Marine algae, such as Undaria pinnatifida, commonly known as "wakame" in Japan, contain valuable bioactive organic compounds including lipids and polysaccharides. However, substandard seaweeds that do not meet strict quality standards are normally discarded as wastes or returned to the sea, a situation which is becoming a serious environmental concern. In this work, hydrothermal treatment of the supercritical carbon dioxide deoiled wakame was investigated using microwave and conventional heating to recover and degrade Undaria polysaccharides (i.e., fucoidan) into highly potent low-molecular-weight components of about 5-30 kDa. Results showed advantages of microwave heating compared to conventional heating, obtaining the target molecular weight at a temperature close to 140 degrees C. Continuous microwave irradiation at constant microwave power in a short irradiation time of 1 min also gave promising results.