conditions (for example, higher susceptibility to contamination by undesired algae and predators, dilution by rain, and fouling by windborne dust and debris), high evaporative water loss, requirement for large expanses of level terrain, and high regulatory hurdles with respect to containment of recombinant strains.

Different kinds of closed photobioreactor systems have been designed and tested. Generally, these systems are in two categories: tubular systems made of rigid or flexible plastic, and flat plate or annular reactors made of rigid materials and typically placed at upright angles to maximize use of light by the cultures. Photobioreactor design is a subject of active research in several algal-biotechnology companies. Because of high capital costs associated with rigid plastics, many of the designs being pursued are focused on tubes manufactured from flexible films. Some press releases have reported the achievement of productivity as high as 170 g/m² per day in novel photobioreactors (for example, GreenFuel Technologies Corporation, 2007), but it will be important to increase understanding of how the calculations were conducted to ensure valid comparisons between the various systems.

The primary advantages of closed photobioreactors are a higher degree of control over some culture conditions (for example, protection from the elements, less water evaporation and outgassing of carbon dioxide [CO₂], and delayed onset of contamination by undesired species and predators), potentially higher productivity as a result of improved use of light, and containment of recombinant strains. The overriding disadvantage of closed photobioreactors is the high capital cost associated with the construction materials, circulation pumps, and nutrient-loading systems. There are other disadvantages:

• Fouling of interior surfaces and difficulty of cleaning them.

• Accumulation of high concentrations of photosynthetically generated oxygen, which leads to photooxidative cell damage.

• Absence of evaporative cooling, which can lead to very high temperatures.

Both types of systems have inherent advantages and disadvantages. It is highly unlikely that one standard system will be applicable for all strains, products, or