TECHNOLOGY FOCUS - BIOLOGY - Preferred Components: The red algae is laver, agar-agar, sea string, or Grateloupiaceae. The brown algae is sea mustard, Laminaria, seaweed fusiforme, gulfweed, Ecklonia stolonifera, rhubarb or Potamogeton oxyphyllus. The green algae is green laver, sea lettuce, Monostroma nitidum or sea staghorn. The extract is: a red alga extract, which is agar, cellulose, carrageenan, xylan or mannan; a green alga extract, which is cellulose, xylan, mannan, starch, fructan or paramylon; or a brown alga extract, which is cellulose, alginate, fucoidan or laminaran.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The heterogeneous catalyst is acidic, and is an ion exchange resin, zeolite, a heteropoly acid, a metal and/or a metal oxide. The metal or the metal oxide is copper, zinc, chrome, nickel, cobalt, molybdenum, tungsten, platinum, palladium, ruthenium, rubidium and/or their oxides.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The hydrolysate is a monosaccharide, a furan compound or an organic acid, where the hydrolysate preferably comprises a compound, which is: galactose, glucose, xylose and/or mannose when the red algae extract is hydrolyzed; glucose, xylose, mannose and/or fructose when the green algae extract is hydrolyzed; or glucose, glucuronic acid, fucose, galactose, xylose and/or mannitol when the brown algae extract is hydrolyzed. The seaweed extract is hydrolyzed in a continuous manner in a batch reactor, a flow reactor or a fixed bed reactor into which the heterogeneous catalyst is loaded. The hydrolyzing step is carried out at 110-200 degrees C (preferably 140-180 degrees C) under a pressure of 1-20 (preferably 1-10) atm in a saturated vapor pressure condition with a weight ratio of seaweed extract to water of 0.1-20% (preferably 0.5-10%) and catalyst to seaweed extract of 0.05-20% (preferably 0.1-15%). EXAMPLE - Powdered agar (4 g) and Amberlyst 70 (RTM: Macroporous polymer based strong acid ion exchange resin) (1 g) were added to water (200 ml) in a 300 ml autoclave. The autoclave was purged with nitrogen, and the mixture was heated to 150 degrees C with stirring at 780 rpm while the internal pressure of the autoclave was adjusted to 10 bar to hydrolyze the agar into monosaccharides (galactose) (55%). The galactose (31 g/l) was subjected to fermentation for 4 days to produce ethanol (88%).