The utilization of microalgae for renewable biofuel production offers a promising alternative to fossil fuels mainly because of their rapid growth and high lipid content. Moreover, cultivating microalgae can help capture carbon dioxide and recycle nutrients, especially when integrated into waste management systems. However, environmental conditions significantly affect the productivity and chemical composition of algal biomass. Among these, the harvest season plays a major role, affecting the physiological conditions of the cells and the accumulation of energy-rich compounds. Understanding how seasonal changes affect the biofuel potential is essential for improving production efficiency and ensuring stable large-scale operations.

In this study, we evaluate how the harvesting season affects the yield and composition of biofuels obtained from naturally occurring microalgae grown in an open raceway pond at a biogas producing facility in Sauquillo de Boñices (Soria, Spain). The carbon dioxide from the biogas upgrading and the nutrients from the digestate liquids were used to stimulate microalgae growth. The algal species primarily consisted of Chlorella sp., Scenedesmus sp., and Ankistrodesmus sp. Samples were collected during four distinct seasons and processed under variable drying conditions. Oil extraction was carried out using the Soxhlet method, and the oils were converted to fatty acid methyl esters (FAMEs) through esterification and transesterification. Our findings show that microalgae collected in winter contained the highest lipid levels and energy yields, while samples from warmer months resulted in lower-quality biofuels due to oxidative degradation and limited lipid accumulation. These observations highlight the importance of seasonal control and integrated biorefinery approaches for achieving efficient and sustainable microalgae-based biofuel production.