CyanoHABs

The cyanobacteria that make up CyanoHABs are just as diverse as the metabolites they produce. Some form obvious blooms/scums, and some less obvious (e.g. benthic). Here is some facts about cyanoHABs and cyanoacteria.

Planktonic Cyanobacteria

Here are some cyanobacteria known to grow in the water column that can be observed by the naked eye as scums or discolored water.

Microcystis

This colonial (coccoid) cyanobacterium is known to produce surface scums due to obligate aerotopes, small gas vesicles that stay filled with air and make them float. The colonies can sometimes be seen by the naked eye, appearing like flecks in the water or "painted water". Microcystis is also the most prominent producer of the cyanotoxin microcystin (MC), the most studied cyanotoxin in the world. Almost all blooms have been shown to produce microcystin, with rare observations of some that are not able to produce the toxins. With Microcystis, it is better to test blooms containing this cyanobacterium rather than guess if it is producing toxin.

Dolichospermum / Sphaerospermopsis / Anabaena

This clade of cyanobacteria used to be just called Anabaena, but molecular research has supported the rationale to split them up into different genera. Some evidence supports that the current Anabaena is benthic, while Dolichospermum (both coiled and straight species) are more planktonic and can move up and down in the water column by compressing their gas vesicles. Sphaerospermopsis can only be distinguished via genetic testing or observation of akinete placement. The observation of these resting cells is integral to morphological identification and distinction of this group. Toxins produced by this group include anatoxins (named after Anabaena originally), saxitoxins, microcystins, cylindrospermopsins, and guanitoxin.

Aphanizomenon / Cuspidothrix / Umezakia (Chrysosporum)

This clade of cyanobacteria, like Anabaena, has split off from Aphanizomenon for the same reasons. Even some Anabaena have moved into this clade, and it could be argued that further modifications are necessary. These cyanobacteria are nearly always straight filaments, with end-cell shape and akinete placement required to tell them apart. Some even form fascicles (e.g., Aphanizomenon flos-aquae), which can help distinguish them microscopically and can appear like blades of cut grass with the naked eye when blooms form. Toxins produced by this clade include anatoxins, cylindrospermopsins, and saxitoxins, but others cannot be fully ruled out.

Raphidiopsis (previously including Cylindrospermopsis)

This cyanobacterium used to be two groups, Raphidiopsis (described first) and Cylindrospermopsis (described later). The latter was found to produce the toxin cylindrospermopsin, ergo the nomenclature. However, upon further study, the genetics could not support their differentiation, so the older name took precedence, and we now refer to all the terminally heterocystic Nostocalean filaments as Raphidiopsis. Other toxins this cyanobacterium is known to produce include saxitoxins. Interestingly, cylindrospermopsin production seems to be limited to strains in the Southern Hemisphere, not here in the US or Europe to a large extent.

Planktothrix

Planktothrix, previously called Oscillatoria and then broken off into its own group due to its planktonic nature and recent molecular evidence supporting it, is a significant potential toxin producer. While it does not form heterocytes or akinetes like the Nostocalean cyanobacteria, it does have aerotopes and can form surface scums. These appear like sticks when looking through the microscope, or sometimes tufts floating when looking macroscopically. They can even turn the water red when blooms of Planktothrix rubescens form. The toxins observed from this group include microcystins, even variants that do not have a standard Adda group, so they can produce harmful toxins that might not show up in certain testing (e.g., ELISA). Also, they can produce anatoxins, and there is one report of saxitoxins.

Woronichinia

This cyanobacterium can sometimes look like Microcystis through the microscope, but has potato-shaped cells instead of circles, which can help distinguish it. Woronichinia is reported to be a microcystin producer, but strong evidence for this is lacking, and it may be that Microcystis hides well in blooms of Woronichinia and can contribute to the toxin presence. It is good to test blooms of Woronichinia for microcystins due to this potential.

Benthic or Epiphytic Cyanobacteria

Cyanobacteria grow everywhere and can grow on rocks, plants, in sediment, and even in trees. Here are some potentially hazardous ones that are frequently observed or cause problems for us, animals, and the environment.

Phormidium / Kamptonema /

Microcoleus / Oxynema

This clade clade is currently undergoing significant revision, expanding as molecular insights reveal a much more complex genetic landscape than previously understood. Once unified under the broad genus Phormidium, researchers now recognize this group as a diverse collection of distinct genera, each with unique morphological characteristics and varying toxicological profiles. This benthic group is particularly hazardous because of its capacity to produce high concentrations of neurotoxic anatoxins.

A significant challenge in managing these cyanobacteria is the risk of analytical oversight; standard Mass Spectrometry (MS) protocols often focus solely on anatoxin-a, potentially missing the presence of potent congeners like dihydroanatoxin-a or homoanatoxin-a. Consequently, the true toxic potential of a site can be severely underestimated if the analysis is not sufficiently broad. These organisms thrive in both stagnant and flowing water environments, where they form dense mats on the bottom. These mats can be detached and transported by hydrogeological events or heavy rainfall, leading to animal intoxications and deaths worldwide as toxic material becomes accessible to wildlife and pets along shorelines.

