A study led by researchers from the CEAB-CSIC has been highlighted by the scientific community as one of the most significant in recent months for its contributions to understanding the self-purification processes occurring in rivers. The research, conducted in Mediterranean rivers, has revealed that the ratios of carbon, nitrogen, and phosphorus, as well as the composition of carbon, control the ability of bacteria to remove nitrates from the water.
Excess nitrates in rivers—resulting from the widespread use of agricultural fertilisers, intensive livestock farming, or insufficient wastewater treatment—pose a serious global problem, leading to reduced water quality and the eutrophication of aquatic ecosystems. Therefore, understanding the various influencing factors is of vital importance. One factor is the self-purification that occurs within rivers, primarily thanks to bacterial communities. Heterotrophic bacteria absorb nitrate, one of the main forms of nitrogen, present in fertilisers used in agriculture, among other sources. That’s why bacterial communities can help to improve water quality, by taking up some of the nitrates entering aquatic environments.
In addition to nitrogen as a nutrient, these bacteria also require organic carbon for growth, and phosphorus, which is essential for many of their biological functions. The influence of these other nutrients on the activity of bacterial communities and their ability to absorb nitrates is not yet fully understood. The study “Carbon-Nutrient Ratios Drive Nitrate Removal in Mediterranean Streams” has shed light on this issue. The research—carried out by scientists from the Centre for Advanced Studies of Blanes (CEAB-CSIC), the Catalan Institute for Water Research (ICRA-CERCA), and the Institute of Environmental Assessment and Water Research (IDAEA-CSIC)— has identified as key factors the nutrient balance and the origin of these nutrients, which condition the bacterial communities’ capacity to absorb nitrates. For its contribution to understanding biogeochemical cycles, the study was recognised by the journal Journal of Geophysical Research of the American Geophysical Union as one of the most relevant in recent months.
The Carbon–Nitrogen–Phosphorus (C:N:P) Balance
This research has shown that the balance between the three elements controls how efficiently microorganisms can capture nitrates. When there is an adequate amount of carbon in the water relative to nitrates, bacteria “work” more efficiently. The balance with phosphorus, in the form of soluble phosphate, is also important: if phosphorus is scarce, the bacteria’s ability to remove nitrates decreases.
The study also revealed that, when it comes to carbon, it’s not just about quantity. Depending on its origin—whether it comes from one type of organic matter or another—the composition of carbon varies. Simpler, more easily degradable forms of organic carbon make it easier for bacterial communities to absorb nitrates.
Xavier Peñarroya, CEAB-CSIC researcher and lead author of the study, explains: “Mediterranean rivers are like open-air laboratories for studying biogeochemical cycles and bacterial processes. In these rivers, nutrient balances and the composition of organic matter are constantly shifting due to large fluctuations in the hydrological connection between the river and groundwater, which breaks down during the dry periods. In this study, we observed that for bacteria to successfully capture nitrate from the river, they require a proper C:N:P balance, and in most of the rivers we studied, phosphorus was lacking.”
Understanding the factors that regulate rivers’ natural purification capacity is key to developing measures that help protect water quality and the health of our ecosystems. For instance, research teams at CEAB-CSIC are already experimenting with the addition of simple plant residues to rivers in order to restore the carbon–nitrogen–phosphorus balance and promote nitrate capture. The study may also prove useful for revisiting current nitrate pollution thresholds. These could be redefined based on the nutrient balance, better reflecting what the ecosystem is actually capable of absorbing (rivers with very little phosphorus or carbon should logically have lower nitrate thresholds due to their reduced self-purification capacity).
