Polyextremophile acidophilic microalgae are a unique class of microorganisms that thrive in extreme environments with extremely low pH levels and high temperatures. These organisms are capable of surviving in very harsh conditions, making them potentially useful, for instance, in the mining and metal recovery industries. The acidity of the growing media hinders the development of bacterial contamination, an impacting problem in microalgal fermentation processes that involves the use of organic carbon. This unique acidic environment represents a huge benefit for food-grade production on an industrial scale. Ongoing research into their molecular biology and metabolic pathways proves that these microalgae are a promising area of study for both basic science and practical applications.
Galdieria sulphuraria is a unicellular red alga belonging to the class Cyanidiophyceae and is found in acidic hot springs with temperatures up to 56°C and a pH range of 0.5 to 4.5. Galdieria is an extremophilic red alga that can grow in extremely acidic and hot environments, such as geothermal springs and volcanic sites, using a range of metabolic pathways. Its growth and metabolism are greatly influenced by the availability of organic carbon sources and the presence of light. One of their unique metabolic pathways is the use of the C4 pathway for carbon fixation, which helps them to survive in low-CO2 environments. Additionally, Galdieria is known for its ability to perform mixotrophic metabolism, which involves utilizing both photosynthesis and heterotrophy to obtain energy and nutrients. This allows them to grow in a wide range of conditions, including those with limited light and nutrients.
Organic carbon sources, such as glucose or pyruvate, are used by Galdieria to generate energy through glycolysis and the tricarboxylic acid (TCA) cycle. Galdieria’s metabolism is characterized by a high rate of respiration, allowing it to thrive in oxygen-rich environments. Studies have shown that the availability and type of organic carbon source can affect the growth and metabolic activity of Galdieria. For instance, when grown on glucose or pyruvate as the sole carbon source, Galdieria has a higher growth rate and produces more pigments than when grown on inorganic carbon sources like CO2.
Furthermore, the type of organic carbon source affects the composition of Galdieria biomass. For example, when grown on sugars like glucose or sucrose, Galdieria produces more lipids and starch, whereas when grown on organic acids like acetate or lactate, it produces more proteins and amino acids. The availability and type of organic carbon source can have significant effects on the growth and metabolism of Galdieria, with important implications for potential biotechnological applications, such as in the biofuels and bioproducts industries.
In this study, we performed a screening of three organic carbon sources to identify the best option and concentration applicable at an industrial scale. Algae growth under photoautotrophic conditions was optimized to boost biomass productivity in the first step. In the second step, different conditions were applied to induce the production of high-value metabolites, like phycocyanin. Glucose at 4.5 g/L was the best option among the three organic carbon sources tested at five different concentrations in presence and absence of light. The increased content of phycocyanin was related to the enhanced growth rate under mixotrophic conditions. Abiotic factors such as the C:N ratio and the light intensity play a role in the enhancement of phycobiliproteins, making Galdieria an exceptional alternative to phycocyanin-producing microalgae like Spirulina. Acidophilic algae are a fascinating subject for scientific investigation and a potential candidate for biotechnological applications.