Planktonic or benthic. Trichomes either solitary and generally straight or aggregated in fine tangled mats, isopolar, markedly constricted at the cross-walls, without branching, sometimes motile by slow gliding and trembling; firm shealths lacking but fine thin diffluent mucilaginous sheaths rare in some species. Cells isodiametric or mainly longer than wide, with or without aerotopes, when present these localised at the poles or in the centre of the cell; end cells sometimes slightly attenuated at the apex, without a calyptra, sometimes with characteristic apical protrusions. Reproduction by trichome fragmentation into many short few- celled motile hormogonia; necridia absent; all cells in the trichome capable of dividing and growing to +- original size before the next division. 

Pseudanabaena catenata – Cosmopolitan species commonly found among decaying vegetation and debris in streams, ponds and reservoirs. African isolated are known to produce the neurotoxins cyanotoxin and anatoxin (Gorham et al., 1982).

Pseudanabaena galeata – Cosmopolitan species common in the plankton of lakes and reservoirs throughout Australia. Also found growing among benthic cyanoprokaryotes assembles in shallow littoral areas of rivers and wetlands; widely distributed in many biotopes throughout north-eastern Australia. Known to produce taste and odour compounds 2-methylisoborneol and Geosmin (Izaguirre & Taylor, 2004).

Pseudanabaena limnetica – Common in the plankton of lakes and reservoirs, also associated with the benthos. Found throughout north-eastern Australia, and considered to be cosmopolitan. Populations from Bjelke-Petersen Dam, south-east Queensland, regularly occur with acuminate apical cells and possibly represent a distinct species.

Pseudanabaena mucicola – Endogloeic within the mucilage of planktonic and benthic populations of colonial cyanoprokaryotes such as Microcystis, Aphanothece, Chroococcus, and Wollea. This species has a cosmopolitan distribution.

*Not known to be toxigenic in Australia. Some spp produce earthy, musty odours

Metabolite produced – MIB/geosmin

**

Characteristics: Cells of Microcystis are arranged in colonies that are

initially spherical, but become irregular or perforated over time. The cells may

be grouped tightly or sparsely within the fine, colourless colonial mucilage.

The mucilage is often not clearly seen in preserved material. Smaller

colonies are microscopic, while larger colonies may be viewed with the naked

eye. Each colony consists of thousands of very small individual cells that are

spherical to sub-spherical without individual mucilage sheaths. Although the

protoplast is a pale blue-green colour, the cells, when viewed through the light

microscope, often appear black as a result of gas vacuoles that are located

within the cells. The gas vacuoles allow the colony to drift through water

layers to find the optimal amount of sunlight. In some species the gas

vacuoles appear glistening or reddish because of the reflection of light.

Species are differentiated by, amongst others, cell-size, presence of gas

vacuoles, the nature of the sheath, and the shape of the colony.

Dimensions: Cells vary from 0.5-9 μm in diameter.

Ecology: Microcystis is usually part of the phytoplankton, but may also form

granular clumps on bottom substrates. Colonies are common in enriched

lakes, ponds and reservoirs or in slow-flowing eutrophic rivers. Microcystis,

like many other cyanobacteria, prefers high water temperatures and usually

form blooms during the summer periods under conditions of adequate

nutrient supply. When environmental conditions are favourable, large

numbers (blooms) can sometimes be seen floating on the surface of the

water, giving a blue-green tinge to the water. It is interesting that, where

blooms of some species (e.g. M. aeruginosa (Kützing) Kützing) occur, the

habitat is completely dominated by this species to the exclusion of almost all

other forms of cyanobacteria.

