Eduardo Bittencourt Sydney, Alessandra Cristine Novak, Julio
Cesar de Carvalho, Carlos Ricardo Soccol

Biotechnology Division, Federal University of Parana, Curitiba, Brazil

4.1 INTRODUCTION

The Framework Convention on Climate Change, signed in Rio de Janeiro in 1992, made global warming a major focus, and the development of technologies for reducing/absorbing greenhouse gases (GhG) gained importance. After another 20 years, at Rio + 20, the final document stated clear concern about emissions and the need to reduce them by 2020.

Rubin et al. (1992) divided the GhG reduction alternatives into three groups: conservation, direct mitigation, and indirect mitigation. Conservation measures reduce electricity con­sumption and thus GhG emissions; direct mitigation techniques capture and remove CO2 emitted by specific sources; and indirect mitigation involves offsetting actions in which GhG producers support reductions in GhG emission.

The concept behind most disposal methods is to offset the immediate effect on the levels of carbon dioxide in the atmosphere by relocation, i. e., by injection into either geologic or oceanic sinks (Stewart and Hessami, 2005). Relocation in ocean and deep saline formations has the capacity for 1012 tons of CO2, whereas global carbon dioxide emissions in 2009 were 33 x 106 tons (Olivier et al., 2011), which means 30,000 years of relocation. Problems related to this issue are the unknown possible environmental problems (such as acidification, for example), costs, and the necessity to concentrate CO2 before relocation (how will it work to transport CO2 emissions, for example?).

Therefore, long-term mitigation technologies for CO2 and other GhG gas removal came to be developed. They can be generally classified into two categories: (1) chemical reaction — based technologies and (2) biological CO2 mitigation.

Chemical reaction-based CO2 mitigation approaches are energy-consuming and costly processes (Lin et al., 2003), and the only economical incentive for CO2 mitigation using the chemical reaction-based approach is the CO2 credits to be generated under the Kyoto Protocol (Wang et al., 2008). For example, CO2 can be instantly absorbed through bubbling it in a hy­droxide solution at 40°Celsius, producing sodium or ammonium bicarbonate. However, the demand for these salts (although high—equivalent to around 14Gt/year) is supplied by the Solvay process, whereas a CO2 process would require previous synthesis of sodium hydroxide.

Biological CO2 mitigation has attracted a good deal attention as a strategic alternative. Microalgae cultivation gained importance because it associates CO2 mitigation and produc­tion of a wide range of commercial bioproducts.

Despite the fact that the existence of microalgae has been known for a long time, studies for its use as industrial microorganisms are relatively recent. Initial studies of microalgae culti­vation began in the late 1940s and early 1950s for its potential as a source of food. Concerns about water pollution in the 1960s increased interest in the use of microalgae in wastewater treatment. The perception in the 1970s that fossil fuels would run out made these microorgan­isms a focus of renewable fuel production. In the 1980s microalgae were used as a source of value-added products, specifically nutriceuticals. In the late 1980s the low cost of oil caused a loss of interest in microalgae-based energy, whereas research with nutraceuticals and biomass for feed continued. In the 2000s, global warming concerns associated with high oil prices made microalgal bioenergy projects popular again.

To create microalgal products, it is necessary to develop mass-cultivation techniques and to understand the physiological characteristics of each strain. There have been extensive stud­ies on process optimization (media and physicochemical parameter optimization, screening and isolation of high CO2 tolerants, search for new valuable products, optimization and development of new vessels and systems for cultivation, for example) to try to overcome the economic issues faced in industrial-scale production of microalgae. Two other aspects are gaining importance: the use of industrial residues (to reduce media costs) and the carbon market (carbon credits as an additional element in the economic evaluation of the process).

The evaluation of nutrient needs in microalgal cultures is an important tool in process development using residues (domestic or industrial), and the quantification of carbon dioxide fixation is of great industrial interest since carbon credits can be traded on the international market and companies may use the process as a marketing strategy.

The rate of carbon uptake is limited by the metabolic activity of microalgae, which is in turn limited by photosynthesis. The ability to identify rates of consumption of nutrients is thus of considerable importance to the understanding of the metabolism of microalgae and to avoid problems in industrial cultivation of such microorganisms.