The most fundamental assumption in microalgae biomass production for biodiesel fuels is that it will be possible to achieve near theoretical solar conversion efficiencies by overcoming the light saturation effect (see Section IV. A.2.c. for a brief discussion). The second most fundamental assumption is that it will be possible to achieve such very high productivities with microalgae cultures high in oils, approaching or even exceeding 50% of lipids by dry weight. This second assumption was tested by this project.

The concept of producing microalgae with a high oil content goes back almost 50 years, to work carried out, and even patented, by Sphoer and Milner (1949), who reported oil contents as much as 80% of the dry weight. Lipid content is affected by many parameters, but most particularly by N (and, for diatoms, Si) limitation, which can result in extraordinarily high lipid contents. However, it appeared from earlier work, and also from a survey of 30 species by Shiffrin and Chisholm (1981), that total productivity declined sharply upon nutrient limitation, resulting in a decline in total lipid productivity, although lipid content increased as a percentage of the cell mass. However, a re-analysis of the data suggested that the evidence for this was not clear-cut, as only rather widely spaced data points had been collected. In fact, an essential assumption in this field is that a “lipid trigger” activates lipid biosynthesis without necessarily reducing photosynthesis, at least for a transitory period (see Section II).

The experimental approach was to first grow the algal cultures under nutrient sufficiency and then to induce deficiency during batch cultivation, using light (single versus two-sided illumination of the 1 — L flasks) as a second variable. In continuous cultures, the growth rates and cellular N contents were dependent on illumination, and there was only a modest increase in lipid content with decreasing cellular N content, with lipid culture productivity maximal at about 5% N biomass. The key experiment was the up-shifting of the light received by the culture (e. g., from single — to double-sided illumination). The results are shown in Figure III. B.U., which demonstrate that cells shifted to a higher light intensity start growing (AFDW increases) at the rate of the higher light level cultures. However, lipid productivity shoots up to a much higher rate than with either of the steadily illuminated cultures. In practical terms, this could be exploited by diluting cultures for lipid induction. This experiment demonstrated the possibility of producing high lipids content by nutrient limitation while achieving a substantial increase in overall lipid productivity. Experiments with continuously diluted cultures, however, did not exhibit such responses, indicating the necessity to carefully control and modulate conditions to maximize lipid production.

Figure III. B.11. Maximizing lipid productivity light shift-up.

A. Top. Biomass yield during light shift.

B. Bottom. Lipid yield during light shift.

(Source: Benemann and Tillett 1987.)