Table of Contents
1 BACKGROUND
1.1 What is Algal Bioenergy?
Algae are aquatic organisms that produce biomass by utilising light and carbon dioxide (CO2). Algae are divided into two groups: macroalgae and microalgae. The large, multicellular algae that are frequently seen growing in ponds are known as macroalgae, and they are measured in inches. These bigger algae have many different ways they can grow. Seaweed refers to the most prominent multicellular algae; an illustration is the enormous kelp plant, which may extend to more than 100 feet long. The microscopic, unicellular algae, known as microalgae, on the other hand, are measured in micrometres and often grow in suspension within a body of water.
Algae are the ‘engines’ that drive global biogeochemical cycles in the aquatic environment and are directly responsible for >50% of current planetary productivity and geologically for fossil fuel resources [1]. However, their use as a source of biofuels is minimal. The search for new forms of biomass for bioenergy production is, and will increasingly be, influenced by the politics of food, water supplies and anthropogenic climate change. Algae that are capable of high levels of productivity and grow in marine ecosystems, or land-based seawater systems, are one of the few realistic sources of future fuels that avoid compromising ecological sustainability and political stability. Technologies are being developed to cultivate and exploit macroalgae for biofuels, including bioethanol and biomethane, and it is envisaged that these will be a commercial reality soon. Microalgal systems present huge potential; however, faced with a range of technical constraints, they will take much longer to develop into a commercial reality.
Because of their relatively high oil content and rapid biomass synthesis, microalgae have long been recognised as possible viable sources for biofuel generation. When compared to terrestrial crops, microalgae develop much more quickly. Algal mass culture can be carried out on non-arable fields utilising non-potable saline water and wastewater. As a result, interest in using microalgae as a substitute for biodiesel biofuel feedstock is growing among academics, businesspeople, and the general public.
Algal biomass primarily consists of lipids/natural oils, proteins, and carbohydrates. Because the bulk of the natural oil made by microalgae is in the form of triacylglycerol, which is the right oil for producing biodiesel, microalgae are the exclusive focus in the algae-to-biodiesel arena. Microalgae can be used in various ways to produce energy in addition to biodiesel. Under specific growing circumstances, several algae species can generate hydrogen gas. Algal biomass can also be anaerobically digested to create methane biogas, which can be burned similarly to wood to produce heat and electricity. Algal biomass may also undergo pyrolysis treatment to have crude bio-oil.
1.2 A Brief Review
The concept of mass cultivation of algae is not new, with studies being undertaken in the 1940s and 1950s in Germany and the United States [2]. The US Department of Energy’s Office of Fuels Development funded Aquatic Species Programme, which ran from 1978 to 1996 and tried to develop the concept further by focusing on microalgal species with a high oil content suitable for biodiesel production. Unfortunately, this project was cancelled, in part due to low oil prices, but in the last ten years, the production of biodiesel from microalgae has again been taken seriously. This is partly because of the escalating cost of petroleum, linked to emerging concerns about anthropogenic global warming and environmental pollution associated with burning fossil fuels.
Most other biomass sources for biofuel need fertile land to be used for growing crops. Due to this rivalry, the amount of food crops that could be grown would constantly be constrained. Some of them are primarily used as food and livestock feed. Algae, on the other hand, thrive on non-arable land, avoiding competing for space with essential food crops. In addition, algae are relatively genetically varied, a sizable portion of their biomass is made up of lipids, and they are not overly particular about the type of water they grow in. The main supply of water and nutrients could alternatively be wastewater that has been tainted with fertilisers. Algal biomass can also be produced quite quickly – with some species having a doubling time of as little as 6 hours.
In an effort to boost the use of alternative fuels, governments all over the world began to subsidise bioethanol and biodiesel in the middle of the 2000s. Microalgae-based biofuels seemed like the ideal substitute due to their adaptability. Businesses worldwide shifted their focus to producing algae biofuels and made sweeping predictions about their potential. By the end of 2009, a company by the name of Algenol intended to produce 375 million litres per year in the Sonoran desert and 3.75 billion litres by the end of 2012. Another business by the name of PetroSun intended to build an algae farm with 445 hectares of saltwater ponds that, in theory, would yield 16 million litres of algal oil annually. Dozens of other companies made similar claims.
The only practicable methods of large-scale production of microalgae are open systems, raceway ponds and closed systems, such as tubular photobioreactors. However, irrespective of the production system envisaged, significant biological and chemical engineering constraints to the commercial-scale production of microalgal biofuels/biodiesel remain to be resolved.
