Microalgae have three main pigments: chlorophyll that absorbs blue light; red carotenoids that absorb blue and green light; and phycobilins that absorb green, yellow, and orange light. These pigments have been used as natural colorants in food products. In many countries biodyes have replaced artificial dyes, which are currently prohibited.

p-carotene is a carotenoid found in all higher plants and algae. p-carotene acts as pro­vitamin A and may be used as a natural food color. Phycolibins are water-soluble pigments and are found only in red algae or cyanobacterias. Most members of cyanophyceae contain blue pigment (phycocyanin), although several species may also contain erythrin. Phycoery- thrin and phycocyanin can be used as natural pigments in food, medicine, and cosmetics, avoiding the use of artificial pigments that are carcinogenic (Richmond, 1990).

Carbon fixation by microalgae is in vogue. In the last decade, more than 4,000 papers were published globally on this subject. Table 4.1 presents some rates of carbon dioxide described in the literature.

Among all species of microalgae, four are most common industrially: Spirulina, Chlorella, Dunaliella, and Haematococcus. Despite not being used industrially, Botryococcus is also largely studied due to its potential use as a source of hydrocarbons. These microalgae’s potential for carbon fixation is discussed next.

The green halophilic alga Dunaliella is the best natural source of p-carotene. This microalgae is marketed in several countries, such as the United States, Australia, and Israel. The biopigment p-carotene is extracted from microalgal biomass and used as a food supple­ment or a natural pigment added to foods, or the dry biomass is marketed in tablets (Wood, 1998).

The biomass of the microalga Dunaliella has demonstrated several biological activities, such as being antihypertensive, bronchodilator, analgesic, muscle relaxant, and anti-edema. The natural p-carotene contains many essential nutrients that are not present in the same pigment produced synthetically (Yousry, 2002). The human body converts p-carotene to vitamin A without forming toxins in the liver. p-carotene has antioxidant activity while avoiding the effects of free radicals.

This microalga is grown in high salinity, with optimal growth in 22% of NaCl. Under these cultivation conditions the microalga culture is axenic and thus poses no problems of contam­ination when kept in open ponds (Wood, 1998).

The concentration of p-carotene accumulated in the cells of Dunaliella overcomes the traditional source of this pigment, and about 14% of the compound may be extracted.

Dunaliella is a eukaryotic green algae that grows in saline sites. Halophilic representatives of microalga have an osmotic mechanism that is different from halophilic bacteria. Dunaliella, which has no cell wall, can be developed with high salt concentration in the cytoplasm by the synthesis of glycerol. This microalga also responds to osmotic stress with the synthesis of glycerol if the high salinity is caused artificially by polyols.

The amount of glycerol produced by the microalga when exposed to saline stress is pro­portional to the concentration of NaCl in the culture.

Open systems can be simply categorized into natural waters (lakes, lagoons, ponds) and artificial ponds or containers. The most commonly used systems include shallow big ponds, tanks, circular ponds, and raceway ponds (Suh and Lee, 2003). Open ponds are much easier to construct and operate than most closed systems. However, major limitations in open ponds include poor light utilization by the cells, evaporative water losses, diffusion of CO2 to the atmosphere, and the requirement of large areas of land. The ponds are usually kept shallow to ensure sufficient light exposure for the microalgae because sunlight can penetrate the pond water to only a very limited depth. Furthermore, contamination by predators, alien microalgae species, and other fast-growing microorganisms restrict the commercial produc­tion of algae in open culture systems. In addition, due to inefficient stirring mechanisms in open cultivation systems, their gas transfer rates are relatively poorer than those of closed systems. All these limitations lead to lower biomass productivities for open systems com­pared with those of closed systems. Nevertheless, the simple operation and easy scale-up for mass cultivation make open systems the first-choice option for microalgae cultivation in industrial applications.

Centrifugation involves the application of centripetal acceleration to separate the microalgae from the culture medium (Harun et al., 2010) and is perhaps the fastest cell-recovery method based on density gradient. The centrifuge disks are easy to clean and sterilize, and centrifugation can be applied to any kind of microalga (Christenson and Sims, 2011).

Heasman et al. (2000) reported that 88-100% of centrifuged cells were viable and the col­lection efficiency was 95-100% at 13,000xg. However, the centrifuge has some disadvantages: The cells are exposed to a high gravitational force, which can alter the cell structure; the re­covery of fragile microalgae biomass requires low-speed centrifugation; the salt contained in the microalgal culture medium can cause rapid corrosion of equipment; and large-scale processes require costly equipment, such as continuous centrifuges (Pires et al., 2012).

The ability of microalgae to produce hydrogen was first reported by Gaffron and Rubin in 1942 (Gaffron and Rubin, 1942). However, the observed emission of hydrogen was transient and the amount was very minimal. In the late 1990s, Melis and co-workers demon­strated that sulfur deprivation changes cellular metabolism and allows algal culture to switch from aerobic photosynthetic growth to an anaerobic physiology state. Switching to anaerobic condition allows microalgal cultures to generate significant amounts of hydrogen for an extended period of time (Melis et al., 2000). This major breakthrough makes sustainable hydrogen production in a microalgal system a possibility. Over the years, extensive studies have been done to understand the physiology and metabolic adaptation resulting from sulfur depletion for better manipulation of biohydrogen production. Here our current understanding of hydrogen production in microalgae is highlighted and possible metabolic engineering/biotechnology strategies for improving hydrogen production are discussed.

