At first sight, the above three trends do not seem to have much in common. And yet: they all have to do with microorganisms. Organic matter is broken down by microorganisms such as bacteria, enhancing the self-cleaning properties of freshwaters. However, the greenhouse gases carbon dioxide and methane may also be produced and released as the microbes break down organic matter.
For this reason, IGB researchers sought to make use of data collected by citizen scientists to find out how the microbial community composition in aquatic sediments changes under light and chemical pollution, and whether this in turn has an impact on greenhouse gas levels.
All freshwaters studied are sources of greenhouse gases
“All the freshwaters were supersaturated with carbon dioxide. To us, this is an indicator that this greenhouse gas is being released into the atmosphere,” explained Dr Katrin Premke, who led the citizen science study at IGB. Generally speaking, a higher carbon dioxide concentration was measured in running waters than in standing waters. If rivers, streams, lakes and ponds are considered separately, however, small standing waters have the highest concentrations. This finding is consistent with the results of previous studies. “Methane concentrations were highest in the small water bodies, falling within the expected range. What surprised us, however, was that the methane concentrations in large water bodies were higher than described in the literature,” remarked Katrin Premke. “This shows that we may be underestimating the role of large water bodies in terms of methane concentrations.”
Antibiotic resistance widespread among bacteria in sediment
Since greenhouse gas levels are directly related to the activities of microorganisms, the researchers also wanted to know the extent to which microorganisms are exposed to chemical stress and antibiotic resistance. For this purpose, they analysed two specific gene sequences (IntI1, blaOX58), which not only allow conclusions to be drawn about anthropogenic stress, i.e. human influence on our environment, but also provide an initial indication of how widespread antibiotic resistance is in our freshwaters. The genetic marker for anthropogenic stress was detected at 85 per cent of the sites, and especially frequently in urban streams and rivers.
Antibiotic resistance was also found at as many as 45 per cent of these sites exposed to human-induced stress. “This highlights the pervasive human impact of residues of pharmaceuticals, such as antibiotics and other trace substances in water bodies, on even the smallest organisms,” stated co-author Dr Christian Wurzbacher of the Technical University of Munich" The degree to which a stressed microbiome affects the function of aquatic ecosystems and our health needs to be clarified in further studies."
Light pollution alters the composition of microorganisms
By cross-referencing the citizen scientists’ geospatial data with satellite imagery, the team was also able to determine the level of light pollution. They found that 75 per cent of the sites studied were affected by excessive artificial lighting at night, particularly along rivers in urban areas.
The microbial community analysis of microorganisms from samples of different sediment surfaces indicated that light pollution altered species composition slightly. However, it could not be proven whether light pollution as a result also has a direct effect on the formation of greenhouse gases. “The analysis only represents a snapshot in very different freshwater contexts. However, based on previous laboratory experiments, we cannot rule out the possibility that light pollution could alter the formation of greenhouse gases by microorganisms in the long term,” stated Dr Franz Hölker, leader of IGB’s light pollution research group who co-initiated the study.
The study highlights the extent to which microorganisms in aquatic sediments are influenced by human stressors. Interestingly, both chemical pollutants and light pollution had a stronger effect on microorganisms in running waters than in standing waters. “This may be related to the fact that the shoreline of running waters in relation to their surface area exceeds that of standing waters. The effects of light pollution, wastewater discharges and urban areas could therefore be more pronounced,” explained Franz Hölker.
Thanks to the large-scale citizen science study, it was possible to conduct a coordinated sampling campaign within a time window of just two weeks, which a research team would never have accomplished on its own. The authors therefore also conclude that the participation of citizen scientists is a promising approach for monitoring freshwaters in large areas, and they expressly thank the many participants for their cooperation.
The greenhouse gas data, satellite data, instructions and manuals for the citizens, and all in-situ derived temperature and pH data can be accessed at the supplementary material. All sequenced amplicon data for this study and their raw reads are available from European Nucleotide Archive (ENA, PRJEB53570). All data that support the findings of this study are available from the corresponding author on request as well. Code and R scripts used to analyse the data are available on Github except for analyses where all settings are fully reported in the Methods.