Engineering strategies for improving the CO₂ fixation and carbohydrate productivity of Scenedesmus obliquus CNW-N used for bioethanol fermentation

Highlights

• Scenedesmus obliquus CNW-N was grown to produce 50% of carbohydrates per dry weight.

• Nearly 80% of carbohydrates in S. obliquus CNW-N was composed of glucose.

• Semi-batch operation resulted in the best CO₂ fixation and carbohydrate production.

• The highest CO₂ fixation and carbohydrate productivity was 1547 mg/L/d and 468 mg/L/d.

Abstract

Engineering strategies were applied to improve the cell growth, CO₂ fixation ability, and carbohydrate productivity of a Scenedesmus obliquus CNW-N isolate. The resulting carbohydrate-rich microalgal biomass was subsequently utilized as feedstock for ethanol fermentation. The microalga was cultivated with 2.5% CO₂ in a photobioreactor on different operation modes. Semi-batch operations with 50% replacement of culture medium resulted in the highest CO₂ fixation rate (1546.7 mg L⁻¹ d⁻¹), carbohydrate productivity (467.6 mg L⁻¹ d⁻¹), and bioethanol yield (0.202 g/g biomass). This performance is better than most reported values in the literature. The microalgal biomass can accumulate nearly 50% carbohydrates, as glucose accounted for nearly 80% of the total carbohydrate content. This glucose-predominant carbohydrate composition of the microalga is well suited for fermentative bioethanol production. Therefore, using the proposed carbohydrate-rich microalgal biomass both as the carbon sink and as the feedstock provides a feasible alternative to current carbon-reduction and bioethanol-production strategies.

Introduction

There is an urgent need to develop effective and sustainable technologies for CO₂ capture, storage, and sequestration, in order to alleviate the adverse effects of CO₂ emissions on the environment, and especially on the climate (Ho et al., 2012a). Biological CO₂ fixation, which is achieved by microalgae through photosynthesis, has emerged as one of the most effective and environmental-friendly methods to mitigate CO₂ emissions (de Morais and Costa, 2007a). Theoretically, photosynthesis can be used to transport atmospheric carbon into a cycle in which no additional CO₂ is created (Kumar et al., 2010). Microalgae can grow about 10–50 times faster than terrestrial plants, and thus achieving a much higher CO₂ fixation rate (Chen et al., 2011). Therefore, there is considerable potential with regard to using microalgae to develop technically- and commercially-feasible processes for the mitigation of CO₂ emissions (Ho et al., 2010a). Moreover, the microalgal biomass produced through photosynthesis can be converted into a wide range of valuable products, such as biofuels, food additives, health-care products, and so on, which represent additional benefits of any microalgae-based CO₂ reduction system (Ho et al., 2011).

Due to the fast growing global demand for fuel energy, alternative biofuels have attracted great attention because they are made from non-toxic, biodegradable, and renewable resources, and their use can lead to a decrease in the emission of harmful pollutants, such as CO₂. Biomass is an excellent renewable resource for the generation of biofuels, such as biodiesel (Ho et al., 2010b) and bioethanol (John et al., 2011). To date, bioethanol has been mostly produced from food crops (e.g., corn and sugarcane) and lignocellulosic materials (e.g. rice straw and switchgrass) (Nigam and Singh, 2011), but this raises the issue of competition with the arable lands needed to produce food, in addition to the high cost associated with conversion of lignocellulosic materials into ethanol (Sun and Cheng, 2002). As a result, more attention has recently been paid to evaluating the feasibility of using microalgae as a so-called “third generation feedstock” for biofuel production. In addition to their favorable characteristics of a fast growth rate and a high CO₂ fixation rate, some microalgae species contain high carbohydrate contents (in the form of starch in the chloroplast and cellulose on inner cell walls), which are suitable for ethanol fermentation (Ho et al., 2013a). In particular, the absence of lignin in microalgae-based cellulose also makes the saccharification process much easier when compared with that of lignocellulosic biomass (Harun et al., 2010). There are also some other advantages associated with using microalgae as potential feedstock, as they can be cultivated on non-arable land, with non-potable water, without seasonal culture limitations, and can be harvested daily (Siaut et al., 2011).

Although microalgae seem to have high potential to become feedstock for ethanol production, there are still some limitations, mainly due to the cost issues, that may hinder their commercial applications. To enhance the economic feasibility of microalgae-based bioethanol production, it is necessary to raise both the biomass productivity and carbohydrates (mainly glucose) content (John et al., 2011). Strategies that achieve a combination of the highest carbohydrate (glucose) content along with the shortest cultivation time can thus lead to an optimization of both carbohydrate productivity and CO₂ removal ability.

A microalga isolate, Scenedesmus obliquus CNW-N, was used in this study to fix CO₂ and also to accumulate cellular carbohydrates for subsequent use in making bioethanol (Ho et al., 2012a). The feasibility of the S. obliquus CNW-N strain with regard to serving as a feedstock for bioethanol production was first evaluated on batch mode in terms of its carbohydrate accumulation and biomass productivity. Moreover, various engineering strategies (namely, semi-batch and continuous operations) were applied to optimize biomass and carbohydrate productivity, and then the related bioethanol production yield was estimated. These processes are reported in detail in the following sections.