Nodularin

(endorsed 2011)

Guideline

Due to the lack of adequate data, no guideline value is set for concentrations of nodularin. However given the known toxicity of nodularin, the relevant health authority or drinking water regulator should be notified immediately if blooms of Nodularia spumigena are detected in sources of drinking water.

General description

Nodularin is a cyclic pentapeptide hepatotoxin produced by and named after the cyanobacterium Nodularia spumigena. Nodularin is structurally similar to microcystins and exerts similar toxicity to microcystin-LR at its main target site in the liver.

Nodularin is found only in the cyanobacterium N. spumigena and up to five toxic variants of the usual structure and one non-toxic variant have been found to date. These variants are not considered here, as they appear to be very rare and the majority of Nodularin found in environmental samples are all of one type. The production of toxins and therefore the presence of toxicity in individual populations of some cyanobacterial species is known to be variable (Chorus and Bartram 1999 Chapter 3). In the case on N. spumigena, however, the majority of the strains tested so far in Australia appear to produce nodularin. It is therefore likely that most blooms of N. spumigena will have some degree of toxicity.

Australian significance

The cyanobacterium N. spumigena occurs primarily in brackish water. It forms blooms in estuarine lakes in Australia, New Zealand and Europe, and can also occur in brackish inland lakes in Australia (Wood 1975). In addition to these saline environments, there are also frequent blooms of toxic N. spumigena in freshwater lakes of the lower River Murray, South Australia (Baker and Humpage 1994). This rare circumstance where N. spumigena blooms in fresh water is of particular importance as the water is used for potable supplies, irrigation and stock watering. Lake Alexandrina in South Australia was the site of the first scientifically documented animal poisoning by N. spumigena, and indeed by any cyanobacterium (Francis, 1878). It is likely that these poisonings and the toxic effects described by Francis were due to nodularin. Low numbers of N. spumigena have also been recorded in the other (freshwater) river systems of the Murray-Darling Basin. The limited geographic scope for blooms of this organism in freshwater in Australia makes the occurrence of nodularin a relatively minor public health threat with respect to drinking water.

Treatment of drinking water

The first line of defence against cyanobacteria is catchment management to minimise nutrient inputs to source waters. Source water management techniques to control cyanobacterial growth include maintaining flow in regulated rivers; water mixing techniques to eliminate stratification and reduce nutrient release from sediments in reservoirs; and the use of algicides in dedicated water supply storages. Caution is necessary in using algicides if a bloom has developed because these agents will disrupt cells and liberate nodularins which are largely intracellular and can otherwise be removed by cell removal as noted below. Once intracellular toxins are released they are much more difficult to manage. Nodularins will eventually be released into the water phase when a developed bloom declines and algal cells lyse, reinforcing the need to prevent blooms as far as possible. Algicide use should be in accordance with local environment and chemical registration regulations. Where multiple intakes are available, withdrawing water selectively from different depths can minimise the intake of high accumulations of cyanobacterial cells at the surface.

Water treatment processes can be highly effective in removing both cyanobacterial cells and nodularin. As with other cyanotoxins, a high proportion of nodularin remains intracellular unless cells are lysed or damaged, and can therefore be removed by coagulation and filtration in a conventional treatment plant (Chorus and Bartram 1999 Chapter 9). It should be noted that using oxidants such as chlorine or ozone to treat water containing cyanobacterial cells, while killing the cells, will also result in the release of free toxin; therefore pre-chlorination or pre-ozonation are not recommended without a subsequent step to remove dissolved toxins.

Nodularin is readily oxidised by chlorine, but has not been evaluated with ozone. Adequate contact time and pH control are needed to ensure optimum removal of these compounds, and this will be more difficult in the presence of whole cells (Chorus and Bartram 1999 Chapter 9). Nodularin is also adsorbed from solution by powdered activated carbon, although it is important to seek advice and carefully select the most appropriate type for toxin removal, as carbons vary significantly in performance for different compounds. Boiling is not effective for destruction of nodularin.

If treatment is instituted in response to the presence of toxin-producing cyanobacteria, the effectiveness of the process needs to be confirmed by testing for toxin in the product water.

