Environmental effects of modified clay flocculation on Alexandrium tamarense and paralytic shellfish poisoning toxins (PSTs)

Marine toxins in environment: Recent updates on depuration techniques

Marine toxins pose a significant safety risk, leading to human intoxications and causing substantial economic losses in seafood-producing regions. The development of rapid, cost-effective, efficient, and reliable approaches for the containment of these substances is therefore crucial in order to mitigate the adverse impact of marine toxins. This research conducted a comprehensive review on the toxicity and influencing factors of marine toxins production. Additionally, depuration technologies, including adsorption, advanced oxidation processes, biodegradation, heating treatment, temporary maintenance and purification, and drug inhibition, were systematically summarized. The study also provided a comparative analysis of the advantages and disadvantages of various depuration technologies and proposed strategies for future 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.

A synthetic and transparent clay removes Microcystis aeruginosa efficiently

Clay-algae flocculation is a promising method to remove harmful algal blooms (HABs) in aquatic ecosystems. Many HAB-generating species, such as Microcystis aeruginosa (M. aeruginosa), a common species in lakes, produce toxins and harm the environment, human health, and the economy. Natural clays, such as bentonite and kaolinite, and modification of these clays have been applied to mitigate HABs by forming large aggregates and settling down. In this study, we aim to examine the impact of laponite, a commercially available smectite clay that is synthetic, transparent, compatible with human tissues, and degradable, on removing HABs. We compare the cell removal efficiencies (RE) of laponite, two natural clays, and their polyaluminum chloride (PAC)-modified versions through clay-algae flocculation experiments. Our results show that the optimum concentrations of laponite, bentonite, kaolinite, PAC-modified bentonite, and PAC-modified kaolinite to remove 80 % of the M. aeruginosa cells from the water column are 0.05 g/L, 2 g/L, 4 g/L, 2 g/L and 0.3 g/L respectively. Therefore, to achieve the same cell removal efficiency, the amount of laponite needed is 40 to 80 times less than bentonite and kaolinite, and 6 times less than PAC-modified kaolinite. We demonstrate that the superior performance of laponite clay is because of its smaller particle size, which increases the encounter rate between cells and clay particles. Furthermore, experiments using water samples from Powderhorn Lake confirmed laponite's effectiveness in mitigating HABs. Our price analysis also suggests that this commercially-available clay, laponite, can be used in the field at a relatively low cost.

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 PO₄^3- (96.22%) and SiO₃^2- (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.

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.

Cation-Exchange Resin Applied to Paralytic Shellfish Toxins Depuration from Bivalves Exposed to Gymnodinium catenatum

The accumulation of marine biotoxins in shellfish and their consumption causes serious food safety problems, threatening human health and compromising the availability of protein-based food. It is thus urgent to develop methodologies for the detoxification of live bivalves, avoiding their economic and nutritional devaluation. In this context, we tested an adsorption mechanism of paralytic shellfish toxins (PST) based on a cation-exchange resin. The first studies using cultures of Gymnodinium catenatum (natural producers of PST) showed a decrease of about 80% in overall toxicity after 48 h. Interestingly, we found that the toxins are adsorbed differently, with toxins' structural features playing a part in the adsorption capacity via steric hindrance, electronic effects, or the extent of positive charge density (e.g., dcSTX). The positive effect of the resin in accelerating PST clearance from live mussels (Mytilus edulis) is not evident when compared to resin-free clearance; nevertheless, relevant information could be gathered that will facilitate further in vivo studies. Several factors appear to be at play, namely the competition of natural substances (e.g., salts, organic matter) for the same binding sites, the blocking of pores due to interactions between molecules, and/or difficulties in resin absorption by mussels. Additionally, the present work revealed the ability of mussels to neutralize pH and proposes bioconversion reactions among the PST molecules.

