Current Biotechnology - Volume 5, Issue 3, 2016

Background: Hydrogen is a clean, versatile fuel and energy carrier which can be produced by a range of renewable technologies for combustion, use in fuel cells, or as a manufacturing feedstock. Despite its attraction and significant technological innovation, commercial feasibility of photobiological hydrogen processes is far from demonstrated. Objective: This review examines direct photobiological biohydrogen systems, with a particular focus on the main obstacles that must be overcome to deliver commercially viable, net energy positive systems. As part of this process the interactions between future photobiological biohydrogen systems and other parts of a renewable energy economy are examined to analyse potential technology integration paths. Results: The primary driver for renewably produced hydrogen is the potential for CO2 emissions reductions. Renewable hydrogen is largely solar driven, either directly (e.g. natural photosynthesis, or bio-inspired devices) or indirectly (e.g. fermentation, electrical hydrolysis). A large market for hydrogen already exists and is supported by extensive infrastructure providing significant opportunities for emerging renewable hydrogen streams. Several key physiological obstacles to efficient photobiohydrogen production have already been overcome, with oxygen tolerance as the most significant remaining problem. Conclusion: A much deeper understanding of photosynthetic biology is required before existing knowledge can be integrated with real world systems. Cross-fertilisation between engineering and biology represents the best path forward for implementation as a robust biotechnology.

Background: The goal of achieving cost-effective biofuels and bioproducts derived from algal biomass will require improvements along the entire value chain, including identification of robust, highproductivity strains and development of advanced genetic tools. Though there have been modest advances in development of genetic systems for the model alga Chlamydomonas reinhardtii, progress in development of algal genetic tools, especially as applied to non-model algae, has generally lagged behind that of more commonly utilized laboratory and industrial microbes. This is in part due to the complex organellar structure of algae, including robust cell walls and intricate compartmentalization of target loci, as well as prevalent gene silencing mechanisms, which hinder facile utilization of conventional genetic engineering tools and methodologies. Conclusion: Recent progress in global tool development has opened the door for implementation of strain-engineering strategies in industrially-relevant algal strains. These developments will facilitate the use of microalgae in a variety of applications relating to biofuels and bioproducts. Here, we review recent advances in algal genetic tool development and applications in eukaryotic microalgae.

Background: Omega-3 long chain polyunsaturated fatty acids (LC-PUFAs), are important constituents of human nutrition and have key roles in maintaining health through their effects on immune system. Although marine fish are the main dietary source of EPA and DHA, the depletion of fish stocks and pollution of the marine environment indicate an urgent need for an alternative and sustainable source of omega-3 LC-PUFAs. Marine microorganisms are the primary producers of omega-3 LC-PUFAs in the aquatic food chain and EPA- and DHA-rich microalgae have been demonstrated to be a promising alternative source to fish oils. Methods: There is increasing interest in the metabolic engineering of microalgae and genetic modification of algal strains is considered to be one of the most promising strategies to produce new sustainable omega-3 oils. The efficient production of high value products such as EPA and DHA from algae is expensive and significant efforts in strain development and cultivation technologies are required to reduce the currently high production costs associated with algal biomass. Results: This review describes the recent advances in metabolic engineering of microalgae towards optimizing the production of omega-3 LC-PUFAs. Conclusion: In the last few years an intense research has been performed to maximize microalgal production of omega-3 LC-PUFAs and several bottlenecks that limit the oil accumulation have been identified. By using metabolic engineering it is possible to overcome these barriers and generate improved strains of microalgae.

Background: Microalgae is considered a promising biofuel feedstock. When algae oil is extracted for fuel production, a significant portion of the lipid-extracted algae (LEA) generated as a co-product is generally re-used on site as a source of energy and nutrients. However, LEA may also be used for other purposes, including as a substitute for animal feeds. Methods: This life cycle assessment (LCA) study investigated the greenhouse gas (GHG) emission impacts of algae biofuel when the LEA co-product is used as a 1:1 (mass basis) substitute for soybean meal, rapeseed meal, fishmeal, or trout feed. All algae cultivation nutrients lost in LEA export were replaced with chemical fertilizers; electricity and heat previously generated on-site were replaced with US grid electricity and with natural gas heat. Important trade-offs were assessed in terms of GHG emissions for algae hydrotreated renewable diesel with LEA displacement credits for the different feed ingredients. Results: This LCA study indicated that the benefit from displacing animal feed does not outweigh the incremental burdens associated with replacing the energetic and nutrient requirements that LEA currently satisfies, resulting in higher GHG emissions for the algae biofuels life cycle. Conclusion: This study demonstrates the importance of decisions made throughout the full value chain of a product in determining the environmental impact of a product, given the regulatory pressure to develop low-carbon fuels.

