Understanding Cyanotoxins: Microcystin, Anatoxin-a & Cylindrospermopsin

Understanding Cyanotoxins: Microcystin, Anatoxin-a & Cylindrospermopsin

Cyanotoxins are among the most significant operational and public-health risks facing drinking water utilities today. As harmful algal blooms (HABs) increase in frequency and scale, utilities are confronted with toxin events that can arise suddenly, persist for weeks, and challenge even advanced treatment systems. Understanding how cyanotoxins work, how they are produced, and how they move through freshwater systems is essential for developing an effective risk-management strategy.

This guide provides a practical breakdown of the three most critical cyanotoxins affecting U.S. drinking water reservoirs: microcystin, anatoxin-a, and cylindrospermopsin.

1. What Are Cyanotoxins?

Cyanotoxins are secondary metabolites produced by certain species of cyanobacteria. Not all blooms are toxic, and not all species produce toxins — but environmental stressors such as heat, nutrient spikes, chemical treatment, and competition can trigger toxin production.

Key properties utilities must consider:

• Toxins are released when cells rupture (lysis), whether naturally or from copper sulfate, peroxide treatments, rapid temperature shifts, or predation.
• Some toxins persist in water for days to weeks, even after a bloom declines.
• Activated carbon, oxidation, and advanced treatment can remove toxins — but effectiveness varies by compound.

EPA Health Advisories require extremely low allowable concentrations, meaning even modest toxin spikes can trigger public notifications.

2. Microcystin: The Most Common Cyanotoxin in the U.S.

Produced by:

Microcystis, Dolichospermum, Planktothrix, and others.

Toxin Type:

Hepatotoxin (liver-damaging)

Why It Matters:

Microcystin is the most frequently detected cyanotoxin in U.S. lakes and reservoirs. It is stable, resilient, and can remain in the water column long after a bloom declines.

Key Characteristics:

• Over 200 variants (congeners)
• Extremely low EPA Health Advisory levels
• Often associated with warm, stagnant, nutrient-rich water
• Released in large quantities when blooms collapse suddenly

Operational Impacts:

• Adsorbs to particulates, requiring optimized coagulation/flocculation
• Responds well to activated carbon (PAC/GAC)
• Requires careful monitoring during and after bloom collapse
• Not destroyed by simple chlorination unless contact times are high

Microcystin events are responsible for most U.S. drinking water HAB advisories.

3. Anatoxin-a: A Fast-Acting Neurotoxin

Produced by:

Dolichospermum, Aphanizomenon, Oscillatoria/Planktothrix strains.

Toxin Type:

Neurotoxin

Why It Matters:

Though less common than microcystin, anatoxin-a presents serious risks due to its rapid onset of neurological effects in animals and recreational users.

Key Characteristics:

• Highly potent, fast-acting
• Breaks down more quickly than microcystin
• Often associated with cooler or early-season blooms
• Can be produced in large amounts by benthic cyanobacteria in rivers

Operational Impacts:

• Responds inconsistently to activated carbon
• More easily oxidized than microcystin
• Often requires multiple sampling locations, especially in flowing systems
• Can occur even when reservoirs appear visually clear

Utilities must be especially vigilant near recreation zones and shallow inflows.

4. Cylindrospermopsin: A Persistent, Emerging Threat

Produced by:

Cylindrospermopsis (also called Raphidiopsis), Aphanizomenon, and others.

Toxin Type:

Cytotoxin (affecting liver, kidneys, and other organs)

Why It Matters:

Cylindrospermopsin is becoming more common in U.S. reservoirs, partly due to warming waters and species migration into northern latitudes.

Key Characteristics:

• Highly water-soluble, meaning it spreads through the water column
• Stable and persistent — can last weeks
• Produced both during growth and after cell rupture
• Difficult to remove with standard treatment
• More common in the Southeast, Southwest, and increasingly the Midwest

Operational Impacts:

• Advanced oxidation processes (AOP) may be required
• Conventional oxidation can be insufficient
• Can bypass treatment if monitoring is infrequent
• Often produced in low-visibility blooms

For utilities, cylindrospermopsin requires heightened surveillance and proactive planning.

5. How Cyanotoxins Behave During Bloom Growth & Collapse

During Active Growth:

• Toxins are primarily inside cyanobacteria cells
• Water samples may show high cell counts but low dissolved toxin levels
• Treatment plants may experience turbidity and filter loading, but not high dissolved toxins

During Bloom Collapse (Natural or Chemical):

This is the highest-risk period for utilities.

When cyanobacteria die:

• Cells rupture (lysis)
• Stored toxins flood the water column
• Dissolved toxin levels can spike quickly
• Carbon demand increases sharply
• Utilities may see sudden Health Advisory exceedances

Chemical treatments such as copper sulfate and peroxides can accelerate cell rupture, making timing critical.

6. Monitoring Strategies for Utilities

Effective monitoring programs combine:

Field Indicators

• Fluorometers (phycocyanin/chlorophyll-a)
• Secchi depth
• Surface scum presence

Laboratory Testing

• Cell identification and enumeration
• ELISA toxin assays
• LC-MS/MS confirmation (for regulatory reporting)

Spatial Monitoring

• Inflow zones
• Shoreline accumulation areas
• Dam forebays
• Mid-lake profiles

Toxins often appear in localized hotspots, meaning single-point sampling can miss critical events.

7. Treatment Considerations for Each Toxin

Microcystin

• PAC/GAC highly effective
• Oxidation with ozone, chlorine, or permanganate (proper CT required)
• Avoid sudden bloom collapse

Anatoxin-a

• Oxidation effective (chlorine, ozone)
• PAC effectiveness varies — jar testing recommended
• Monitor both benthic and pelagic sources

Cylindrospermopsin

• Best removed via AOP (UV/peroxide or ozone)
• Highly soluble — requires dissolved-phase monitoring
• PAC/GAC effectiveness variable

Utilities often benefit from multiple parallel treatment barriers.

8. Takeaways for Water Managers

• Microcystin, anatoxin-a, and cylindrospermopsin represent the most significant cyanotoxin risks for drinking water providers.
• Each toxin has unique behavior, persistence, and treatment challenges.
• Toxin spikes most often occur during sudden bloom collapses.
• Effective management requires continuous monitoring, pre-bloom planning, and multi-barrier treatment strategies.
• As HABs intensify nationwide, utilities should expect more frequent toxin-related operational demands.

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