sites, and research is being conducted on various designs. A combination of closed and open systems will probably be used in many cases—enclosed bioreactors for inoculum generation and open ponds as final production units.

To reduce the volume of water handled during cell harvesting, a flocculent (such as alum or various ionic polymers) is typically added to the cells to facilitate their concentration in a settling tank; the biomass is concentrated further with continuous centrifugation, which is an expensive process because of the capital and operating costs. For some filamentous strains, strainers or filters can be used to collect the cells. Clearly, additional research and development to improve the biomass-harvesting process will have a great effect on production costs.

Harvesting of naturally occurring algae would eliminate the need for a photobioreactor, but harvesting appreciable amounts of biomass would require filtering large quantities of water and extensive operating-expense outlays. In addition, environmental groups strongly resist this approach because of potential unpredictable environmental consequences of ocean and lake fertilization.

Cultivation of the dominant strains of photosynthetic organisms in the locale of the installed system might be the easiest way to maximize the productivity of the system. However, those strains might not be optimal for biofuel production. Most fuels under development require the use of microorganisms that have high lipid content, which might not be an attribute of random local strains. Many researchers in the field therefore believe that commercial production strains will be initially selected on the basis of superior product formation and processing attributes and then developed through dedicated strain-improvement programs.

Various information and tools are available for the genetic modification of photosynthetic microorganisms, including whole genome sequences, systems for gene introduction, and protocols for random and directed mutagenesis.