Macroalgae biomass can be converted by biological methods. Fermentation is a biological conversion method of the biomass to produce energy carriers like hydro­gen, ethanol and biogas. All living organisms obtain the energy necessary to sustain life, from the oxidation of organic substances by molecular oxygen, in the process of respiration. Under anaerobic conditions (low oxygen concentrations), many organisms, including yeast, obtain the energy from the process of fermentation. Fermentation normally occurs under anaerobic conditions but also occurs some­times when oxygen is present (Prescott et al. 2002).

In alcoholic fermentation, characteristic of many yeast species, the fermentation process starts with one molecule of the six carbon sugar and terminates with two molecules of the two-carbon alcohol-ethanol and two molecules of CO2. The major source for energy production in the yeast Saccharomyces cerevisiae (S. cerevisiae) is glucose.

Glycolysis is the general pathway for conversion of glucose to pyruvate, whereby production of energy in form of ATP is coupled to the generation of intermediates and reducing power in form of NADH for biosynthetic pathways (Fig. 13.1). Two principal modes of the use of pyruvate in further energy production can be distin­guished: respiration and fermentation. As the figure depicts, the input of anaerobic fermentation is sugar and the product of respiration is glycerol, ethanol, CO2 and succinate. The fermentation pathway is being utilised in bioethanol production (Feldmann 2005).

Yeast has been the main component in fermentation process. It converts complex material such as sugar to simpler material, that is, ethanol. S. cerevisiae is well — known yeast commonly used in fermentation and capable of galactose fermentation (Goh and Lee 2010). It is often used in research as a model eukaryotic organism because it is easy to manipulate and culture (Chen 2008). It is widely used in the fermentation industry such as wine, bread and beer production. It can withstand temperatures as low as 1-3% and as high as 40%. The optimum temperature for its growth will be 28%. S. cerevisiae can survive in extreme condition such as acidic condition (pH 3.0 or lower) (Presscott et al. 2002). Therefore, it is also known as extremophiles.