What Makes Blue-Green Algae Dangerous?—Speaking of Chemistry


Pond scum, blue-green algae, cyanobacteria.
You’ve probably seen this pesky, greenish stuff floating on lakes and ponds. It’s usually just a nuisance, but it can release all sorts of deadly poisons into the environment. A small amount of cyanobacteria isn’t necessarily
anything to worry about, but when conditions are right, the bacteria reproduce uncontrollably
into what’s called an algal bloom. These huge swaths of blue-green bacteria are sometimes
large enough to cover entire lakes. Blooms can block sunlight and deplete oxygen in the
water, disrupting everything from aquatic ecosystems to the fishing industry to drinking
water systems. These blooms sprout all over the world and
grow quickly thanks to their favorite nutrients, phosphorus and nitrogen. These elements are
in great supply thanks to pesticides and fertilizers in agricultural runoff, and detergents from
water treatment plants. Climate change is also doing wonders for the blooms — warmer
waters, flash flooding, and hotter, wetter summers all help algal blooms explode. Although some algal blooms are just eyesores,
some can become harmful algal blooms. Some algae produce what are called cyanotoxins that can injure or kill land-dwelling creatures, including pets, livestock, and even humans. Toledo,
Ohio, for example, shut down its entire drinking water system in 2014 because of an algal bloom
on Lake Erie that produced a toxin called microcystin. So these toxins are dangerous and the blooms
creating them are a growing global problem. Here to introduce you to
the nefarious toxins is Speaking of Chemistry’s own Lauren Wolf, showing off her mad structure
drawing skills. Microcystin has a large cyclic peptide ring
with seven amino acids that can vary — we’re showing the lysine and arginine version here.
This one’s the most common microcystin and one of the most toxic. Different amino acids
change the potency of a microcystin molecule. Ingesting enough microcystin can cause bleeding
in the liver and lead to a quick death. Some studies suggest the molecules also promote
tumor growth. Anatoxin-a is a much smaller molecule, but
it’s still a powerful toxin. It causes respiratory failure almost immediately, which is why it’s
sometimes called VFDF, or very fast death factor.
Anatoxin-a has a structure similar to that of cocaine — in fact, cocaine is a precursor
for anatoxin-a that’s made in the lab. Cylindrospermopsin is an alkaloid toxin with
lots of nitrogen and a complex multi-ring system. This molecule sent nearly 150 children
and young adults to clinics in Palm Island, Australia, in 1979. According to one report,
their symptoms included “vomiting, anorexia, and enlarged, tender livers,” and some even
had bloody diarrhea that lasted up to three weeks. People called the incident the Palm Island mystery disease because it was only after many years that researchers figured out that a cyanotoxin caused the outbreak. And finally, saxitoxin prevents sodium ions from passing
through protein channels vital to the nervous system. Although sodium channel blockers like
saxitoxin can be used in drugs like anesthetics and heart medications, saxitoxin itself isn’t
a very helpful molecule. It paralyzes the respiratory system and kills within minutes.
It’s so lethal, in fact, that the U.S. military started developing it as a chemical weapon.
Don’t worry, it was never used and stockpiles of it have since been destroyed. However,
the U.S. did give saxitoxin to one spy plane pilot in case of capture. This pilot was actually
was shot down over the USSR in 1960, but he refrained from using the saxitoxin, and who
can blame him? Let us know in the comments if you have questions
about cyanotoxins or algal blooms. To do a little reading on your
own, head on over to C&EN for Janet Pelley’s great article on the topic. And a big thanks
to her for helping with this episode, and to Jessica Marshall who has reported on the
topic and provided some of the great photos in the video. Thanks for watching!