Published 22 June 2018
Master of Environmental Science Candidate Kansas Keeton shares her research findings on toxic cyanobacteria in the Hawkesbury-Nepean catchment irrigation water.
Over the past several decades, as a result of warming and pollution from anthropogenic climate change, Australian freshwater bodies have experienced increases in the density and distribution of harmful cyanobacterial blooms (cyanoHABs). Cyanobacteria, or blue-green algae, are the planet’s oldest photosynthetic organisms and are the only known bacteria to produce oxygen (Walls, Wyatt, Doll, Rubenstein, & Rober, 2018). Their extensive evolutionary history has allowed them to dominate in a range of aquatic environments, from acidic freshwater to alkaline seas. Not only can they outcompete other ecologically essential bacteria and phytoplankton, they are capable of releasing highly toxic compounds, called cyanotoxins, as a by-product of photosynthesis,1 and have been known to cause death in rural farm animals, wildlife, and even humans.2 Exposure routes to toxins include drinking water or accidental ingestion of water involved in recreational activities, however, a prominent but often overlooked pathway is consumption of fruits and vegetables that have been irrigated with water containing cyanotoxins.3 The toxicity of cyanobacteria, their accumulation in crops, and the use of untreated irrigation water mark a need for research to determine the presence of toxin-producing cyanobacteria in the Hawkesbury-Nepean catchment, a major irrigation supplier for the city of Sydney.
A sizable percentage of cyanobacteria are toxic. The most common cyanotoxins are hepatotoxins (from Greek hepato-, liver) and neurotoxins (from Greek neuro-, nervous system). The hepatotoxins consist of microcystin, nodularin, and cylindrospermopsin, named after their initial discovery in cyanobacterial genera Microcystis, Nodularia, and Cylindrospermopsis, respectively, however, they are produced by a range of other cyanobacteria as well. Neurotoxins include anatoxin-A and anatoxin-A(s), saxitoxin, and β-N-methylamino-L-alanine (BMAA), the latter of which is believed to be involved in degeneration of the brain from Alzheimer’s.4 A complete list of ailments is outlined below in Table 1. These potentially fatal toxins highlight a great need to understand how their presence in irrigation water may threaten public health, as cyanotoxins have been found to bioaccumulate in the roots and shoots of many crops.
There is extensive scientific literature on the accumulation of cyanotoxins in plants and crops irrigated with water containing cyanobacterial blooms. Certain crops, such as lettuce, may retain actual colonies of toxic cyanobacteria on their edible parts after spray irrigation with water containing cyanoHABs, as irrigation water is very rarely treated for cyanobacteria or toxins, and these colonies are not removed with the washing and rinsing typical of household vegetable preparation.5 Additionally, microcystins have been found to remain stable in lettuce, carrots, and green beans when cooked, meaning boiling, frying, or steaming does not degrade toxins prior to consumption.6 Absorption of cyanotoxins has also occurred in many crops, including lettuce, rice, tomato, apple, wheat, carrots, green beans, and brassica vegetables.7
The Hawkesbury-Nepean catchment, a major supplier of potable and irrigation water for the city of Sydney, has been known to experience cyanoHABs since 1993,8 but irrigation sources have not previously been tested. I conducted a two-season study to determine the presence of toxic cyanobacteria in six irrigation sources from the Hawkesbury-Nepean River (Figure 1).
Water samples were collected, the organisms filtered, and their DNA extracted for use in PCR amplification and Next-Generation Sequencing to detect the presence of genes involved in the biosynthesis of cyanotoxins and identify toxic cyanobacterial species. An extensive bloom was observed during autumn in one major irrigation source, “Brewer’s Lane,” in which cylindrospermopsin-producing genes and a high number of unidentified cyanobacterial species were detected (as seen in the featured image above). The bloom was associated with prominent levels of turbidity, alkalinity, dissolved organic matter, dissolved solids, warm surface temperatures, and vertical stratification of the water column.
Genes involved in microcystin and saxitoxin production were also discovered among the samples and varied on a spatiotemporal scale. Microcystin-producing genes were detected in all samples except Brewer’s Lane and Yarramundi in the summer but were found in all samples except Brewer’s Lane in autumn. The isolation of Brewer’s Lane from the river explains why it was the only site in which microcystin-producing genes were not present and cylindrospermopsin-producing genes were. The presence of the latter genes, detected only in autumn, also indicated a temporal shift in water quality properties that were favourable to the proliferation of additional species of cyanobacteria as indicated by toxin-producing genes.
