A taste of Acacia difficilis

I’ve just spent 2 days at the World Science Festival Brisbane. It was a wonderful experience that I’d really recommend and I’ll be posting about different things I learnt, and today’s post is no exception.

A taste of Acacia difficilis

Ethnobotany: the scientific study of the traditional knowledge and customs of a people concerning plants and their medical, religious, and other uses.

Imagine you’re an Aboriginal kid from the Larrakia people, walking along beside a waterhole just after dawn. You can see the fish darting below the reflection of tall wattles and golden sunlight. They’ve lost their stunning yellow flowers and the long seed pods are beginning to harden. You’ve never eaten those seeds, but you’re curious, why not feed them to a fish and see what happens? 

You crack several pods open and smile at the shiny black seeds. You throw them into the water, as many as you can, watching the tiny ripples emanate across the brown water. The fish are curious, bobbing their little heads up to the surface with their curious gaping mouths. You watch as they explore the little seeds, but they quickly become bored and disappear, even as you throw in a couple more seeds. Time to leave.

It’s not until late afternoon the next day that it comes back to you. The men race back into the village shouting excitedly, something has killed all the fish! As you race alongside your friends to the waterhole, you want to grin. The seeds have killed the fish, so they’re not edible, but imagine how easy it is fish this way! However, your grin is wiped away as the men cautiously survey the small fish they’ve fetched out of the water. They’re worried that whatever killed the fish could kill the person who eats the fish.

They collect the fish to return to the village, they will feed the fish to the young man who was caught stealing stored berries. If he dies, no matter. If he lives, it will be a rich feast indeed. 

12 years later the boy is a man. He leads his first fishing trip with a handful of crushed seed. They say it is magic and he wins fear and respect as day after day he brings back fish to feed the whole village. 

That’s just a little artistic licence, but the seeds from the wattle, Acacia difficilis, were indeed used as a fishing tool by the Aborigines of Larrakia Country (around Darwin). This wattle grows around Queensland and Northern Territory and is an incredible example of Aboriginal plant science, representing an incredible energy-efficient method of fishing. All you do is collect seeds, crush them and scatter on the surface of waterholes, return 36 hours later and collect the dead fish from the surface!

The Science

These seeds contain a compound called rotenone (C23H22O6) and occurs naturally in the stems, leaves and seeds of many plants. It works by inhibiting a certain step in the electron transport chain of aerobic cellular respiration within mitochondria, resulting in excess electrons that reduce oxygen to its radical which can damage parts of the cell and also means that the cell never gets that energy it’s trying to produce.

So said more simply, the electron transport chain is simply a series of reactions involved when cells make energy. These reactions all involve electrons being moved down to more electronegative atoms (atoms that “want” electrons more) and occur within the mitochondria. As rotenone disrupts one of these electron-transferring reactions, it leaves a “spare” electron. As oxygen is second only to fluorine in electronegativity (electron-wanting), this electron comes along and attaches itself to oxygen. Pretty much, we end up with an oxygen radical, either a peroxide or superoxide, both unstable compounds that have 1 or 2 extra electrons respectively, as opposed to normal O2 which has a neutral charge of 0.

These radicals are highly reactive because the electrons have a lot more energy in a radical and they want to reach the lowest energy state possible, so they react. With pretty much anything they can. That includes: DNA, parts of the mitochondria and other really important bits that you don’t really want destroyed.

So now you might be wondering, why does it kill fish and not humans? Firstly, we have to consider that fish are a lot smaller than humans, so we’d need a lot more of toxin to kill a human than a fish. However, more importantly, it affects fish a lot more because they’re up taking it through their gills directly into the bloodstream while it’s not nearly as easily absorbed in the human gastrointestinal tract.

So there you have it. Ethnobotany, some smart indigenous people who discovered an extremely efficient fishing method! And I like to think of the person who might have discovered it walking back to his tribe and saying, “hey guys, I can fish for all of you while sitting around all day!”


2 thoughts on “A taste of Acacia difficilis

    1. Brilliant questions! Some of the most common rotenone-containing plants are from the pea family (Leguminosae) and also the aptly named, Florida fish-poison tree! Rotenone is definitely commercially extracted and is used as a pesticide for both fish and insects as it tends to affect them the most without damaging other species or the environment as it degrades within 5-6 days under exposure to sunlight. It’s been used in America to control fish overpopulation and is even applied in some head lice treatments. It’s also a very useful research tool for studying oxygen consumption and produces effects very similar to Parkinson’s disease.


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