Domoic acid-producing diatom responsible for amnesic shellfish poisoning. Spring blooms in California and Mediterranean coastal waters with moderate TEP production and unique silica-frustule fouling behaviour.
Silica-walled cells with a potent neurotoxin that challenges conventional pretreatment assumptions
Pseudo-nitzschia is a pennate diatom with elongated cells (30–100 µm length, 1–8 µm width) enclosed in a silica frustule of two overlapping valves. The frustule is ornamented with rows of poroids (pores) that facilitate nutrient exchange. Unlike unarmoured dinoflagellates, the rigid silica wall makes Pseudo-nitzschia cells resistant to shear-induced lysis. This mechanical stability means cells survive pumping and mixing intact, passing through screens as needle-like particles that can penetrate deeper into pretreatment systems.
Domoic acid (DA) is a water-soluble tricarboxylic amino acid that acts as a potent glutamate receptor agonist, causing neurological damage in mammals. DA is produced by at least 14 Pseudo-nitzschia species, with production triggered by nutrient stress (particularly iron or phosphorus limitation) and high light. Intracellular DA concentrations reach 1–50 pg/cell. DA is significantly smaller (mol. wt. 311 Da) than brevetoxins and may show 50–80% rejection by polyamide RO membranes, depending on fouling state and pH. Product water monitoring is essential during blooms.
Cells connect end-to-end via silica spines to form chains of 2–20+ cells. Chain length increases under favourable conditions and decreases under stress. The chain structure affects settling and flotation behaviour: short chains and single cells remain suspended in the water column, while long chains form aggregates that settle or float depending on gas vacuole content. Chain fragmentation during pumping creates numerous single cells that challenge filtration.
The physical oceanography of domoic acid events in eastern boundary upwelling systems
Eastern boundary upwelling systems (California Current, Humboldt Current, Benguela Current, Canary Current) supply the nutrient-rich water that fuels Pseudo-nitzschia blooms. Equatorward winds drive surface water offshore, replacing it with cold, nutrient-laden deep water. Upwelling events last days to weeks and occur seasonally, creating predictable bloom windows.
Pseudo-nitzschia outcompetes other phytoplankton under high nitrate, low silicate, and low phosphate conditions. The silicate-to-nitrate ratio is particularly important: when silicate is depleted, diatom growth slows, but Pseudo-nitzschia continues producing domoic acid. Iron limitation also triggers DA production. These nutrient dynamics make bloom prediction possible from nutrient monitoring.
Optimal growth occurs at 12–18 °C, making Pseudo-nitzschia a cool-water species compared to tropical HAB dinoflagellates. Spring blooms in California (March–June) coincide with peak upwelling and increasing day length. Mediterranean blooms occur in spring (March–May) and autumn (September–November) during mixing events.
Offshore Pseudo-nitzschia blooms are transported to coastal waters by wind-driven surface currents and internal waves. Once nearshore, cells accumulate in retention zones created by bathymetry and coastline geometry. The lag between offshore bloom development and coastal arrival is typically 3–7 days, providing a window for alert and response.
The unique fouling profile of diatoms: rigid silica particles plus sticky polysaccharides
The silica frustules of Pseudo-nitzschia are essentially inert, rigid particles that do not adhere to membrane surfaces directly. However, needle-shaped frustules (1–8 µm wide, 30–100 µm long) can penetrate into feed channel spacers and accumulate at spacer-membrane junctions, creating localised flow disruption. Frustule fragments from dead cells add to particulate loading. While not chemically adhesive, frustule accumulation increases feed-channel pressure drop and creates sites for organic fouling deposition. Acid CIP (pH 2) does not dissolve silica; only alkaline CIP with surfactant can dislodge frustule accumulations.