Post-ecological effect and risk assessment of using modified clay in harmful algal bloom mitigation: An attempt based on the responses of zooplankton Brachionus plicatilis and bivalve Mytilus edulis

The modified clay is the only worldwide-accepted practical method for mitigating algal bloom. Is it ecologically safe? To evidence it, a simulative bloom-occurring system of Karenia mikimotoi was set up, and the sentinel organisms of rotifer Brachionus plicatilis in sea surface and blue mussel Mytilus edulis on the benthos were respectively included. The organisms' physiological responses were determined as the indicators to reflect the ecological impacts when clay settled from surface to the bottom during the mimic bloom-mitigating process. Modified clay at a concentration of 0.1 g/L effectively removed the K. mikimotoi at an 81% removal rate, and its addition would not significantly strengthen the negative impacts on population dynamics and reproductive activities of B. plicatilis induced by sole K. mikimotoi within the first 2 h. Even an alleviation was observed at 2 d indicated by the increase of survival rate, egg and larva production after clay addition compared with those of 2 h. When the clay particles settled to benthos, the physical damage to the gills and digestive glands of M. edulis were found via the tissue and SEM observation, especially in higher treatment groups of 0.5 and 1.0 g/L, and filtering rate, digestive enzymes, condition index, water content and mortality were also influenced. However, little impact was found in group of 0.1 g/L. Risk assessment based on the adverse outcome pathway (AOP) model further revealed that the complete key event-key event relationship-adverse outcome pathway was only clearly observed in 0.5 g/L and 1 g/L groups but not in 0.1 g/L group, inferring the small ecological risk of 0.1 g/L. The integrated biomarker response (IBR) based on the mussel's physiological responses further backed up the AOP outcoming. The combined results from rotifer to bivalve emphasized on one conclusion that modified clay at 0.1 g/L was effective and ecologically safe in coastal bloom mitigation.

Healthier Communities of Phytoplankton and Bacteria Achieved via the Application of Modified Clay in Shrimp Aquaculture Ponds

The composition and stability of microbial communities in aquaculture water are crucial for the healthy growth of shrimp and present considerable risk to aquatic ecosystems. The modified clay (MC) method has been proposed as an efficient and safe solution for the mitigation of harmful algal blooms (HABs). Currently, the effects of MC on microbial communities in aquaculture water remain unknown. Here, we adopted the MC method to regulate shrimp-culture water quality and evaluated the effects of MC on the composition and stability of phytoplankton together with bacteria communities through high-throughput sequencing. On the one hand, a prominent change in the composition of microbial community was observed, with green algae becoming the most abundant genera and pathogens being infrequent in the MC-treated pond, which was more conducive to the growth of shrimp than that in the control pond. Moreover, MC could increase the diversity and stability of the microbial community structure in the water column, which had a higher anti-interference ability, as demonstrated by the analysis of the diversity and molecular ecological network. Taken together, MC could reduce the possibility for the occurrence of HABs and maintain a stable microbial community, which is beneficial for the health and high yield of shrimp.

An eco-environmental assessment of harmful algal bloom mitigation using modified clay

The application of modified clay (MC) to mitigate harmful algal blooms (HABs) is becoming a widespread practice in China because of its low cost, high efficiency and environmental friendliness. Due to its success in China, this technology has also gained international recognition in recent years. Nevertheless, the eco-environmental impacts of this technology still remain to be a concern to many customers and government agencies, which has motivated scientists in both China and many other countries to evaluate its potential effects on nontarget aquatic organisms, water quality and the benthic environment. This paper reviews the results from these studies both in the laboratory and in the field, which showed that MC does not harm nontarget aquatic organisms and has no negative impact on water quality or the benthic environment. Additionally, findings show that MC can alter nutrient cycling and reduce algal toxins in water bodies. Furthermore, researchers also found that MC affects cyst formation and germination in sedimentary environments. This review is expected to provide scientific guidance for mitigating HABs in China and worldwide using clay or MC.

