Algae and algal toxins

Chapter 11 Algae and algal toxins

11.1 Introduction Many impoundment and storage reservoirs are rich in nutrients from farming and

the discharge from sewage treatment plants upstream (Chapter 5 ). Given adequate light, algae can quickly establish themselves and become a problem. Algal development is worse in the summer due to thermal stratification, with the lower zones of the reservoir (i.e. hypolimnion) becoming anaerobic. This causes ammonia, phosphorus and silica, all algal nutrients, to be released from the bottom

sediments, which encourages even more algal growth (Sections 4.3 and 5.5 ).

11.2 Problems associated with algae There are two specific problem areas for the water supply engineer associated

with algal blooms. The first is an operational problem. The physical presence of the algae makes water treatment more difficult because the algae must be removed from the finished water. If small algae do get through the filter, then algae will enter the distribution system and cause several problems. The water will be coloured and turbid, the algae will decay causing taste and odour problems, and finally the algae will either become the source of food for micro-organisms growing on the walls of the supply pipes, or the source of food for larger animals

infesting the supply system (Chapter 24 ). In practice, algae reduce the rate of flow through the treatment plant by blocking microstrainers; the larger algae blocking rapid sand filters and the smaller species blocking the finer slow sand filters. In severe cases the supply may be drastically reduced or stopped altogether.

The second problem area is one of metabolism. The algae do two things. They all produce carbon dioxide during respiration, and this can cause a severe alteration of the pH of the water. This is most often caused by large crops, known as blooms, of blue-green algae. These changes in pH severely disrupt the coagulation process, resulting in the loss of coagulant, often aluminium sulphate, into the finished water and a reduction in quality in terms of colour and turbidity. Certain algae also release extracellular products, some of which are toxic, others that increase the organic matter content of the water or form

11.2 Problems associated with algae

halogenated by-products that are carcinogenic (Chapter 18 ). Taste and odour

problems can cause treatment plants to close during the summer for up to two month s (Section 14.2 ) (Nationa l Rivers Auth ority, 1990 ).

11.2.1 Toxins There has been much interest in toxins that are released by large blooms of algae,

not only in reservoirs but also in coastal waters. It is the blue-green algae that are responsible for toxins in drinking water. Despite their name, these algae are actually

a group of bacteria capable of photosynthesis. Therefore the blue-green algae are also referred to as cyanobacteria (Hunter, 1991 ). It is not a new phenomenon for algal blooms to release toxins that can kill livestock, domestic animals and even fish and birds that have drunk contaminated water (Codd et al., 1989 ), with the earliest recorded report over 110 years ago. In the British Isles three algae are known to produce toxins in freshwater, Microcystis aeruginos, Anabaena flos-aquae and Aphanizomenon flos-aquae (Falconer, 1989 , 1991 ). Cyanotoxins that affect either the liver or nervous system include microcystins, anatoxin a, cylindrospermopsins and saxitoxins. Blue-green algae also produce endotoxins that affect the gastrointestinal tract such as lipopolysaccharides. Two major types of toxins are produced by blue-green algae. Neurotoxins, such as anatoxin A, which cause paralysis of the skeletal and respiratory muscles, can result in death in as little as five minutes. These are mainly alkaloids produced by species of the genera Anabaena, Aphanizomenon and Oscillatoria. Hepatotoxins, such as microcystins, nodlarines and cylindrospermopsins, cause severe and often fatal liver damage. These are peptides, mainly different types of microcystin, and are produced by many common species of the genera Microcystis, Oscillatoria, Anabaena and Nodularia (Chorus and Bartram, 1999 ). The two most important toxin groups that have been associated with human poisoning are the microcystins and cylindrospermopsins (Falconer and Humpage, 2006 ). Other toxins, associated

with different species of blue-green algae, are also produced (Table 11.1 ). While

some species are ubiquitous, others have a more restricted distribution. When there is sunlight, a nitrogen source (e.g. nitrates from farm runoff or a sewage treatment plant effluent) and a phosphorus source (mainly from sewage treatment plants), then algal growth will develop rapidly to very high densities,

i.e. eutrophication. The toxins are thought to be released on decay (lysis), rather than as extracellular products released while active. Decay can occur naturally, but massive releases of toxins only occur when the algae are killed by the addition of chemicals such as copper sulphate.

Cyanobacterial poisoning occurs in humans and animals in three ways: through contact with contaminated water, by the consumption of fish or other

species taken from such waters, or by drinking contaminated water (Hunter, 1991 ). A major occurrence of toxic algae took place at Rutland Water in Leicestershire, which is one of the largest reservoirs in Western Europe. During

Algae and algal toxins

Table 11.1 Details on the four most common toxins produced by blue-green algae in Australia. Reproduced from Department of Natural Resources, Mines and Water ( 2006 ) with permission from Department of Natural Resources and Water, Queensland Government

Toxin type

Organism producing toxin Hepatotoxins

Potential health effects

Microcystis aeruginosa Microcystins

Damages liver cells, leading to

pooling of blood and finally liver

Microcystis flos-aquae

Nodularia spumigena Neurotoxins

failure

Anabaena circinalis Saxitoxins

Interfere with the function of the

nervous system. Death by paralysis of the respiratory system as a result of muscle failure

