Seasonal and intra-thallus variations of energy content and chemical composition were assessed in an intertidal population of Macrocystis in southern Chile. Phylloid protein and lipid from cultured material were compared with seasonal variation in native Macrocystis. Furthermore, populations in northern and southern Chile and Falkland Islands were compared with various intra-/inter-cultivar genotypes of Chilean Macrocystis. Energetic values did not show seasonal or intra-thallus variations, with the exception of pneumatocysts, which had high levels of ash (49.9% DW) and low values of total energy (8.3% DW). Seasonal patterns were detected in protein and carbohydrate composition, with opposite trends. Likewise, holdfasts contained high amounts of protein (21.0% DW), and phylloids were high in soluble carbohydrates (4.5% DW). Lipids instead showed two peaks per year in an intertidal population and reached up to 0.4% DW. Alginic acid was the major organic compound in intertidal Macrocystis (46.8% DW), with differences on seasonal and intra-thallus levels. Mannitol content, in contrast, was erratic and lower than in other Laminariales (< 5% DW). In general, protein and lipid content in our cultivars were 20% higher than in natural populations. Our experimental results indicate the possibility to manipulate the chemical composition of Macrocystis thalli through inter-/intra-specific crosses. This will be a basis, upon which selected genotypes can open new perspectives to Macrocystis mariculture industries in Chile.