The current forecasts of 3.8 billion litres of algae biofuel production are nothing more than a pipe dream because we are nowhere near producing 3.8 million gallons of fuel annually [3]. Most businesses working towards this objective have failed, and those still operating have switched to employing algae to create more valuable nutraceuticals, food items, colouring agents, cosmetics, etc. What went wrong with this technology that seemed so promising? To understand that, we need to look at the process of obtaining fuel from algae.
2 MICROALGAE BIOLOGY
2.1 Habitat, Growth Forms and the Evolution of the Algae
Writing a section on microalgae biology is a daunting task, given their incredible diversity. The following paragraphs only cover selected aspects of their biology and highlight particular features to illustrate microalgal diversity and evolution and how this is reflected in their biology. “Algae” is not a taxonomic term but a convenient and valuable collective common name for all the apparently primitive, plantlike organisms which contain chlorophyll a, carry out oxygenic photosynthesis (usually), and are not specialised land plants (the embryophytes) like mosses, ferns, coniferous trees, and flowering plants.
We will define the microalgae as microscopic eukaryotic, unicellular, colonial, and filamentous algae, as well as the oxygenic photosynthetic bacteria (prokaryotes) the cyanobacteria. Microalgae are placed in two domains/superkingdoms (Bacteria and Eukaryota) and, in the Eukaryota, in the “supergroups” Archaeplastida (Glaucophyta, Rhodophyceae, Chloroplastida), Chromalveolata (Cryptophyceae, Haptophyta, Stramenopiles, Alveolata), Rhizaria (Chlorachniophyta), and Excavata (Euglenozoa). The algae include organisms ranging from multicellular kelps, which can reach tens of meters in length, to microscopic unicells such as the cyanobacterium Synechococcus (0.5–1.5μm) and the green alga Ostreococcus tauri (0.5–1.3μm).
Microalgae grow much faster and possess high oil content compared with terrestrial crops, which take a season to sprout and only contain a maximum of about 5 per cent dry weight of oil [4]. They frequently grow by a factor of two every day. Some microalgae can double every three and a half hours during peak growth. Microalgae typically have an oil concentration of 20% to 50%, while certain strains can have an oil content of up to 80%. For this reason, the algae-to-biofuel industry is concentrating on microalgae.
Figure 1. Pictures of a variety of microalgae. (A) The colonial Pediastrum sp. in a sample of soil (Chlorophyta, Chlorophyceae, Sphaeropleales); (B) The unicellular Micrasterias radiosa (Charophyta, Conjugatophyceae, Desmidales). Note the central nucleus between the two identical halves of the cell, and the pyrenoids in the chloroplast (look like raised nodules); (C) Synura petersenii (Ochrophyta, Synuriophyceae, Synuraceae); (D) Aplanospore of the snow alga, Chlamydomonas sp. (Chlorophyta, Chlorophyceae, Chlamydomonadales)—note the ornamented cells wall and the high content of carotenoids; (E) Pseudogoniochloris tripus (Ochrophyta, Mischococcales, Pleurochloridaceae); (F). Cryptomonas sp. (Cryptophyta, Cryptophyceae, Cryptomonadales). Note the patterned region near the centre of the cells. Source: https://doi.org/10.1016/B978-0-12-811405-6.00003-7
Algae are vital members of the biota. They play an essential role in the global carbon cycle, using light energy to produce the organic matter which supports all life on the earth. Algal photosynthesis contributes around 50% of the approximately 11–117PgC assimilated into organic matter annually [5]. Most of this inorganic carbon is fixed by the microalgae in the open ocean, which occupies about 70% of the world’s surface area. Concurrent with CO2 fixation, they also produce O2 (=oxygenic photosynthesis).
Microalgae also have an important role in global nitrogen cycles. All heterocystous cyanobacteria and many nonheterocystous ones fix N2 into organic nitrogen and are the major contributors to global biological nitrogen fixation. A single genus, the nonheterocystous marine planktonic cyanobacterium, Trichodesmium, is responsible for a significant part of the total global nitrogen fixation. Nitrogen-fixing cyanobacteria directly contribute to global food production, especially rice.
