Degradation of paralytic shellfish toxins during flocculation of Alexandrium pacificum by an oxidized modified clay: A laboratory experiment.

Paralytic shellfish toxins (PSTs), produced by Alexandrium pacificum in the marine environment, are a group of potent neurotoxins which specifically block voltage-gated sodium channels in excitable cells. During the toxigenic A. pacificum blooms outbreaks, PSTs can be accumulated through the food chain and finally enter the human body, posing a significant threat to human health and safety. This study experimented with a novel type of oxidized modified clay, potassium peroxymonosulfate modified clay (PMPS-MC), which could remove A. pacificum cells as well as reduce intracellular and extracellular PSTs toxicity rapidly. For the extracellular PSTs, its content decreased to below the detection limit rapidly through oxidative degradation within 15 min of 10 mg/L PMPS-MC treatment. Whereafter, although the residual cells in water column and some viable cells in flocculated sediment continued to secrete toxins, the extracellular PSTs content and toxicity in the PMPS-MC treatment groups remained significantly lower than those in the control group. For the intracellular PSTs, PMPS-MC might induce the transformation of more toxic GTX1&4 to less toxic GTX2&3 and C1&2, resulting in intracellular PSTs toxicity reduced within 15 min. In addition, intracellular PSTs content and toxicity in the PMPS-MC treatment groups were consistently lower than the control group within 48 h, possibly by inhibiting the A. pacificum cells growth. These results will provide a scientific basis for the field application of modified clay to control A. pacificum blooms.

The synthesis of an acrylamide copolymer and its synergistic effects on clay flocculation of red tide organisms.

The modified clay (MC) method is a common emergency treatment technology for red tides, and the selection of surface modifiers is the key to the MC technology. A cationic polymeric modifier, the copolymer of dimethyl diallyl ammonium chloride and acrylamide (P (DMDAAC-co-AM), PDA) was optimized via a visible-light-induced polymerization technique. The PDA-modified clay (PDAMC) was prepared with strong salt tolerance and achieved efficiencies of 86% at the concentration of 50 mg L-1, and the dose was 90% lower than that of aluminum polychloride-modified clay (PACMC). While polyacrylamide and commercial PDA can achieve efficiencies of only 25 and 67%, respectively, but high doses were required. This is because PDA changed the surface charges of clay particles from negative to positive, which promotes the formation of the polymer-chains bridging network to overcome the difficulties of curling in seawater. According to the analysis of flocculation parameters and spatial conformation of PDAMC, the high salinity tolerance of the PDAMC was attributed to the synergistic processes of charge neutralization and the three-dimensional network bridging. Therefore, this study has developed a highly effective flocculant material used in seawater and provided an important reference for the management of red tide organisms.

The molecular mechanisms and environmental effects of modified clay control algal blooms in aquacultural water.

Modified clay (MC) technology is an effective method for controlling harmful algal blooms (HABs). Based on field experience, a bloom does not continue after treatment with MC, even though the residual HAB biomass accounts for 20-30% of the initial biomass. Laboratory studies using unialgal cultures have found that MC could inhibit the growth of the residual algal cells to prevent HABs. Nevertheless, the phytoplankton in field waters is diverse. Therefore, unclassified complex mechanisms may exist. To illustrate the molecular mechanisms through which MC controls HABs in the field and verify the previous laboratory findings, a series of experiments and bioinformatics analyses were conducted using bloom waters from aquacultural ponds. The results showed that a 72.29% removal efficiency of algal biomass could effectively control blooms. The metatranscriptomic results revealed that the number of downregulated genes (131,546) was greater than that of upregulated genes (24,318) at 3 h after MC addition. Among these genes, several genes related to DNA replication were downregulated; however, genes involved in DNA repair were upregulated. Metabolism-related pathways were the most significantly upregulated (q < 0.05), including photosynthesis and oxidative phosphorylation. The results also showed that MC reduced most of the biomass of the dominant phytoplankton species, likely by removing apical dominance, which increased the diversity and stability of the phytoplankton community. In addition to reducing the pathogenic bacterial density, MC reduced the concentrations of PO43- (96.22%) and SiO32- (66.77%), thus improving the aquaculture water quality, altering the phytoplankton community structure (the proportion of Diatomea decreased, and that of Chlorophyta increased), and inhibiting phytoplankton growth. These effects hindered the rapid development of large phytoplankton biomasses and allowed the community structure to remain stable, reducing HAB threats. This study illustrates the molecular mechanisms through which MC controls HABs in the field and provides a scientific method for removing HABs in aquacultural waters.

