACC10292^T appeared in a relatively broad algicidal range. HYD0802-MK36, as a direct attacker, killed M. aeruginosa cells when direct cell (bacterium)-to-cell (cyanobacteria) contact happens. KACC10292^T, as an indirect attacker, released algicidal substance which is located in cytoplasm. Interestingly, algicidal activity of KACC10292^T was enhanced according to co-cultivation with the host cyanobacteria, suggesting that quantity of algicidal substance released from this bacterium might be increased via interaction with the host cyanobacteria.

Toxicological effects of hypoxanthine on Heterosigmaakashiwo: Mechanism of growth inhibition and change in hemolytic toxin content

Heterosigmaakashiwo is an algal species that causes harmful algal blooms (HABs) with strong hemolytic toxicity on coastal aquatic organisms. This study investigated the mechanism of growth inhibition and changes in hemolytic toxin contents in algal culture after exposure to hypoxanthine, a compound secreted by algicidal bacterium Bacillus sp.strain B1. An algal inhibition rate of 86% was observed with 1.0 mM hypoxanthine treatment on day 15. The levels of superoxide dismutase and catalase in algal cell culture increased while that of glutathione decreased during the treatment. In addition,the level of hemolytic toxin contents increased on day 3 under hypoxanthine treatment, and significantly decreased on days 6, 9, 12, and 15. Twelve fatty acids in H.akashiwo were detected by GC-MS, and the changes in the contents of C16, C18, C18:4ω3, and C20:5ω3 in the treatment group were consistent with the change in hemolytic toxin content. The four fatty acids were tested for hemolysis and it was observed that the hemolysis rate of 25 μg/mL C18:4ω3 and 5 μg/mL C20:5ω3 reached more than 80%, but C16 and C18 exhibited no hemolytic capability.Therefore, our results showed that hypoxanthine inhibited the growth of H. akashiwo through the changes of levels of antioxidants and hemolytic toxin content in the cultures, and fatty acids C18:4ω3 and C20:5ω3 were contributors to hemolytic toxins. The results confirmed that hypoxanthine is a potential algal inhibitor to prevent HABs.

Effects on photosynthetic and antioxidant systems of harmful cyanobacteria by nanocrystalline Zn-MOF-FA

Metal-organic frameworks (MOFs) constitute new class of materials recently used by researchers in the field of controlling cyanobacteria. However, the use of MOFs in combination with allelochemicals for cyanobacteria inhibition had not been investigated before. The present study is aimed towards the investigation of the effect and mechanism of cyanobacteria inhibition by combining MOF with allelochemical (ferulic acid, FA) for the first time. In this study, the results showed that the synergistic effect of Zn²⁺ and FA from Zn-MOF-FA could inhibit cyanobacteria to a greater extent than the corresponding dosage of Zn²⁺ and FA. The inhibition ratio of Microcystis aeruginosa has been found to be more than 50% when the Zn-MOF-FA concentration exceeds 2 mg·L^-1 after four days exposure. Zn-MOF-FA at 1 mg·L^-1 did not completely inhibit M. aeruginosa, and the inhibition effect has been of only temporary type. The inhibitory effect of Zn-MOF-FA on algae has mainly been attributed to the hindrance of electron transfer and energy capture in the photosynthetic system and the oxidative damage caused by reactive oxygen species (ROS).

The potential of prodigiosin for control of Prorocentrum donghaiense blooms: Algicidal properties and acute toxicity to other marine organisms at various trophic levels

Prorocentrum donghaiense, a marine dinoflagellate, causes harmful algal blooms (HABs) characterised by the highest outbreak frequency and most extensive coverage among similar species in the East China Sea. Highly efficient and ecofriendly biocontrol strategies should be developed for HAB control. Prodigiosin is an efficient biological algicide that demonstrated strong algicidal activity towards P. donghaiense. However, the mechanism of its toxicity to P. donghaiense is unknown. These factors were investigated to evaluate potential use of prodigiosin for control of P. donghaiense blooms. Photosynthetic electron transport rate, maximum quantum yield and respiration rate of P. donghaiense decreased significantly upon exposure to prodigiosin, indicating that prodigiosin rapidly exerted adverse effects on the chloroplasts and mitochondria. Furthermore, a significant increase in dichlorofluorescein fluorescence intensity indicated an overproduction of reactive oxygen species (ROS). The antioxidant system of P. donghaiense scavenged ROS; however, an increase in malondialdehyde concentrations indicated that excessive ROS were still able to initiate lipid peroxidation. Thus, ROS production resulted in the formation of lipids with a reduced degree of unsaturation. Lipid peroxidation decreased lipid fluidity and rigidified the membrane system, causing serious functional destruction of the membrane. Flow cytometry analysis indicated that prodigiosin arrested the cell cycle of P. donghaiense. However, surviving algal cells were able to repair the damaged functions and resume the cell cycle after prodigiosin was removed by photodegradation. Otherwise, P. donghaiense cells lost their membrane integrity and died. To begin an evaluation of ecological safety of prodigiosin, we tested four marine organisms at various trophic levels. The results of these tests indicated that Chlorella vulgaris, Photobacterium phosphoreum, Artemia salina and Lateolabrax japonicus were less sensitive to prodigiosin than P. donghaiense. Toxicity to all five organisms declined after prodigiosin was exposed to sunlight for 6 h. Considering the toxic doses of prodigiosin to various organisms and its photodegradation characteristics, we suggest that prodigiosin has potential in controlling P. donghaiense blooms but should be applied at night, in small doses, with multiple applications.

