5.6 Cyanobacteria

Cyanobacteria are true bacteria, although they are often called “blue-green algae” because they resemble green algae in morphology, habitat and photosynthetic ability (and contain chlorophyll). They occur as single cells, filaments or colonies and their buoyancy can be metabolically regulated to allow optimal access to light and nutrients. Cyanobacteria inhabit all surface waters and become a problem only when present in excessive numbers (commonly known as “algal blooms”). Such blooms occur when temperatures are high, with long sunny days, high levels of phosphorus and to a lesser extent nitrogen in the water, low stream flows and calm conditions that permit the cells to migrate to the surface. These conditions occur sporadically, although in many parts of Australia blooms are more likely in late spring through to autumn. In addition, eutrophication (nutrient enrichment) associated with increased agriculture and urbanisation has increased the occurrence of cyanobacterial blooms.

Cyanobacterial toxins of concern in Australian drinking waters are summarised briefly in Table 5.8 with more details included in the fact sheets in Part V. For recent changes in cyanobacteria nomenclature see Chorus and Welker (2021). No human deaths have been recorded from ingesting the toxins directly produced by cyanobacteria. However, adverse outcomes may result from drinking water containing cyanotoxins, with extended exposure potentially leading to more serious health impacts (Chorus and Welker 2021).

Table 5.8 Cyanobacteria of concern in Australian drinking water

Toxin	Cyanobacteria species	Disease
Cylindrospermopsin	Cylindrospermopsis raciborskii, Chrysosporum (formerly Aphanizomenon) ovalisporum	Damage to liver, kidneys, lungs, heart, stomach, adrenal glands, the vascular system and the lymphatic system
Microcystins	Microcystis aeruginosa	Liver damage
Nodularin	Nodularia spumigena	Liver damage
Saxitoxin	Dolichospermum circinale (formerly Anabaena circinalis)	No direct evidence of disease in humans from exposure in drinking water

Direct contact with toxic or non-toxic species of cyanobacteria may cause skin rashes or eye irritation due to adverse reactions to components in the cell walls of the organisms. This could occur through showering or bathing in water containing blooms or scums. In one study 11-15% of people were found to be sensitive to contact with cyanobacteria (Pilotto et al. 2004).

Management of cyanobacteria is based on a combination of preventive measures including:

• source protection to reduce nutrient inputs

• management of reservoir conditions (e.g. to minimise stratification)

• treatment to remove cyanobacterial cells (and cell bound toxins)

• disinfection with oxidising disinfectants to inactivate cell-free toxins.

More details are provided in the fact sheets in Part V.

To date, most interest in cyanobacteria has focussed on planktonic species. However, potential concerns have been identified with the production of cyanotoxins and taste and odour compounds by benthic species (Gaget et al. 2017). There is limited information available on the public health significance of these species in drinking water supplies (Chorus and Welker 2021).