Novel technologies for microalgae cultivation and subsequent lipid extraction

World economy is strictly linked to the availability of fossil fuels, which nowadays meet the world’s growing energy demand. However, the intensive exploitation of fossil fuels as main source of energy is currently recognized to be not sustainable due to the continuous depletion of available resources as well as to their contribution to environmental pollution and greenhouse gases emissions (Ahmad et al., 2011). Moreover global warming (GW) induced by increasing concentrations of greenhouse gases (GHG) in the atmosphere has become today an important environmental concern. The major anthropogenic sources of GHG are transportations, energy sectors and agriculture which are responsible in European Union (EU) for more than 20%, 60% and 9% of emissions, respectively (EEA 2004, 2007). A worldwide problem has become the depletion of petrochemical fuels and the continuous rise in oil prize that call us to make a global effort in order to find alternative energetic sources. Currently, many options are being studied and implemented in practice to meet the sustainability goals agreed under the Kyoto Protocol (1992) with different degrees of success. Wind, geothermal, solar (either thermal or photovoltaic), hydroelectric, ocean wave, carbon sequestration and bio fuels energy are been developed as more sustainable alternative energy sources compared with the combustion of fossil fuels (Dewily and Van Langenhove, 2006; Schiermeier et al., 2008). The use of fossil fuels is now widely accepted as unsustainable, due to depleting resources and the accumulation of GHG in the environment that have already exceeded dangerously high thresholds. For this reason, the production of renewable sources of energy such as biofuels is recognized to be critical to fulfill a sustainable economy and face global climate changes (Cheng and Timilsina, 2011). Therefore, biofuels deriving from feedstocks such as plants, organic wastes or algae could help to reduce the world’s oil dependence (Naik et al., 2010). In fact, biomass feedstocks are intrinsically renewable since they are produced through a natural process, i.e. photosynthesis that is continuously replenished by sunlight. Moreover, biofuels would mitigate global warming problems since all the CO2 emitted during their burning can be fixed by plants used as biomass feedstock, through photosynthetic mechanisms. On the other hand, first and second generation biofuels are characterized by several drawbacks which can limit their exploitation as alternative source of energy. One important goal for the gradual replacement of fossil fuels by renewable energy sources, as a measure for transportation emissions reduction, is the use of biofuels which are seen as real contributors to reach those goals, particularly in the short term. Today the most common biofuels are biodiesel and bio-ethanol, which can replace diesel and gasoline, respectively. In EU biodiesel represent 82% of total biofuels production (Bozbas, 2008) and is still growing in Europe, South America and United States, based on political and economic objectives. The first generation biofuel production systems (starch- and sugar-based ethanol production crops) demonstrated the feasibility of generating liquid transportation fuels from renewable sources, but at initially low energy-conversion efficiencies and high cost. However since vegetable oil produced by crops of first generation may also be used for human consumption, this can lead to an increase in price of food-grade oils, causing the cost of biodiesel to increase and preventing its usage. Plants that produce high levels of cellulose and hemicellulose biomass (which can be converted into sugars using advanced enzyme catalysts) are being developed as second generation biofuel production systems. These biofuel crops do not compete directly with food production, require less agronomic inputs and have lower environmental impacts than first generation biofuels. Morevor the use of biodiesel from second generation crops may also be advantageous since they do not require arable lands and do not affect biodiversity deriving from the cutting of existing forests and the use of potential invasive crops that may disrupt the biological integrity of local ecosystems and important ecological areas (Scarlat et al., 2008; RFA, 2008). However the main drawback of second generation biofuels is that they cannot be produced at a rate which coud meet the growing energy demand of the transopration sector. This is due to the fact that they are produced from feedstocks such as wastes or agricultural residues whose production is constrained by the original productive process from which they come. Morevoer second generation biofuels process have not still attained the economic sustainability. Although biofuels are still more expensive than fossil fuels their production is increasing in countries around the world also encouraged by policy measures and biofuels targets for transport (COM, 2006). A transition to a third generation biofuels, such as microalgae, is than needed since low-cost and profitable biodiesel should be produced from low-cost feedstocks in order not to compete with edible vegetable oils and should have lower environmental impacts. Thus transition can also contribute to a reduction in land requirements due to their higher energy yields per hectare as well as to their non-requirement of agricultural land. Concerning potential feedstock microalage are among the more interesting possibilities currently being investigated and implemented at pilot scale or even at industrial scale. Their use as a possible solution to the problem of GW is desirable since this group of fast-growing unicellular organisms shows several advantages which make them one of the most promising and attractive renewable sources for a fully sustainable and low-carbon economy portfolio. Between their advantages: widespread availability, absent (or very reduced) competition with agricultural land, utilization of cheap and abundant nutrient sources (sunlight, carbon dioxide, water), high oil and biomass yields, high quality and versatility of the by-products, generation of biomass for biofuel production with concomitant CO2 sequestration and suitability for wastewater treatments and other industrial plants (Vilchez et al., 1997; Olguín, 2003; Mulbry et al., 2008; EABA, 2012). The high potentiality of algae based biofuels is confirmed by the number of recent papers available in the literature related to the use of microalgae in the energy sector (Usui and Ikenouchi, 1997; Borowitzka, 1999; Kargi and Ozmihçi, 2004; Chisti, 2007), by the growing investments of private companies (Solazyme, Ocean Nutrition Canada, Cellana, AlgaeLink) and governments (US Dep. Energy, 2010) in algae-related research activity as well as by the increasing number of filed patents (Burton and Cleeland, 2008; Wu and Xiong, 2009; Cao and Concas, 2010; Parsheh et al., 2010; Rispoli et al., 2011). Despite this growing interest, the current microalgae-based technology is still not widespread since it is affected by technical and economic constraints that hinder its full scale-up (Chen et al., 2011). Therefore, great R&D efforts are currently undertaken to produce biodiesel at competitive costs and with the required quality starting from microalgae feedstock. In particular given the potential benefits of microalgae, their cultivation should be studied and optimized to make them competitive as fuel producing systems in the global market (Debska et al., 2010). The main technical barriers are related to the fact that photosynthetic efficiency, growth rate and lipid content of microalgae are still low if compared to the rate of fuel demand of the transportation market. In order to overcome such drawback, scientific community is moving on three main directions. The first one is the identification of cultivation conditions and photobioreactors configurations that maximize lipid productivity and CO2 fixation by means of a reduced number of known microalgae (Yoo et al., 2010; Yeh et al., 2011). Another research line is targeted to the identification of new microalgae strains which are intrinsically characterized by high growth rates and high lipid content (de la Vega et al., 2011). A futher attractive scientific challenge to face this problem is the genetic manipulation of existing strains in order to increase their photosynthetic efficiency and/or to regulate their metabolism in order to achieve an abundant production of lipids coupled with high biomass accumulation (León-Bañares et al., 2004). Finally the identification of novel techniques to improve lipid extraction from microalgae is one of the main target to be achieved in order to make the technology economically sustainable. Along these lines the present PhD activity has been focused on two different lines of research which share the common target of identifying suitable strategies to increase the lipid productivity of the current microalgal technology. Specifically, in the first line of activity, a novel cell disruption technique for the enhancement of lipid extraction yields from C. Vulgaris is proposed. In the second line of activity the exploitation of iron-based strategies to increase lipid synthesis in C. Vulgaris, is investigated