Method of identification and detection

Animal bioassays (mouse tests) have traditionally been used for detecting the presence of the entire range of cyanotoxins including nodularin. These tests provide a definitive indication of toxicity, although they cannot be used for precise quantification of compounds in water. A number of techniques are available for determining nodularin in water (Chorus and Bartram 1999 Chapter 13). These include screening techniques based on enzyme-linked immunosorbent assays (ELISA), protein phosphatase inhibition assays, and quantitative techniques such as high performance liquid chromatography (HPLC). The analytical techniques based on liquid chromatography (HPLC, liquid chromatography with mass spectrometry) offer good quantitative information on toxin concentrations, especially as chemical standards for nodularin are commercially available.

Cyanobacteria are detected by light microscopy, identified using morphological characteristics, and counted per standard volume of water (Hotzel and Croome 1999). Practical keys for their identification are provided in Baker and Fabbro (2002).

Health considerations

There are no reports of human health effects from consumption of water containing nodularin and/or N. spumigena. In addition, there are no human or animal studies of toxicity by oral exposure to nodularin. Nodularin is at least as hepatotoxic as microcystin for intraperitoneal exposure in experimental animals and, given its identical mode of action, can be regarded as presenting at least the same risk to human health as microcystin if ingested in drinking water. Nodularin is also known to accumulate in mussels in estuaries, and the consumption of contaminated shellfish therefore represents a potential alternative route of human exposure (Falconer et al. 1992).

Derivation of guideline

There are insufficient animal toxicity data to establish a guideline value for nodularin.

As there are some similarities between the toxicity of nodularin and microcystins, the guideline for microcystins (see Microcystins Fact Sheet) could be used to derive cell numbers of N. spumigena that provide a preliminary indication of the potential hazard. The only available monitoring data for nodularin in fresh water indicated that the upper range for cell numbers of N. spumigena was 50,000-80,000 cells/mL, and this correlated with nodularin levels of 1.0-1.7 mg/L (Heresztyn and Nicholson 1997). Based on these limited data, nodularin levels of around 1.3 mg/L would be associated with cell densities of 40,000-100,000 cells/mL (biovolume of 9.1 to 22.7 $\text{mm}^3\text{/L}$; based on a mean cell volume of 227 $\text{mm}^3$).

Notification procedure

It is recommended that a notification procedure be developed by water and health authorities. A tiered framework should be considered. Initial notification to health authorities could be provided when numbers of N. spumigena reach 30% of the density equivalent to 1.3 μg/L nodularin (12,000 cells/mL; biovolume 2.7 $\text{mm}^3\text{/L}$;), while an alert could be provided when cell numbers are equivalent to 1.3 μg/L nodularin (40,000 cells/mL; biovolume 9.1 $\text{mm}^3\text{/L}$).

In all cases, cell numbers should only be used as preliminary signals and as triggers for toxin testing to enable assessment of potential health risks.

References

Baker PD, Fabbro LD (2002). A Guide to the Identification of Common Blue-Green Algae (Cyanoprokaryotes) in Australian Freshwaters. CRCFE Identification Guide No. 25, Cooperative Research Centre for Freshwater Ecology, Albury.

Baker P, Humpage AR (1994). Toxicity associated with commonly occurring cyanobacteria in surface waters of the Murray-Darling Basin, Australia. Australian Journal of Marine and Freshwater Research, 45:773-786.

Chorus I, Bartram J (eds) (1999). Toxic Cyanobacteria in Water. A guide to their public health consequences, monitoring and management. E&FN Spon, London.

Falconer IR, Choice A, Hosja W (1992). Toxicity of the edible mussel (Mytilus edulis) growing naturally in an estuary during a water-bloom of the blue-green alga Nodularia spumigena. Environmental Toxicology and Water Quality, 7:119-123.

Francis G (1878). Poisonous Australian lake. Nature, 18:11-12.

Heresztyn T, Nicholson BC (1997). Nodularin concentrations in Lakes Alexandrina and Albert, South Australia, during a bloom of the cyanobacterium (blue-green alga) Nodularia spumigena and degradation of the toxin. Environmental Toxicology and Water Quality, 12:273-282.

Hotzel G, Croome R (1999). A Phytoplankton Methods Manual for Australian Freshwaters. LWRRDC Occasional Paper 22/99, Land and Water Resources Research and Development Corporation, Canberra.

Wood GW (1975). An Assessment of Eutrophication in Australian Inland Waters. Australian Water Resources Council Technical Paper No.15. Australian Government Publishing Service, Canberra.