Chemical mutation of Stichococcus sp. for the selection of strains with lower chlorophyll content and higher biomass production

Sunlight is the engine that drives life in the ocean, but for the tiny, single-celled plants known as microalgae, too much of a good thing can become a problem. These microscopic organisms rely on a green pigment called chlorophyll to capture light energy and convert it into food and growth. However, when these cells grow crowded together in a large tank, their high concentration of green pigment creates a barrier. The cells on the outside absorb almost all the light, casting a deep shadow that starves the cells in the middle. This phenomenon, often called the "shadow effect," limits how much biomass the entire culture can produce, making it difficult to grow these organisms efficiently on a large scale for fuel or other products.

Scientists have long sought ways to bypass this limitation by finding or creating strains of microalgae that do not need as much chlorophyll to survive. If a strain could grow with less green pigment, it would allow light to penetrate deeper into the culture, enabling more cells to grow simultaneously and increasing the total harvest. A team of researchers at the Technical University of Crete in Greece recently tackled this challenge by working with a specific type of rod-shaped microalgae called Stichococcus. They aimed to create a version of this organism that naturally produces less chlorophyll but grows faster and produces more valuable chemicals, such as oils and sugars, than its wild counterparts.

To achieve this, the researchers used a technique called chemical mutagenesis. They exposed a culture of the wild Stichococcus to a chemical substance known as ethyl methanesulfonate, which acts like a randomizer for the organism's genetic code. This process introduces small, unpredictable changes to the DNA of the cells. The goal was not to design a specific change, but to generate a wide variety of random mutations and then sift through the survivors to find the ones that happened to have less chlorophyll. The team carefully adjusted the concentration of the chemical to ensure that only about five percent of the cells survived the treatment, a level of stress that encourages the emergence of new traits without wiping out the entire population.

After the chemical treatment, the researchers plated the surviving cells on solid agar dishes and waited for them to grow into visible colonies. They looked for colonies that appeared lighter in color than the deep green of the original wild strain. From these, they selected two specific strains, which they named EMS1 and EMS2, because they appeared the palest. To confirm that these lighter colors were a permanent genetic change and not just a temporary reaction, the team grew these strains for three generations in liquid seawater. They then compared the performance of these new mutants against the original wild type under identical conditions, measuring how much biomass they produced, how much chlorophyll they contained, and how much oil and carbohydrates they stored.

The results showed a clear advantage for the mutated strains. The best performer, strain EMS1, grew significantly faster and produced more total biomass than the wild type. By the end of the two-week experiment, the EMS1 culture reached a density of 2.47 grams per liter, whereas the wild type reached only 2.20 grams per liter. More importantly, the EMS1 strain contained significantly less chlorophyll. While the wild type held about 11 milligrams of chlorophyll per gram of dry biomass, the EMS1 mutant held only about 6.29 milligrams. This reduction in pigment confirmed that the mutation successfully altered the organism's ability to harvest light, potentially allowing it to grow more efficiently in dense cultures where light penetration is limited.

Beyond just growing faster and having less green pigment, the mutant strain also showed promise for producing valuable industrial materials. The researchers found that the EMS1 strain stored more lipids, which are fats that can be converted into biofuels, and more carbohydrates, which can be used for other chemical products. The lipid content in the EMS1 strain rose to 21 percent of its dry weight, compared to 15 percent in the wild type. Similarly, the carbohydrate content increased to 44 percent of the dry weight, up from 28 percent in the original strain. The amount of protein in the cells remained roughly the same across all groups, indicating that the mutation specifically boosted the production of energy-storing compounds without sacrificing the basic structural components of the cell.

The study concludes that this chemical mutation process is a viable method for improving microalgae strains for large-scale production. The EMS1 strain, with its lower chlorophyll content and higher yield of biomass and valuable chemicals, represents a significant step forward. The researchers plan to use this specific strain in future experiments involving biofilm photobioreactors, systems where the algae grow on solid surfaces rather than floating freely. By using a strain that allows light to travel deeper into the culture, they hope to capture carbon dioxide from industrial smoke and convert it into high-value products more efficiently, turning a waste gas into a resource with lower cultivation costs.