Microalgae are a key component of aquatic food webs in their role as primary producers and play several roles in global geochemical cycles. Microalgae have many industrial applications as well. In aquaculture, microalgae are essential feeds for the larvae and juveniles of crustaceans, molluscs and fish and as feed throughout the life cycle for bivalve molluscs. Several species are grown on a large scale and used as nutritional supplements (mainly Chlorella and Arthrospira) and as sources of high-value fine chemicals such as β-carotene, astaxanthin, and long-chain polyunsaturated fatty acids.
Microalgae can be found in almost all environments and habitats on earth, from the polar regions to the equator (freshwater, seawater, salt lakes, soil, rocks, trees, etc.). They may be free floating (planktonic) or grow attached to substrates such as plants and macroalgae (epiphytic), rocks (epilithic) or other hard substrata, sand grains (episammic), etc. Microalgae may also form mats on sediment surfaces (epipelic). Microalgae have a diverse range of morphologies and sizes, the smallest being the marine green alga Ostreococcus tauri, with an average cell size of 0.8μm. Microalgae may be unicellular or multicellular, or colonial. Fig. 1 illustrates some of the structural diversity of the microalgae. Some are nonmotile, whereas others move using flagella or other means. The flagella of algae have a generally consistent internal microtubular structure but show great variation in external morphology, mode of insertion, and root systems.
Microalgae include some of the most extremophilic organisms on earth. For example, the green alga Dunaliella salina can grow at salinities from about 1.5× seawater to NaCl saturation (∼5M NaCl=∼11× seawater salinity) and over an extensive temperature range from Haptophyta The haptophytes are characterised by having not only flagella but also a flagella-like organelle, the haptonema, which has a role in food capture and transport to the cell for phagocytosis. They are also characterised by having organic body scales. One haptophyte family, the Coccolithophyceae, also has calcified scales, the coccoliths. The form of the coccoliths is very species-specific. Several bloom-forming species are found in this group [6]. The calcified marine unicell, Emiliania huxleyi, is the most intensively studied member of the coccolithophorid microalgae because (1) it is easily cultured and (2) it is the most abundant coccolithophore species in the ocean and can form massive blooms in temperate and subpolar regions, producing up to 108 cells L−1. Together with other ecologically significant species, the coccolithophores contribute up to half of the ∼1.6Pgy−1 of CaCO3 produced in the pelagic zone. Coccolithophores influence surface-ocean biogeochemistry by fixing a considerable amount of C through photosynthesis (the biological C pump) and by releasing CO2 during coccolith formation (the carbonate counter-pump). Coccolithophores also contribute to global S cycling through their production of dimethylsulfoniopropionate. An exciting aspect of E. huxleyi is that this species harbours a genome constituted by core genes plus genes distributed variably amongst strains, which support considerable intraspecies variability. This enables E. huxleyi to form large seasonal blooms in temperate waters and subpolar regions under various environmental conditions. Aside from their importance in the oceans and global biogeochemical cycles, coccolithophorids such as E. huxleyi also have potential industrial applications as carbon sinks for CO2 bioremediation, as sources of lipids for nutrition or biofuels or as sources of microfine CaCO3. Eustigmatophyceae Eustigmatophytes are a distinct lineage of ochrophyte (stramenopile) algae with a relatively small number (∼30) of described species. Eustigmatophytes are distinguished from other ochrophytes by a suite of cytological features (not all are necessarily present in all taxa): a pigmented lipid body (reddish globule), a swelling at the base of the anterior flagellum associated with an extraplastidial stigma (eyespot), lamellate vesicles (with a putative reserve product), and plastids without a girdle lamella and lacking continuity with the nuclear envelope. Also characteristic is the lack of chlorophyll c and violaxanthin as the dominant xanthophyll. Reproduction occurs primarily via autosporogenesis, but many members of this class form zoospores with an anterior mastigoneme-bearing flagellum and a (sometimes missing) posterior bare flagellum. Sexual reproduction has not been directly observed, but genomic evidence suggests its presence in some species. Several species are easy to culture and accumulate large amounts of lipids, including polyunsaturated fatty acids making the eustigmatophytes of particular interest for potential commercial applications, such as the production of biofuels, pigments, and long-chain fatty acids [7]. Species of Nannochloropsis have become very popular for studies of lipid production for biofuels and/or the production of long-chain polyunsaturated fatty acids, and attempts are