Bioactive peptides usually contain 3-20 amino acid residues, and their activities stem from both their amino acid composition and sequence (Pihlanto-Leppala, 2000). Usually such short chains of amino acids are inactive within the sequence of the parent protein, but they become active upon release during gastrointestinal digestion or during food processing, including

fermentation. Examples of bioactive peptides obtained by enzymatic hydrolysis of algal proteins (Kim and Wijesekara, 2010) are shown in Table 10.3 together with their characteristic physiological roles.

Savage et al. demonstrated hydrothermal liquefaction to produce a crude bio-oil from wet algae paste and then hydrothermal catalytic upgrading of the biocrude to produce hydro­carbon product in high yield. This work provides new results on the liquefaction pathways and kinetics and on the roles and effectiveness of different upgrading catalysts for removing heteroatoms from algae and reducing the viscosity of the biocrude (Savage et al., 2012b).

Duan et al. reported the catalytic hydrotreatment of crude bio-oil produced from the hy­drothermal liquefaction of microalgae (Nannochloropsis sp.) over Pd on C (5% Pd/C) in super­critical H2O (SCW) at 400°C and 3.4 MPa high-pressure H2. Longer reaction times and higher catalyst loadings did not favor the treated oil yield due to the increasing amount of gas and coke products formation but did lead to treated bio-oil with higher HHV (41-44 MJ kg-1) than that of the crude feed. Highest HHV of treated oil (ca.44 MJ kg-1) was obtained after 4 h using an 80% intake of catalyst on crude bio-oil. The product oil produced at longer reaction times and higher catalyst loadings, which was a freely flowing liquid as opposed to being the vis­cous, sticky, tar-like crude bio-oil material, was higher in H and lower in O and N than the crude feed, and it was essentially free of S (below detection limits). Typical H/C and O/C molar ratio ranges for the bio-oils treated at different reaction times and catalyst loadings were 1.65-1.79 and 0.028-0.067, respectively. The main gas-phase products were unreacted H2, CH4, CO2, C2H6, C3H8, and C4H10. Overall, many of the properties of the treated oil obtained from catalytic hydrotreatment in SCW in the presence of Pd/C are very similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011a).

Duan and Savage determined the influence of a Pt/C catalyst, high-pressure H2, and pH on the upgrading of a crude algal bio-oil in supercritical water (SCW). The SCW treatment led to product oil with a higher heating value (ca.42 MJ kg-1) and lower acid number than the crude bio-oil. The product oil was also lower in O and N and essentially free of sulfur. Including the Pt/C catalyst in the reactor led to freely flowing liquid product oil with a high abundance of hydrocarbons. Overall, many of the properties of the upgraded oil obtained from catalytic treatment in SCW are similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011b).

Studied LCAs use three different kinds of energy carriers: electricity obtained by direct combustion of the biomass, biodiesel by sequential or direct triglycerides esterification, and biogas by anaerobic digestion.

An adequate mixture should provide a high concentration of biomass, enable the circula­tion of liquid, keep the cells in suspension, eliminate thermal stratification, optimize the dis­tribution of nutrients, improve gas exchange, and reduce the shading and photoinhibition of microalgae. Turbulent flow is essential for maximum production of microalgae in open ponds. In raceway cultures, velocities of 5.0 cm. s-1 are sufficient to eliminate thermal strat­ification and maintain most species of algae in suspension.

Several mixing systems are used in microalgal cultures, depending on the type of reactor. In open pond systems, paddlewheels are used to induce turbulent flow. In stirred-tank photobioreactors, impellers are used to mix the algal cultures. In tubular photobioreactors, mixing can be carried out directly or indirectly through airlift systems (Ugwu et al., 2008).

The main costs of growing microalgae arise from the mixing and mass transfer in cultures (using paddlewheels, impellers, and airlifts) because of the energy consumed. For the race­way pond, the mixing cost is €0.08 per kg DW (dry weight), for the tubular reactor it is €1.27, and for the flat panel reactor it is €3.10 per kg DW (Norsker et al., 2011).

The mechanical stirrers (paddlewheel) provide optimal efficiency of mixing and gas transfer, but they cause significant hydrodynamic stress. Gas injection (bubbling) by airlift or impellers causes low hydrodynamic stress, good transfer of gas, and a reasonable mixing efficiency (Richmond and Cheng-Wu, 2001).

In closed photobioreactors, where the mixing is carried out by impellers or airlift, the increase of the speed of the gas bubbles enlarges the diameter of the bubbles (Ugwu et al.,

2008) . The bigger the bubbles, the lower the exchanges of gases with the liquid.

