Dig a teaspoon into the nearest clod of earth and what you come out of it will contain more microorganisms than there are humans on Earth. We know this from laboratory studies that analyze samples of soil taken from microbial nature to determine what forms of microscopic life exist in the world beneath our feet.
The problem is, studies like this can’t actually tell us how this underground kingdom of fungi, flagellates, and amoebae operates in the soil. Because they remove soil from its environment, these studies destroy the delicate structures in mud, water, and air in which soil microbes reside.
This prompted my lab to develop a way to spy on these underground workers, indispensable in their role as organic matter recycling agents, without disturbing their micro-habitats.
Our study revealed darkness, thanks to the cities in which soil microbes reside. We have found mazes of tiny highways, skyscrapers, bridges and rivers that are traversed by microorganisms to find food or to avoid becoming someone’s next meal. This new window into what is happening underground could help us better appreciate and preserve the Earth’s increasingly damaged soils.
In our study, we developed a new type of “cyborg soil”, half natural and half artificial. It’s made up of micro-technical chips that we’ve either buried in nature or surrounded by soil in the lab for enough time for microbial cities to emerge in the mud.
Fleas literally act like windows on the basement. A transparent spot in the otherwise opaque soil, the chip is cut to mimic the pore structures of actual soil, which are often odd and counterintuitive to the scale at which microbes undergo them.
Different physical laws become dominant at the micro scale compared to what we know in our macro world. Water clings to surfaces and bacteria at rest are jostled by the movement of water molecules. Air bubbles form impassable barriers for many microorganisms, due to the surface tension of the water around them.
Once we implanted our chips in the ground, we could watch the microbes scroll by as they make their decaying movements, revealing their interactions, food webs, and how different microbes design their ever-changing surrounding micro-habitats.
When we searched our first chips, we encountered all the variety of single-celled organisms, nematodes, tiny arthropods, and species of bacteria that exist in our soils. Fungal hyphae, which burrow like plant roots underground, quickly grew deep within the pores of our cyborg soil, creating a direct living connection between real soil and our chips.
This meant that we could study a phenomenon known only from laboratory studies: the “fungal highways” along which bacteria “hitchhike” to disperse in the ground. Bacteria usually disperse in water. So, by filling some of our chips with air, we were able to observe how bacteria infiltrate new pores by following the groping arms of fungal hyphae.
Unexpectedly, we also found large numbers of protists – enigmatic single-celled organisms that are neither animal, plant, nor fungus – in the spaces around hyphae. Obviously, they too are hitchhiking on the fungal highway – a phenomenon so far completely unexplored.
Because we investigated several hundred possible paths in our cyborg soil chips, including several thousand individual pore spaces, we were also able to quantify that this occurs often. This shows that hyphae must be an important vector for the dispersal of a wide variety of swimming microorganisms, giving them a significant advantage when foraging in underground micro-cities.
In our study, we also wanted to explore how and by what means microbial cities are designed. One way to do this was to observe how minerals from the soil seep into our shavings, creating pockets of real space in the soil within the man-made structures that we had placed in the soil.
As our chips began to dry, we witnessed how water is sucked through the pores of the soil: a tsunami of water movement to which soil microorganisms are regularly exposed as rain and the sun alter their little worlds. The resulting patterns in soil minerals looked like a riverbed system in our macro world.
And it’s not just physical forces that shape the habitat of soil microbes. With their powerful hyphal tips, fungi often act as “ecosystem engineers”, opening passageways and blocking others with their cells. They are responsible for most of the streets, avenues and bridges of the microbial metropolis.
Read more: The Secret Life of Mushrooms: How They Use Clever Strategies to Forage Underground
More surprisingly, we have found that other less “strong” organisms also modify the microscopic structure of soils. A ciliate, for example, which has small hair-like extensions for locomotion, can bulldozer the ground with its vigorous foraging as well.
Soil, science and society
Our cyborg soil study ultimately helps link field ecology with controlled laboratory studies. It combines the benefits of studying realistic and complex communities of soil organisms while carefully controlling and adjusting factors such as nutrient intake or temperature so that we can see how soils and their microbes respond to changes. above ground.
But there is another advantage. We believe that observing the hidden world of soils and their fascinating inhabitants could help people emotionally engage with this vital ecosystem. Other ecosystems have long had charismatic animals to represent conservation initiatives. Floors, on the other hand, are always associated with dirt and grime. Yet soils support 95% of our food production. They store more than twice the amount of carbon that the biosphere and the atmosphere combined.
Read more: Treated like dirt: Urban soil is often overlooked as a resource
We want to show that when you dig your teaspoon into the earth, you are digging the upper parts of an exciting secret metropolis that contains a quarter of Earth’s species. The adorable organisms in your spoon aren’t dirty – they silently provide vital ecosystem services that sustain all life above the ground. These urban dwellers are in urgent need of better protection.
Author: Edith Hammer – Associate Lecturer, Department of Biology, Lund University