Microalga–Macroalga Dry Biomass Blending of Tetradesmus obliquus and Pterocladiella capillacea as a Low-Cost Strategy to Tune Biodiesel Fuel Properties

Rising fossil-fuel dependence and its associated environmental burden have intensified interest in third-generation algal biodiesel feedstocks. This study screened six locally sourced algal strains (Chlamydomonas hedleyi, Nostoc commune, Tetradesmus obliquus, Ulva intestinalis, Pterocladiella capillacea, and Sargassum latifolium) from Egyptian freshwater and marine sites for lipid content and fatty acid methyl ester (FAME) profiles, and then evaluated direct dry-biomass powder blending of the high-lipid green microalga T. obliquus with the red seaweed Pterocladiella capillacea at 1:1, 1:2, and 2:1 (w/w) ratios as a low-cost strategy for upgrading biodiesel feedstock quality. T. obliquus (TET) accumulated the highest lipid content among the monocultures (26.50% dry weight), while P. capillacea (PTE) led the macroalgae (7.80%). The gravimetric lipid yield of the blends followed a predictable mass-balance trend, peaking at 18.70% for the 2TET:1PTE ratio. GC-MS-based FAME profiling showed that blending shifted the fatty acid distribution from strongly saturated (up to 94.54% SFA in Nostoc commune) toward a more balanced saturated/unsaturated composition, with the 2PTE:1TET blend reaching 30.59% monounsaturated fatty acids. Biodiesel quality indices estimated from the FAME profiles indicated that all blends met the kinematic viscosity (4.52–4.71 mm2/s) and density (0.873–0.880 kg/L) limits of EN 14214, with cetane numbers (55.59–57.63) matching both the EN 14214 and ASTM D6751 standards. The 2:1 PET:TET blend offered the best overall trade-off, combining a low cold filter plugging point (1.73 °C), a moderate iodine value (71.20 g I2/100 g), and a favorable cetane number (57.63). These findings support direct biomass co-blending of a high-lipid microalga with an abundant, low-cost macroalga as a scalable route for tuning biodiesel fuel-quality indices without additional extraction or refining steps, although the estimated properties reported here require experimental engine-bench validation before industrial application.