We develop a “Green cycle” to cultivate microalgae cells from lab scale to commercial scale, which means a novel heterotrophy-autotrophy route for microalgae. By introducing this “Green cycle” into the cultivation platform, it will significantly shorten the algal growth period and enable the autotrophic stage to inoculum with high cell density.
Figure 1. A simple illustration of “Green cycle”.
Q: What is “Green cycle” for microalgae cultivation?
A: “Green cycle” means a heterotrophy-autotrophy route for microalgae. Firstly, one will have to select a heterotrophy-capable strain, which you may screen based on the literature or test on lab bench. It usually starts with an inoculum in autotrophic stage, and after a few series of well-controlled phototrophic growth, cells which show its normal color are induced into heterotrophic stage without light. After serial heterotrophic cultivation, cells are accumulating to a higher density in the enclosed system and start to reduce its chlorophyll content. Once we deplete the carbon source in the heterotrophic phase, we dilute the cell density to a certain fold and start to recover the microalgae to autotrophic state. During recovery, lipids, protein, and chlorophyll content in the cell start to accumulate. Finally, the recovered cells are subject to photo-autotrophic growth.
Figure 2. During the heterotrophic stage, cells with low chlorophyll start to increase (chlorophyll content showing as APC fluorescent intensity). The left column shows two population counts for the heterotrophy group and control group, for the same volume that pass through the channel, cell numbers in heterotrophy group are about 9 folds than those in control group. The right column shows the FITC and APC fluorescent intensity versus the cell size. FSC shows particle’s front scattering which usually indicate the cell size.
Figure 3. During the recovery stage, protein (showing in the horizontal axis as PE fluorescent intensity) starts to accumulate in the cell. Horizontal axis shows the PE-A fluorescent intensity which indicates protein content while the vertical axis shows the cell counts which share the similar PE-A fluorescent intensity.
Q: Why is “Green cycle” important for microalgae cultivation?
Microalgae cultivation in a commercial scale is often a tedious process since it is subject to a series of culture containers. Since Oswald analyzed a variety of utilitarian design for large open ponds system and later earthrise farm firstly introduces the raceway ponds for Spirulina in 1977, microalgae cultivation system have evolved for many decades. Nowadays, large microalgae companies such as Sapphire Energy, Earthrise farm, Cyanotech (NYSE: CYAN), etc. all use large raceway ponds (>1 acre), regardless of the end product they are interested in. However, in these large raceway ponds, algal cells are vulnerable to biological predation and chemical contamination. Therefore we want to start with high cell density and necessarily shorten the photoautotrophic time. This scheme did not only reduce the time and cost that is required to cultivate microalgae from small volume to large open raceway ponds, but also introduce a higher cell density into open system or final culture system which will effectively avoid complexity.
Figure 4. A view of large open raceway ponds