Like many other plants, marimo, through the process of photosynthesis, absorb sunlight through their green surface and use the light's energy to create nourishment. This nourishment is used by the whole of the marimo. As marimo become larger, they require more nourishment than they are able to produce. Therefore if large round marimo don't remain in areas which receive plenty of sunlight allowing them to actively photosynthesize available light into energy, they will die. Strong sunlight able to penetrate shallower waters and reach the lake's bottom is essential if spherical marimo are to survive and grow.
Ironically, however, as spherical marimo become larger their size dictates that they reside in deeper waters to remain completely submerged in water. If for some reason sunlight reaching the lake's bottom decreases, large marimo living in deeper areas of the lake will be adversely affected. If the light necessary to sustain growth is not sufficient and this condition continues for an extended period of time, the marimo's cells will begin to die and the spherical form will collapse.
Insufficient sunlight is believed to be primarily to blame for the continual disappearance of large round marimo in Lake Akan. An overabundance of nutrition in the water has lead to an increase in the amount of plankton and other aquatic plants which intercept the sun's rays before they are able to reach the lake's bottom.