Oscillatoria / Tenebriella

The genus Oscillatoria is a filamentous cyanobacterium named for its characteristic oscillating movement, a gliding motion used to orient itself toward light. While the genus Planktothrix was once included in this group, it has since been distinguished by its use of aerotopes (gas vacuoles) for buoyancy. In contrast, Oscillatoria generally lacks these vacuoles and remains benthic, dwelling on the bottom or attached to substrates. However, these organisms can rise to the surface when oxygen bubbles from photosynthesis become trapped in their mucilage, causing mats to lift and float.

Beyond its ecology, Oscillatoria is a major concern for water quality due to its production of geosmin and MIB, compounds that create unpleasant "earthy" tastes and odors. More significantly, the genus can produce potent cyanotoxins, including microcystins (liver toxins) and anatoxins (neurotoxins). Because these toxins pose serious risks to humans and animals, the presence of Oscillatoria mats requires close monitoring, especially when they detach and wash into shallow areas.

Aetokthonos hydrillicola

The epiphytic and colonial cyanobacterium Aetokthonos hydrillicola has been a subject of intense study for decades, yet its role in producing lethal neurotoxins was only recently fully understood. Its name, derived from the Greek for "eagle-killer," reflects a long-standing ecological mystery. Scientists collaborating globally eventually discovered that this cyanobacterium—which grows on the invasive aquatic plant Hydrilla verticillata—is the source of aetokthonotoxin (AETX), the driver behind Avian Vacuolar Myelinopathy (AVM). This disease causes lesions in the brain and spinal cord, leading to the deaths of bald eagles, waterfowl, and other wildlife.

The production of this neurotoxin requires a specific environmental trigger: the element bromine. Ironically, humans have inadvertently fueled this toxicity; bromine is often introduced into waterways through the use of certain herbicides, such as diquat dibromide, used to control the very Hydrilla the cyanobacteria colonize. When bromine is present, A. hydrillicola synthesizes the lethal toxin, which then bioaccumulates up the food chain. As this cyanobacterium spreads through infested waterways, the threat to apex predators, including eagles and even large mammals like mountain lions, has become an urgent concern for conservationists.

Microseira wollei

The benthic cyanobacterium Microseira wollei (formerly Lyngbya wollei) is a prominent fixture in freshwater springs across the Southeast, particularly in Florida, though its range extends into reservoirs and lotic systems as far north as Canada. Known for its thick, mucilaginous sheaths, it forms dense, coarse mats often referred to as "mermaid hair." These mats play a complex role in the ecosystem; for example, invasive fish like tilapia have been observed using the resilient filaments for nesting. However, the organism is best known for producing a unique suite of neurotoxins originally dubbed Lyngbya wollei toxins (LWTs).

Chemically, these toxins share the same carbamate alkaloid backbone as other saxitoxins, but they are distinguished by the addition of an acetyloxy group. This structural nuance is significant because the group can easily fall off when hydrolyzed, a process that effectively converts the LWTs into more potent saxitoxin derivatives, such as decarbamoyl gonyautoxins (dcGTX) and decarbamoylsaxitoxin (dcSTX). Given the formal taxonomic shift from Lyngbya to Microseira, many in the scientific community argue that it is time for this class of neurotoxins to be renamed to reflect its current classification and to align with the standard nomenclature used for other cyanotoxins.

Nostoc

The genus Nostoc is a remarkably versatile cyanobacterium found globally, from aquatic benthos to terrestrial environments where it attaches to tree branches or damp soil. In the environment, it often appears as dark, gelatinous clumps—sometimes nicknamed "witch’s butter"—but under a microscope, its beaded filaments look strikingly like Anabaena. It is unique for its ability to form these organized, jelly-like colonies that protect the filaments within a thick, "snot-like" mucilage.

While many Nostoc species are consumed as food in Asia and generally considered safe, scientific understanding has shifted since strains from the Baltic Sea were found to produce a unique and genetically distinct set of toxins. These strains produce acetyloxy-Adda microcystins. This specific chemical modification is a major concern for water quality because most standard laboratory tests are designed to find the common "Adda" group. Because the acetyloxy version is structurally different, it can easily slip past routine screenings undetected, even though it remains just as toxic. This "hidden" nature makes Nostoc a complex organism to monitor, as it may harbor dangerous compounds that standard methods simply aren't looking for.

Anabaena

In the field, benthic Anabaena typically forms thin, felt-like mats or fuzzy coatings on submerged rocks and sediments. Unlike their planktonic relatives, they do not possess gas vacuoles (aerotopes) and therefore do not form surface blooms on their own. However, like other benthic cyanobacteria, these mats can occasionally detach and float to the surface if oxygen bubbles from photosynthesis become trapped within the mat's structure.

From a water quality perspective, benthic Anabaena is a potent producer of cyanotoxins. It is particularly notorious for synthesizing anatoxins, powerful neurotoxins that can act so rapidly they are often associated with sudden dog poisonings when pets lick or ingest mats washed up on shorelines. Some benthic strains also have the genetic capacity to produce microcystins (liver toxins) and saxitoxins (paralytic toxins). Because these mats are concentrated on the bottom, they can harbor high levels of toxins even when the overhead water column appears clear, making them a "hidden" risk for recreational users and local wildlife.