Understanding how algae fight back when water is cleaned

Cleaning water before it reaches our taps often involves adding chlorine to kill harmful algae. But algae have a clever trick—they wrap themselves in sticky layers called extracellular polymeric substances, or EPS. These layers act like shields, protecting the algae from chlorine’s effects. Scientists wanted to see how well different types of EPS hold up when faced with two common chlorine treatments: free chlorine and monochloramine. There are two main types of EPS. The loosely bound kind, called LB-EPS, is more like a flexible outer layer filled with proteins. The tightly bound kind, TB-EPS, acts like a tougher, sugar-based coating mixed with humic substances. When chlorine is added, the reaction speeds depend on which type of EPS is present. Free chlorine reacts faster with TB-EPS, gobbling it up quicker than LB-EPS. Monochloramine, on the other hand, works more slowly but affects both types more evenly. What’s interesting is how chlorine changes the shape of proteins inside the EPS. The structure weakens, making the algae easier to separate from their protective layers.Algae don’t just sit back and take the chlorine treatment quietly. Without their EPS, they experience stress inside their cells. Tests showed that free chlorine increases harmful superoxide levels inside algae, especially when LB-EPS or TB-EPS is removed. Monochloramine causes even higher stress but triggers stronger antioxidant responses from the algae. This suggests that the EPS layers don’t just block chlorine—they also help algae manage the damage chlorine causes. The findings show that EPS plays a big role in how well chlorine can work. If water treatment plants understand this better, they might adjust how they add chlorine to make it more effective. The goal isn’t to destroy the algae completely but to weaken them enough so they can’t cause problems downstream.