A high concentration of oxygen produced by photosynthesis inhibits microalgal growth. The supply of gas with the turbulent labor regime in closed photobioreactors is one solution to reduce this negative effect. However, depending on the microalgal species, high turbulence can cause damage to cells due to stress and high energy consumption (Pires et al., 2012). Low mixing results in an accumulation of toxic compounds in stagnant areas. In open ponds, oxygen has low solubility and rapid outflow since the photobioreactors are low in height.

Open systems are currently still the preferable choice for microalgal production on a large scale, especially when they are designed to produce low-priced products, such as biofuels. However, due to the requirements of good manufacturing practice (GMP) guidelines, pro­duction of high-value products from microalgae for application in pharmaceuticals and cosmetics seems feasible only in well-controlled photobioreactors with closed system ope­rations. Therefore, several closed systems (photobioreactors) for microalgae cultivation are discussed here.

The term closed systems refers to photobioreactors that have no direct exchange of gases and contaminants between the cultivation systems and the outside environment. The necessary gas exchange is performed through a sterilized gas filter, to avoid contamination inside the culture system. Therefore, closed systems are characterized by the minimization of con­tamination over open systems. Besides the typical drawback of high equipment cost, closed- system photobioreactors do have several major advantages over open systems (Singh and Sharma, 2012): (1) Photobioreactors could minimize contamination and allow axenic algal cultivation of monocultures; (2) photobioreactors offer better control over conditions such as pH, temperature, light, CO2 concentration, and so on; (3) using photobioreactors leads to less CO2 loss and prevents water evaporation; (4) photobioreactors permit higher cell con­centrations; and (5) photobioreactors permit the production of complex biopharmaceuticals.

There are several types of closed systems designed and developed for the cultivation of microalgae, including vertical (tubular) columns, flat plate photobioreactors, and horizontal tubular photobioreactors. The detailed descriptions of those cultivation systems are provided here. In addition, their advantages and weaknesses are summarized and compared in Table 2.2.

Vibrating screens are commonly used in industries such as the paper or food industry as a material separating or sorting device. They are also used in municipal wastewater treatment plants to concentrate sludge. Earlier harvesting of Coelastrum algae by vibrating screen was reported (Mohn, 1980). Higher algae solids concentration of 7-8% has been harvested under batch operations in comparison with lower algal solids contents of 5-6% when operated in continuous mode. In a study by the Food and Agriculture Organization of the United Nations (Habib et al., 2008), vibrating screens were used for harvesting Spirulina, which are multicellular and filamentous blue-green microalgae belonging to two separate genera, Spirulina and Arthrospira. In the commercial Spirulina production as food for humans and domestic animals and fish, vibrating screen filtration used for harvesting achieved very high algal biomass removal efficiency of up to 95% for harvesting up to 20 m3/hour, from which algal slurry of 8-10% biomass solid contents were produced. Compared with the inclining screens counterpart with a filtration area of 2 to 4 m2/unit, the vibrating screens required only one-third of the area.

In electroflotation or electrolytic flotation, fine gas bubbles are formed by electrolysis. The formed hydrogen gas attaches to fine algal particles, which float to the surface, where they are removed by a skimmer. Instead of a saturator, a costly rectifier supplying 5-20 DC volts at approximately 11 Amperes per square meter is required. The voltage required to maintain the necessary current density for bubble generation depends on the conductivity of the feed suspension. Further discussion of research on electroflotation is presented in Section 5.3.7.

Photosynthates provide an endogenous source of acetyl-CoA by activated acetyl-CoA synthetase in the stroma, from free acetate, or from the cytosolic conversion of glucose to pyruvate during glycolysis (Somerville et al., 2000; Schwender and Ohlrogge, 2002). This acetyl-CoA is preferentially transported from the cytosol to the plastid, where it is converted to the fatty acid and subsequently to TAG, which again is transported to the cy­tosol and forms the lipid bodies (Figure 8.1). The acetyl-CoA pool will be maintained through the Calvin cycle, glycolysis and pyruvate kinase (PK) mediated synthesis of py­ruvate from PEP, which occur in the chloroplast in addition to the cytosol. The first reaction of the fatty acid biosynthetic pathway towards the formation of malonyl-CoA from acetyl — CoA and CO2 is catalyzed by the enzyme Acetyl-CoA carboxylase (ACCase). (Ohlrogge and Browse, 1995). Figure 8.2 illustrates the conversion of acetyl-CoA to malonyl-CoA by utilizing ATP. During this process, seven molecules of acetyl-CoA and seven molecules of CO2 form seven molecules of malonyl-CoA. This malonyl Co-A undergoes synthesis of long carbon-chain fatty acids through repeating multistep sequences, as represented in Figures 8.2 and 8.3. A saturated acyl group produced by this set of reactions becomes the substrate for subsequent condensation with an activated malonyl group (Ohlrogge and Browse, 1995).

