Blooms of the toxic dinoflagellate Karenia mikimotoi could threaten the survival of marine life, and modified clay (MC) is considered a promising method for the control of harmful algal blooms. Here, using marine medaka as the model organism, the toxicity of K. mikimotoi before and after MC disposal was investigated. The results showed that only a certain density of intact K. mikimotoi cells could cause obvious damage to fish gills and lead to rapid death. A systematic analysis of morphology, physiology, and molecular biology parameters revealed that the fish gills exhibited structural damage, oxidative damage, osmotic regulation impairment, immune response activation, and signal transduction enhancement. MC can flocculate K. mikimotoi rapidly in water and reduce its toxicity by reducing the density of intact algae cells and hemolytic toxicity. The results indicate that MC is an effective and safe method for controlling K. mikimotoi blooms.

Phaeocystis globosa is an important harmful algal species that is globally distributed. Previous studies have indicated that P. globosa preferentially uptake nitrate, but the underlying mechanism is still unclear. To further verify this preference and reveal the underlying mechanism, the assimilation rates of nitrate and ammonium by P. globosa at different concentrations was quantitatively studied by using a nitrogen stable isotope (¹⁵N) tracer technique, and the regulatory mechanism was determined from the physiological and biochemical responses. The findings revealed that the preferential assimilation of nitrate by P. globosa was influenced by the ambient ammonium concentration. When the ambient concentration of ammonium was less than approximately 3.5 μmol·L⁻¹, the assimilation rates of nitrate form P. globosa were as high as 1.05 × 10⁻⁵ μmol·L⁻¹·h⁻¹·cell⁻¹. Even though the nitrate assimilation in P. globosa was inhibited at ammonium concentrations greater than this threshold, nitrate assimilation could not be completely suppressed. The activity of NR and the expression of related genes in P. globosa were also affected by ammonium. In addition, ¹⁵N signals originally labeled nitrate accumulated in ammonium. This indicated that P. globosa was likely to reduce excess nitrate to ammonium and subsequently release it into the substrate, which might be an important energy dissipation mechanism for P. globosa. These results expand the classical understanding of the utilization of nitrogen nutrients by marine phytoplankton, and offer valuable resources for comprehending the mechanism of harmful algal blooms.

On the basis of field experience, a bloom does not continue after treatment with modified clay (MC), even though the residual harmful algal bloom (HAB) biomass accounts for 20–30% of the initial cells. This interesting phenomenon indicates that, in addition to causing flocculation, MC can inhibit the growth of residual cells. Here, from a cell morphology perspective, Aureococcus anophagefferens was used as a model organism to explore this scientific issue and clarify the mechanism by which MC mitigates harmful algal blooms (HABs). The results showed that, at an ~70% removal efficiency, neutral clay (NC) could not effectively inhibit the growth of residual cells, although it caused various forms of damage to residual cells, such as cell deformation, cell breakage, decreased extracellular polysaccharides (EPS), increased cell membrane permeability, and increased cytoplasmic granularity, due to physical collisions. After modification, some physical and chemical properties of the clay particle surface were changed; for example, the surface electrical properties changed from negative to positive, lamellar spacing increased, hardness decreased, adhesion chains increased, adhesion improved, and the number of absorption sites increased, enhancing the occurrence of chemical and electrochemical effects and physical collisions with residual cells, leading to severe cell deformation and chemical cell breakage. Thus, MC effectively inhibited the growth of residual cells and controlled HABs.

Harmful algal blooms pose a pervasive threat to coastal ecosystems and aquaculture worldwide, with estuarine waters being hotspots of HAB outbreaks. Modified clay (MC) is one of the few emergency mitigation measures with demonstrated field effectiveness, yet its flocculation efficiency remains highly uncertain across estuarine and coastal waters with fluctuating salinity. Here we combine particle image velocimetry, algae-removal experiments, and Derjaguin–Landau–Verwey–Overbeek (DLVO) calculations to resolve how salinity (0‰–34‰) governs the flocculation and algae-removal performance of polyaluminium chloride-modified clay (PAC-MC), and we develop a high-salinity modified clay (MC-HS) to overcome the identified limitation. As salinity decreases, floc strength, recovery coefficient, and fractal dimension all decline, and removal of the raphidophyte Heterosigma akashiwo drops from 35.2% at 30‰ to 8.2% at 5‰ (0.1 g/L). DLVO calculations trace this decline to the MC–MC self-flocculation energy barrier, which rises to ≈33 kT at S = 5‰ and vanishes only above a critical coagulation concentration of ≈7‰. Because MC–algae adhesion remains barrier-free at all salinities, it is this self-flocculation barrier, not cell capture, that limits removal. MC-HS, prepared by simply incorporating NaCl into PAC-MC, creates a transient local high-salinity micro-zone upon dosing, compresses the electric double layer, and restores flocculation without raising bulk salinity; across 5–30‰ its removal efficiency shows no significant salinity dependence and reaches 5.4-fold that of PAC-MC at the lowest dosage and salinity tested. The local ionic-atmosphere compression strategy demonstrated here removes this barrier without altering bulk water chemistry, offering a practical basis for harmful algal bloom emergency control in salinity-variable waters.