PEER-REVIEWED REVIEW ARTICLE bioresources.com Potential of the Micro and Macro Algae for Biofuel Production: A Brief Review Renganathan Rajkumar,* Zahira Yaakob, and Mohd Sobri Takriff The world seems to be raising its energy needs owing to an expanding population and people’s desire for higher living standards. Diversification biofuel sources have become an important energy issue in recent times. Among the various resources, algal biomass has received much attention in the recent years due to its relatively high growth rate, its vast potential to reduce greenhouse gas (GHG) emissions and climate change, and their ability to store high amounts of lipids and carbohydrates. These versatile organisms can also be used for the production of biofuel. In this review, sustainability and the viability of algae as an up-coming biofuel feedstock have been discussed. Additionally, this review offers an overview of the status of biofuel production through algal biomass and progress made so far in this area. Keywords: Microalgae; Macroalgae; Biomass; Lipid; Biofuel; Oil production; Bioconversion; Algaculture; Wastewater treatment; Malaysia Contact information: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, University Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia; * Corresponding author: [email protected] INTRODUCTION The energy requirements of the global community are rising year by year. Currently, fossil fuels are a prominent source of transportation fuels and energy. The world’s demand for oil is expected to rise 60% from the current level by 2025 (Khan et al. 2009). In view of the increasing oil demand and the depleting oil reserves, development of innovative techniques for the production of biofuels from novel renewable biomass feedstock sources are gaining importance all over the world. Production of biofuels from traditional agricultural crops such as corn, oil palms, and soybeans using arable lands and fresh water will greatly impact food production. Biomass, whether terrestrial or aquatic, is considered a renewable energy source. Relative to alternative energy sources, the aquatic biomass represents the strategy that is most ready to be executed on a large scale without any economic or environmental penalty (Aresta et al. 2005). Among these, algae are endowed with a unique adaptability to grow in diverse habitats, either in marine or fresh waters (IEA Report 1994). In the past, research mainly focused on their usage as food, animal feed, bio-fertilizer, and in aquaculture. Algae have received a great deal of attention as a novel biomass source for the generation of renewable energy. Apart from other biomass sources, algae contains a high biomass yield per unit of light and area, can have a lot of starch or oil content, does not require fresh water or agricultural land, and the requirements for nutrients can be fulfilled by either wastewater or seawater. Algae produce an array of organic molecules, particularly carbohydrates and lipids. These biomolecules can be used to extract a fuel Rajkumar et al. (2014). “Algal biofuel production,” BioResources 9(1), Pg # to be added 1 PEER-REVIEWED REVIEW ARTICLE bioresources.com known as biofuel. Algae are both unicellular and multicellular autotrophic aquatic life forms. Microalgae can provide several different kinds of renewable biofuel. These include methane produced by the anaerobic digestion of the algal biomass (Spolaore et al. 2006), biodiesel synthesized from the micro algal oil (Thomas 2006), and biohydrogen produced by a photobiological mechanism (Gavrilescu and Chisti 2005). The idea of producing microalgal biofuel is not a new one (Kapdan and Kargi 2006), but it is now being viewed seriously in view of the increasing price of petroleum. Serious interest is also motivated by concern about global warming that is associated with the use of fossil fuels (Sawayama et al. 1995). Macroalgae are generally fast growing and are able to reach sizes up to 60 m in length (McHugh 2003). Growth rates of macroalgae far exceed those of terrestrial plants. For example, brown algae biomass of the average productivity was approximately 3.3 to 11.3 kg dry weight m−2 yr−1 for non-cultured algae and up to 13.1 kg dry weight m−2 over 7 month for cultured algae compared with 6.1 to 9.5 kg fresh weight m−2 yr−1 for sugar cane, a most productive land plant (Kraan 2010). They are seasonally available in the natural water basins. Cultivation of macroalgae at sea, which does not require arable land and fertilizer, offers a possible solution to the energy crisis. Macroalgae are mainly utilized for the production of food and the extraction of hydrocolloids, and it is possible to produce ethanol from algae (Goh and Lee 2010). Macroalgal biomass contains high amounts of sugars (at least 50%), which can be used in ethanol fuel production (Wi et al. 2009). This review explores the opportunities for energy products, encompassing both fresh and marine habitat macro- and microalgae. This paper also discusses the variety of algal resources and their environment, along