Pseudo-nitzschia produces moderate quantities of TEP (200–1,000 µg Xeq/L during blooms), lower than C. polykrikoides but still significant. The AOM is rich in polysaccharides associated with the extracellular polymeric sheath and released during chain fragmentation. Because diatom TEP is often bound to silica spines, it has a composite particulate-colloidal character that responds differently to coagulation than pure polysaccharide TEP from dinoflagellates.
Diatom-derived TEP provides a nutritious, structured matrix for bacterial colonisation. The silica frustules within the biofilm create a composite fouling layer with both organic and inorganic components. This composite layer is more resistant to standard alkaline CIP than organic-only biofilms because the silica skeleton maintains structure even after organic digestion. Extended soak times and mechanical shear from high-flow flushing improve removal.
Domoic acid is highly water-soluble and polar, with limited adsorption to activated carbon. RO rejection of DA is moderate (50–80%) and variable. During bloom conditions, DA concentrations in raw seawater can reach 1–10 µg/L. Even 50% rejection leaves 0.5–5 µg/L in permeate — below acute toxicity but above the precautionary limit of 0.5 µg/L applied in some jurisdictions. Post-treatment with UV oxidation or AOP is recommended during confirmed DA events. Brine discharge monitoring may be required.
Cool-water blooms with distinct fouling and toxin challenges
The narrow width of Pseudo-nitzschia cells (1–8 µm) allows them to pass through coarse screens (10–50 mm) and even some fine screens (1–3 mm) if oriented lengthwise. Intact cells reach DAF and multimedia filters. Because cells are denser than water, they tend to settle in quiescent zones rather than float. DAF must be optimised for removal of slightly negatively buoyant particles.
Ferric chloride at 5–15 mg/L as Fe effectively removes Pseudo-nitzschia cells and TEP. The silica frustule surface is negatively charged at seawater pH, facilitating charge neutralisation by Fe³+. Jar testing confirms optimal dose because cell density and chain length vary. Polymer aid improves settling and flotation of silica-containing floc.
RO fouling from Pseudo-nitzschia is moderate compared to C. polykrikoides events. Normalised differential pressure increases 5–15% per week. CIP intervals contract to 6–10 weeks. The composite organic-silica fouling layer requires alkaline CIP with surfactant for organic removal, followed by high-flow flush for frustule dislodgement. Acid CIP alone is ineffective.
Domoic acid passage through RO creates potential product water quality concerns. GAC has limited effectiveness for DA removal. UV/H₂O₂ AOP at 500–1,000 mJ/cm² achieves >90% DA destruction. Permeate DA monitoring by LC-MS/MS during bloom events confirms safety. Regulatory limits for DA in drinking water are not yet universal but are under development in several jurisdictions.
Addressing both the silica particulate and the domoic toxin in cool-water upwelling systems
Key differences: Rigid silica frustules require different CIP chemistry than organic fouling. Domoic acid is more water-soluble and harder to adsorb than brevetoxins. Cooler temperatures slow biological fouling kinetics but do not eliminate them.
Ferric chloride at 8–20 mg/L as Fe with anionic polymer (0.1–0.3 mg/L) removes cells and TEP. DAF hydraulic loading 5–10 m/h with recycle ratio 8–12%. Because diatom cells are denser than dinoflagellates, flotation efficiency may be slightly lower; verify with jar tests. Saturator air dosing 6–8 g/m³ ensures adequate bubble supply.
UV/H₂O₂ AOP in post-treatment at 500–1,000 mJ/cm² with 5–10 mg/L H₂O₂ achieves >90% domoic acid destruction. GAC provides limited benefit but improves overall organic removal. Permeate DA monitoring by LC-MS/MS during bloom events. Brine discharge dilution modelling ensures environmental compliance.
Alkaline CIP (pH 11–12, NaOH + surfactant, 60–90 minutes) digests organic matrix. High-flow flush (120% design flow, 30 minutes) dislodges silica frustules. Repeat alkaline CIP if pressure drop remains elevated. Acid CIP is not effective for silica. Normalised performance must recover to >95% baseline before return to service.
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