Of the several species of toxic cyanobacteria identified among the samples, the dominance of BMAA-producing marine species Prochlorococcus marinus was of particular interest as it was thriving in a freshwater environment, which is highly unusual and has not been discovered in any known literature. Other toxic species included microcystin-producing Microcystis panniformis and Calothrix parietina. The presence of toxin-producing cyanobacteria in irrigation sources from the catchment indicates a high probability that crops are being irrigated with water containing cyanotoxins, but further research is needed to confirm this, including determining the presence and concentration of toxins in water as bioaccumulation is directly related to concentration.9
This study highlights the need for frequent monitoring programs as it identified bacterial genes involved in the synthesis of microcystin, cylindrospermopsin, and saxitoxin, as well as the presence of toxic cyanobacterial species. Further research and greater replication is needed to confirm these results and may consider the inclusion of benthic cyanobacteria, or those that dwell in bottom sediments, to obtain a holistic picture of total cyanobacteria in the water column. Additionally, factors that may be contributing to the proliferation of Prochlorococcus marinus should be investigated to determine why a marine cyanobacterial species is dominant in a freshwater environment. This study contributes to an existing dataset on the biogeography of toxic cyanobacteria in Australia and provides a platform on which future research may be conducted to assist in making informed managerial decisions on the safety of using untreated water from the Hawkesbury-Nepean catchment as an irrigation source.
1. Codd, G. A., Morrison, L. F., & Metcalf, J. S. (2005). Cyanobacterial toxins: risk management for health protection. Toxicology and Applied Pharmacology, 203(3), 264-272.
2. Bittencourt-Oliveira, M. d. C., Cordeiro-Araújo, M. K., Chia, M. A., de Toledo Arruda-Neto, J. D., de Oliveira, Ê. T., & dos Santos, F. (2016). Lettuce irrigated with contaminated water: Photosynthetic effects, antioxidative response and bioaccumulation of microcystin congeners. Ecotoxicology and Environmental Safety, 128, 83-90.
3. See for example, Baker, P., & Humpage, A. (1994). Toxicity associated with commonly occurring cyanobacteria in surface waters of the Murray-Darling Basin, Australia. Marine and Freshwater Research, 45(5), 773-786; Davis, J. R., & Koop, K. (2006). Eutrophication in Australian rivers, reservoirs and estuaries–a southern hemisphere perspective on the science and its implications. Hydrobiologia, 559(1), 23-76; Gaget, V., Humpage, A. R., Huang, Q., Monis, P., & Brookes, J. D. (2017). Benthic cyanobacteria: A source of cylindrospermopsin and microcystin in Australian drinking water reservoirs. Water Research, 124, 454-464.
4. Esterhuizen, M., & Downing, T. G. (2008). β-N-methylamino-l-alanine (BMAA) in novel South African cyanobacterial isolates. Ecotoxicology and Environmental Safety, 71(2), 309-313.
5. Codd, G. A. (2000). Cyanobacterial toxins, the perception of water quality, and the prioritisation of eutrophication control. Ecological Engineering, 16(1), 51-60; Corbel, S., Mougin, C., Nelieu, S., Delarue, G., & Bouaicha, N. (2016). Evaluation of the transfer and the accumulation of microcystins in tomato (Solanum lycopersicum cultivar MicroTom) tissues using a cyanobacterial extract containing microcystins and the radiolabeled microcystin-LR (14C-MC-LR). Science of the Total Environment, 541, 1052-1058.
