The biological production of hydrogen fuel has been a topic of research since at least the 1970s. Hydrogen gas can be produced from biomass sources like agricultural and forest residues, consumer waste and other specific agricultural crops. Specifically, hydrogen fuel is produced by a process called gasification, where biomass is processed into a combustible gas and then burned, or by pyrolysis, a related process which can lead to hydrogen gas suitable for fuel-cell applications. One continuing subject of research regards the production of unwanted co-products in both of these processes. The presence of other contaminant gases often depend on specific composition of the biomass source, which can be difficult to control. Another source for biological production of hydrogen fuel is algae. In the late 1990s it was discovered that if algae are deprived of sulfur they will switch from the production of oxygen, as in normal photosynthesis, to the production of hydrogen. Experimental algae farms are attempting to make algae an economically feasible energy source.