The primary ingestion route of microcystins into the human body is via the oral route, with the toxin being absorbed as it passes through the small intestine, with some evidence to support partial absorption through the stomach when ingested. The toxin then enters the blood stream, where it makes its way to the liver ⁽²⁴⁾
. Although, the liver is adversely affected with the ingestion of the toxin, the extensive research effort onto microcystins has identified adverse effects to the following organs. It should be noted that the toxicological impacts were tested using rodents.
- Colon and gastrointestinal tract ⁽²⁵⁾ ⁽²⁶⁾
- Kidneys ⁽²⁷⁾ ⁽²⁸⁾
- Reproductive organs ⁽²⁹⁾ ⁽³⁰⁾
- Heart ⁽³¹⁾
- Lungs ⁽³²⁾ ⁽³³⁾
- Skin ⁽³⁴⁾ ⁽³⁵⁾ ⁽³⁶⁾
Nodularins are similar to microcystins in both structure and resulting hepatotoxic effects, with only five amino acids in their molecular structure (cyclic pentapeptitde) as opposed to the seven in microcystins (cyclic heptapeptide) ⁽ˢᵉᵉ ᶠᶦᵍ. ²⁾. However, unlike Microcystins which are produced by a wide range of cyanobacterium, nodularins are produced primarily by Nodularia spumigena. Only 10 structural variants of Nodularin have been identified, compared to the 80 congeners of Microcystins described. Currently, nodularin has only been recorded in N. spumigena strains in Australia, the Baltic Sea, and New Zealand ⁽³⁷⁾.
Fig. 2: Example of one structural assortment of the ring-shaped nodularin chemical structure, with characteristic amino acids coloured ⁽⁵⁷⁾
Nodularin acts upon protein phosphatases in the liver which can lead to cytoskeletal and liver damage in significant doses ⁽³⁸⁾
. In addition, nodularins have been identified as carcinogens in rodents, acting as strong tumour promoters particularly within the liver ⁽³⁹⁾
. The pathway of nodularin toxicity has been identified as similar to microystin uptake, entering orally and accumulating predominantly in the liver via the small intestine. Symptoms of nodularin toxicity have been closely related to Microcystin impacts, however the exact mechanisms of its toxicity (particularly in humans) still require further studies ⁽³⁴⁾.
Listed below are some of the additional effects nodularins pose and the references for further information:
- Neurotoxicity ⁽⁴⁰⁾
- Reproductive toxicity ⁽⁴¹⁾ ⁽⁴²⁾
- Genotoxicity ⁽⁴³⁾ ⁽⁴⁴⁾
- Potential carcinogenicity ⁽⁴⁵⁾ ⁽⁴⁶⁾
Nodularin exposure is not as widespread as microcystin, as it rarely occurs in freshwater that is commonly used for drinking purposes. Instead, nodularin populations largely occur in brackish/saline water bodies, including saline and coastal lakes. From this, major exposure to nodularins is through oral consumption, including the accidental swallowing of contaminated water during recreational activities and seafood consumption. There have been no confirmed incidents of human intoxication from nodularin, but there have been numerous reports of livestock deaths ⁽³⁴⁾