Soil is the living, breathing skin of the earth – it is literally alive. Every handful of soil is home to billions of microscopic organisms. Bacteria, fungi, protozoa, nematodes, and arthropods inhabit soils in every corner of the planet. For these microorganisms, soils serve as the ideal habitat, full of cracks and crevices and pores dug out by growing roots. Within soils, films of water bridging grains of sand and assorted mixtures of decaying plant material also serve as habitat. In a habitat this diverse, it is not surprising that the organisms occupying these spaces are just as diverse.
Soils are one of the most diverse habitats on Earth. One gram of soil contains up to 1 billion bacterial cells from thousands of different taxonomic groups. Soil ecologists have estimated that we have only identified about 1% of all the microorganism species living in the soil! There is so much life still to discover below ground, and the organisms we have identified continue to amaze us – some of them even glow. So, yes, there are glow-in-the-dark soil organisms.
Yes, they glow, but there’s so much more to it than that! First of all, what does it even mean for an organism to “glow in the dark”? Biologists use the term bioluminescence to describe the phenomenon of organisms glowing in the dark. Bioluminescence is the ability of organisms to convert chemical energy into light. This light can come in many colors, but it is usually green when emitted from organisms living on land, and blue when in the ocean. So who glows green below ground? Surprisingly, a lot of different soil organisms have the ability to luminesce, including bacteria, fungi and collembola (a.k.a., springtails). The question now remains: Why do these soil organisms glow in the dark?
Bacteria – Bacterial luminescence is much less common in soils than in the ocean, but that doesn’t mean it isn’t important. Bacteria can colonize and infect nematodes (microscopic worms that live in the soil). When the bacteria inside the nematode glows, this may make their nematode host more attractive to predators. Then when the predator consumes the nematode, the bacteria goes with it and is able to colonize the predator. In this way, luminescence serves a mechanism of dispersal — that is it allows bacterial populations to expand and explore new habitats (both inside and outside of other organisms). Another reason bacteria might luminesce is to signal back and forth between other bacteria within the same community. This phenomenon is known as quorum sensing where bacteria signal for other bacteria to start producing a certain chemical compound, for example, that will benefit the larger bacterial community. In this case, bacteria communicate through light.
Fungi – Fungal luminescence is more common than bacterial luminescence in soils. Like bacteria, fungi may use the glowing light as a means of expanding their population. The glow may attract insects that spread fungal spores, the reproductive part of the fungus. The more attractive a fungus’ glow, the more likely it is for insects to spread its spores. This could be especially important for fungi that release their spores at night. Another reason fungi may luminesce is in order to attract predators of their predators. For example, if a fungus is being eaten by another organism, say, a microarthropod, the green glow may attract a predator or parasite of that microarthropod. In this way, fungi can glow to fend off predators and protect themselves from being eaten.
Collembola – Collembola are microscopic arthropods that are also known as springtails. Their nickname comes from the powerful tail-like structure, known as a furcula, that they use to move (or spring) around soils and leaf litter. Soil ecologists have observed luminescence in collembola, but like much of the below ground world, the reason for the glow is still unknown.
Like any characteristic of an organism, it is important to consider that luminescence could just be a by-product of another biological process, and not have any real ecological significance at all. A soil organism could undergo a biochemical reaction that produces a green glow in the process, and, well, that’s that. While it might be hard to believe that an organism would glow for no real reason, we cannot rule it out completely.
Do soil organisms glow in the dark? Yes! But to answer the question of why they glow, we will have to keep digging.
By Yamina Pressler, Colorado State University