A New Perspective: Revealing the Algicidal Properties of Bacillus subtilis to Alexandrium pacificum from Bacterial Communities and Toxins.

Algicidal bacteria are important in the control of toxic dinoflagellate blooms, but studies on the environmental behavior of related algal toxins are still lacking. In this study, Bacillus subtilis S3 (S3) showed the highest algicidal activity against Alexandrium pacificum (Group IV) out of six Bacillus strains. When treated with 0.5% (v/v) S3 bacterial culture and sterile supernatant, the algicidal rates were 69.74% and 70.22% at 12 h, respectively, and algicidal substances secreted by S3 were considered the mechanism of algicidal effect. During the algicidal process, the rapid proliferation of Alteromonas sp. in the phycosphere of A. pacificum may have accelerated the algal death. Moreover, the algicidal development of S3 released large amounts of intracellular paralytic shellfish toxins (PSTs) into the water, as the extracellular PSTs increased by 187.88% and 231.47% at 12 h, compared with the treatment of bacterial culture and sterile supernatant at 0 h, respectively. Although the total amount of PSTs increased slightly, the total toxicity of the algal sample decreased as GTX1/4 was transformed by S3 into GTX2/3 and GTX5. These results more comprehensively reveal the complex relationship between algicidal bacteria and microalgae, providing a potential source of biological control for harmful algal blooms and toxins.

Profiles of and variations in aluminum species in PAC-MC used for the removal of blooming microalgae.

Polyaluminum chloride modified clay (PAC-MC) is a safe and efficient red tide control agent that has been studied and applied worldwide. Although it is well known that the distribution of hydrolytic aluminum species in PAC affects its flocculation, little is known about the influence of particulars aluminum species on the microalgae removal efficiency of PAC-MC; this lack of knowledge creates a bottleneck in the development of more efficient MCs based on aluminum salts. The ferron method was used in this study to quantitatively analyze the distributions of and variations in different hydrolytic aluminum species during the process of microalgae removal by PAC-MC. The results showed that Ala, which made up 5%-20% of the total aluminum, and Alp, which made up 15%-55% of the total aluminum, significantly affected microalgae removal, with Pearson's correlation coefficients of 0.83 and 0.89, respectively. Most of the aluminum in the PAC-MC sank rapidly into the sediments, but the rate and velocity of settlement were affected by the dose of modified clay. The optimal dose of PAC-MC for precipitating microalgae was determined based on its aluminum profile. These results provide guidance for the precise application of PAC-MC in the control of harmful algal blooms.

Programmed cell death induced by modified clay in controlling Prorocentrum donghaiense bloom.

Modified clay (MC), an effective material used for the emergency elimination of algal blooms, can rapidly reduce the biomass of harmful algal blooms (HABs) via flocculation. After that, MC can still control bloom population through indirect effects such as oxidative stress, which was initially proposed to be related to programmed cell death (PCD) at molecular level. To further study the MC induced cell death in residual bloom organisms, especially identifying PCD process, we studied the physiological state of the residual Prorocentrum donghaiense. The experimental results showed that flocculation changed the physiological state of the residual cells, as evidenced by growth inhibition and increased reactive oxygen species production. Moreover, this research provides biochemical and ultrastructural evidence showing that MC induces PCD in P. donghaiense. Nuclear changes were observed, and increased caspase-like activity, externalization of phosphatidylserine and DNA fragmentation were detected in MC-treated groups and quantified. And the mitochondrial apoptosis pathway was activated in both MC-treated groups. Besides, the features of MC-induced PCD in a unicellular organism were summarized and its concentration dependent manner was proved. All our preliminary results elucidate the mechanism through which MC can further control HABs by inducing PCD and suggest a promising application of PCD in bloom control.

Molecular Mechanism of Modified Clay Controlling the Brown Tide Organism Aureococcus anophagefferens Revealed by Transcriptome Analysis.