Notes: In contrast to M. aeruginosa colonies, which are highly irregular and

clathrate when mature, M. flos-aquae (Wittrock) Kirchner occurs in nearly

globular colonies. M. wesenbergii (Komárek) Komárek also has intensely

lobed colonies, but it can be distinguished from M. flos-aquae and

M. aeruginosa by a smooth, very firm and colourless mucilage with the outer

margin of the colony clearly delimited and extending 3-6 μm beyond the cell

aggregations. The presence of gas vacuoles (which appear black or dark

brown under the light microscope) easily distinguishes Microcystis from

Aphanocapsa Nägeli (which lacks gas vacuoles).

Problems: Microcystis is a common cause of algal blooms, sometimes

secreting chemicals that inhibit other algae. Because of the presence of gas

vacuoles that render them buoyant, they produce surface scums and cause

a great deal of disturbance in lakes and reservoirs. Dense growths may lead

directly or indirectly to the death of fish through suffocation (as a result of

oxygen depletion) or by poisoning. Microcystis can produce a polypeptide,

called microcystin (named after Microcystis), which is toxic to animals

ingesting contaminated water. It has also been implicated in human

illnesses, such as necrosis of the liver (from ingestion) and severe dermatitis

(from skin contact). Blooms of Microcystis can also impart taste and odour to

the water and interfere with recreational activities.

Microcystis blooms often form during warm, calm weather in lakes and ponds with relatively high nutrient concentrations (nitrogen or phosphorous) or low nitrogen to phosphorus ratio (N:P<15)

Recent work suggests that high total phosphorus (TP) or total Nitrogen (TN) concentrations are better predictors of bloom formation than N:P ratios.

Microcystis does not fix atmospheric nitrogen, but is very efficient in taking up ammonium and urea, and often blooms when nitrogen concentrations are limiting to other algae. Microcystis blooms may follow blooms of nitrogen fixing Aphanizomenon.
Microcystis can use low concentrations of the herbicide glyphosate as a phosphorous source. The gas vesicles in Microcystis cells provide a mechanism to move up and down in the water column, which increases access to nutrients and other growth factors.

Microcystis blooms usually contain other types of cyanobacteria, especially Aphanizonmenon, Dolichospermum, Gloeotrichia, and Woronichinia. Microcystis colonies may be surrounded by short filaments of Pseudanabaena and inactive Chlamydomonas cells.

​

Microcystis cells may produce anatoxins (nerve toxin), Microcystins (liver toxins), lipopolysaccharides (skin irritants), and BMAA ( beta-Methylamino-L-alanine; nerve toxin). These toxins are released into the ambient environment when the cell wall is disrupted (cell lysis).

The toxin microcystin was first isolated from Microcystis aeruginosa.

Microcystins are rapidly degraded by naturally occurring but specialised bacteria. If the specialised bacteria are not present, Microcystins can persist in the aquatic environment for months. Anatoxins are rapidly degraded by sunlight and at pH levels that are slightly above neutral. At low pH levels, and in the absence of light, anatoxins may persist in the aquatic environment for a few weeks.

BMAA can bioaccumulate in zooplankton and fish, so this nerve toxin can contribute to health risks long after the toxic bloom has died back. Higher water temperatures and light appear to be associated with increased toxin production.

Not all Microcystis blooms result in the release of toxins.

*Potentially toxic – Microcystins

Sulphurous, earthy

Metabolite produced – Beta cyclo-citral, iso-propyl mercaptan, disulphides (aeruginosa) Disulphides, Geosmin (flos-aquae), Disulphides, iso-propyl mercaptan(Wesenbergii)

The cells of microcystis are small – only a few microns in diameter – and lack individual mucilage sheaths. The cells are usually arranged in colonies that are initially spherical but become irregular or perforated over time. The cells may be grouped tightly or sparsely within the fine, colourless colonial mucilage. The colonies are free-floating and may be composed of clustered subcolonies. Smaller colonies are microscopic, while larger colonies may be viewed with the naked eye.

The cells are spherical or hemispherical after dividing. The protoplast is a pale blue-green, but overall the cells appear dark or brownish due to the presence of gas vesicles. This feature easily distinguishes microcystis from aphanocapsa under light microscopy.