A research team from the Instituto de Agricultura Sostenible de Córdoba (IAS-CSIC) has developed a method to study with greater precision how certain bacteria interact with agricultural plastic waste. For the first time, they have differentiated the microbial communities that live directly on this material, which they have called plastic plane or plastiplane, from those that grow in the soil that sticks to it, the plastisphere. This advancement lays the groundwork for better understanding the impact of these residues on terrestrial ecosystems and opens the door to identifying microorganisms capable of degrading them.
Additionally, this work, funded by European funds from the SOPLAS project, framed within the Marie Skłodowska-Curie actions of the Horizon 2020 program, and in collaboration with the Ministry of University, Research and Innovation of the Andalusian Government, through the ‘Qualifica’ program, could be useful in designing strategies to decompose these wastes more effectively and sustainably, known as bioremediation. «When we talk about plastic pollution, we mainly think of environments like seas or rivers, and we hardly focus on soils, where there is also significant contamination,» highlights Giovana Macan, a researcher at the Instituto de Agricultura Sostenible de Córdoba and author of the study, to the Fundación Descubre, an entity dependent on the Ministry of University, Research and Innovation.
Another novelty compared to previous research was the collection of samples of real conventional plastic, made from polyethylene, a material present in a multitude of everyday objects, such as bags, containers, or caps. The fragments came from several fields in Baza (Granada), an area characterized by intensive horticultural agriculture, where the use of plastic mulch is common. This type of sheet covers the soil to retain moisture, control temperature and weeds, thus increasing crop yield. However, the canvases break down over time, generating both visible or macroplastics, as well as other particles smaller than five millimeters known as microplastics. These accumulate in the soil and are difficult to remove, which is why the United Nations Environment Programme considers it one of the most concerning contaminants of the 21st century, not only due to its persistence but also its environmental ubiquity, being detected in terrestrial ecosystems, aquatic environments, food, and even living organisms like the human body.
Plastisphere and plastic plane
The IAS-CSIC began to address this issue with the SOPLAS training network, which covers 14 doctoral projects of young Europeans to study the interaction of plastic in agricultural soil, each with different techniques and objectives, but with mutual cooperation. Under its auspices, another work was presented at this center that traced for the first time the spread of plastic during tillage.
Laboratory of Soil Microbiota Biology and Ecology.
Now, in the study titled ‘Unravelling the plastisphere-soil and plastic plane microbiome of plastic mulch residues in agricultural soils’ and published in the journal Applied Soil Ecology, the experts have developed a protocol to separate two zones in the collected samples: the direct surface of the material, which they have called plastiplane, and the one with soil residue attached, the plastisphere. This distinction is key, as both compartments present different conditions and can harbor different bacterial families. «We have extrapolated two concepts from microbiology: the rhizoplane, which corresponds to the root surface, where a more specialized microbial community resides, and the rhizosphere, which includes the soil that surrounds it and is under its influence, different from the rest of the terrain, favoring the growth of certain microorganisms,» explains Dr. Blanca Landa, principal researcher of the study at IAS.
The team designed a methodology based on sequential washes, to extract the most adhered soil, and sonication, a technique that uses ultrasonic waves to detach the microorganisms directly attached to the plastic surface. Using optical and scanning electron microscopes, which allow the visualization of very small particles, they visually confirmed that both fractions had been separated correctly. For the subsequent analysis, they combined traditional cultivation techniques with high-throughput genetic sequencing, which directly analyzes DNA to determine the complete composition of the microbial community. The results showed that the plastisphere contained a greater richness of bacteria, but the plastiplane also harbored a specific community, with some genera present in both compartments. Among them, some have already been described as potential degraders of this contaminating material.
Future applications
In addition to identifying the present bacteria, the scientists isolated some specimens of interest for future research on bioremediation. «If these microorganisms survive on the plastic surface, it is very likely that they are using it as a nutrient source. If confirmed, we could cultivate them in the laboratory on different plastic substrates and observe if they are capable of partially or completely decomposing them,» suggests Macan.
Being based on real samples, the methodology can be adapted to other types of materials or agricultural conditions. In fact, the team has applied it in subsequent studies with fragments from blueberry fields in Huelva, and other experimental biodegradable plastics being developed at the Instituto Hortofrutícola de la Mayora (Málaga), derived from cellulose or tomato residues.
