Stepwise Cathodic Potential as a Double-Edged Sword for Anammox: Enzyme-Level Inhibition versus Multi-Pathway Compensation in a Biocathode

Cleaning wastewater without adding extra carbon is a holy grail for environmental engineers. Traditional methods rely on bacteria that eat organic food to strip nitrogen from water, but this requires adding carbon sources that can be expensive and create sludge. A more efficient alternative uses a special group of bacteria called anammox organisms. These microbes are autotrophs, meaning they do not need organic food; instead, they combine ammonium and nitrite to turn nitrogen into harmless gas. However, these bacteria are notoriously slow growers and have very specific demands. They need a precise balance of ingredients, and they are extremely sensitive to their chemical environment. If the conditions are wrong, they stop working or die, leaving the water dirty.

To solve the problem of getting the right ingredients, scientists have turned to electricity. By placing an electrode in the water, they can use electrical current to drive chemical reactions that create the nitrite the anammox bacteria need. This approach, known as a bioelectrochemical system, offers a way to remove nitrogen without adding carbon. But there is a catch. The electrical voltage required to make the necessary ingredients is often too high for the anammox bacteria to tolerate. It is like trying to feed a delicate plant with a fire hose; the energy needed to create the food destroys the plant that eats it. This creates a fundamental conflict: the electricity that helps make the food also poisons the eater.

Researchers at the Yancheng Institute of Technology and other institutions set out to resolve this conflict by testing a new strategy. Instead of keeping the electrical voltage steady, they applied it in a stepwise pattern, switching the voltage up and down in short intervals. They wanted to see if they could find a rhythm that allowed the bacteria to thrive while still using electricity to generate the necessary chemicals. They built small reactors containing anammox bacteria and subjected them to nine different voltage levels, cycling between a specific test voltage and a high voltage every thirty minutes for three hours at a time. They monitored the water chemistry, the bacterial communities, and the proteins inside the cells to understand exactly what was happening.

The results revealed that the electrical voltage acts as a double-edged sword. At lower voltages, specifically around a specific positive value, the bacteria worked at their peak efficiency, removing nearly all the ammonium and nitrite from the water. This was the sweet spot where the electricity helped without causing harm. However, as the voltage increased, the bacteria's performance dropped sharply. At a certain high level, their ability to clean the water was cut in half. The researchers found that this drop was not because the bacteria were dying or washing away. In fact, the number of these bacteria remained high even when they were not working. The problem was internal; the high voltage was damaging the tiny molecular machines inside the cells that perform the chemical work. It was as if the workers were still present in the factory, but their tools had been broken by the heat.

Yet, the story did not end with failure. When the voltage was pushed even higher, the system unexpectedly recovered some of its cleaning ability. This rebound happened because other types of bacteria, which are usually less important, woke up and took over the job. These different microbes could handle the high voltage and used the electricity to convert nitrate into nitrite through a different path. They also started recycling nitrogen within the system, turning waste back into usable ingredients. This created a backup plan that kept the water clean even when the primary anammox bacteria were struggling. The researchers observed that the system was not relying on a single method but was shifting between a dominant anammox pathway and a complex network of alternative routes depending on the electrical conditions.

The study also clarified that the electricity itself was not acting as a direct source of electrons for the bacteria in the way some had hoped. The amount of charge transferred was far too small to account for the nitrogen removal. Instead, the changing voltage acted more like a signal, telling the microbial community to switch gears. It triggered a reorganization of the bacterial population and the proteins they produced. At moderate voltages, bacteria that produce nitrate became more active, while at high voltages, bacteria capable of reducing nitrate to ammonium became more prominent. This dynamic shift allowed the system to compensate for the loss of anammox activity.

Based on these findings, the researchers proposed a new operating strategy called stepwise-potential-assisted anammox denitrification. This approach involves running the system at the low, safe voltage for a longer period to let the anammox bacteria do their main work, and then briefly switching to a higher voltage to let the backup bacteria clean up the leftovers and regenerate ingredients. This two-stage cycle avoids the damage caused by keeping the voltage high for too long while still taking advantage of the compensatory pathways that kick in at higher levels. The study showed that this method could be reversed; if the system was pushed too hard and the bacteria stopped working, bringing the voltage back down allowed them to recover their function within a few days.

This work suggests that the key to using electricity for wastewater treatment is not just finding the right number, but finding the right rhythm. By treating the voltage as a tool to manage the microbial community rather than just a power source, engineers can navigate the narrow window where anammox bacteria are happy. The research highlights that biological systems are resilient and capable of finding alternative paths when the primary route is blocked, provided the conditions are managed with enough precision to prevent permanent damage. The next steps will involve testing this rhythmic approach over longer periods and in real-world wastewater to see if it can be scaled up for practical use.