Effect of carboxymethyl chitosan on the detoxification and biotransformation of paralytic shellfish toxins in oyster Ostrea rivularis

Economic bivalve ingested toxic algae causes frequent human poisoning events. To explore new compounds that can accelerate the depuration of toxins in shellfish, we investigated the detoxification of the paralytic shellfish toxins (PSTs) and the biotransformation pathway of PSTs during detoxification by the application of three treatments to a toxic bloom, Alexandrium minutum (A. minutum). The detoxification effect of Platymonas subcordiformis (PS) mixed with carboxymethyl chitosan (CMC) group is significantly better than the starving group in each oyster tissues. The toxicity of viscera which occupied 78.95% of total toxicity reduced to 155 MU/100g after 13 days' depuration experiment. And adding CMC could significantly achieve rapid detoxification and effectively reduce the STX to 0.07 μmol/100 g in viscera. Meanwhile, PSTs underwent biotransformation during the depuration process, which mainly manifested as GTX1/4→GTX2/3→STX, GTX2→dcGTX2. This study explored a new strategy for toxin depuration in shellfish.

Shellfish Toxin Uptake and Depuration in Multiple Atlantic Canadian Molluscan Species: Application to Selection of Sentinel Species in Monitoring Programs

Shellfish toxin monitoring programs often use mussels as the sentinel species to represent risk in other bivalve shellfish species. Studies have examined accumulation and depuration rates in various species, but little information is available to compare multiple species from the same harvest area. A 2-year research project was performed to validate the use of mussels as the sentinel species to represent other relevant eastern Canadian shellfish species (clams, scallops, and oysters). Samples were collected simultaneously from Deadmans Harbour, NB, and were tested for paralytic shellfish toxins (PSTs) and amnesic shellfish toxin (AST). Phytoplankton was also monitored at this site. Scallops accumulated PSTs and AST sooner, at higher concentrations, and retained toxins longer than mussels. Data from monitoring program samples in Mahone Bay, NS, are presented as a real-world validation of findings. Simultaneous sampling of mussels and scallops showed significant differences between shellfish toxin results in these species. These data suggest more consideration should be given to situations where multiple species are present, especially scallops.

Using flocculation and subsequent biomanipulation to control microcystis blooms: A laboratory study

The frequent occurrence and long-term duration of Microcystis harmful algal blooms (HABs) are of great concern. Chemical flocculation is thought to be an effective way to deal with the HABs, while the application of the flocculants at a high dosage pose potential adverse impacts to the aquatic ecosystems. In this study, an alternative approach is proposed that involves the employment of polyaluminum chloride (PAC) combined with the Daphnia magna (D. magna) to achieve sustainable HABs removal efficiency with an acceptable ecological risk. It was found that under a dense Microcystis HABs (algal density of 1.5 × 10⁷ cells/ml), a PAC dosage of 30 mg/l triggered >95% algae removal, but the released Al³⁺ caused 90% mortality of planktonic D. magna. Reducing the PAC dosage to 15 mg/l resulted in a slightly lower algal removal efficiency (>90%). In addition the reduced PAC dosage benefited the proliferation of the remaining unicellular algal cells, which tended to form a large colony during the 25-day experiment. Incubation of D. magna following flocculation with 15 mg/l PAC effectively grazed the remaining algal cells, meanwhile increasing the D. magna density by approximately 40-folds, and enlarging the body size by 1.37-1.50 times. This result implied that the released Al³⁺ was not detrimental to the D. magna. Flocculation with a reduced dosage is sufficient for colonial and large algal cells mitigation, which creates a window time for the biomanipulation of the residual tiny algae. Hence, the subsequent addition of D. magna triggered the sustainable removal of the HABs cells. The present study provides an environmentally friendly strategy for cleaning up the green tides without obvious detrimental effects on the aquatic ecosystem.

