Microalgal Strains for Mass Culture: Source and Genetic Improvements

The immediate issue arises of how to isolate, select, improve, and maintain the algal strains required for large-scale, low-cost microalgae cultivation. The ASP spent considerable effort in this area, with the isolation, screening, maintenance, laboratory studies, outdoor cultivation, and genetic improvement of microalgal strains (see Section II). In general, laboratory results were not predictive of outdoor performance. In addition, the strains most successfully maintained outdoors were those that spontaneously arose in and then dominated the ponds, often for considerable periods. Indeed, one conclusion from the outdoor culture work was that strains maintained in laboratory culture are, in general, not very competitive in open ponds.

What was attempted in this context, the mass culture of specific, selected and productive algal strains in large open ponds for long periods of time, has only been accomplished in algal mass cultures in a few cases, and is still rare in most industrial or environmental microbiology applications. In the case of microalgae mass cultures, only a few strains, Spirulina, Dunaliella, Scenedesmus, and Chlorella, have been successfully mass cultured at a commercial or large (>0.1 ha) scale. In the most successful cases, Spirulina and Dunaliella are maintained in open ponds through the use of chemically selective media, containing high bicarbonate and high salinity, respectively. Scenedesmus was mass cultured at the pilot scale in Germany and Chekoslovakia, and other countries, with the cultures obtained from isolates that invaded and dominated the ponds. Commercial Chlorella production, using selected strains, has suffered from culture instabilities, requiring frequent inoculation and short production runs, greatly increasing the costs of the process. Thus, commercial-scale production of microalgae does not provide a good guide for this problem.

In the case of industrial microbiology, only the traditional fermentations (e. g., ethanol, vinegar, cheese production) use selected strains that can be inoculated into and maintained in the production system, which must be relatively “clean” to avoid rapid contamination, but do not need to be sterilized. In environmental and agricultural microbiology it has not yet been possible to inoculate desired microbes (e. g., pollutant degraders, N fixers) into the open environments and demonstrate their survival and efficacy.

Within this context, the demonstration of the ability to mass culture at least some algal strains on a relatively long-term and reliable basis by ASP-supported projects in California, Hawaii and New Mexico, must be considered a significant advance and accomplishment. These results provide a fundamental basis for future developments and improvements in this technology. However, a basic issue still to be resolved is the source of the microalgal strains to be used in

outdoor cultures. The results of the ASP Program suggest that one choice would be to allow the production system to self-select the organisms. Strains that naturally invade potential production sites could be screened for subtle combinations of fast growth, competitiveness in high densities, and adaptation to prevailing environmental conditions. In this context, most of the critical parameters—temperature, light intensity, pH fluctuations—can be modeled rather easily at a modest scale. Thus, it should be possible to select such strains in downscaled models that would allow much better control than possible in large ponds over the selective conditions desired.

One factor essentially impossible to model or scale down is the biotic environment itself, that is, invasions by other microalgae, predation by grazers, infection by viruses, and other obvious or hidden biological effects that result in decreased productivities or even loss of culture. However, it appears from the experience with outdoor ponds, that these biotic effects are usually consequences of, not fundamental reasons for, loss of culture competitiveness. Further, some techniques have been developed to counteract such problems, for example rotifer grazing. In general, these problems will have to be dealt with when the technology has advanced to the point where large-scale culture efforts can be justified. That is, after high productivity cultures can be demonstrated at smaller scales, starting with laboratory simulations.

It is thus recommended that small-scale systems, mimicking as much as possible the outdoor environment, be used as selection devices for microalgae strains suitable for outdoor algal mass cultures. Suitability for mass culture can be established at a relatively small scales (<200 m[10]). Such selected “wild type” algal strains, would, of course, not necessarily exhibit the high biomass and lipid productivities required for the purposes of biodiesel production. Thus, considerable R&D will be required to genetically improve such strains. The techniques used to increase photosynthetic efficiencies or to optimize lipid quantity or quality, achieved with laboratory strains, must then be applied to the isolated strains suitable for algal mass culture.

Thus the recommendation for future R&D in this field is for a parallel track effort:

1. Demonstrate the feasibility to achieve with laboratory systems the high solar conversion efficiencies and lipid productivities required for biodiesel production.

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