The biodegradability of the green alga <I>Staurastrum arctiscon</I> was studied, using a natural bacterial community sampled from Lake Biwa, to understand the generation characteristics of dissolved organic carbon (DOC) from the cytoplasm and extracellular polysaccharides (EPS). The biodegradation test revealed that 31% of the particulate organic carbon (POC) of the cytoplasm remained after 200 days of biodegradation and 2-4% of the initial cytoplasm POC was transformed into DOC. Moreover, it was clearly shown that 29-35% of the initial POC of the EPS was transformed into DOC and stably remained after 77 days of biodegradation. The morphology changes of <I>S. arctiscon</I> observed in the biodegradation experiments by staining EPS with fluorescently labeled wheat germ agglutinin (WGA) revealed that the fluorescence intensity gradually decreased over time and disappeared after 21 days of biodegradation, implying the partial degradation of the EPS. However, the EPS were still present even at 100 days, suggesting that some of them were comparatively recalcitrant to biodegradation. Consequently, our results suggest that phytoplankton-derived organic matter may play an important role as a source of DOC and POC in Lake Biwa.