Microalgae have long been under development as renewable energy resources and other useful products (Benemann and Weissman, 1993). Almost 20,000 species are known. Unicellular algae such as the species Chlorella and Scenedes — mus have been produced by continuous processes in outdoor light at high photosynthesis efficiencies. Chlorella has been reported to be produced at a rate as high as 1.0 dry t/ha-day. This corresponds to an annual rate of 401dry t/ha-year presuming growth can be sustained (Retovsky, 1966). These figures are probably in error, but there is no theoretical reason why yields cannot achieve very high values because the process of producing algae can be almost totally controlled. Also, production is not composed only of surface growth. Algae are produced as slurries in lakes, ponds, and custom-designed raceways so that the depth of the biomass-producing area as well as plant yield per unit volume of water are important parameters. The nutrients for algae production can be supplied by municipal biosolids and other wastewaters. It should be pointed out that most unicellular algae are grown in fresh water, which tends to limit their energy applications to small-scale algae farms. The high water content of unicellular algae also tends to limit the conversion processes to biological methods. But this can be an advantage in some cases where the particular microalgae exudes triglycerides without cell destruction so that the product oil is continuously formed and can be easily recovered from the water surface.

Macroscopic multicellular algae, or seaweeds, have also been considered as renewable energy resources for many years. Some of the candidates are the giant brown kelp Macrocystis pyrifera (Bryce, 1978; North, 1971; North, Gerard, and Kubawabara, 1981), the red benthic alga Gracilaria tikvahiae (LaPointe and Hanisak, 1985; Ryther and DeBusk, 1982), and the floating, brown pelagic algae Sargassum natans and S. ftuitans (LaPointe and Hanisak, 1985). Giant brown kelp has been studied in great detail and is harvested commercially off the California coast. Because of its high potassium content, giant brown kelp was used as a commercial source of potash during World War I and is used today as a commercial source of organic gums, thickening agents, and alginic acid derivatives. Off the East Coast, Laminaria seaweed is harvested for the manufacture of alginic acid derivatives. In tropical seas not cooled by upwelled water, species of the Sargassum variety of algae may be suitable as renewable energy sources. Several species of Sargassum grow naturally around reefs sur­

rounding the Hawaiian Islands. Unfortunately, only a small amount of research has been done on Sargassum and little detailed information is available about this alga. A considerable amount of data on yields and growth requirements are available, however, on the Macrocystis and Laminaria varieties. Again, the very high water content of macroscopic algae suggests that biological conver­sion processes rather than thermochemical conversion processes should be used for synfuel manufacture. The manufacture of co-products from macro­scopic algae, such as polysaccharide derivatives, along with biofuel might make it feasible to use thermochemical processing techniques on intermediate process streams.