November 29, 2020

I, Science

The science magazine of Imperial College

On the left, a chemical map of the yabby burrow: on the right, a photo of another burrow, showing the colourful banding of the chemistry.

Your world’s a toxic, dark hole

Your home is your only place of sanctuary. Yet, lurking in the walls is a silent menace. It is a noxious, suffocating, toxic presence, creeping out of cracks and threatening to overwhelm you if you let your guard down. If you venture out to collect food, you face attack: your only option is to flee back to your house, then fight off the danger within.

Fortunately, there are several tools in your arsenal. The strongest is what we all love: good, clear water. The dark menace hates it, because it is transformed into a benign cloud by the oxygen in the water. You’ve developed your defence carefully: your house is designed to flush water through it as you move, creating a protective bubble around you. This chemical battle never ends – your life is a war.

Sound familiar? Of course not. We are lucky enough to live in a mostly friendly atmosphere. However, for animals living in sediments (the ground under a water body), the chemicals surrounding them are downright dangerous. They must engage in an ongoing struggle to keep their immediate surroundings livable, while doing all of the usual things – eating, building shelter, and of course, securing the attentions of the opposite sex.

These animals are important for us to study for a number of reasons. For starters, we know very little about how they interact with their environment. More than that, though, their actions – digging, mixing and manipulating – mean the exchange of chemicals from the water to the sediment is much more complex and dynamic than a simple model would assume. Most important chemical cycles – carbon, nitrogen, sulfur and so on – have a substantial part of their global cycle taking place at the interface between water and the soil or sediment, and it’s in these places that the local residents have their biggest say.

On the left, a chemical map of the yabby burrow: on the right, a photo of another burrow, showing the colourful banding of the chemistry.

That’s where my research came in. Working with my supervisors, I improved a technique for imaging 2D distributions of the highly toxic chemical sulfide (S2-). I also was able to develop a similar technique for looking at dissolved iron, another important sediment chemical. It’s the combination of these two chemicals, into iron sulfide (FeS), which creates the dark black you see in the photos above. When the animals flushed oxygen-rich water into the holes, a thin layer of iron oxide – the rusty orange colour in the photo on the right – would form, creating a very visual clue to the state of the sediment chemistry.

The sulfide method worked a lot like taking a film photograph, but rather than the film reacting to light to form a picture, the sulfide chemical bound straight onto my potassium iodide infused ‘film’ to create an image.

When we applied the technique, we were able to capture dramatic pictures that gave us an insight into the world inhabited by burrowing creatures. The yabby you can see hiding in the top picture was one of our stars: he survived in a sulfide-rich environment for a month, giving us a great sequence of images showing how the whole mini-environment was gradually changed by the presence of his well-tended burrow.

In the rainbow image, the colours represent how intense the sulfide concentration was near the burrow (red was highest). He mostly lived in the central chamber and down the left side, and the plume you can see in the top right is where he started digging a new burrow, which released a stream of sulfide into the overlying water. He had to pump fresh water in, almost non-stop, to neutralise the sulfide creeping into his home.

We take our (mostly) friendly chemical environment for granted, but there are animals living in all kinds of crazy and extreme locations around the world. Their fight for survival is not only an epic tale for each and every one of them, but collectively, they help to shape the world we live in. And we have the audacity to call them shrimp!

By David Robertson