Background: Continuous culture is a method with significant benefits that is underused in the area of microalgae. Despite this, with the spread of these cultures into many different areas, such as biofuel production, pigment production for the food and pharmaceutics industries, effluent treatment, biosorption of heavy metals, it is necessary and imperative to make advancements in the technology in terms of knowledge and use. Therefore, it is necessary to identify its fundamental basis and to fully understand its operation in order to be able to successfully maximise important parameters such as biomass or metabolite productivity. Methods: This article presents the fundamental concepts and applies them clearly and simply in continuous culture photobioreactors operated as light-limited chemostats, using a Scenedesmus obliquus culture. Results: This article presents the basis and applications of a simple light-limited chemostat method, which is the most commonly-used type of operation for achieving high levels of biomass productivity. The usefulness of operation curves: microalgae concentration(X) - dilution rate (D) and volumetric productivity of cells (Qx) - D and how to build them are also presented, along with the operational and environmental design parameters that affect them. Conclusion: The construction of operation curves for the microalga in question must be carried out in a laboratory with day-night cycles of temperature and irradiance to obtain critical D values and the optimal D at which to successfully operate the outdoor culture.

Background: Photosynthetic diatom microalgae have significant capacity for biosynthesis of energydense biofuel molecules, as well as unique co-products not found in other algae, including metal oxide nanomaterials for advanced material applications, and glucosamine biopolymers or monomers for nutraceutical and biomedical applications. Diatoms biomineralize soluble silicon to nanostructured biosilica, and require dissolved silicon (Si) as a required substrate for cell wall biosynthesis and division. Objectives: To exploit their silicon metabolism for eliciting the biosynthetic pathways of selected products, a two-stage cultivation process is developed to induce high levels of lipid and chitin production by the centric marine diatom Cyclotella within a bubble-column photobioreactor under conditions were light and CO2 delivery are not limiting. Methods and Results: The two-stage batch cultivation process synchronized Cyclotella diatom cells to silicon-starved state in Stage I and reduced the time to silicon depletion in Stage II biomass production by surge uptake of dissolved silicon. Lipid and chitin production were elicited at silicon depletion but not at nitrogen depletion. Stage II product yields associated with the biomass were 34 wt% total lipid and 16 wt% chitin, with 60% of total biomass carbon allocated to these two products. From this information, a material balance on the diatom-based photosynthetic bio-refinery for production of the nutraceutical glucosamine with co-production of biodiesel and biosilica illustrated the productivity of this biological production system. Conclusion: The diatom-based photosynthetic biorefinery has significant potential as a future platform for biofuels and unique co-products.

Background: The cultivation stage of a microalgae-based process has the highest energy burden, and a significant portion of this consumption is related to bioreactor aeration. Thus, the objective of this work was to assess the aeration energy requirements in heterotrophic microalgal bioreactors applied to poultry and swine slaughterhouse wastewater treatment. Methods: The experiments were performed in a bubble column bioreactor, operating at 25ºC, pH of 7.5, 100 mg/L of inoculum, absence of light and flow rate per unit volume (Q/V) of 0.5, 1.0 and 1.5 VVM (volume of air per volume of wastewater per minute). Based on this, experimental data estimated the aeration energy requirements and the net energy ratio of the system. Results: The results showed slight performance gains in cell growth and substrate consumption in flow rates per unit volume of 1.5 VVM. A markedly similar behavior was observed under the conditions of 0.5 and 1.0 VVM. Thus, Q/V of 0.5 VVM can be considered the equilibrium condition (kinetic performance vs energy consumption) for the operation of bioreactor. Conclusion: Based on experimental data, a power density of 9.68 W/m3 was estimated for the treatment of agroindustrial wastewater, resulting in a positive energy balance (NER>1).