After a time, cells have trouble transferring electrons to the environment because the concentration of electrons inside the tree andoutside the tree are about the same. Sulphur becomes the next element used by soil organisms as an electron accepter. Sulphur respiration can cause hydrogen-sulfide to be produced, which is highly toxic to tree roots. Sulphur respiration also causes many compounds to bond with sulfides making them insoluble and unusable. At the end of the respiration process with no oxygen, comes fermentation.
Fermentation is the transfer of electrons to carbon.
Compared with normal respiration, fermentation respiration consumes twenty times the amount of food for the same amount of energy produced. See Table 1. Various hydrocarbons, alcohols, fatty acids, phenolic acids, sulfur compounds, and cyanogenic compounds are produced. Also generated are methane, carbon-dioxide, hydrogen, and nitrogen gas. These materials escape as gas bubbles, dissolve in the water, or float to the water surface.