Switching Harmful Algal Blooms to Submerged Macrophytes in Shallow Waters Using Geo-engineering Methods: Evidence from a 15N Tracing Study

Switching the dominance from algae to macrophytes is crucial for lake management of human-induced eutrophication. Nutrients from algal sources can be utilized in the process of transition from algal blooms to macrophytes, thereby mitigating eutrophication. However, this process rarely occurs in algal bloom dominated waters. Here, we examined the hypothesis that the transition of algal blooms to macrophytes and the transfer of nutrients from algae at different temperatures (8 and 25 °C) can be facilitated by using a geo-engineering method. The results showed that the combination of flocculation and capping treatment could not only remove Microcystis aeruginosa blooms from eutrophic waters but also facilitate algal decomposition and incorporation into a submerged macrophyte ( Potamogeton crispus) biomass. The flocculation-capping treatment could trigger algal cell lysis. As compared with the control groups, the photosynthesis and respiration rate of algae were inhibited and chlorophyll-a (Chl- a) concentrations were significantly reduced in the flocculation-capping treatment groups. The ¹⁵N tracing study revealed that 3.3% and 34.8% of algae-derived nitrogen could be assimilated by Potamogeton crispus at 8 and 25 °C, respectively. The study demonstrated that the flocculation-capping method can facilitate the switchover from algae- to the macrophyte-dominated state, which is crucial for restoring the aquatic ecosystem.

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.

Reconstructing clear water state and submersed vegetation on behalf of repeated flocculation with modified soil in an in situ mesocosm experiment in Lake Taihu

The geo-engineering approach of modified soil flocculation has been widely applied to mitigate algal blooms and eutrophication in relatively small lakes. Nevertheless, its potential ecological risks and feasibility should be examined and identified prior to its application in large natural lakes given the multiple functions of these water bodies in human health and welfare. In situ mesocosm experiments on modified soil flocculation were performed in Lake Taihu during summer 2010 and 2011. Chitosan-modified kaolinite (CMK) soil was used to flocculate algal blooms and improve water transparency to facilitate the re-establishment of the submersed macrophyte Vallisneria natans in this shallow eutrophic lake. Moreover, the ecological effects of CMK soil were assessed. Results showed that repeated additions of CMK (0.3g/L for each time) improved water quality in terms of Chl-a, TN, and TP concentrations; TN/TP ratio; turbidity; redox conditions; and nitrification and denitrification activities. These effects lasted for 48days. After the fourth dose of CMK, the biomass of all phytoplankton categories, except for that of Cryptophyta, decreased by >90% (ca. 1-2×10⁶cell/L or 0.38-0.55mg/L of wet weight). Zooplankton biomass markedly decreased after the first CMK addition, and copepods became dominant. These effects, however, did not last for the long term. Most importantly, submersed V. natans was restored successfully when water clarity and quality were improved through repeated CMK flocculation. Nevertheless, the indices of carbohydrate depletion and free amino acid accumulation indicated that the plant experienced physiological stresses. The reestablishment of V. natans reinforced the positive effects of repeated CMK dosing on water quality, and promoted a clear water state. V. natans is recommended for vegetative restoration in shallow eutrophic lakes given its facile transplantation, high stress tolerance, and physiological traits, which can be applied as indices of post-flocculation effects. In summary, the combination of repeated CMK dosing and revegetation of V. natans can feasibly improve water quality and initiate the restoration of a clear water state in shallow eutrophic lakes.

Application of activated carbon to accelerate detoxification of paralytic shellfish toxins from mussels Mytilus galloprovincialis and scallops Chlamys farreri

Contamination of economic bivalves with paralytic shellfish toxins (PST) occurs frequently in many parts of the world, which potentially threatens consumer health and the marine aquaculture economy. It is the objective of this study to develop a suitable technology for accelerating detoxification of PST from shellfish using activated carbon (AC). The adsorption efficiency of PST by eight different AC materials and by different particle sizes of wood-based AC (WAC) were tested and compared. Then WAC particles (37-48µm) were fed to mussels Mytilus galloprovincialis and scallops Chlamys farreri previously contaminated with PST through feeding with dinoflagellate Alexandrium tamarense ATHK. Results showed that the maximum adsorption ratio (65%) of PST was obtained by WAC. No significant differences in adsorption ratios were found between different particle sizes of WAC. The toxicity of mussels decreased by 41% and 68% after detoxification with WAC for 1 d and 3 d, respectively. Meanwhile, the detoxification ratio of mussels was approximately 3 times higher than that of scallops. This study suggests that the WAC could be used to accelerate the detoxification of PST by shellfish.

