Potential Impact of Cyanobacteria on Crop Plants

Cyanobacteria (blue-green algae) can produce compounds in surface water that are harmful to both plants and humans. In 2001, Oregon began monitoring cell counts of potentially harmful cyanobacteria and nearly 40 advisories were issued in the last five years (oregon.gov/DHS/ph/envtox/maadvisories.shtml). However, there is no monitoring for the toxins produced by these bacteria in the state of Oregon, though they are of potential concern to Oregon growers.

What are Cyanobacteria?

Cyanobacteria are photosynthetic bacteria that are common in all freshwater systems. Many are not a problem but a number of species may produce toxins that can affect both plant and human health. Microcystis, Anabaena, Aphanizomenon, Lyngbya, Nodularia, Planktothrix, Nostoc, and Cylindrospermopsis are common and can produce toxins (cyanotoxins).

What Toxins are Produced?

The cyanotoxins that have been detected in freshwaters of Oregon include microcystin, anatoxin-a, and cylindrospermopsin.

The toxin, microcystin-LR, has been a focus of research because it is widespread and is produced by many species of cyanobacteria. This toxin is hydrophobic, which may allow it to diffuse across cell membranes, though the exact mechanism of uptake is not known.

How does Microcystin-LR Affect Plants?

Plants are generally not killed by cyanotoxins but plant growth may be inhibited6 and result in a yield reduction. The concentration of microcystin-LR (5 µg/liter) studied was below the 8 µg/liter limit recommended for recreational bathing. In other words, bodies of water that would not hurt swimmers could be damaging to crop plants.

Microcystin is a potent inhibitor of key regulatory enzymes (protein phosphatases) in both animals and plants.4 Protein phosphatases in plants regulate important cellular processes such as carbon and nitrogen metabolism, tissue development and photosynthesis. It has been shown that plant seedlings can take up microcystin,1, 7 inhibiting plant development, root growth and photosynthesis.9, 5, 2, 3 Necrotic lesions on leaves are also observed and likely due to microcystin-induced stress.9 Plant exposure to microcystin results in the generation of reactive oxygen species such as hydrogen peroxide, and if the plant’s antioxidant capacity is overwhelmed then cells die. Plants can detoxify microcystin but no studies have been performed to determine the length of time necessary to completely break down the toxin.

Is Human Health at Risk when Growing or Consuming Crops in Oregon?

Any direct contact between contaminated waters and the human body exposes one to toxic effects. Overhead irrigation of crops with water containing cyanotoxins poses an inhalation risk to field workers. Exposure symptoms include rashes, asthma and dry sporadic cough with vomiting on the days of, and after, exposure.2 Exposure in rare cases can cause death.

Much less is known about human health risks through the consumption of plant products exposed to cyanotoxins. Currently there are no regulatory limits for microcystin loads in plant tissue but there are recommendations set by the World Health Organization. Six-day-old plants of green pea, sugar pea, chickpea, green and red mung bean, snap bean, soybean, alfalfa, lentil, wheat and corn were exposed to either purified microcystin or cyanobacterial laden lake water.8 All of these crops were found to accumulate toxin and peroxides in shoots and roots to high enough levels that consumption of a small amount of plant tissue would exceed recommendations set by the World Health Organization (Table 1). In another study, water containing cyanobacteria was applied to the shoots of four different crops. The contaminated water ran off of rape and ryegrass, where absorption of microcystin was not observed, while lettuce and clover retained microcystin.4 Though there is currently limited evidence, it appears that when certain crops are exposed to environmentally realistic concentrations of microcystin-LR, this toxin can accumulate to levels where even a small serving of plant product (0.7 to 9 oz) would exceed the World Health Organization recommended consumption limit. This consumption limit is based on acute poisoning3 and does not consider the potential of microcystin as a carcinogen through chronic exposures.5

References

1 Abe, T., Lawson, T., Weyers, J.D.B., and Codd, G.A. 1996. Microcystin-LR inhibits photosynthesis of Phaseolus vulgaris primary leaves: Implications for current spray irrigation practice. New Phytologist 133(4):651-658.

2 Codd, G. A., Bell, S.G., Kaya, K., Ward, C.J., Beattie, K.A., and Metcalf, J.S. 1999. Cyanobacterial toxins, exposure routes and human health. European Journal of Phycology 34(4):405-415.

3 Chorus, I., and Bartram, J. 1999. Toxic Cyanobacteria in Water: A Guide to Their Public Health Consequences, Monitoring and Management., London: E & FN Spon. 416 pp.

4 Crush, J.R., Briggs, L.R., Sprosen, J.M., and 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.

5 Grosse, Y. 2006. Carcinogenicity of nitrate, nitrite, and cyanobacterial peptide toxins. Lancet Oncology 7(8):628-629.

6 Mackintosh, C., Beattie, K.A., Klumpp, S., Cohen, P., and Codd, G.A. 1990. Cyanobacterial microcystin-LR is a potent and specific inhibitor of protein phosphatase-1 and phosphatase-2a from both mammals and higher-plants. Febs Letters 264(2):187-192.

7 McElhiney, J., Lawton, L.A., and Leifert, C. 2001. Investigations into the inhibitory effects of microcystins on plant growth, and the toxicity of plant tissues following exposure. Toxicon 39(9):1411-1420.

8 Peuthert, A., Chakrabarti, S., and Pflugmacher, S. 2007. Uptake of Microcystins-LR and -LF (cyanobacterial toxins) in seedlings of several important agricultural plant species and the correlation with cellular damage (Lipid peroxidation). Environmental Toxicology, 22(4):436-442.

9 Pflugmacher, S., Jung, K., Lundvall, L., Neumann, S., and Peuthert, A. 2006. Effects of cyanobacterial toxins and cyanobacterial cell-free crude extract on germination of alfalfa (Medicago sativa) and induction of oxidative stress. Environ. Toxicology and Chemistry 25(9): 2381-2387.

10 Smith, R. D., Wilson, J.E., Walker, J.C., and Baskin, T.L. 1994. Protein-phosphatase inhibitors block root hair-growth and alter cortical cell-shape of Arabidopsis roots. Planta 194(4):516-524.

Table 1. Accumulation of microcystin in 12 crops and the calculated consumption limit assuming a 60 kg (132 lb) person and using the WHO daily consumption limit of 40ng/kg. All shoot load data from8, except lettuce 4.

Crop

Shoot load

(ng toxin/g wet wt)

Consumption Limit (g)

Consumption Limit (oz)

Green Pea

27

88.9

3.1

Sugar Pea

28

85.7

3.0

Chick Pea

12

200.0

7.1

Green Mung bean

30

80.0

2.8

Red Mung Bean

30

80.0

2.8

Snap Bean

40

60.0

2.1

Soy Bean

10

240.0

8.5

Alfalfa

125

19.2

0.7

Lentil

12

200.0

7.1

Corn

37

64.9

2.3

Wheat

125

19.2

0.7

Lettuce

79

30.4

1.1

4 & 8 See References