Algae, which are simple photosynthetic organisms found in nearly every aquatic environment, release a wide variety of volatile compounds that give them a noticeable smell. The odor is highly variable, ranging from a fresh, sweet earthiness to a strong, intensely foul, or fishy scent. This odor is a direct byproduct of the organism’s metabolism while living or the chemical compounds released during its decomposition. The specific smell depends on the dominant algal species and the compounds they produce.
The Chemical Origins of Algae Odor
The characteristic smells associated with algae are caused by specific Volatile Organic Compounds (VOCs) that are released into the water and the air. These compounds are produced by different species of algae, particularly the group known as cyanobacteria, or blue-green algae. The most well-known of these odor-causing molecules are the terpenoid compounds Geosmin and Methylisoborneol (MIB).
Geosmin is responsible for the distinct, damp soil or earthy smell often detected in freshwater bodies or tap water. It has an extremely low odor detection threshold, meaning humans can detect it at minute concentrations. Methylisoborneol (MIB) produces a musty or slightly moldy odor and is also produced by certain cyanobacteria species. Both Geosmin and MIB are stored within the algal cells and are released into the water when the cells naturally die or are broken apart.
Marine environments are characterized by a different primary odor compound: Dimethyl Sulfide (DMS). This sulfur compound is the source of the familiar, sometimes sweet, “sea air” smell at the coast. DMS is a breakdown product of a larger molecule called Dimethylsulfoniopropionate (DMSP), which is synthesized by marine phytoplankton. The DMS gas is released into the atmosphere, where it plays a role in the global sulfur cycle and helps sea birds locate feeding grounds.
Odor Profiles of Freshwater Versus Marine Algae
The environment in which algae grow dictates the type of odor a person is likely to encounter. Freshwater environments, such as lakes and slow-moving rivers, are often dominated by cyanobacteria, which produce the earthy and musty smells. A healthy freshwater pond or lake may have a faint, earthy aroma primarily due to the continuous, low-level production of Geosmin and MIB. However, when these algae experience a rapid growth phase, the resulting “pond scum” smell can become dense and swampy.
Marine water, in contrast, typically has a more complex sulfurous odor profile. The pleasant, crisp smell of the sea is due to the release of DMS by living phytoplankton. When marine algae, fish, or other aquatic life die and decompose, the odor changes dramatically to a foul, acrid, or intensely “fishy” scent. This putrid odor is caused by the formation of Trimethylamine (TMA), a breakdown product of biological material that is released during decay. Therefore, the smell of fresh sea air indicates living marine algae, while a strong fish market odor suggests material is actively decaying.
Identifying Smells Associated With Algal Toxicity and Decay
While many algae odors are harmless, certain smells can act as a warning sign for potential hazards or severe water quality issues. When a large mass of algae dies and begins to decompose in an environment with little oxygen, it can lead to the production of Hydrogen Sulfide gas. This gas is easily recognized by its powerful, rotten-egg smell and signals severe anaerobic decomposition. This decomposition can be hazardous due to extremely low oxygen levels in the water.
Certain Harmful Algal Blooms (HABs) can also produce odors that are described as intensely foul, chemical, or even like spilled paint or petroleum. These smells can sometimes be a sign that a bloom is actively producing toxins, known as cyanotoxins. If the water has an unusually strong, non-natural, or metallic odor, it is prudent to avoid contact with the water completely. The smell is a general indicator of a dense, potentially toxic bloom or a significant ecological problem.