underway to commercialise this alga. Nannochloropsis species are unicellular, planktonic, with either 2–4μm diameter subspherical or 3–4×1.5μm cylindrical cells. They contain a yellow-green chloroplast with the main pigments being chlorophyll a and the xanthophylls, violaxanthin and vaucheriaxanthin. Botryococcus braunii (Chlorophyta, Trebouxiophyceae, Botrycoccaceae) This colonial green microalga is found in freshwater and brackish ponds and lakes around the world, where it often can be found in large floating masses. It has been widely studied ever since the discovery that this alga accumulates long-chain hydrocarbons and ether lipids, similar in many ways to crude oil, in its extracellular matrix. It is these hydrocarbons that reduce the density of the colonies, thus leading to their floating on the water’s surface. Currently, 17 species of Botryococcus are recognised, but B. braunii remains the most widely studied. Strains of B. braunii differ in the types of hydrocarbons they synthesise and are classified into races. Of these, the Race B species is of particular interest due to its ability to accumulate up to 86% of dry-weight botryococcenes and methylsqualenes, both of which can be readily converted to biofuels [8]. B. braunii strains usually grow as different-sized colonies, although in some cultures, they may also occur almost exclusively as single cells. Relatively little is known of the life cycle of Botryococcus. Tetraselmis spp. (Chlorophyta, Chlorodendrophyceae, Chlorodendraceae) Tetraselmis spp. (earlier name Platymonas) are unicellular flagellates with elliptical or almost spherical, slightly flattened cells with an invagination at the anterior end from which arise four equal flagella in two opposite pairs. The cells are surrounded by close-fitting theca of fused organic scales. The stellate scales which make up the theca are produced in the Golgi and then secreted to the exterior of the cell. The flagella are covered by square/diamond-shaped scales in 24 rows, overlaid by 24 double rows of scales. Two rows of hair-shaped scales project from opposite sides of the flagella. Asexual division occurs in the nonmotile stage within the parental periplast, and sexual reproduction has never been observed. Vegetative thick-walled cysts are known in several species, and these germinate by division into four cells. Marine/euryhaline and freshwater species are known. Some species of Tetraselmis occur in plankton; others are benthic, colonising sand, and a few occur as endosymbionts in metazoans, for example, in the acoel turbellarian flatworm Symsagittifera (=Convoluta) (Serôdio et al., 2011). Tetraselmis is easy to culture, and several species, such as Tetraselmis chui, Tetraselmis suecica, and Tetraselmis tetrahele, are widely used as a feed in aquaculture (De Pauw and Persoone, 1988). More recently, euryhaline strains of Tetraselmis, which can grow over a very wide salinity range, have gained interest as potential, sustainable sources of lipids for biofuels Photobioreactors (PBRs) are robust cultivation systems that employ effective cultivation techniques providing all the essential elements, i.e., light, nutrients, temperature and mixing, for the healthy growth of microalgae. The operating conditions are monitored, regulated, and controlled for the high yield of algal biomass [9]. Figure 2. Diagram of a flat-plate photobioreactor. Source: https://www.researchgate.net/publication/336349730_Application_of_Microalgae_in_Biodiesel_Production Microalgae can go through different metabolisms like autotrophic, heterotrophic and mixotrophic. Autotrophic culture is the most common mode of microalgae cultivation. In this cultivation process, microalgae directly convert the inorganic carbon present into organic matter through the photosynthetic process. Most microalgal cells efficiently harness solar energy and utilise CO2 as the carbon source, contributing to the abatement of CO2. Hence autotrophic mode is considered an environmentally sustainable and economical mode of microalgae cultivation. In the heterotrophic mode of cultivation, microalgae use organic carbon for their growth and do not require light. Therefore acetate, glycerol, and volatile fatty acids (VFAs) may be utilised as organic carbon sources for biomass synthesis. In heterotrophic cultivation, the biomass yield is better than in autotrophic cultivation, but the organic carbon is expensive compared to CO2. Accordingly, organic carbon must be utilised from wastewater to overcome this limitation. Mixotrophic cultivation is to counter the limitations of autotrophic and heterotrophic cultivation modes. So, in this mode of cultivation, microalgae utilise inorganic carbon through photosynthesis and organic carbon. The mixotrophic cultivation mode is divided into two stages. In the first stage, the growth is heterotrophic, and the microalgae cells consume organic carbon present in the medium; when it is consumed up to a specific limit, autotrophic growth is induced, and organic carbon assimilation starts taking place. The selection and design of PBRs for microalgae cultivation depends