The effects and mechanisms of polystyrene and polymethyl methacrylate with different sizes and concentrations on Gymnodinium aeruginosum

In this study, Gymnodinium aeruginosum was exposed to polystyrene (PS) and polymethyl methacrylate (PMMA) of three particle sizes (0.1 μm, 1.0 μm and 100 μm) and two concentrations (10 mg/L and 75 mg/L) for 96 h. The density of algae cells, the endpoints that reactive oxygen species (ROS), total protein (TP), malondialdehyde (MDA), superoxide dismutase (SOD) and catalase (CAT), scanning and transmission electron microscopy (SEM and TEM) were used to explore the toxicity mechanism to the microalgae. At a concentration of 75 mg/L, the 96 h inhibition ratios (IR) with particle sizes of 0.1 μm, 1.0 μm and 100 μm on G. aeruginosum were 55.9%, 63.7% and 6.0% for PS, respectively, and 3.0%, 4.1% and -0.6% for PMMA, respectively. The most significant changes in ROS, TP, MDA, SOD and CAT were observed at 75 mg/L 1.0 μm of PS when treated for 96 h. When exposed to nanoplastics (NPs) and microplastics (MPs), the algae cells were damaged, and the antioxidant system was activated. Extracellular polymeric substance (EPS) could help to detoxify the algae. In general, PS was more toxic than PMMA. The toxicity of small MNPs (0.1 μm and 1.0 μm) was related to the concentrations, while large MNPs (100 μm) did not.

Effects of allelochemical artemisinin in Artemisia annua on Microcystis aeruginosa: growth, death mode, and microcystin-LR changes

To investigate the effects of an allelochemical artemisinin extracted from Artemisia annua (A. annua) on cell growth, death mode, and microcystin-LR (MC-LR) changes of Microcystis aeruginosa (M. aeruginosa), a series of morphological and biochemical characteristics were studied. The results showed that artemisinin could inhibit the growth of M. aeruginosa and reduce the content of phycobiliprotein. Under the allelopathy of artemisinin, algae cells deformed due to swelling, which caused cell membranes to rupture and cell contents to leak. FDA/PI double-staining results showed that 15.10-94.90% of algae cells experienced the death mode of necrosis-like. Moreover, there were 8.35-14.50% of algae cells undergoing programmed cell death, but their caspase-3-like protease activity remained unchanged, which may mean that algae cells were not experiencing caspase-dependent apoptosis under artemisinin stress. Attacked by artemisinin directly, both intracellular and extracellular MC-LR increased sharply with the upregulation of mcyB, mcyD, and mcyH. The upregulation multiple of mcyH suggested that M. aeruginosa could accelerate transportation of algal toxin under adverse conditions of artemisinin. Artemisinin not only can inhibit the growth of M. aeruginosa but it also causes the accelerated release and increase of microcystin-LR. These imply that the application of artemisinin should be reconsidered in practical water bodies.

Killing effect of deinoxanthins on cyanobloom-forming Microcystis aeruginosa: Eco-friendly production and specific activity of deinoxanthins

Cyanobacterial blooms caused mainly by Microcystis aeruginosa could be controlled using chemical and biological agents such as H₂O₂, antagonistic bacteria, and enzymes. Little is known about the possible toxic effects of bacterial membrane pigments on M. aeruginosa cells. Deinococcus metallilatus MA1002 cultured under light increased the production of several carotenoid-like compounds by upregulating two deinoxanthin biosynthesis genes: crtO and cruC. The deinoxanthin compounds were identified using thin-layer chromatography, high-performance liquid chromatography, and liquid chromatography-mass spectrometry. D. metallilatus was cultured with agricultural by-products under light to produce the deinoxanthin compounds. Soybean meal, from six tested agricultural by-products, was selected as the single factor for making an economical medium to produce deinoxanthin compounds. The growth of axenic M. aeruginosa PCC7806, as well as other xenic cyanobacteria such as Cyanobium gracile, Trichormus variabilis, and Dolichospermum circinale, were inhibited by the deinoxanthin compounds. Scanning electron microscopic images showed the complete collapse of M. aeruginosa cells under deinoxanthin treatment, probably due to its interference with cyanobacterial membrane synthesis during cellular elongation. Deinoxanthins appeared to be nontoxic to other non-cyanobacteria such as Acinetobacter, Pseudomonas, Methylobacterium, and Bacillus species, suggesting that it can be a novel candidate for preventing cyanobacterial blooms through its specific activity against cyanobacteria.

