4.1.1 Carbon Dioxide’s Role in Photobioreactors

An important issue in most photobioreactors and the first step in CO2 fixation is the diffusion of CO2 from the gas phase to the aqueous phase. The solubility of CO2 in the culture media de­pends on depth of the pond, the mixing velocity, the productivity of the system, the alkalinity, and the outgassing. It has been reported (Becker, 1994) that only 13-20% of the supplied CO2 was absorbed in raceway ponds when CO2 gas was bubbled into the culture fluid as a carbon source. Binaghi et al. (2003) achieved a maximum value of 38% efficiency of carbon utilization in Spirulina cultivation. Gas-liquid contact time and gas-liquid interfacial area are, therefore, two key factors to enhance the gas-liquid mass transfer. In addition, high oxygen tension is problematic, since it promotes CO2 outgassing and competes with CO2 for the CO2-fixing enzyme (RuBisCO).

The capacity for carbon dioxide storage in a growth medium is important because it deter­mines the amount of CO2 that may be used for medium saturation, leading to high growth rates and in-process economics. Since CO2 reacts with water, producing carbonic acid and its anions, chemical equilibrium will have a significant impact on the amount of carbon dioxide stored. pH is the major determinant of the relative concentrations of the carbonaceous system in water and affects the availability of carbon for algal photosynthesis in intensive cultures (Azov, 1982).

The absorption of CO2 into alkaline waters may be accelerated by one of two major uncatalyzed reaction paths: the hydration of CO2 and subsequent acid-base reaction to form bicarbonate ion, and the direct reaction of CO2 with the hydroxyl ion to form bicarbonate. The rate of the former reaction is faster at pH values below 8, whereas the latter dominates above pH 10. Between pH 8 and 10, both are important.

Microalgae can fixate carbon dioxide from different sources, including CO2 from the atmo­sphere, from industrial exhaust gases (e. g. furnaces flue gases), and in form of soluble carbon­ates. Traditionally, microalgae are cultivated in open or closed reactors and aerated with air or air enriched with CO2. Industrial exhaust gases contain up to 15% of carbon dioxide in their composition, being a rich (and cheap) source of carbon for microalgae growth.

In microalgae cultivation, high concentrations of CO2 are not usually used because it may result in decreasing the pH, since unutilized CO2 will be converted to HCO3 . Shiraiwa et al. (1991) and Aizawa and Miyachi (1986) reported that an increase in CO2 concentration of sev­eral percent resulted in the loss of a carbon concentration mechanism (CCM), and any further increase was always disadvantageous to cell growth. Most processes use air enriched with CO2 (2-5% CO2 final concentration), but some studies using high CO2-resistant strains are being described in scientific literature.

If there is not enough CO2 gas supply, algae will utilize (bi)carbonate to maintain its growth. When algae use CO2 from bicarbonate, an increase of pH is observed (a growth in­dicator), even reaching growth-inhibition pH values. To overcome pH fluctuation, the CO2 gas injection should be controlled in such a way that photosynthesis rates are balanced with enough and continuous availability of dissolved carbon. Interesting studies about isolation and selection of strains with high CO2 absorption capacity, which is an important step no matter the process in development, are available in scientific literature. Maintaining constant CO2-free concentration in the media will keep carbon uptake constant.

The ability to accumulate DIC has been shown to occur in many algae and cyanobacteria (Williams and Colman, 1995). Whereas CO2 can diffuse into algal cells and is the substrate for carbon fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase (RubiscO), it forms a small proportion of the total available inorganic carbon. The largest proportion of total DIC available to microalgae consists of ionic HCO3 , which has a low capacity for diffusion across cell membranes (Young et al., 2001). A number of eukaryotic microalgae have devel­oped mechanisms that permit the use of HCO3 for photosynthesis (Miller and Canvin, 1985). Access to the larger pool of HCO3 is assumed to involve one or both of two basic processes:

1. In some green algae, the use of HCO^ has been correlated with the presence of external carbonic anhydrase (CA) activity (Aizawa and Miyachi, 1986). In these cases external CA is thought to facilitate the use of HCO^ by maintaining equilibrium between HCO^ and CO2, and thereby maintaining the supply of CO2 to a specific transporter (Aizawa and Miyachi, 1986).

2. Direct HCO^ transport via a transmembrane bicarbonate transporter, which has been demonstrated even in cells that have external CA activity (Williams and Turpin, 1987). The involvement of transmembrane ATPase proteins was also reported in DIC uptake by chlorophytes (Ramazanov et al., 1995).