upon various factors such as mixing strategy, orientation, surface area to volume ratio (S/V), illumination, air/CO2 supply, accumulation of oxygen and temperature. PBRs can be closed, or open systems having various advantages and disadvantages but closed PBRs have the upper hand due to better control of the operational parameters. The prospects and limitations of both types of systems are summarised in Table 1. Table 1. Advantages and disadvantages of open and close PBRs. Source: https://iopscience.iop.org/article/10.1088/1757-899X/1142/1/012004 It is almost impossible to decouple the retention time of microalgae from the dilution rate in an ordinary PBR merely using settling, but this can be achieved by using a membrane photobioreactor. Using MPBR serves two purposes, i.e., they help to hold the biomass inside the reactor (preventing washout) and assist CO2 transfer. MPBR provides complete retention of microalgae cells while the medium passes through (as permeate), thereby enhancing biomass concentration in the bioreactor. The biomass concentration can be better controlled by partly returning the retentate. As the biomass is concentrated in the retentate stream, the permeate can be reused as a feed medium. The membrane will act to separate solid and liquid, thereby helping to isolate the microalgal cells from the effluent. Furthermore, the bioreactor’s hydraulic retention time and solids retention time can be controlled independently during the culture, and the microalgal biomass concentration is not affected by the hydraulic loading and the growth of algal cells. Consequently, improved performance in terms of nutrients/pollutant removal and algal biomass productivity can be achieved in membrane PBRs. Compared with the open pond, major advantages of the PBR include more facile control of the operating parameters, algal species, less contamination, higher biomass concentrations in the broth, and higher oil productivity. However, issues associated with PBR include oxygen removal and reactor overheating. As a by-product of photosynthesis from algae, oxygen can inhibit the photosynthesis process at concentrations beyond air saturation. To overcome the overheating issue, cooling water spray may be necessary, and the water consumption associated can be significant. The most diffused shape for algae ponds is the raceway pond. Raceway Ponds (RWPs) typically represent “open systems,” that is, reactors in which the culture is in direct contact with the external environment (sun radiation, wind, contaminants, etc.). No devices are interposed between algae and the environment; culture temperature is usually not controlled. RWPs are made by a closed-loop channel in which propulsion systems allow algae mixing and transport. RWPs equipped with paddle wheels have been used since the 1950s and, still today, represent the most common case. In open ponds, propulsion systems must be used to move water along the channel, promoting mixing in this way [10]. Algae are maintained in suspension, and flow is mixed through the paddle wheel and the curves (bends) to allow for CO2 and nutrient distribution and ensure light-dark cycles for the microorganisms. These systems are characterised by a flat area in which the culture medium is confined by lateral walls. The raceway shape is usually stretched in long straight channels connected by two bends. After a curve, or in other positions, a paddle wheel is placed. The typical ratio between length and width is up to 20 (Figure 3). Figure 3. Diagram of an open raceway pond. Source: https://www.researchgate.net/publication/336349730_Application_of_Microalgae_in_Biodiesel_Production Currently, photobioreactors and open ponds/raceways are the two major systems for algae growth, both of which have their own strength and weakness. The advantages of open ponds include lower capital and operational costs and thus are more widely used than the PBR. RWPs are generally considered the cheapest and easiest way to develop and operate large-scale production of microalgae. For future microalgal-based biofuels, whether lipid- or biomass-based photoautotrophic, or photosynthetic, growth is extremely attractive as it removes costs associated with the provision of a carbon-based substrate. Many algal species can utilise a significant fraction of solar energy (up to 10% of the total solar energy can be fixed into biomass) through the process of photosynthesis. Like any other plant, they require CO2 and nutrients to grow. This is coupled with the fact that they can exploit a variety of habitats, everything from brackish water to hypersaline environments, which are not favourable for the growth of terrestrial biomass. In addition, they can sequester nutrients from effluent/wastewater streams, thus removing/minimising competition for potable/irrigation water and conventional chemically derived fertilisers. Furthermore, algae cultivation could be directly coupled to waste CO2 generated as an industrial by-product. If employed on a large scale, they could contribute to stripping greenhouse gas from the atmosphere, potentially generating ‘carbon credits’. Triglycerides (TAGs) lack a charge and form the majority of microalgal-neutral lipids. Phospholipids and glycolipids comprise most of the polar lipids, where glycerides in one or more of the fatty acids have been replaced with a polar group. The algal TAGs have been a major focus for biofuel production because they can be transformed into low viscosity and low melting point esters (methyl esters), low sulphur diesel substitutes (biodiesel) or catalytically converted to hydrocarbons as gasoline substitutes. When algal cells are actively growing, most lipids are in the form of glycolipids and phospholipids, but once the cells enter a less active phase of growth, the overall lipid profile begins to change; TAGs start to get stored and become the dominant lipid. Figure 4. TAG synthesis in algae. Source: https://doi.org/10.1002/9780470015902.a0023715 Figure 4 briefly outlines the pathways involved in algal TAG synthesis. Initially, lipid biosynthesis occurs in the chloroplast and fatty acids are produced, which act as precursors for membrane lipids and storage lipids in the form of TAGs. The first step in the pathway is the conversion of acetyl CoA and CO2 to malonyl CoA through a two-step process. The malonyl CoA is considered the key carbon donor for subsequent steps in the pathway. The enzymes catalysing the pathway’s final steps influence the type of lipids produced. Microalgae can be induced to produce as much as 60% of their biomass in extractable oils by limiting the nutrient components of the medium or by varying other environmental conditions (light, salinity or temperature). Under stress conditions, the photosynthetic membrane is rapidly degraded within the algal cell, and TAGs are accumulated in the cytoplasm in oil-rich bodies. Like in other plants, nitrogen availability during growth has been shown to affect an organism’s lipid content and composition. The key to lipid accumulation lies in allowing the nitrogen supplied to the culture to become exhausted. Carbon continues to assimilate via photosynthesis because nitrogen is no longer available; the cell cannot produce proteins to support growth, and the fixed carbon is converted into storage lipids. For instance, the fatty acid content of the oleaginous alga Parietochloris incisa increases, following nitrogen starvation, from 16% to 36% (of dry weight). There are several critical challenges to achieving high TAG accumulation within microalgae, including strain selection and productivity levels. Cell selection using flow cytometry has demonstrated that it is possible to select one overproducing algal cell in a population of 10 000 cells (Cooksey et al., 1987). Alternatively, it may be necessary to produce mutants from ‘wild-type’ strains to generate high-producing cells. Productivity levels remain the key to success, with high cell densities affecting the cost of harvesting and the ‘footprint’ of any biofuel production system. Although there have been significant developments and high-profile success in this sector, in reality, biodiesel has not yet been produced on a large scale from microalgae. To date, most successful large-scale production systems have focused on algae capable of growing in extreme conditions in open pond systems, for example, Arthrospira/ Spirulina for dietary supplements cultivated at high pH and Dunaliella salina for b-carotene production at high salinity. The compounds these algae are grown for are produced at relatively low yields, but their production is commercially viable because they are valuable. For biofuels generated from microalgal biomass to be economically successful, it has been estimated that a dry-weight biomass yield of 100 t ha yr21 would be needed [11]. Yields of 60 t ha yr21 have already been reported for Pleurochrysis carterae and D. salina in field-scale production systems, so this should be technically achievable. Irrespective of the crop used, any large-scale replacement of conventional oil-based fuels requires a vast area. With their high growth rates, algae could be substantially more productive than their alternatives, such as oil palm, which produces 4–5 t ha yr-1. To date, there are few commercial-scale producers of algal biofuel. Major challenges in the commercialisation of algal biofuel production include nutrients and carbon source supply, pond contamination, low harvesting and extraction efficiencies [12]. In recent years, water has been increasingly regarded as one of the most critical factors affecting the sustainability of algal biofuel development. It may become a limiting factor for the up-scaling of algae growth, especially when the algae biofuel process relies heavily on freshwater input. Because water is one of the most critical parameters in the sustainable development of algae, it is essential to clearly understand water consumption of the algal biofuel process, especially in regions with limited water availability. The highest water consumption rate occurs at the algae growth stage for the open pond systems. This is consistent with those of ethanol and soybean biodiesel in that the biomass growth