Compositional variations of brown seaweeds Laminaria digitata and Saccharina latissima in Danish waters

Around Denmark, Laminaria digitata and Saccharina latissima are particularly common macroalgae species and are considered as prospective candidates for biorefineries. In this study, the carbohydrate composition and protein levels of L. digitata and S. latissima from three different sites in Denmark were compared for 1 year, and compositional variations of wild L. digitata harvested in August from the North Sea was monitored for 3 years. Glucan levels of L. digitata were consistently higher than those of S. latissima irrespective of harvest site and time of the year. Glucan levels in wild L. digitata from Kattegat peaked in October with 37.0% by dry weight compared to 22.6% by dry weight in wild S. latissima (Kattegat) and were accompanied by lower ash contents (18.5% w/w in L. digitata versus 26.5% w/w in S. latissima). Alginate contents were almost constant throughout the year, but mannuronic/glucuronic acid ratios differed between species and location from 1.33 to 3.64. Wild L. digitata harvested from the North Sea in August contained >50% glucans by weight and had low ash contents for three consecutive years (2012-2014). Compositional variation of the seaweeds was mainly related to season but also varied with species, location, and within populations. Among environmental variables (temperature, salinity, phosphate, nitrate, ammonia), only temperature was found to correlate with the chemical composition of the seaweeds. Amino acid profiles were dominated by glutamic acid, aspartic acid, and alanine and varied with season, especially for L. digitata from the North Sea, and location. Total nitrogen contents fluctuated more between samples than the actual protein contents; hence, application of a common N-to-protein factor cannot be recommended.