Comparative Nutrient Removal and Biomass Production by Chaetoceros neogracile, Isochrysis galbana, and Tetraselmis chuii in Saline Aquaculture Wastewater

The ocean is a vast, living system, but the waters surrounding fish farms tell a different story. When fish are raised in large numbers, the water they live in becomes a soup of waste: uneaten food, fish droppings, and dissolved chemicals like nitrogen and phosphorus. If this water is released back into the ocean without treatment, it can trigger a chain reaction that depletes oxygen and harms marine life, a process known as eutrophication. For decades, scientists have looked for ways to clean this water naturally. One promising approach involves using microscopic plants called microalgae. These tiny organisms act like underwater vacuum cleaners, absorbing the excess nutrients to fuel their own growth. The challenge, however, is finding the right kind of algae that can survive the salty, variable conditions of fish farm water while growing fast enough to be useful.

In a recent study, researchers set out to solve this puzzle by testing three different types of marine microalgae against real wastewater from a fish farm. The team, led by Aisha Khan at the University of Karachi, wanted to see which species could grow the most biomass—essentially, the most plant material—and which could remove the most pollution. They tested three specific strains: Chaetoceros neogracile, a type of diatom that requires silica to build its shell; Isochrysis galbana, a tiny, round alga often used as food for marine larvae; and Tetraselmis chuii, a larger, flat alga that is relatively easy to harvest. To ensure the results were reliable, the researchers did not just dump the algae into raw, undiluted waste. Instead, they created a series of mixtures, diluting the wastewater with clean seawater to create four different strengths: 25%, 50%, 75%, and 100% wastewater. They also included a control group where the algae grew in standard laboratory food to see how the waste compared to ideal conditions, and another group with just wastewater and no algae to measure natural changes.

The experiment ran for two weeks in glass vessels, with the water kept at a steady temperature and exposed to a consistent cycle of light and dark. The researchers checked the water every few days, counting the number of algae cells and measuring how much nitrogen, phosphorus, and organic matter had disappeared from the water. They also weighed the dried algae at the end to calculate exactly how much new plant material had been produced. The results revealed a clear trade-off: the algae that grew the fastest were not necessarily the ones that cleaned the water the best.

Tetraselmis chuii proved to be the champion of growth. When placed in water that was half wastewater and half clean seawater, this species produced the highest amount of biomass, reaching a final weight of 0.870 grams per liter. It grew steadily and efficiently, turning the nutrients in the water into new cells faster than the other two species in that specific mixture. However, when the goal was purely about cleaning the water, a different species took the lead. Isochrysis galbana, when grown in water that was 75% wastewater, removed the highest percentage of pollutants. It cleared out 92% of the ammonium, 76% of the nitrate, and 87% of the phosphate. It also reduced the chemical oxygen demand—a measure of organic pollution—by 63%. This means that while Tetraselmis chuii was better at making plant food, Isochrysis galbana was better at scrubbing the water clean.

The study also highlighted that the concentration of the wastewater mattered significantly. In the strongest, undiluted wastewater, the algae struggled more, growing slower and removing fewer nutrients compared to the diluted mixtures. This suggests that the high levels of ammonia and other chemicals in the raw waste can stress the algae, limiting their ability to do their job. Interestingly, the researchers found that the best performer for one task was not the best for the other. The species that produced the most biomass did not remove the most nutrients, and vice versa. This finding is crucial because it means that a fish farm cannot simply pick one "super algae" and expect it to do everything perfectly. The choice of which algae to use depends entirely on the farm's goal: if the priority is to harvest the algae for feed or fuel, Tetraselmis chuii in a 50% mixture is the better choice. If the priority is to clean the water before releasing it, Isochrysis galbana in a 75% mixture is the superior option.

Despite these promising results, the researchers are careful to note that this was a controlled laboratory experiment. The study was conducted in small glass vessels over a short period, which is very different from the complex, open-air systems used in real-world fish farming. The team explicitly states that these findings are not yet ready for immediate commercial use. Before any of these algae strains can be used to treat wastewater on a large scale, further testing is needed. Future work must validate these results in pilot-scale systems that run continuously, test how the algae perform across different seasons, and ensure that the harvested algae are safe from contaminants like heavy metals or pathogens. For now, the study provides a clear map of the potential and the limitations, showing that while microalgae offer a powerful tool for cleaning our oceans, the right tool depends on the specific job at hand.