An alternative or addition to the production of biodiesel is the production of bio-ethanol from the carbohydrates and starches in the algal cells. De­pending upon the strain and composition of the algal species significant yields of ethanol can be produced from algal biomass [78-81]. Strains with filamentous cells such as Spirulina and Spirogyra are considered most promising due to the higher percentage of carbohydrate in their make­up. The conventional process of producing bioethanol using hydrolysis and fermentation is well understood for many feedstocks but optimal con­version has not yet been achieved for algal biomass. Similarly to lipid extraction, the first stage in the process is the disruption of the biomass cells which can be carried out using numerous techniques including bead­beating, autoclaving, microwaving and acid or alkali treatment. Once the cells have been disrupted the carbohydrates and starches can be converted into sugars using enzymatic or acid hydrolysis. Following hydrolysis the sugars are then be fermented with yeast (typically S. cerevisiae or S. baya — nus) which will provide a broth of up to 17% (v/v) ethanol depending upon the concentration of sugars (AB Mauri, personal correspondence). The next step to produce bioethanol is to distil the broth to produce an ethanol concentration of around 98% (v/v) then further refinement of the ethanol produces a fuel which can be used as an additive to conventional engines or up to a maximum of 85% in specialised E85 engines [82].

As the concept of converting algal biomass into bioethanol is relatively under-researched most studies have simply focussed upon investigating what ethanol recoveries are possible. In an early study by Hirano et al. [79] a variety of freshwater and marine algae was selected for testing. Chlo — rella vulgaris was found to contain a high proportion of starch (37%) and a recovery of 65% of ethanol from the starch was obtained using enzymatic hydrolysis followed by fermentation with S. cerevisiae. An overall recov­ery of 24% from the biomass was therefore obtained. Using the strain Chlorococum spp, a conversion efficiency of about 38% of the ethanol was obtained [78], which can be considered promising however this was an optimal value and no consideration was given to the energy requirement of processing. What is interesting from this research is that when the lipid content of the biomass was recovered prior to fermentation, ethanol yields were far higher [78]. This suggests that biomass could provide both diesel and ethanol, maximising potential recoveries. Nguyen et al. [83] found in several studies that yields of up to 29% ethanol recovery efficiency were possible using Chlamydomonas reinhardtii. The studies mentioned above prove that high ethanol yields from algal biomass are possible but further studies are necessary to assess the viability in terms of energy balance, economics and environmental impacts.

Alternative methods of ethanol production have been investigated which focus upon intracellular ethanol production in which algae produce ethanol under dark, anaerobic conditions. The species which are capable of the process are cyano-bacteria and include the species: Chlamydomo­nas reinhardtii, Oscillatoria limosa, Microcystis, Cyanothece, Cicrocys — tis aeruginosa and Oscillatoria spp. [84]. The process requires the algae to be cultivated in a closed environment with the addition of CO2 under which conditions, it is believed that concentrations of between 0.5 and 5% ethanol can be produced. Hirano et al. [79] investigated this phenomenon using Chlamydomonas reinhardtii and Sak-1 isolated from salt water, and a maximum yield of 1% , w/w produced by C. reinhardtii was reported. The ethanol-water mix can then be extracted and treated further to produce highly concentrated ethanol for fuel use. The benefits of the process are that no other organisms (e. g., enzymes and yeast) are required for hydro — lysis/fermentation and the algae remains unaffected and can continue to grow without a requirement for harvesting. The energy requirements are likely to be lower than those necessary for conventional fermentation of biomass however the two methods need to be directly compared. In their study Luo et al. [84] show that the whole process provides a positive en­ergy balance with the greatest surplus of energy when the maximum etha­nol concentration is produced. Additionally the greenhouse gas emissions compare well to emissions via gasoline production but to reach 20% of the emissions from gasoline (a government aim) would require further reduc­tions in the process chain.

Bioethanol production from algal biomass is still very much in its infancy, the concept is proven but the viability is not. Further life-cycle analyses are required to understand the potential of the concept. Post lipid processing and intracellular ethanol production look promising as energy consumption is minimised, further research will establish viability.