Transcriptional Analysis of Microcystis aeruginosa Co-Cultured with Algicidal Bacteria Brevibacillus laterosporus

Harmful algal blooms caused huge ecological damage and economic losses around the world. Controlling algal blooms by algicidal bacteria is expected to be an effective biological control method. The current study investigated the molecular mechanism of harmful cyanobacteria disrupted by algicidal bacteria. Microcystis aeruginosa was co-cultured with Brevibacillus laterosporus Bl-zj, and RNA-seq based transcriptomic analysis was performed compared to M. aeruginosa, which was cultivated separately. A total of 1706 differentially expressed genes were identified, which were mainly involved in carbohydrate metabolism, energy metabolism and amino acid metabolism. In the co-cultured group, the expression of genes mainly enriched in photosynthesis and oxidative phosphorylation were significantly inhibited. However, the expression of the genes related to fatty acid synthesis increased. In addition, the expression of the antioxidant enzymes, such as 2-Cys peroxiredoxin, was increased. These results suggested that B. laterosporus could block the electron transport by attacking the PSI system and complex I of M. aeruginosa, affecting the energy acquisition and causing oxidative damage. This further led to the lipid peroxidation of the microalgal cell membrane, resulting in algal death. The transcriptional analysis of algicidal bacteria in the interaction process can be combined to explain the algicidal mechanism in the future.

Advances in the use of microalgal-bacterial consortia for wastewater treatment: Community structures, interactions, economic resource reclamation, and study techniques

The rise in living standards has generated a demand for higher aquatic environmental quality. The microalgal community and the surrounding organic molecules, environmental factors, and microorganisms, such as bacteria, are together defined as the phycosphere. The bacteria in the phycosphere can form consortia with microalgae through various forms of interaction. The study of the species in these consortia and their relative proportions is of great significance in determining the species and strains of stable algae that can be used in sewage treatment. This article summarizes the following topics: the interactions between microalgae and bacteria that are required to establish consortia; how symbiosis between algae and bacteria is established; microalgal competition with bacteria through inhibition and anti-inhibition strategies; the influence of environmental factors on microalgal-bacterial aggregates, such as illumination conditions, pH, dissolved oxygen, temperature, and nutrient levels; the application of algal-bacterial aggregates to enhance biomass production and nutrient reuse; and techniques for studying the community structure and interactions of algal-bacterial consortia, such as microscopy, flow cytometry, and omics. PRACTITIONER POINTS: Community structures in microalgal-bacterial consortia in wastewater treatment. Interactions between algae and bacteria in wastewater treatment. Effects of ecological factors on the algal-bacterial community in wastewater treatment. Economically recycling resources from algal-bacterial consortia based on wastewater. Technologies for studying microalgal-bacterial consortia in wastewater treatment.

Cyanobacterial Harmful Algal Blooms in Aquatic Ecosystems: A Comprehensive Outlook on Current and Emerging Mitigation and Control Approaches

An intensification of toxic cyanobacteria blooms has occurred over the last three decades, severely affecting coastal and lake water quality in many parts of the world. Extensive research is being conducted in an attempt to gain a better understanding of the driving forces that alter the ecological balance in water bodies and of the biological role of the secondary metabolites, toxins included, produced by the cyanobacteria. In the long-term, such knowledge may help to develop the needed procedures to restore the phytoplankton community to the pre-toxic blooms era. In the short-term, the mission of the scientific community is to develop novel approaches to mitigate the blooms and thereby restore the ability of affected communities to enjoy coastal and lake waters. Here, we critically review some of the recently proposed, currently leading, and potentially emerging mitigation approaches in-lake novel methodologies and applications relevant to drinking-water treatment.

The algicidal efficacy and the mechanism of Enterobacter sp. EA-1 on Oscillatoria dominating in aquaculture system

The global escalation and intensification of cyanobacterial blooms require powerful algaecides. This study investigated the algicidal efficacy and mechanism of EA-1 against Oscillatoria. Bacteria EA-1, identified as Enterobacter, was isolated with high algicidal activity against harmful cyanobacteria. Results showed that a complete removal of Oscillatoria was observed within 3 days with the initial Chl-a concentration of 1.74 mg/L. Physiological responses of Oscillatoria revealed that EA-1 induced severe lipid peroxidation and the ultimate decline of antioxidant enzyme activities. Moreover, the contents for both intracellular protein and carbohydrate of each algae cell increased first and then decreased. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) analysis clarified that the possible process of Oscillatoria lysis included the breach of cross wall, followed by the disruption of photosynthetic membrane and incipient nucleus, and the ultimate outflow of inclusion. Confocal laser scanning microscopy (CLSM) analysis illustrated the degradation process of incipient nucleus in Oscillatoria.

