Earthworms are very special because they keep the soil healthy all over the world. But what these animals look like from the inside and how their metabolism is shaped by the environment as well as by symbionts and parasites has so far been difficult to determine. Now a research team has developed a method that combines chemical and anatomical analyzes – and thus offers completely new insights into the inner workings of the earthworm.
Earthworms can be found in almost all soils around the world, where they take on important functions as ecosystem engineers. For example, they drill huge systems of ducts into the earth, so that humus is mixed in, the soil is loosened and water can drain into deeper soil layers. In addition, the worms transport organic material into the earth, eat it and then feed on soil bacteria and fungi with their excrement. In their body they harbor symbiotic microbes and also small animal parasites that their immune system has to fight.
Chemistry and anatomy combined
However, it was not previously known which interactions exactly take place inside an earthworm. A research team led by Benedikt Geier from the Max Planck Institute for Marine Microbiology (MPIMM) has now tried to get a closer look at the anatomy and metabolism of the earthworm. The problem: The understanding of the chemical interactions between the small animals and the microorganisms living in their body has so far been extremely limited because there are hardly any suitable methods. In order to uncover the basic processes of animal-microbe symbiosis and parasites, one must first understand which chemical substances are produced where by the individual partners. To do this, it must be illustrated how molecules are distributed at the micrometer level. In addition, it is almost impossible to correctly interpret the chemical images of previous methods if it is unknown whether, where and with which useful or pathogenic microbes or even animal parasites a tissue is infected.
To overcome these difficulties, Geier and his colleagues have combined two high-resolution imaging methods into a new method. “In our study, we present chemo-histotomography, a special three-dimensional representation of the chemistry and anatomy of millimeter-sized animals and their parasites on a cellular level,” says Geier. Chemo-histotomography consists on the one hand of micro-computed tomography, which does not intervene in the organs of the examination subject, but nevertheless enables a 3D representation of the tissue by reconstructing a large number of X-ray images of the sample. The scientists also used MALDI imaging mass spectrometry. With this, micrometer-sized, natural distributions of metabolic products can be visualized and the chemical profiles assigned to their place of production and, possibly, to their producer.
Detailed insight into symbiosis processes
With the help of this combination of methods, the research team was able to create micrometer-accurate cross-sections and longitudinal sections of individual tissue sections of an earthworm and examine them both anatomically and chemically. “This progress enables us to take an earthworm out of the environment and to create a 3D atlas of its chemical and physical interactions with microorganisms that occur naturally in its tissue,” summarizes Geier’s colleague Manuel Liebeke. With the help of the 3D model, the scientists can now use the metabolic products in the earthworm to understand how it defends itself chemically against parasites and how these in turn protect themselves against the earthworm’s immune response.
For example, the accumulation of lombricine in the muscle tissue of earthworms – a substance that serves as a chemical energy store – became visible. Geier and his team also found that the pigment protoporphyrin is only found in the back of the outermost skin layer of earthworms and that the metabolites in the intestine vary greatly along the longitudinal axis. The new method also confirms that metabolic products between animals and their symbiotic microbes are not only exchanged locally. One example is the gut-brain axis, in which chemical compounds are produced in the gut by microbes, which then reach the host’s brain and thus influence fundamental processes.
Can also be used on mussels, corals and the like
“This method offers a new way of visualizing products of the metabolism in small animal symbioses and thus spatially assigning the chemistry to the animal host and its microbial partners in the micrometer range,” sums up Geier. In doing so, it outperforms comparable applications that have been developed for medical research on mice with a resolution that is up to two orders of magnitude higher. Therefore, the new technology could presumably also enable research on insects or corals in the future. Geier and his colleagues are currently testing the method on deep-sea mussels.
Source: Max Planck Institute for Marine Microbiology, Article: Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2023773118