Two-Stage Photobioreactor Cultivation Process for Enhancing Lipid Production From Diatom Cells by Controlled Silicon Limitation

Diatoms are a class of single-celled algae which make silica cells walls. As a consequence, these organisms require dissolved silicon as a substrate for cell division. Therefore, manipulation of soluble silicon delivery to the culture offers a route to control cell cycle and lipid production. A two-stage, semi-continuous photobioreactor cultivation process was developed to induce the production of lipid-rich algal biomass from the centric diatom Cyclotella. In the first stage of the process, algal cells were grown up to high density until all of the dissolved silicon in the feed medium was consumed. The cells were in the silicon-starved state when the cell density was constant for 24 hours and the dissolved silicon concentration was zero. During silicon starvation, the total lipid content increased from 7 wt% in the dry algal biomass to over 40 wt%. Then, about one-half of the total culture volume was harvested, and the culture volume was re-charged with fresh medium containing dissolved silicon. The silicon was rapidly consumed by the silicon-starved cells under a surge uptake mechanism. The silicon concentration rapidly decreased towards zero, so that silicon starvation was re-established within the diatom cells after 24 hours. Consequently, high lipid content in the algal biomass was maintained at 30 wt% as the culture grew back to the target cell density. This study illustrates how bioreactor cultivation strategies can be used to enhance lipid production by algal cultures.