Effects of modified clay used for the control of harmful algal blooms on Alexandrium pacificum cysts

Cyst formation plays an important role in the resistance of dinoflagellates to adverse environments, and cyst germination is considered one of the causes of harmful algal blooms (HABs). Among the methods for mitigating HABs, modified clay (MC) is considered a promising strategy because of its high efficiency and low environmental impacts. The typical HAB species Alexandrium pacificum was focused on in this study to clarify the effects of MC on cyst formation and germination. The results showed that more than 90% of the vegetative cells were removed under the 0.6 g/L MC treatment. The vegetative cell density was monitored over 90 d and increased slightly to the peak at 10 d after the cell removal experiment, but persistent growth was not observed. The amount of cysts was maximal at 20 d after removal, however, most of the cysts were temporary cysts that subsequently disappeared. After 80 d, all the remaining cysts were resting ones. The total density of resting cysts was higher under MC concentrations of 0.2 and 0.4 g/L and lower under concentrations of 0.6, 0.8 and 1.0 g/L compared with that in the control. The total formation rate of resting cysts was 29.6% in the control group, and the lowest formation rate in the experimental groups was 15.5% at 0.6 g/L MC. The total germination rate of resting cyst decreased as the MC concentration increased, and approximately 68.0% of the resting cysts in the control group germinated successfully, whereas the addition of MC reduced the germination rate to as low as 12.4%. Our results indicated that the application of appropriate MC concentrations may provide an effective mitigation strategy for A. pacificum blooms because it does not leave more residual cysts, which can act as "seeds" for the initiation of HABs.

Mitigation of harmful algal blooms using modified clays: Theory, mechanisms, and applications

Clay dispersal is one of only a few mitigation methods for harmful algal blooms (HABs) ever applied in the field; however, low flocculation efficiency has always been the most significant drawback associated with natural unmodified clays. This review discusses key factors affecting the flocculation efficiency, based on results obtained in studies of the mechanisms underlying interactions between clay particles and HAB organisms. It further elaborates clay surface modification theory and methods for improving removal efficiency of HAB cells, followed by descriptions of various modified clays successfully prepared with removal efficiencies of HAB cells that are up to hundreds of times greater than natural clays and have lower dosing requirements of 4-10t/km². Presently, modified clays are the most widely used method for the mitigation of HAB in the field in China. This review also evaluates potential ecological effects of modified clay disposal on water quality, typical aquatic organisms, benthic environments, and ecosystems. Both laboratory and field results have demonstrated that modified clays markedly can actually improve water quality after treatment and pose no negative effects on aquatic ecosystems.

An investigation of the space distribution of Ulva microscopic propagules and ship-based experiment of mitigation using modified clay

Previous studies suggested that the removal of Ulva microscopic propagules (UMP) from cradle water might restrict the formation and expansion of green tides in the Yellow Sea, China. In this study, the distribution characteristics of UMP in the southern Yellow Sea was investigated, and then a flocculation experiment of UMP using modified clay (MC) was conducted at a selected station of the research cruise. The results indicated that the distribution of green algae thalli is one of the main factors that directly influence UMP distribution. UMP density was strongly negatively correlated with the distance between the sampling station and the centre of the area containing floating Ulva (r=-0.618***, n=83). >80% of the UMP was removed from the water column after MC application at a concentration of 0.1g/L, and MC applied at a concentration of 0.5g/L reduced the germination rate to 0.3%.