with the manufacture systems that have been demonstrated for use, as well as algal mass cultivation. BACKGROUND History and the Prospects of Research for Algal Biofuel Production Biofuel production and the environment have been crucial issues in today’s world. Several researchers have described the need for biofuels and the kinds of materials that can serve this purpose (Naika et al. 2010; Antoni et al. 2007). Based on productivity per unit area, algae constitute one of the most effective raw materials that could be exploited for the biofuel production. Algal biomass is capable of producing a host of end products including energy, chemicals, food, cosmetics, fertilizer, and agents for wastewater treatment and/or CO2 sequestration. This could reduce production costs, since there would be a variety of products to serve as sources of revenue, as the cost-effectiveness of these is crucial for the economic and commercial viability of these algal products. Algal biomass can be used as raw material for biofuel production via pyrolysis (bio-oil), or for bio-gas and bio-ethanol generation through fermentation. Macro and micro algae for bioenergy production should satisfy several criteria as listed below (Carlsson et al. 2007): i) they should be highly productive; ii) they should be easily harvestable; iii) they should be able to withstand water currents in the open ocean; and iv) they should be produced at a cost that is equal or lower than the other available sources. Scientific research has been started on the utilization of the various species of algae in waste water/seawater treatment in order to transform them into biofuels by means of Rajkumar et al. (2014). “Algal biofuel production,” BioResources 9(1), Pg # to be added 2 PEER-REVIEWED REVIEW ARTICLE bioresources.com various technological processes ranging from the esterification to anaerobic digestion (Kraan 2010). Macroalgae Macroalgae constitute the most important component in the marine ecosystems that serve for the marine bioresources preservation by preventing eutrophication and pollution (Notoya 2010). Macroalgae belong to the lower plants, in that they do not have roots, stems, and leaves. Instead, they are composed of a thallus (leaf-like) and sometimes a stem and a foot. Some species enclose gas-filled structures to help in buoyancy. They can grow very fast and in sizes of up to tens of meters in length (Luning and Pang 2003). Macroalgae differ in various aspects, such as morphology, longevity, and ecophysiology. Based on their pigmentation, they are classified into Phaeophyta (brown), Rhodophyta (red), and Chlorophyta (green) algae (Chan et al. 2006). In their natural environment, macro-algae grow on rocky substrates and form stable, multi- layered, perennial vegetation, capturing almost all available photons. Approximately 200 species of macroalgae are used worldwide, about ten of which are intensively cultivated, such as the Phaeophyta, Laminaria japonica and Undaria pinnatifida, the Rhodophyta, Eucheuma, Gracilaria, Porphyra and Kappaphycus, and the Chlorophyta, Enteromorpha and Monostroma (Luning and Pang 2003). Figure 1 shows examples of some commercially exploited macroalgae. A B C D Fig. 1. Some commercially exploited macroalgae A) Gracilaria dura; B) Acanthophora spicifera; C) Hypnea esperi; D) Padina pavonica The world production of macroalgae reached 8 million tons in 2003 (McHugh 2003). Many countries have now embarked on establishing large scale macroalgae Rajkumar et al. (2014). “Algal biofuel production,” BioResources 9(1), Pg # to be added 3 PEER-REVIEWED REVIEW ARTICLE bioresources.com cultivation in their territories. Recent research (www.unbsj.ca/sase/biology/chopinlab) has shown the potential of macroalgae for large-scale culture in the Atlantic waters of Canada, France (Kaas 2006), Germany (Buck and Buchholz 2004), Ireland (Kraan et al. 2000), Isle of Man, UK (Kain et al. 1990), and Spain (Peteiro and Freire 2009). In Asian countries such as China, India, Philippines, South and North Korea, Indonesia, and Japan, macroalgae is being cultivated for various needs such as food, feed, chemicals, cosmetics, and pharmaceutical products (Carlsson et al. 2007). Importance of Macroalgal Biomass With substantial processing required for fossil fuels and the higher cost of vegetable oils, there has been a great deal of interest in the algal culture. Apart from that, algal biofuel production presents the following advantages: 1. Production of biofuel from the macroalgae cultivation in seawater is a new approach, since 70% of the earth’s surface is covered by water. Macroalgae possess a unique life cycle. They are more productive in view of the fact that more than five harvests can be made in a year. 2. In addition, macroalgae can succeed in salty water with only sunlight and available nutrients from the seawater. They do not need any chemical fertilizer. Thus, large amounts of energy and money could be saved. These characteristic features favor the sustainability of the production of macroalgae-based bioethanol. 3.