Polytech-CyanoSPO is a chemically rationalized BG11 derivative overcoming precipitation and toxicity for robust cyanobacterial growth

In the quiet corners of freshwater lakes and rivers, microscopic plants called cyanobacteria are constantly at work. These single-celled organisms are the engines of the water world, capturing sunlight to create energy and releasing oxygen, much like the trees on land. However, some of these tiny plants can become dangerous. When conditions are right, they can multiply rapidly, forming thick, green scums known as harmful algal blooms. These blooms can choke out other life in the water and release poisons that threaten drinking water supplies, pets, and people. To understand how these blooms start, how they produce toxins, and how to stop them, scientists must be able to grow these organisms in the laboratory. They need a controlled environment where they can watch the cells divide and study their behavior without the chaos of a natural lake.

For decades, the standard recipe for feeding these freshwater cyanobacteria in a lab has been a mixture called BG11. It is a well-known formula, trusted by researchers everywhere, containing water, salts, and essential nutrients like iron, nitrogen, and phosphorus. The idea is simple: provide the food the cells need, and they will grow. But for a specific group of toxic cyanobacteria, this standard recipe has failed completely. When scientists tried to grow three particular species—Planktothrix agardhii, Shackletoniella antarctica, and Pseudanabaena catenata—in this standard mixture, the cells did not just grow slowly; they stopped growing entirely. They turned pale, their internal machinery broke down, and the cultures died. This failure created a major roadblock. Without a way to grow these specific toxic strains in the lab, researchers could not study what triggers them to release their poisons or how to predict when a bloom might happen in the wild.

A team of researchers at Peter the Great St. Petersburg Polytechnic University decided to investigate why this standard food was failing and to build a better one. They discovered that the problem was not a lack of food, but rather how that food was delivered. In the standard mixture, certain ingredients reacted with each other to form solid clumps that the bacteria could not eat, while other ingredients created a chemical environment that was actually toxic to the cells. To fix this, the scientists developed a new, modified recipe they named Polytech-CyanoSPO. This new mixture did not invent new nutrients; instead, it swapped out four specific ingredients for different forms of the same nutrients that the bacteria could actually use.

The first change addressed the iron, a vital mineral needed for the bacteria to perform photosynthesis. In the old recipe, the iron was mixed with a compound that broke down quickly in the light, causing the iron to turn into a solid rust-like sludge that settled to the bottom of the container. The bacteria could not reach it. The new recipe replaced this with a different iron salt paired with a special molecule that acts like a protective cage, holding the iron in a liquid, dissolved state so the cells could grab it easily. The second change involved the nitrogen source. The old recipe used a form of nitrogen that brought along sodium, a salt that can stress the cells and disrupt their internal balance. The new recipe swapped this for a form of nitrogen paired with potassium, a mineral the bacteria need to function, effectively turning the food into a source of both energy and essential minerals without the stress.

The third adjustment tackled the phosphorus, another key nutrient. In the standard mixture, the way the phosphorus was dissolved made it likely to stick to calcium and magnesium, forming invisible solids that the bacteria could not absorb. The new recipe used a different form of phosphorus that kept the water slightly acidic, preventing these solids from forming and keeping the phosphorus available for the cells to eat. Finally, the team changed the source of a trace metal called cobalt, removing a chemical form that could be harmful and replacing it with a safer, more stable version. These four changes were small in number but massive in their effect.

When the researchers tested their new mixture against the old one, the results were dramatic. In the standard BG11 mixture, the three toxic species barely survived. Their numbers did not increase, their green color faded, and they produced almost no oxygen, a sign that their energy engines had stalled. In contrast, the cultures grown in the new Polytech-CyanoSPO mixture exploded with life. Over a period of fourteen days, the number of cells in the new mixture grew ten to thirty times higher than in the old one. The water in the new cultures became so rich with oxygen that it held more than the air above it could normally dissolve, a clear sign that the bacteria were photosynthesizing at full speed.

The scientists also measured what was left in the water after the experiment. In the cultures that failed, the water was still full of unused nitrogen and phosphorus, proving that the bacteria were too sick to eat it. In the successful cultures, the water was nearly empty of these nutrients, showing that the healthy bacteria had consumed almost everything available. The new mixture also kept the water at a stable, neutral pH level, whereas the old mixture became too alkaline, which further poisoned the cells. By solving these chemical puzzles, the researchers have provided a reliable way to grow these difficult, toxic strains. This breakthrough means that scientists can now study these organisms in the lab with confidence, opening the door to understanding how they produce their toxins and how to protect water supplies from the dangers of harmful algal blooms.