This new instalment of my Mad Mud Love series is not about the butterfly effect. But, when I first thought about how to approach this post, I kept coming back to the idea that such a tiny creature could have such a devastating effect. Microbial mats may not cause tornados, but they do have a profound effect on the places where they live. One of the changes that this complex combination of microorganisms and sediment produce is that they may be strong enough to change the way water behaves in certain places. By doing that, they can change the characteristics of those places.
Sometimes, when we arrived at Paso Seco, Argentina, we saw that on the edges of small ponds of leftover seawater, there were small crystals of different minerals: gypsum, calcite, and halite (or rock salt, or in its most processed form, table salt).
Following the links above, you’ll find professional photos of the pure minerals. Below, I’ll show you what we find in the field. Not so pretty, but it’s a start, right?
When considering only the climate of the area and the overall physical characteristics of the field site, we shouldn’t have been able to find these minerals on the surface. The texture of the soil was not impermeable as it was rich enough in sand, so seawater that came in after storms should have been able to percolate through it. However, the water didn’t. Well, some of it did, but a lot of the water stayed on the flat for several days. So, we spent some time scratching our heads over how come sometimes we would find these crystals, and sometimes we wouldn’t.
We realized that the crystals would form because the stagnant seawater would evaporate thanks to the very windy, semi-desert climate that we have there. As this water is very rich in the necessary ions to form the crystals, all the materials were there to generate the crystal. However, we still couldn’t understand why the water remained.
That’s when our interest in microbial mats paid off. We started to pay attention to the behaviour of the water in the places where there were microbial mats and where there weren’t.
As you may have noticed when you walk at the beach, sand doesn’t have much of an issue with water and lets it flow pretty fast between its grains. However, in the places where the microbial mats were present, we noticed that the speed at which water flowed was sluggish, almost as if the ground was made of clay. We obtained measurements and realized that the microbial mats were the ones that were giving the water enough time on the ground to evaporate and leave the crystals behind. But how did the microbial mats do that?
Why are you asking me? Aren’t you supposed to be the one that’s telling me how this stuff works?
Remember from the previous post in this series that these microorganisms secrete (that is to say, they produce and discharge to the outside) molecules of organic nature (known as EPS)? Well, these substances can act similarly to a spider web.
You mean that this EPS thing is sticky and it catches flies?
Well, not flies, at least not that I know of. But, the EPS is sticky and, as you can see in the electronic microscope picture below, it even looks similar to a web or net.
This interaction makes the whole system more cohesive, which means that the microbes, the sediments, and the EPS act more like a whole than as single items. Also, the EPS has hygroscopic properties (the capacity to absorb moisture), which helps make the microbial mat more impermeable. Thus, they stop the water from percolating as fast as it would otherwise do.
That way, microbial mats help make beautiful crystals that fill people with wonder (at least me!) in this place that shouldn’t have these types of crystals. Some of those minerals remain trapped inside the mats, marking the arrival of water and its evaporation. Others dissolve again when a new batch of seawater invades the flat.
In a distant future, those remaining crystals (calcite) will still be there like candle marks, showing the passage of time. So, microbial mats may not be tornado-producing butterflies, but they do show that with a little sticky effort on their part, they can make considerable changes in the extensive system where they live.
This article is the second in the Mad Mud Love series. Keep tuned to this website or subscribe to my mailing list in order to keep yourself updated. The list is sent on a monthly basis with a digest of new posts and sometimes updates on my fiction writing or new scientific papers published.
Also, you can find the research paper here.