6. Lee, S., Jiang, X., Manubolu, M., Riedl, K., Ludsin, S. A., Martin, J. F., & Lee, J. (2017). Fresh produce and their soils accumulate cyanotoxins from irrigation water: Implications for public health and food security. Food Research International, 102, 234-245
7. See for example, Crush, J. R., Briggs, L. R., Sprosen, J. M., & Nichols, S. N. (2008). Effect of irrigation with lake water containing microcystins on microcystin content and growth of ryegrass, clover, rape, and lettuce. Environmental Toxicology, 23(2), 246-252; Gutiérrez-Praena, D., Campos, A., Azevedo, J., Neves, J., Freitas, M., Guzmán-Guillén, R., Vasconcelos, V. (2014). Exposure of Lycopersicon Esculentum to microcystin-LR: Effects in the leaf proteome and toxin translocation from water to leaves and fruits. Toxins, 6(6), 1837-1854; Jianzhong Chen et al., 2010; Chen, J., Han, F. X., Wang, F., Zhang, H., & Shi, Z. (2012). Accumulation and phytotoxicity of microcystin-LR in rice (Oryza sativa). Ecotoxicology and Environmental Safety, 76, 193-199; Kittler, K., Schreiner, M., Krumbein, A., Manzei, S., Koch, M., Rohn, S., & Maul, R. (2012). Uptake of the cyanobacterial toxin cylindrospermopsin in Brassica vegetables. Food Chemistry, 133(3), 875-879.
8. Kerr, R., Church, T., & Root, M. (1996). Hawkesbury-Nepean River: Eutrophication Upstream of Wisemans Ferry & the Impact of Discharges from South & Eastern Creeks, July 1990-June 1995: Technical Report: Environment Protection Authority.
9. Machado, J., Campos, A., Vasconcelos, V., & Freitas, M. (2017). Effects of microcystin-LR and cylindrospermopsin on plant-soil systems: A review of their relevance for agricultural plant quality and public health. Environmental Research, 153, 191-204.
Table 1. References
Baker, P., & Humpage, A. (1994). Toxicity associated with commonly occurring cyanobacteria in surface waters of the Murray-Darling Basin, Australia. Marine and Freshwater Research, 45(5), 773-786.
Ballot, A., Fastner, J., Lentz, M., & Wiedner, C. (2010). First report of anatoxin-a-producing cyanobacterium Aphanizomenon issatschenkoi in northeastern Germany. Toxicon, 56(6), 964-971.
Codd, G. A. (2000). Cyanobacterial toxins, the perception of water quality, and the prioritisation of eutrophication control. Ecological Engineering, 16(1), 51-60.
Corbel, S., Mougin, C., & Bouaïcha, N. (2014). Cyanobacterial toxins: Modes of actions, fate in aquatic and soil ecosystems, phytotoxicity and bioaccumulation in agricultural crops. Chemosphere, 96, 1-15.
de Figueiredo, D. R., Azeiteiro, U. M., Esteves, S. M., Gonçalves, F. J., & Pereira, M. J. (2004). Microcystin-producing blooms—a serious global public health issue. Ecotoxicology and Environmental Safety, 59(2), 151-163.
Gutiérrez-Praena, D., Campos, A., Azevedo, J., Neves, J., Freitas, M., Guzmán-Guillén, R., Vasconcelos, V. (2014). Exposure of Lycopersicon Esculentum to microcystin-LR: Effects in the leaf proteome and toxin translocation from water to leaves and fruits. Toxins, 6(6), 1837-1854.
Kittler, K., Schreiner, M., Krumbein, A., Manzei, S., Koch, M., Rohn, S., & Maul, R. (2012). Uptake of the cyanobacterial toxin cylindrospermopsin in Brassica vegetables. Food Chemistry, 133(3), 875-879.
Kurmayer, R., & Christiansen, G. (2009). The genetic basis of toxin production in cyanobacteria. Freshwater Reviews, 2(1), 31-50.
Machado, J., Campos, A., Vasconcelos, V., & Freitas, M. (2017). Effects of microcystin-LR and cylindrospermopsin on plant-soil systems: A review of their relevance for agricultural plant quality and public health. Environmental Research, 153, 191-204.
Mohamed, Z. A., & Al Shehri, A. M. (2007). Cyanobacteria and their toxins in treated-water storage reservoirs in Abha city, Saudi Arabia. Toxicon, 50(1), 75-84.
Pearson, L., Mihali, T., Moffitt, M., Kellmann, R., & Neilan, B. (2010). On the chemistry, toxicology and genetics of the cyanobacterial toxins, microcystin, nodularin, saxitoxin and cylindrospermopsin. Marine Drugs, 8(5), 1650-1680.
Kansas Keeton recently completed a Master of Environmental Science by Coursework from the University of Sydney. Her research interests include food security, water quality, hydrology, and public health, which were incorporated in her research thesis on toxic cyanobacteria in irrigation water that she undertook in July 2017. Kansas is returning to the United States to pursue a career in environmental science.
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