stages have much higher water consumption rates. Open ponds tend to have much higher water consumption (216 to 2000 gal/gal) than the PBR (94 to 272 litres/l; 3790 litres/l if considering the evaporation of cooling water added to maintain the reactor temperature). Water conservation technologies are highly process-dependent and can occur at various algal biofuel stages. Unlike biomass growth stages of the ethanol and biodiesel processes, technologies for algae biofuel process are still undergoing significant development and improvement, which can affect water consumption rates in the future. An additional long-term goal is the production of hydrogen by microalgae [13]. Both eukaryotic microalgae and cyanobacteria are capable of producing hydrogen through direct or indirect biophotolysis. For direct biophotolysis to take place, water-splitting and ferredoxin-reducing reactions of the photosynthetic pathway are coupled to a hydrogenase, resulting in electron transfer from water to protons to produce H2. This process is very short-lived and is inhibited by the accumulation of oxygen. Currently, producing hydrogen from microalgae is not economically viable, as the conversion rates of gas evolution need to be improved from approximately 1% to at least 5%. One option in the longer term is to couple H2 production with a biorefinery approach, where more than one saleable product is generated. Advances in systems biology and bioengineering to increase hydrogen generation rates will help further improve the efficiencies of the biological energy generation process. The focus of most R&D efforts on algal biofuels has been on oil-producing algae, but all the components of microalgae have the potential to generate biofuels. Figure 5. Facilities at the University of Valencia are supported by the Climate-KIC‘s Microalgae Biorefinery 2.0 project. Source: https://algaerefinery.eu/ Algal biomass not only contains oils; indeed, many algal taxa contain very low lipid levels and have significant quantities of proteins, carbohydrates and other nutrients. The residual biomass after biodiesel production, or alternatively highly productive nonoleagenous algae, has the potential to be used as feedstock to produce methane by anaerobic digestion (AD), alcohol or even burnt to produce energy. A key strategy to make the economics more attractive will be integrating energy production with other processes, that is, a biorefinery approach where the source of biomass being processed will generate not one but several different products, including bioenergy. For example, macroalgal biomass subjected to AD produces VFAs, such as acetic and lactic acids, which can potentially be used as novel building blocks for chemicals and fuels. The rapid advances in genomics may well provide the building blocks for the future exploitation of microalgae. Much emphasis has been placed on the enhancement of the productivity of lipids. However, a more recent exciting development has been the genetic engineering of Cyanobacteria to produce ethanol, butanol and isoprene. The question has to be asked whether this technology can be expanded to include other microalgal species. [1] Stanley, M. S., & Day, J. G. (2014). Algal bioenergy. eLS. [2] https://imperialbiosciencereview.com/2020/09/04/the-past-present-and-future-of-algal-biofuels/ [3] Duan, D., 2019, Algae Biofuel–What Happened After the Hype?, viewed 29 August 2020, https://www.labroots.com/trending/chemistry-and-physics/14258/algae-biofuel-what-happened-hype [4] Chisti, Y. 2007. Biodiesel from microalgae. Biotechnology Advances 25:294-306. [5] Borowitzka, M. A. (2018). Biology of microalgae. In Microalgae in health and disease prevention (pp. 23-72). Academic Press. [6] https://hab.whoi.edu/impacts/impacts-golden-algae/ [7] Pilátová, J. (2013). The Potential Use of the Eustigmatophyceae In the Production of Biofuels. [8] Fon-Sing, S., & Borowitzka, M. A. (2016). Isolation and screening of euryhaline Tetraselmis spp. suitable for large-scale outdoor culture in hypersaline media for biofuels. Journal of applied phycology, 28(1), 1-14. [9] Ahmad, I., Abdullah, N., Koji, I., Yuzir, A., & Muhammad, S. E. (2021, April). Evolution of Photobioreactors: A Review based on Microalgal Perspective. In IOP Conference Series: Materials Science and Engineering (Vol. 1142, No. 1, p. 012004). IOP Publishing. [10] https://www.nrel.gov/docs/legosti/old/2840.pdf [11] Carlsson A, Beilen van J, Mo¨ller R, Clayton D and Bowles De (2007) Micro- and Macroalgae – Utility for Industrial Applications Bioproducts, E. R. t. E. P.o. S. R.-. and Crops, f. N.-f., CNAP, pp. 4–9. Berkshire, UK: CPL Press. [12] Tu, Q., Lu, M., Thiansathit, W., & Keener, T. C. (2016). Review of water consumption and water conservation technologies in the algal biofuel process. Water Environment Research, 88(1), 21-28. [13] https://www.jpost.com/environment-and-climate-change/article-7038113 MICROALGAL CULTIVATION TECHNOLOGIES
3.1 PBRs
3.2 Open Ponds
4 PRODUCTION TAKEAWAYS
4.1 The Potential of Microalgal-Derived Biofuels
4.2 Potential and Actual Yields
4.3 Production Challenges
5 REFERENCES
