immobilization of cells brings several advantages over current suspension biopro­cessing, such as (1) providing flexibility to the photobioreactor designs; (2) increasing reaction rates arising from higher cell density; (3) enhancing oper­ational stability; (4) avoiding cell washouts; (5) facilitating cultivation and easy harvesting of microorganisms; (6) minimizing the volume of growth medium as the immobilized cellular matter occupies less space; (7) easier handling of the products;

(8) permitting the easy replacement of the algae at any stage of the experiment;

(9) protecting the cell cultures from the harsh environmental conditions such as salinity, metal toxicity, variations in pH, and any product inhibition; and

(10) allowing continuous utilization of algae in a non-destructive way. Enhanced survival rates of immobilized cells in toxic environments provide a significant alternative to achieve sufficient bioremediation of chemically contaminated envi­ronments. It is also important to stress that continuous biomass production, opportunity for product recycling, and nearly spontaneous biomass harvesting will have the potential to outweigh the difficulties and added costs associated with applying the technology on a larger scale.

Conventional wastewater treatment methods are mostly focused on the separa­tion of pollutants from the liquid effluents with a requirement for a further stage to eliminate them. Developing integrated wastewater treatment processes that elimi­nate the undesired portion of the wastewater while converting it into valuable products is important in developing sustainable processes for the future. immobi­lization of algal cells is important in the development of an integrated process while simplifying the harvesting of biomass and providing the retention of the high-value algal biomass for further processing.

There are, however, technical issues to address, such as the hybridization of different polymers for creating more efficient and stronger immobilization matrix for algal cells. Immobilization of viable algae inside three-dimensional gel lattices also faces several limitations given that the encapsulating materials can have high volume-to-surface ratios. As a consequence, algal viability decreases since the light, nutrients, or reactants have to diffuse far into these materials to reach the algal cells. One of the other restrictions for the gel-entrapped cultures is their lower growth rates compared to their free-living counterparts. Such drawbacks can be addressed by optimizing the immobilization processes, that is, by choosing different encap­sulating materials with lower volume-to-surface ratios such as thin films. Over­coming the difficulties of the current technology will increase the applicability of immobilized algae systems for various industrial applications.

Current immobilization projects have been often confined to the laboratory in providing an effective proof-of-concept rather than quick-install industrial proto­types. For larger scale wastewater treatment and biofuel production bioprocesses, the cost of immobilization matrix becomes a significant parameter that needs to be improved by further innovative designs and additional profits through generating valuable by-products.

Discovering the optimal microalgae-bacteria combinations for co-immobilization processes can also be a good alternative for large-scale wastewater treatment prac­tices, since algal cultures in nature are usually associated with bacteria.

Application of innovative composite materials for use as the algal immobiliza­tion matrices can have a significant contribution to the economic and environmental development by sustainable utilization and recovery of the local resources, while bringing valuable strategies for solving important environmental issues.

Potentials of Exploiting Heterotrophic Metabolism for Biodiesel Oil Production by Microalgae

James Chukwuma Ogbonna and Navid R. Moheimani

Abstract The current prices of microalgae oils are much higher than oils from higher plants (vegetable oils) mainly due to the high cost of photoautotrophic cultivation of microalgae. However, many strains of microalgae can also grow and produce oil using organic carbons, as the carbon source under dark (heterotrophy) or light conditions (mixotrophy). Lipid productivities of most strains of microalgae are higher in culture systems that incorporate heterotrophic metabolisms (presence of organic carbon source) than under photoautotrophic conditions. This is because for many strains, cell growth rates and final cell concentrations are higher in het­erotrophic cultures than in photoautotrophic cultures. Furthermore, in some cases, the oil contents of the cells are also higher in cultures incorporating heterotrophic metabolisms. It has also been reported for some strains that the quality of oil produced in the presence of organic carbon sources are more suitable for biodiesel oil production than those produced under photoautotrophic conditions. Thus, het­erotrophy can be used to reduce the cost of biodiesel oil production, but the effectiveness of the various organic carbons in supporting cell growth and oil accumulation depends on the strain and other culture conditions. Use of waste­waters for cultivation of microalgae can further substantially reduce the cost of production (since they contain carbon, nitrogen, and other nutrients) and also reduce the requirement for freshwater. Generally, many factors such as nitrogen limitation, phosphate limitation, silicon limitation, control of pH, and low tem­perature can be used to increase oil accumulation, although their effectiveness depends on the strain and other culture conditions.

J. C. Ogbonna (H)

Department of Microbiology, University of Nigeria, Nsukka, Nigeria e-mail: james. ogbonna@unn. edu. ng

N. R. Moheimani

Algae R&D Center, School of Veterinary and Life Sciences, Murdoch University, Murdoch, WA 6150 Australia

© Springer International Publishing Switzerland 2015

N. R. Moheimani et al. (eds.), Biomass and Biofuels from Microalgae,

Biofuel and Biorefinery Technologies 2, DOI 10.1007/978-3-319-16640-7_3

1.1 Introduction

Interest in production of biodiesel continues to be sustained because, unlike fossil diesel which is non-renewable and associated with various environmental problems, biodiesel is biodegradable, renewable, non-toxic, and emits less gaseous pollutants. The cost of biodiesel will determine to what extent it will be able to replace or complement fossil diesel production. Vegetable oil remains a major source of oil for large-scale industrial biodiesel production. However, the cost of vegetable oil is high, and waste oils often contain large amounts of free fatty acids which are difficult to convert to biodiesel through transesterification. Microalgae oil has a high potential for biodiesel production as it contains large proportions of fatty acid triglycerides, and the composition of the oil can be controlled by varying the culture conditions (Jiang and Chen 2000; Widjaja et al. 2009; Wen and Chen 2001a, b; Zhila et al. 2005). Microalgae oil is characterized by lower oxygen content, higher calorific value, and higher H/C ratio which make it more suitable for biodiesel, as compared to terrestrial plant oils (Miao and Wu 2004, 2006).

However, the cost of microalgae biodiesel is still too high to compete with the fossil diesel. The cost of microalgae cultivation accounts for 60-75 % of the total cost of the microalgae biodiesel fuel (Krawczyk 1996). It has been estimated that the cost of production of a liter of oil ranges from $1.40 to $1.81, depending on the type of photobioreactor used, and assuming that the biomass contains 30 % oil by weight (Azimatun-Nur and Hadiyanto 2013). Reduction in the cost of microalgae oil requires improvement in growth rate, oil content of the cells, and reduced cost of construction and operation of bioreactors. Reports from various studies have shown that it is already very difficult to increase cell growth rates and productivities in photoautotrophic cultures. However, many strains of microalgae can grow het — erotrophically, using various organic carbons in dark. Heterotrophic cultures can be used to overcome most of the problems associated with photoautotrophic cultures. Generally, in comparison with photoautotrophic cultures, higher cell densities are achieved in heterotrophic cultures, with consequent reduction in the cost of downstream processing. Thus, heterotrophic cultures can be used to significantly reduce the cost of microalgae biodiesel production. The feasibility of exploiting heterotrophy for efficient biodiesel oil production is discussed in this chapter.