Current methods for risk assessment of persistent chemicals in aquatic sediments do not take into account the effect of bacterial communities on degradation. Now, a new study by researchers at the Department of Environmental Science and international colleagues, published in Environmental Science and Technology shows that the composition of bacterial communities present in aquatic sediments influences how long chemicals can persist in them. These findings highlight the importance of understanding the role bacterial community composition has in environmental persistence of chemicals, with implications on how to better regulate their production and use in the future.
Persistent chemicals degrade slowly and remain for a longer time in the environment, which makes them potentially hazardous to humans and the environment. Bacterial communities are known to degrade chemicals in a process called biodegradation. To determine how fast organic chemicals degrade in aquatic sediments, the standard method OECD 308 has frequently been used. However, this method does not account for the contribution of bacterial communities.
“Bacterial communities have been a “black box” when studying biodegradation in the environment. It has been assumed that they are a homogeneous group, which, in combination with the challenges in studying bacterial communities, has led to that they are not included in standard methods for persistence assessment,” says Claudia Coll, former PhD student at the Department of Environmental Science and currently Postdoc at the Swiss Federal Institute of Aquatic Science and Technology in Switzerland, and leading author of the study. “Now, with relatively new DNA/RNA techniques in place we can find out who is who in bacterial communities. We are realizing that communities are very different and that there are thousands of bacteria we don’t know anything about, such as their ability to degrade environmental pollutants.”
The researchers used an experimental setup based on OECD 308 to study the relationship between bacterial community composition and biodegradation rates of ten common micropollutants in sediment from two rivers, one in Sweden and one in Germany, collected upstream and downstream of a wastewater treatment plant in each river. The ten micropollutants tested were commonly found in wastewater, and included the artificial sweetener acesulfame-K, caffeine and eight pharmaceuticals. Sediment bacteria were identified using 16S rRNA sequences that are specific for each bacteria. The team found that the slower a chemical degraded in the environment, the higher the variability in its degradation rate between the sediment samples, suggesting the involvement of bacterial communities in the degradation process.
“Sediment from the German river sampled downstream from the wastewater treatment plant showed the fastest degradation of the tested chemicals after accounting for differences in bacterial mass. The sediment sample from the German river also had a completely different bacterial community than the other test sediments. As a consequence, bacterial community composition of the sediment was associated with its capacity for degrading chemicals, but we found no effect of bacterial diversity on degradation efficiency,” says Anna Sobek, Associate Professor at the Department of Environmental Science who is also a senior author and project leader of the study. “Diversity is a parameter that has been highlighted as an important driver in many other studies, but the composition of the bacterial community provides more information on who is there.”
The team’s findings show that the environmental persistence of a chemical can be highly variable depending on the bacterial community composition of the receiving environment, continues Sobek. “The variability in degradation half-lives in our experiment was enough to affect the persistence assessment of a compound and thus highlights the limitations of current methods used in regulation of chemicals to measure environmental persistence,” concludes Sobek.