On our last post, we discussed the impacts of HABs, such as hypoxia and reduction of light in the water column, however we left discussion of algal toxins aside as that opens a whole can of worms that deserves its own post.
Harmful algal blooms can generally be divided into three categories (Hallegraeff, Anderson, & Cembella, 2004).
- HABs caused by non-toxic microalgae, which lead to mostly-harmless water discolorations, but in certain conditions may grow dense enough to cause indiscriminate kills of fish and invertebrates through oxygen depletion. Which was covered in the previous post.
- Species that produce potent toxins that cause a variety of gastrointestinal and neurological illnesses to humans and animals up the food chain.
- And lastly species that are generally non-toxic to humans and terrestrial animals, but harm fish and invertebrates by damaging or clogging their gills (Svendsen, Andersen, Hansen, & Steffensen, 2018).
Fortunately for us, of the three categories, the first is the most prevalent, with the toxin producing algal blooms represented by the second and third categories accounting for less than 1% (Figure 1) of HABs (NOAA, 2020). In this post, we will largely be discussing the second category of HAB.
The type of algae responsible for producing toxins varies from case to case depending on the location and environment, in general, freshwater toxic HABs tend to be caused by cyanobacteria, whereas marine toxic HABs tend to be caused by dinoflagellates or diatoms. Although exceptions do exist, with studies of cyanobacterial toxins being conducted in marine environments (Botana, Vasconcelos, & Leão, 2014)
One of the cyanobacteria that produces toxins is Microcystis aeruginosa, which produces a type poison known as microcystin (clearly whoever named it was the most creative person in their lab). However, despite what its name implies, it is also produced in many other genera of toxic cyanobacteria such as Nostoc and Anabaena. Microcystins act as a hepatotoxin, causing severe liver damage and failure. Cyanobacteria also have the capacity produce neurotoxins. In fact, Anabaena circinalis, which commonly found in Singapore (Yeo & Gin, 2013) produces Anatoxin-a otherwise known as….
‘Very Fast Death Factor (VFDF)’,
I swear I didn’t make that name up. Without boring you with the details; it interferes with neurotransmitter receptors, causing loss of nervous coordination, which doesn’t sound too scary until you realize it can prevent you from breathing, because your diaphragm isn’t moving, leading to death by respiratory arrest. The danger cyanobacterial toxins are further exacerbated by the fact that some of them such as microcystins are heat stable and cannot be broken down easily by boiling drinking water.
The danger of algal toxins is not confined to freshwater ecosystems unfortunately. In marine environments toxic algal blooms may produce vibrant discolorations known as ‘red tides, which are predominantly caused by dinoflagellates or diatoms that produce neurotoxins such as saxitoxins and domoic acid respectively, which can fatally poison higher vertebrates such as livestock and humans. In these toxins travel readily up the food chain, leading to toxic effects when contaminated animals are eaten. In the case of saxitoxins, they are concentrated within bivalves such as clams which are filter feeders, when these shellfish are eaten, they cause the illness known as paralytic shellfish poisoning (PSP), named after the taxa it was first isolated in, before it was later found to be produced by dinoflagellates.
With all the terrible things associated with algal blooms what can we do to stop them? Find out more in part 5!
References
Botana, L., Vasconcelos, V., & Leão, P. (2014). Marine Cyanobacterial Toxins. Seafood and Freshwater Toxins, (May), 1073–1090. https://doi.org/10.1201/b16662-45
Hallegraeff, G. M., Anderson, D. M., & Cembella, A. D. (2004). Manual on Harmful Marine Microalgae.
Svendsen, M. B. S., Andersen, N. R., Hansen, P. J., & Steffensen, J. F. (2018). Effects of harmful algal blooms on fish: Insights from Prymnesium parvum. Fishes, 3(1), 1–12. https://doi.org/10.3390/fishes3010011
US Department of Commerce National Oceanic and Atmospheric Administration. (2020). Are all algal blooms harmful? Retrieved October 2, 2020, from https://oceanservice.noaa.gov/facts/habharm.html
Yeo, B. H., & Gin, K. Y.-H. (2013). Cyanophages infecting Anabaena circinalis and Anabaena cylindrica in a tropical reservoir . Bacteriophage, 3(3), e25571. https://doi.org/10.4161/bact.25571