Microalgae for biofuel production

Abstract

Microalgae have been used commercially since the 1950s and 1960s, particularly in the Far East for human health foods and in the United States for wastewater treatment. Initial attempts to produce bulk chemicals such as biofuels from microalgae were not successful, despite commercially favorable conditions during the 1970s oil crisis. However, research initiatives at this time, many using extremophilic microalgae and cyanobacteria (e.g., Dunaliella and Spirulina), did solve many problems and clearly identified biomass productivity and harvesting as the two main constraints stopping microalgae producing bulk chemicals, such as biofuels, on a large scale. In response to the growing unease around global warming, induced by anthropogenic CO₂ emissions, microalgae were again suggested as a carbon neutral process to produce biofuels. This recent phase of microalgae biofuels research can be thought to have started around 2007, when a very highly cited review by Chisti was published. Since 2007, a large body of scientific publications have appeared on all aspects of microalgae biotechnology, but with a clear emphasis on neutral lipid (triacylglycerol) synthesis and the use of neutral lipids as precursors for biodiesel production. In this review, the key research on microalgal biotechnology that took place prior to 2007 will be summarized and then the research trends post 2007 will be examined emphasizing the research into producing biodiesel from microalgae.

Introduction

One of the major themes of the 21st century to date is the need to replace fossil fuels with fuels based on renewable energy to mitigate the rise in atmospheric CO₂, which is a major component in anthropogenic global warming. Renewable energy requires the use of biomass produced from recently fixed CO₂. The term “fixed CO₂” refers to CO₂ absorbed by a photosynthetic organism (for oxygenic photosynthesis this is a plant, alga or cyanobacterium) and converted into sugar with oxygen as a by-product. The CO₂ fixation reaction is shown below:

$${\mathrm{6CO}}_{\mathrm{2}}+\mathrm{12}\mathrm{NADPH}+{\mathrm{H}}^{+}+\mathrm{18}\mathrm{ATP}+\mathrm{18}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to {\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{12}}{\mathrm{O}}_{\mathrm{6}}+{\mathrm{6H}}_{\mathrm{2}}\mathrm{O}+\mathrm{12}{\mathrm{NADP}}^{+}+\mathrm{18}\mathrm{ADP}+{\mathrm{P}}_{\mathrm{i}}$$

ATP and NADPH production is via electron transport driven by light energy absorbed by photosystems 1 and 2. The fixation of CO₂ to sugars (C₆H₁₂O₆) takes place in the Calvin cycle with the key CO₂ fixation reaction catalyzed by ribulose bisphosphate carboxylase (Rubisco). Utilizing light energy to fix CO₂ into biomass and then using the biomass (or components of the biomass) as a fuel can potentially lead to a carbon neutral fuel. The term “recently” indicates that the biomass has been grown in the recent past and this is to distinguish it from fossil fuel, in which the biomass was produced several 100 million years ago.

First-generation biofuels utilized crop plants as very well-established sources of recently produced biomass. The Brazilian model, set up originally in the 1970s, uses sugar cane waste as the source of its feedstock to produce bioethanol (Goldemberg, 2007). More recently, both the United States and Europe attempted to copy the Brazilian model, but using crops such as wheat as the biomass source. This led to a “food vs fuel” debate and claims that turning arable land to fuel production was increasing food prices (Rosillocalle & Hall, 1987). In response, second-generation biofuels utilize lignocellulose (inedible to humans) waste from agricultural crops or use non-crop plants grown specifically for lignocellulose such as switchgrass or Miscanthus. In either case, there is no direct competition with food crops, but to produce fuel from the chemically recalcitrant lignocellulose is an expensive process due to heating the biomass in the presence of acids—costly and environmentally unfriendly (Himmel et al., 2007). This brings us to third-generation biofuels based on microalgal biomass and the subject of this review. Microalgae do not compete for agricultural land and their simple morphology (single cells or filaments) makes extraction of fuel precursors easier and more environmentally friendly than lignocellulose. Microalgae in the oceans are responsible for over 45% of global CO₂ fixation (Falkowski et al., 2004) and this makes them very good candidates to produce biofuels that are carbon neutral.

During the oil crisis of the 1970s, which kick started the Brazilian first-generation biofuel industry, attempts were made to utilize microalgae for biofuel production. However, the modern era of microalgal biofuels began with the review by Chisti published in 2007 in Biotechnology Advances. This highly cited review article (5081 citations on Web of Science at 3rd September 2019) stated the case for using microalgae as a source of biodiesel that could replace fossil fuel diesel (Chisti, 2007). As noted above, this was not a new idea, but the Chisti review was comprehensive and was published at a time when global environmental concerns about greenhouse gases were recognized by the intergovernmental panel on climate change (Metz, Davidson, Bosch, Dave, & Meyer, 2007). The organization of this review will be to treat the Chisti review as a “before and after” marker. The first section will examine the literature prior to 2007 and then the following section will look at the progress made since 2007. Section 4 will complete the review by examining the future prospects for microalgal biofuels.