FIGURE 8.3 Sequential chain elongation steps and formation of precursor molecules (palmitic acid) from CO2

Kuan-Yeow Show1, Duu-Jong Lee2

xDepartment of Environmental Science and Engineering, Fudan University,

Shanghai, China

^Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan

9.1 INTRODUCTION

Extensive effort is being made globally to exploring renewable energy sources that could replace fossil fuels in mitigating global warming and other environmental issues. Hydrogen is a promising fuel alternative to conventional fossil fuels because it releases energy ex­plosively without air pollutants in combustion. Most of the hydrogen in use currently is produced through thermochemical processes via electricity generation from fossil fuels. Because the current hydrogen fuel is based on the use of nonrenewable fossil-fuel resources, a major issue related to conventional hydrogen production is sustainability.

Biohydrogen production is deemed a key development in creating a sustainable energy supply and a promising alternative to fossil fuels. Hydrogen production via biological pro­cesses is carried out largely at ambient temperatures and pressures and hence is less energy intensive than chemical or electrochemical ones. As a desired green energy product of natural bioconversion, biohydrogen metabolism is primarily the domain of bacteria and microalgae. Within these groups, it involves many taxonomically diverse species, a variety of enzymes, and metabolic pathways and processes (Schulz, 1996; Vignais et al., 2001; Weaver et al., 1980). Biological processes use the enzyme hydrogenase or nitrogenase as a hydrogen-producing protein. This enzyme regulates the hydrogen metabolism of prokaryotes and some eukaryotic organisms, including green algae. The function of nitrogenase as well as hydrogenase is linked with the utilization of metabolic products of photosynthetic reactions that generate reductants from water.

Current development of algal hydrogen production is focusing on biophotolysis and photosynthesis-hydrogen production using various microbial species. Sunlight is necessary for hydrogen production by photosynthetic microorganisms. Photoautotrophic green microalgae and cyanobacteria use carbon dioxide and sunlight as the respective sole carbon and energy sources. The reducing power for cellular photosynthesis and/or biophotolysis comes from water oxidation under light irradiation (Ghirardi et al., 2000; Schutz et al.,

2004) . This chapter examines the perspectives and state-of-the-art of algal hydrogen research in the context of pathways of hydrogen production, bioreactor design and operation, and eco­nomic evaluation. Prospects and challenges in algal hydrogen production are also outlined.

Phenols (sometimes called phenolics) are a class of chemical compound consisting of a hy­droxyl group (-OH) directly bound to an aromatic hydrocarbon group. The simplest of this class is phenol, the parent compound used as disinfectant and for chemical synthesis. Phlorotannins are an extremely heterogeneous group of phenolic compounds in terms of structure and degree of polymerization; accordingly, they provide a wide range of biological activities (Holdt and Kraan, 2011). Green and red macroalgae possess low concentrations of phenols (Mabeau and Fleurence, 1993) compared to brown macroalgae that are particularly rich in phlorotannin. Typical phenolic contents vary from 1-14% of dry macroalga biomass. Such polyphenols as fucol, fucophlorethol, fucodiphloroethol G, and ergosterol as well as phlorotannin are abundant in brown macroalgae and possess strong antioxidant effects. The concentration of polyphenols exhibits seasonal variations and shows a significant time correlation with the algal reproductive state, besides being affected by a number of other parameters such as location and salinity (Holdt and Kraan, 2011).

Polyphenols entail a cosmetic and pharmacological value owing to their antioxidative activity; they also have shown other favorable effects, e. g., protection from radiation as well as antibiotic and antidiabetic qualities. Several of these effects were tested in bacteria, cell cultures, rodents, and even humans, namely with regard to sexual performance and desire. Certain polyphenols may work as preventative medicines due their several bioactivities (see Table 10.6); in particular, phlorotannins are candidates for development of unique natural antioxidants for further industrial applications in functional food, cosmetic, and pharmaceu­tical formulations (Li, Qian et al., 2009).

For their extraction, several methods can be applied using Soxhlet-based solvent extraction or ultrasonic extraction, as discussed elsewhere (Mahugo Santana, Sosa Ferrera et al., 2009).