The data and experiences in mitigating harmful algal blooms (HABs) by modified clay (MC) show that a bloom does not continue after the dispersal of the MC, even though the density of the residual cells in the water is still high, at 20-30% of the initial cell density. This interesting phenomenon indicates that in addition to flocculation, MC has an additional control mechanism. Here, transcriptome sequencing technology was used to study the molecular mechanism of MC in controlling HABs. In residual cells treated with MC, the photosynthetic light reaction was the most affected physiological process. Some genes related to the light harvesting complex, photosystem (PS) I and PS II, were significantly up-regulated ( p < 0.05), and several transcripts increased by as much as 6-fold. In contrast, genes associated with the dark reaction did not significantly change. In addition to genes associated with photosynthesis, numerous genes related to energy metabolism, stress adaptation, cytoskeletal functioning, and cell division also responded to MC treatment. These results indicated that following treatment with MC, the normal physiological processes of algal cells were disrupted, which inhibited cell proliferation and growth. Thus, these findings provide scientific proof that HABs are controlled by MC.

Phycosphere bacterial composition and function in colony and solitary Phaeocystis globosa strains providing novel insights into the algal blooms.

Phycosphere bacteria can regulate the dynamics of different algal blooms that impact marine ecosystems. Phaeocystis globosa can alternate between solitary free-living cells and colonies and the latter morphotype is dominate during blooms. The mechanisms underlying the formation of these blooms have received much attention. High throughput sequencing results showed that the bacterial community composition differed significantly between colony and solitary strains in bacterial composition and function. It was found that the genera SM1A02 and Haliea were detected only among the colony strains and contribute to ammonium accumulation in colonies, and the genus Sulfitobacter was abundant among the colony strains that were excellent at producing DMS. In addition, the bacterial communities of the two colony strains exhibited stronger abilities for carbon and sulfur metabolism, energy metabolism, vitamin B synthesis, and signal transduction, providing inorganic and organic nutrients and facilitating tight communication with the host algae, thereby promoting growth and bloom development.

BTXs removals by modified clay during mitigation of Karenia brevis bloom: Insights from adsorption and transformation.

Harmful algal blooms (HABs), especially those caused by toxic dinoflagellates, are spreading in marine ecosystems worldwide. Notably, the prevalence of Karenia brevis blooms and potent brevetoxins (BTXs) pose a serious risk to public health and marine ecosystems. Therefore, developing an environmentally friendly method to effectively control HABs and associated BTXs has been the focus of increasing attention. As a promising method, modified clay (MC) application could effectively control HABs. However, the environmental fate of BTXs during MC treatment has not been fully investigated. For the first time, this study revealed the effect and mechanism of BTX removal by MC from the perspective of adsorption and transformation. The results indicated that polyaluminium chloride-modified clay (PAC-MC, a typical kind of MC) performed well in the adsorption of BTX2 due to the elevated surface potential and more binding sites. The adsorption process was a spontaneous endothermic process that conformed to pseudo-second-order adsorption kinetics (k2 = 6.8 × 10-4, PAC-MC = 0.20 g L-1) and the Freundlich isotherm (Kf = 55.30, 20 °C). In addition, detailed product analysis using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) indicated that PAC-MC treatment effectively removed the BTX2 and BTX3, especially those in the particulate forms. Surprisingly, PAC-MC could promote the transformation of BTX2 to derivatives, including OR-BTX2, OR-BTX3, and OR-BTX-B5, which were proven to have lower cytotoxicity.

Key biogeochemical processes and source apportionment of nitrate in the Bohai Sea based on nitrate stable isotopes.

Excessive nitrate input is one of the primary factors causing nearshore eutrophication. This study applied the nitrate stable isotope techniques to analyse the biogeochemical processes and sources of nitrate in the Bohai Sea (BHS). The results showed that intensive NO3- assimilation probably occurred at surface in summer, while nitrification primarily occurred in the Yellow River diluted water. In autumn, regional assimilation and nitrification were still identified. For avoiding the interference from assimilation, the isotopic fractionations were further calculated as correction data for the quantitative analysis of nitrate sources. The river inputs were identified as the primary source of nitrate in the BHS in summer and autumn, accounting for >50 %, and the atmospheric deposition was the secondary source. This study provides quantitative data for evaluating the significance of river inputs to the nearshore nitrate, which will be beneficial to policy formulation on the BHS eutrophication control.