Botryococcus braunii Kützing (Trebouxiophyceae, Chlorophyta), a green colonial microalga, is a cosmopolitan species that occurs worldwide in both freshwater and brackish-water environments and robustly produces liquid hydrocarbons. The hydrocarbon content of B. braunii colonies is much higher than that of any other oil-producing microalgae and over 50% of the dry weight. Therefore, B. braunii has received considerable attention as a potential source of renewable fuels.Many studies have been performed to understand its growth characteristics, ecology, genomics, develop culturing methods, and identify mass cultivation systems. In this review, I summarized biological characters and mass cultivation studies of Botryococcus. In addition, I introduced the possibility of Botryococcus hydrocarbon utilization.

Botryococcus braunii is a green microalga known for its especially high hydrocarbon production. We studied on 1) Effective hydrocarbon extraction process from B. braunii, 2) Extraction mechanism with a nonpolar solvent, and 3) Effects of culture medium salinity on the extractability. The results are as followings. 1) A solvent extraction technique was investigated from the concentrated algae slurry via thermal pretreatment. Efficient extraction was achieved by preheating the slurry to a temperatures below 100 °C. This procedure did not require a highly energy consuming drying process. 2) It was found that algae colonies are coated with polymers and the solvent can not approach inside the colony. The polymers are transferred to sol and released into the water phase by increasing the temperature. Then the hydrocarbon can be extracted by the solvent. The main components of the polymers were found to be galactose and arabinose. 3) B. braunii was cultured in media by supplying seawater. After a certain period of culture in the media with an osmotic pressure corresponding to less than 1/4-seawater, hydrocarbon extraction rates exceeding 90% were obtained by simply mixing intact wet algae with the solvent without any pretreatments. Additionally, there was an increase in the colony sizes and flotation of the alga.

This article presents the status of development of microalgae production for renewable jet fuel. A special species of improved Botoryococcus braunii is cultivated in the pond for producing jet fuel. The species has great characteristics of large growth rate compared with wild species, high content of oil and large particle size. The open-pond cultivation has been succeeded through the scaled-up examinations using pond ranging from 100 m²- to 1500 m²-area in Japan without contamination troubles that other species attacked to it. In the meantime, the open-pond cultivation was also succeeded in low latitudes. The growth rate was larger than that in Japan, that revealed the strength of solar radiation has the tremendous effect of enhancement of growth on the species.

Biofuel made from microalgal biomass have been attracting attention as renewable fuels. However, a significant reduction of oil production cost as well as an establishment of a stable supply system is required for an activation of algae biofuel industry. Production process of microalgae fuel can be divided into five major processes, cultivation (algal biomass production), harvesting of microalgal cells (including dehydration), oil extraction, reforming and refining. Technology developments of each process are required to reduce the production cost. Here, we review the advantages of “attached cultivation system”, which is recently attracting attention as an efficient next generation biomass production system for microalgae, after reviewing the status and essential problems of the current biomass/biodiesel production systems. We also introduce our recent studies on the isolation and selection of microalgae suitable for biomass/biodiesel production using attached culture system.

To cope with upcoming shortage of fossil fuels and the environmental threat of global climate change, exploiting biofuels produced from microalgae is a possible solution ¹⁾. Many microalgal species are known to produce and store oil in response to nutritional and environmental conditions ^2)～4). Euglena, which is well known in Japan as Midorimushi, is one of such oil producing microalgae. Euglena gracilis accumulates paramylon (β-1,3-glucan in a crystalline form) as reserve polysaccharide ⁵⁾, while, in the absence of oxygen, paramylon is converted to wax ester ⁶⁾. The wax ester produced in E. gracilis is characteristic in that it is mainly composed of myristyl-myristate, C14:0-C14:0, which is suitable to be used as the feedstock of biodiesel and bio-jet fuel ⁷⁾. Since the achievement of outdoor mass-cultivation of E. gracilis in 2005, Euglena has been cultivated and supplied as food source material. By taking the experience of industrial cultivation into account, biofuel made from E. gracilis is one of the most feasible algal biofuel. In this review, the characteristics and the history of industrial use of the microalga Euglena are summarized. In addition, recent progress in research and development of Euglena breeding and the bio-jet fuel project is described.

Global warming has become a serious environment issues which cause several kinds of disaster to our earth. Most of the totally emitted greenhouse which affect global warming is carbon dioxide (CO₂). CO₂ emissions into the atomosphere mainly from fossil fuels has been increased dramatically over the past 50 years. For this reason, carbon neutral biofuels have attracted much interest as an alternative fuels. Microalgae can utilize sunlight energy for absorbing CO₂ and also have the ability to produce lipids. It is known that microalgae are superior to terrestrial plants in terms of efficiency to produce biofuels. But we still have many challenges for stepping up the microalgal technology to practical use. Establishment of stable culture process is one of the most important keys for improving cost performance. We have selected open racewayponds systems for cultivating microalgae because of its economical use. We chose Pseudococcomyxa sp. KJ strain as a microalga to produce biofuels. It can grow fast and has the ability to accumulate lipids in the cell body under nitrogen starvation conditions. We have developed two-stage cultivation system for algal biomass and lipid production. KJ strain was grown up in a semi-batch culture in the first stage, and the grown microalgae were transferred to second stage culture ponds. Transferred microalgae were cultivated without nitrogen fertilizer for the purpose of promoting lipids synthesis in its cell. As KJ strain can be cultivated under acidic conditions, it has less potential risk of contamination due to other microalgae or protozoa from outdoor environment. But it is difficult to cultivate specific alga for extended periods of time even if we chose KJ strain. Microorganism present in the raceway ponds were analyzed by molecular biology techniques and predators which cause predation damage to KJ strain were clarified. We have succeeded to cultivate Pseudococcomyxa sp. KJ strain for two months with two-stage cultivation system in outdoor ponds by using sodium chlorite which inhibit the growth of bacteria and the predator.