Microalgae provide various potential advantages for biofuel production when compared with 'traditional' crops. Specifically, large-scale microalgal culture need not compete for arable land, while in theory their productivity is greater. In consequence, there has been resurgence in interest and a proliferation of algae fuel projects. However, while on a theoretical basis, microalgae may produce between 10-and 100-fold more oil per acre, such capacities have not been validated on a commercial scale. We critically review current designs of algal culture facilities, including photobioreactors and open ponds, with regards to photosynthetic productivity and associated biomass and oil production and include an analysis of alternative approaches using models, balancing space needs, productivity and biomass concentrations, together with nutrient requirements. In the light of the current interest in synthetic genomics and genetic modifications, we also evaluate the options for potential metabolic engineering of the lipid biosynthesis pathways of microalgae. We conclude that although significant literature exists on microalgal growth and biochemistry, significantly more work needs to be undertaken to understand and potentially manipulate algal lipid metabolism. Furthermore, with regards to chemical upgrading of algal lipids and biomass, we describe alternative fuel synthesis routes, and discuss and evaluate the application of catalysts traditionally used for plant oils. Simulations that incorporate financial elements, along with fluid dynamics and algae growth models, are likely to be increasingly useful for predicting reactor design efficiency and life cycle analysis to determine the viability of the various options for largescale culture. The greatest potential for cost reduction and increased yields most probably lies within closed or hybrid closed -open production systems.

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The metabolic and enzymatic bases for growth tolerance to ethanol (4%) and H2 (2 atm [1 atm = 101.29 kPaJ) fermentation products in Clostridium thermohydrosulfuricum were compared in a sensitive wild-type strain and an insensitive alchohol-adapted strain. In the wild-type strain, ethanol (4%) and H2 (2 atm) inhibited glucose but not pyruvate fermentation parameters (growth and end product formation). Inhibition of glucose fermentation by ethanol (4%) in the wild-type strain was reversed by addition of acetone (1%), which lowered H2 and ethanol production while increasing isopropanol and acetate production. Pulsing cells grown in continuous culture on glucose with 5% ethanol or 1 atm of H2 significantly raised the NADH/NAD ratio in the wild-type strain but not in the alcohol-adapted strain. Analysis of key oxidoreductases demonstrated that the alcohol-adapted strain lacked detectable levels of reduced ferredoxin-linked NAD reductase and NAD-linked alcohol dehydrogenase activities which were present in the wild-type strain. Differences in the glucose fermentation product ratios of the two strains were related to differences in lactate dehydrogenase and hydrogenase levels and sensitivity of glyceraldehyde 3-phosphate dehydrogenase activity to NADH inhibition. A biochemical model is proposed which describes a common enzymatic mechanism for growth tolerance of thermoanaerobes to moderate concentrations of both ethanol and hydrogen.Thermophilic anaerobic bacteria have potential uses as new biocatalysts for the production of industrial ethanol because they can directly ferment inexpensive substrates such as cellulose, hemicellulose, and starch (18, 20, 28-30, 34, 36). Thermophiles lack industrial utility because they produce low concentrations of ethanol (<2.0% wt/vol). Herrero and co-workers (8-11) have studied the problem of ethanol tolerance in Clostridium thermocellum and concluded that low ethanol tolerance (<3% [wt/vol] ethanol) was a combined result of general solvent effects on membrane fluidity and a specific inhibition of enzymes involved in sugar phosphate metabolism.We previously demonstrated (21) that the wild-type strain 39E of C. thermohydrosulfuricum, which ferments starch, had low ethanol tolerance (i.e., no growth was achieved at 2% [wt/vol] ethanol); however, an ethanol-tolerant strain, 39EA, was selected which was tolerant of >4% (wt/vol) ethanol at 60°C and produced ethanol under these conditions. These studies concluded that direct enzymatic modifications could account for the higher ethanol tolerance of C. thermohydrosulfuricum at 60°C rather than those indirectly caused by membrane disruption in C. thermocellum. In addition to displaying low ethanol tolerance, wild-type strain 39E of C. thermohydrosulfuricum also differs from C. thermocellum (18) by being very sensitive to growth inhibition by hydrogen, another end product of saccharide fermentation (1, 31).The purpose of the present paper is to describe the biochemical basis for both hydrogen tolerance and ethanol tolerance in C. thermohydr...

Managing organic waste streams is a major challenge for the agricultural industry. Anaerobic digestion (AD) of organicwastes is a preferred option in the waste management hierarchy, as this processcangenerate renewableenergy, reduce emissions from wastestorage, andproduce fertiliser material.However, Nitrate Vulnerable Zone legislation and seasonal restrictions can limit the use of digestate on agricultural land. In this paper we demonstrate the potential of cultivating microalgae on digestate as a feedstock, either directlyafter dilution, or indirectlyfromeffluent remaining after biofertiliser extraction. Resultant microalgal biomass can then be used to produce livestock feed, biofuel or for higher value bio-products. The approach could mitigate for possible regional excesses, and substitute conventional high-impactproducts with bio-resources, enhancing sustainability withinacircular economy. Recycling nutrients from digestate with algal technology is at an early stage. We present and discuss challenges and opportunities associated with developing this new technology.

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