Cylindrospermopsis Cylindrospermopsin

Non-specific toxins

Relatively slow-acting toxin that

damages a number of organs of the

raciborskii Aphanizomenon

body including liver, kidney and

ovalisporum

thymus

Potentially produced by all Dermatotoxic

Endotoxins

Associated with outbreaks of

blue-green algae lipopolysaccharides

gastroenteritis, skin and eye irritation

and hay fever in humans who come into contact with blue-green algae during recreational activities

late August and mid-September 1989, blooms of Anabaena and Aphanizome- non were followed by Microcystis aeruginosa. Twenty sheep and 15 dogs are thought to have been killed by drinking the water, which is part of the supply network for 1.5 million people. Another toxic bloom also occurred in 1989 at Rudyard Lake in Staffordshire. This time army cadets who had been canoeing and swimming through the algal bloom had influenza-type symptoms, two being admitted to hospital (Turner et al., 1990 ). A survey in Florida found that

75 out of 167 surface waters used for drinking water supply contained toxic cyanobacterial blooms during 1999. This resulted in some finished waters containing toxins with maximum concentrations recorded for microcystins of

1 1 1 (Burns, 2003 ). Cylindrospermopsin had been recorded earlier from Australia where 138

children and 10 adults were poisoned after a Cylindrospermopsis bloom in a reservoir (Richardson, 2003 ). The presence of algae can cause other toxicity problems. For example halogenated acetonitriles are developmental toxicants that are produced during chlorination or chloramination when algae are present.

11.2.2 Taste and odour The cyanobacterial blooms also cause strong tastes and odours. Pressdee and Hart

( 1991 ) report that the Ardleigh Reservoir (Anglian Region) has had to close for

11.4 Treatment

supply for two months every year since 1978 because of blooms of Microcystis aeruginosa. So at the time when water demand is at its highest and supplies are reaching their lowest, algal blooms can cause havoc with water distribution and supply. Algae produce mainly earthy or musty odours such as geosmin and

2-methylisoborneol (Section 8.4 ). However, dimethyltrisulphide, dimethyldisul-

phide and methyl mercaptan are also produced when algae decompose producing

a range of odours similar to rotten cabbage to fishy (Table 8.1 ; Figure 8.2 ).

11.3 Standards Microcystin-LR is the most frequently encountered algal toxin at concentrations

high enough to be a danger to consumers and water users. As little data exist for the other toxins there is currently insufficient data to derive specific guideline

values. While the World Health Organization (WHO, 2004) has set a guideline

for microcystin-LR of 0.001 mg l 1 , neither the EC or the USA have set

standards for algal toxins; although the US Environmental Protection Agency have included it in their Contaminant Candidate List (Appendix 7). This is in contrast with other countries, for example in Australia a drinking water

1 expressed as

microcystin-LR toxicity equivalents. In the Netherlands, where this is also a serious problem, maximum

1 of

1 for other algal groups. These

levels can be dealt with successfully by conventional treatment, but do not relate to the concentrations of the toxins. Water supply companies monitor reservoirs and other surface water resources daily for blue-green algae. Using an inverted microscope, they scan for the toxin-producing species and when these are identified in significant numbers action is taken. Once the density of blue-green algae exceeds 1000 cells per ml in surface waters then there is a risk of cyanotoxins being present in harmful concentrations.

Immunoassay kits are used to screen suspected water for microcystin-LR, but accurate measurements requires analysis by high-performance liquid chromatog- raphy after extraction using 75% aqueous methanol. All toxins are peptide- related, highly polar and have relatively high molecular weights making them difficult to measure in environmental samples. The presence of microcystins appears associated with the presence of algal derived tastes and odours in drinking water caused by the compounds geosmin and 2-methylisoborneol. It may be

possible to use these compounds as indicators for the toxin (Section 8.4 ).

11.4 Treatment Treatment of algal toxins from water requires filtration to remove whole cells,

followed by oxidation of the toxin by ozone or chlorine at sufficient

Algae and algal toxins

concentrations and contact time. Activated carbon is effective under controlled conditions (Hart et al., 1992 ). Granular activated carbon and ozone are both particularly effective and the use of activated carbon in conjunction with ozonation is, as expected, very effective (Himberg et al., 1989 ; AWWA, 2002 ).

This is discussed furt her in Se ction 16.2 . Cya notoxins quickl y degrade or are oxidized in surface waters once they have been released after cell lysis. The biological component of slow sand filters also removes the toxins, but other processes including coagulation, sedimentation, oxidation and chlorination will also contribute to their removal. In a study of 677 source and finished waters 80% tested positive for microcystin-LR. However, only two of the samples of finished drinking water exceeded the WHO guideline value demonstrating that water treatment was effective in removing algal toxins (Carmichael, 2001 ).

Control options are currently directed at the blue-green algae rather than at the toxins. None of the current methods are reliable. The obvious method is to reduce the nutrient loading to the reservoir to reduce the crop of algae. Destratification of deeper lakes may eliminate some species but encourage others, whereas biological control methods such as using fish to graze the algae are difficult to operate and unreliable (Parr and Clarke, 1992 ). Decomposing straw has been shown to inhibit both green and blue-green algae (Gibson et al., 1990 ), although the feasibility of using barley straw to control algae in small reservoirs is still being examined. Currently preferred operational management techniques include selective abstraction, for example varying the depth of abstraction in reservoirs, and using air flotation to remove cells from raw waters

(Sectio n 4.3 ).