CARBOHYDRATE DEGRADATION AND METHANE PRODUCTION DURING FERMENTATION OF LAMINARIA-SACCHARINA (LAMINARIALES, PHAEOPHYCEAE)

Anaerobic digestion of the brown alga Laminaria saccharina (L.) Lamour . harvested in spring and autumn was carried out at controlled laborat ory conditions in stirred fermentor systems. Due to the normal seasona l variations, the autumn material had a much higher content of carbohy drates such as mannitol and laminaran. Both batch and semi-continuous feeding conditions were investigated for periods up to 800 h, with ino culum provided from previous kelp fermentations. In batch cultures, th e methane yield from the autumn material was doubled compared to that of the spring material. Semi-continuous conditions gave more similar m ethane yields for both raw materials, 0.22 and 0.271 CH4 per g VS for spring and autumn material, respectively. In all experiments, mannitol and laminaran were reduced to less than 5 % of the initial values wit hin 24-48 hours after inoculation, whereas 30 % of the alginate conten t was detectable even after 30 days. Viscometry revealed that this mat erial was severely depolymerized, and alginate lyase activity was foun d to develop rapidly in all cultures. Although mannitol and laminaran were fermented much faster than alginate, the total accumulated methan e yields seemed to be determined by the total carbohydrate content of the raw material during extended semi-continuous feeding.