Cell Death in Cyanobacteria: Current Understanding and Recommendations for a Consensus on Its Nomenclature

Cyanobacteria are globally widespread photosynthetic prokaryotes and are major contributors to global biogeochemical cycles. One of the most critical processes determining cyanobacterial eco-physiology is cellular death. Evidence supports the existence of controlled cellular demise in cyanobacteria, and various forms of cell death have been described as a response to biotic and abiotic stresses. However, cell death research in this phylogenetic group is a relatively young field and understanding of the underlying mechanisms and molecular machinery underpinning this fundamental process remains largely elusive. Furthermore, no systematic classification of modes of cell death has yet been established for cyanobacteria. In this work, we analyzed the state of knowledge in the field of cyanobacterial cell death. Based on that, we propose unified criterion for the definition of accidental, regulated, and programmed forms of cell death in cyanobacteria based on molecular, biochemical, and morphologic aspects following the directions of the Nomenclature Committee on Cell Death (NCCD). With this, we aim to provide a guide to standardize the nomenclature related to this topic in a precise and consistent manner, which will facilitate further ecological, evolutionary, and applied research in the field of cyanobacterial cell death.

A review on control of harmful algal blooms by plant-derived allelochemicals

Harmful algal blooms (HABs) have several negative impacts on aquatic ecosystem, and even harm to humans. Utilization of allelochemicals to inhibit microalgal overgrowth is an environment-friendly approach for controlling HABs. This paper demonstrated the development of allelochemicals with algicidal effects, including the development history of allelopathy, the application methods, the reported allelopathic plants and their derived allelochemicals. Allelopathy is a promising strategy to control HABs as the effectiveness of allelochemicals on inhibiting microalgae cells has been discovered and confirmed for many years. The proposed allelopathic mechanisms and species-selective properties were expounded as well. Moreover, this paper further proposed suggestions for the further research and development of allelopathy strategy for HABs control.

Prodiginines Postpone the Onset of Sporulation in Streptomyces coelicolor

Bioactive natural products are typically secreted by the producer strain. Besides that, this allows the targeting of competitors, also filling a protective role, reducing the chance of self-killing. Surprisingly, DNA-degrading and membrane damaging prodiginines (PdGs) are only produced intracellularly, and are required for the onset of the second round of programmed cell death (PCD) in Streptomyces coelicolor. In this work, we investigated the influence of PdGs on the timing of the morphological differentiation of S. coelicolor. The deletion of the transcriptional activator gene redD that activates the red cluster for PdGs or nutrient-mediated reduction of PdG synthesis both resulted in the precocious appearance of mature spore chains. Transcriptional analysis revealed an accelerated expression of key developmental genes in the redD null mutant, including bldN for the developmental σ factor BldN which is essential for aerial mycelium formation. In contrast, PdG overproduction due to the enhanced copy number of redD resulted in a delay or block in sporulation. In addition, confocal fluorescence microscopy revealed that the earliest aerial hyphae do not produce PdGs. This suggests that filaments that eventually differentiate into spore chains and are hence required for survival of the colony, are excluded from the second round of PCD induced by PdGs. We propose that one of the roles of PdGs would be to delay the entrance of S. coelicolor into the dormancy state (sporulation) by inducing the leakage of the intracellular content of dying filaments thereby providing nutrients for the survivors.

Ambigols from the Cyanobacterium Fischerella ambigua Increase Prodigiosin Production in Serratia spp

When a library of 573 cyanobacteria extracts was screened for inhibition of the quorum sensing regulated prodigiosin production of Serratia marcescens, an extract of the cyanobacterium Fischerella ambigua (Näg.) Gomont 108b was found to drastically increase prodigiosin production. Bioactivity-guided isolation of the active compounds resulted in the two new natural products ambigol D and E along with the known ambigols A and C. Ambigol C treatment increased prodiginine production of Serratia sp. ATCC 39006 (S39006) by a factor of 10, while ambigols A and D were found to have antibiotic activity against this strain. The RNA-Seq of S39006 treated with ambigol C and subsequent differential gene expression and functional enrichment analyses indicated a significant downregulation of genes associated with the translation machinery and fatty acid biosynthesis in Serratia, as well as increased expression of genes related to the uptake of l-proline. These results suggest that the ambigols increase prodiginine production in S39006 not by activating the SmaIR quorum sensing system but possibly by increasing the precursor supply of l-proline and malonyl-CoA.