Microcystis aeruginosa is a type of freshwater cyanobacteria, also known as blue-green algae, that exists in aquatic environments globally. These single-celled organisms are typically small, measuring only a few micrometers in diameter, and can form large colonies visible to the naked eye. While a natural component of many freshwater ecosystems, Microcystis aeruginosa becomes a concern when it rapidly proliferates, forming dense, often discolored, aggregations known as harmful algal blooms (HABs).
Formation of Harmful Blooms
The growth of Microcystis aeruginosa into harmful blooms is driven by environmental factors. A primary trigger is eutrophication, the enrichment of water bodies with excess nutrients (particularly phosphorus and nitrogen). These nutrients often originate from human activities, such as agricultural runoff containing fertilizers and discharges from wastewater treatment facilities, acting as a direct food source for the cyanobacteria. Microcystis species are efficient at nutrient uptake and thrive in such nutrient-rich, or eutrophic, waters.
Warm water temperatures contribute to the proliferation of these blooms, with growth rates increasing as temperatures rise. While Microcystis aeruginosa can grow at temperatures above 15°C, its highest laboratory growth rates are observed around 32°C. This organism also benefits from abundant sunlight for photosynthesis; its cells contain gas-filled vesicles that allow it to regulate buoyancy and position itself to access light. Stagnant or slow-moving water, along with high water column stability and decreased lake flushing, further promotes bloom formation by allowing the cyanobacteria to accumulate at the surface.
Toxins and Health Risks
The danger associated with Microcystis aeruginosa blooms stems from their ability to produce toxins, primarily a group of compounds called microcystins. Not all blooms produce toxins, and toxin concentrations vary, making visual assessment alone insufficient to determine risk. Microcystin-LR is one of the most commonly encountered variants of these toxins.
Human exposure to microcystins can occur through several routes, leading to various health issues. Ingesting contaminated water can cause gastrointestinal illness, including vomiting, nausea, headaches, and diarrhea. Higher exposure levels can lead to liver damage. Skin contact with bloom-affected water may result in irritation, rashes, and blistering. Inhaling aerosolized toxins, which can occur near dense blooms, can cause respiratory issues such as pneumonia and fever.
Pets, especially dogs, face severe health risks due to ingesting contaminated water or licking bloom material from their fur after swimming. Microcystin poisoning in dogs can lead to severe liver failure, internal hemorrhaging, and kidney damage. These effects can progress rapidly and, despite aggressive treatment, can result in death. Livestock and wildlife have also experienced illness and fatalities after exposure to microcystin-contaminated water.
Impact on Aquatic Ecosystems
Beyond direct health risks, Microcystis aeruginosa blooms damage aquatic ecosystems. The dense surface scum formed by these blooms blocks sunlight from penetrating the water column. This light reduction suppresses and kills submerged aquatic plants, which are a foundation for aquatic food webs and provide habitat. The decline of these plants disrupts the ecological balance.
When the cyanobacterial bloom dies, decomposition consumes large amounts of dissolved oxygen in the water. This leads to hypoxia (low oxygen) or anoxia (no oxygen), creating “dead zones” where most aquatic life cannot survive. Such oxygen depletion can result in fish kills and negatively affects the survival of other aquatic organisms important to the food web. The presence of these blooms has been linked to declines in pelagic fish populations in certain regions.
Detection and Management Strategies
Authorities employ various methods to detect and monitor Microcystis aeruginosa blooms and their associated toxins. Monitoring involves field sampling for laboratory analysis, including counting cyanobacterial cells and testing for toxin concentrations. Satellite imagery can also be used for broad-area bloom detection. The World Health Organization (WHO) provides risk levels and suggests limits for cell counts and microcystin levels in drinking and recreational water.
Management strategies for Microcystis aeruginosa blooms fall into two main categories: prevention and direct intervention. Long-term prevention focuses on reducing nutrient loads entering water bodies, such as improved agricultural practices and upgraded wastewater treatment facilities. Encouraging the replacement of lawns with native plants can also help reduce the need for fertilizers and water usage. Increasing flushing or flow rates in waterways, for example, by releasing water from dams, can help reduce bloom conditions by increasing mixing and flow.
Direct intervention methods for existing blooms include the application of algaecides like hydrogen peroxide, which can reduce Microcystis aeruginosa growth and microcystin production. Other approaches involve adsorbents that can remove cells and toxins, or specific algicidal bacteria that target cyanobacteria. Physical methods such as mechanical removal or ultrasonic devices are also sometimes employed.