Does Algae Have Mercury in It? A Comprehensive Guide

December 23, 2025 •

3 min read

Algae can absorb mercury, which is a major entry point for this toxic element into the aquatic food web. Mercury levels in algae depend on the algae's source and cultivation methods.

Quick Summary

Algae absorbs mercury from its environment, and the amount varies based on its origin and how it is cultivated. Supplements from controlled indoor facilities are generally free from contaminants, unlike wild marine algae.

Key Points

Algae Absorbs Mercury: Algae absorb mercury from their environment, which is the start of heavy metal accumulation in the food chain.
Sourcing Matters: Algae from polluted waters can contain significant mercury, while cultivated algae for supplements are grown in clean systems.
Supplements are Generally Safe: High-quality algae supplements are tested for heavy metals and are generally mercury-free.
Chlorella May Act as an Adsorbent: Certain algae, such as chlorella, may help remove heavy metals from the body, though the efficacy of this process should be discussed with a doctor.
Check for Testing: Prioritize products from brands that offer third-party testing and COAs to verify low heavy metal levels.
Dilution in Blooms: In nutrient-rich environments with algal blooms, the mercury concentration per algal cell can be lower.
Wild Seaweed Requires Caution: Some seaweeds harvested from the ocean may contain low mercury levels, though generally below a hazardous threshold.

Mercury in Algae: Sources and Risks

Mercury is a naturally occurring element that enters aquatic environments through human activities, such as industrial processes. Bacteria convert mercury into methylmercury, a more toxic form.

Algae absorb mercury from the water, acting as the initial point of entry into the food chain. This absorption is a key step in bioaccumulation, where mercury concentrations increase at each level. Therefore, the mercury level in a species of algae is directly related to the mercury levels in its environment.

Bioaccumulation: How Mercury Moves Up the Food Chain

Absorption by Algae: Algae absorb both inorganic and organic mercury directly from the water.
Source Matters: Wild marine algae from polluted areas have a higher risk of containing mercury.
Dilution Effect: Algal blooms in freshwater lakes can decrease the concentration of methylmercury per algal cell, reducing mercury bioaccumulation.
Transfer to Higher Organisms: When fish and zooplankton consume algae, they ingest the mercury. The mercury then concentrates in larger predatory fish.

Wild vs. Cultivated Algae: A Comparison

The primary difference for consumers lies between wild-sourced and cultivated algae. The mercury risk differs significantly between these two types. Wild seaweed, especially from areas with industrial runoff, can have detectable heavy metal levels. In contrast, companies that grow supplements such as spirulina and chlorella use closed, freshwater systems and test regularly for contaminants. This controlled cultivation avoids exposure to marine pollutants.

Algae Supplements and Mercury Exposure

Consumers of algae supplements should pay attention to sourcing and testing. Algae oil is a plant-based source of omega-3s, and a safer alternative to some fish oils, which may contain mercury. The closed fermentation systems used to produce high-quality algae oil prevent contamination.

Some algae varieties are known for their heavy metal adsorption capabilities. Chlorella is often marketed for its detoxifying properties, as it can bind to and help remove heavy metals from the body. Studies on spirulina show that mercury levels in commercial products are low and below safety regulations.

Comparison Table: Wild vs. Cultivated Algae


Feature	Wild-Sourced Marine Algae (e.g., Seaweed)	Cultivated Algae (e.g., Spirulina, Chlorella)
Sourcing Environment	Open oceans, rivers, and coastal areas; susceptible to environmental contamination.	Closed-loop systems, freshwater tanks, or ponds; controlled, contaminant-free environment.
Risk of Mercury	Can be moderate to high, depending on the harvest location's proximity to pollution sources.	Very low to negligible, as the water source is clean and monitored.
Heavy Metal Testing	Often less rigorous or variable; may or may not be tested by distributors.	Reputable brands conduct regular and rigorous testing for heavy metals and other contaminants.
Primary Use	Food ingredient (seaweed wraps, snacks), natural whole food.	Dietary supplement (capsules, powders), omega-3 source.
Example Products	Various seaweeds, hijiki (can have high arsenic).	Chlorella tablets, Spirulina powder, algae oil supplements.

How to Reduce the Risk of Mercury Exposure from Algae

Consumers can minimize mercury exposure by following these steps:

Choose Cultivated Algae: Select supplements from algae grown in controlled, indoor facilities to avoid environmental contamination.
Look for Third-Party Testing: Seek brands that perform independent, third-party testing and publish the results. Certifications from organizations like IFOS or USP confirm purity and potency.
Check Certificates of Analysis (COA): A COA provides information on heavy metal levels. Reputable manufacturers often provide these upon request.
Consider the Source: Brands should be transparent about their sourcing. Products harvested from the ocean have a higher potential for contamination compared to those grown in a lab.

Conclusion

Algae can contain mercury. The risk is extremely low for consumers of supplements like spirulina and chlorella, when products are sourced from reputable, controlled environments. Choose cultivated products from transparent manufacturers who conduct third-party testing for metal purity to ensure you receive the nutritional benefits without environmental risks.

Frequently Asked Questions

Mercury poisoning from algae is unlikely with supplements from reputable sources. These products are grown in controlled environments and tested for contaminants. The risk is associated with fish, or wild-harvested algae from polluted waters.

Yes, high-quality algae oil is a mercury-safe alternative to fish oil. Algae oil is produced in closed fermentation systems, avoiding exposure to ocean pollutants like mercury. Fish can accumulate mercury over their lifespan, requiring fish oils to undergo purification to remove contaminants.

Commercial spirulina products generally do not contain mercury. Studies show low levels in some products, all below safety regulations. This is because spirulina is grown in controlled, non-marine environments, which eliminates oceanic mercury contamination.

Chlorella is marketed for its ability to bind to and remove heavy metals, including mercury, from the body. Some studies suggest it can act as a detoxifying agent. The efficacy should be discussed with a healthcare professional.

To ensure your supplement is safe, look for brands that provide third-party testing results. COAs should be available, confirming the product meets purity standards. Choose products from companies that state their algae are grown in controlled environments.

Mercury is a bigger problem for fish due to biomagnification. As mercury moves up the food chain, its concentration increases. Algae absorb mercury, but fish and predators eat algae (or other fish), leading to a higher mercury concentration over time.

All algae can absorb mercury from their environment. The amount absorbed depends on the level of contamination in the water where they grow. Algae grown in clean freshwater systems have negligible mercury levels compared to wild algae from polluted waterways.