Since 1940, the most widely used plastics have been polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(ethylene terephthalate) (PET), and poly(vinyl chloride) (PVC). Despite advances, plastics processing and manufacturing generate two major problems: the use of nonrenewable resources to obtain their raw materials and large quantities of waste generated for disposal.

Biodegradable plastics degrade completely within three to six months when attacked by microorganisms, depending on the environmental conditions. The polyhydroxyalkanoates (PHAs) are natural polyesters consisting of units of hydroxyalkanoic acids with similar prop­erties to petrochemical plastics (Jau et al., 2005).

The polyhydroxyalkanoates are produced as a reserve of carbon and energy accumulated within the cells of various microorganisms such as microalgae. Among the PHAs, polyhydroxybutyrate (PHB) and its copolymer polyhydroxybutyrate-co-valerate (PHB — HV) are synthesized by cyanobacteria when exposed to specific conditions of cultivation (Sharma et al., 2007).

The degradation rate of PHB and PHB-HV depends on many factors, some related to the environment, such as temperature, moisture, pH, and nutrient supply, and others related to the biopolymer itself, such as composition, crystallinity, additives, and surface area. Due to its physical and chemical properties, PHB is easily processed in equipment commonly used for polyolefins and synthetic plastics (Khanna and Srivastava, 2005).

The first photosynthetic microbe to be isolated and grown in pure culture was the fresh­water microalga Chlorella vulgaris. It is a spherical unicellular eukaryotic green algae that presents a thick cell wall (100-200 nm) as its main characteristic. This cell wall provides mechanical and chemical protection, and its relation to heavy metals resistance is reported, which explains why C. vulgaris is one of the most used microorganisms for waste treatment.

The uptake of carbon by C. vulgaris cells is done through the enzyme carbonic anhydrase, which catalyzes the hydration of CO2 to form HCO3 and a proton. Hirata and collaborators (1996) studied carbon dioxide fixation by this microalga, which showed important variations comparing cultivation under fluorescent lamps and sunlight. In the first case the estimated rate of carbon dioxide fixation was 865 mg CO2 L-1 d-1; in a sunlight regimen the estimated rate achieved 31.8 mg CO2 L-1 d-1. Winajarko et al. (2008) achieved a transferred rate of

441.6 g CO2 L-1 d-1 under the same cultivation conditions as Hirata et al. (1996). According to Sydney et al. (2011), in experiments using classic synthetic media and a 12-h light/dark regimen, C. vulgaris biofixation rate of carbon dioxide is near 250 mg L-1 day-1.

Carbon fixation by Chlorella vulgaris is variable and depends, among other factors, on the concentration of CO2 in the gaseous source. Yun et al (1997) cultivated C. vulgaris in 15% of carbon dioxide and achieved a fixation of 624 mg L-1 day-1; Scragg et al. (2002) achieved a fixation of 75 mg L-1 day-1 under CO2 concentration of 0.03%. In the same study, Scragg tested a medium with low nitrogen and the fixation rate was 45 mg L-1 day-1, suggesting that nitrogen also influences carbon uptake rate.

Some studies (Chinassamy et al., 2009; Morais and Costa, 2007) indicate that the best concentration of CO2 in the gas supplied to C. vulgaris growth is about 6%.

The efficiency of a photobioreactor depends on the integration of capture, transport, distri­bution, and use of light by the microalga through photosynthesis (Zijffers et al., 2008). The main feature of the photobioreactor that influences the exposure of microalgae to light is the surface/volume ratio. Some materials used for construction of reactors are glass, fiber­glass, Plexiglas, polyvinyl chloride (PVC), acrylic-PVC, and polyethilene (Wang et al., 2012).

The particularity of each of these materials should be individually evaluated prior to their application in the construction of photobioreactors. Glass is hard, transparent, and suitable for the construction of small-scale photobioreactors. However, this material requires many connections for the construction of large-scale photobioreactos, which increases the produc­tion cost. Fiberglass and PVC can be used in open ponds, where the light reaches the surface of the culture that is open, but cannot be used for tubular photobioreactors because they are nontransparent materials.

Another important feature of the photobioreactor’s building material is the ability to pre­vent the formation of biofilms. Biofilms are not difficult to clean, but they can dramatically reduce the transmission of light, even to the microalgae in open photobioreactors. In open ponds, biofilms may promote the contamination of crops. The photobioreactor should also be constructed to facilitate the control of operating parameters, not have a high cost of con­struction and operation, facilitate the harvesting of the biomass, and minimize power con­sumption during the process (Wang et al., 2012). The photobioreactor must allow the cultivation of several microalgal species.

The marked advantage of these open ponds is their simplicity, resulting in low production costs and low operating costs. Operation is very simple for this system, which only has a giant rotating mixer at the center of the pond to avoid the precipitation of algal biomass. Although this is indeed the simplest among all the microalgae cultivation techniques, it has a major drawback: The environment in and around the ponds is not completely under control. Bad weather conditions can stunt algae growth due to the lack of environment control. For example, high temperatures as well as insufficient or excessive sunlight intensities are critical factors affecting the efficiency of microalgae growth (Norsker et al., 2011). In addition, con­tamination from bacteria or other foreign microorganisms often results in the predominance of undesirable species over the desired algae growing in the pond. Rainy conditions are also a common contamination source, since the rain may flush down enormous microorganisms into the ponds from the air. Therefore, finding an appropriate cultivation location is crucial to the success of such open systems. Even though there could be many disadvantages with the simple pond system, the simple operation and the high scale-up availability of simple ponds are still very attractive factors and these ponds are often utilized for industrial production of microalgae (Borowitzk, 1999).