Purple bacteria, such as Chromatium, Thiospirillum, Thiocapsa and Chlorobium, metabolize sulphide from their nutrient solution. This is determined by iodometric titration with sodium thiosulphate. Following gravimetric CO[2] determination, Van Niel asserted that two H[2]S molecules are used up per assimilated CO[2] molecule. Analogous to higher plants, protons reduce CO[2] to organic carbon. The oxidised rests of the hydrogen donors are O[2] and 2S respectively.