The continued development of a bioeconomy capable of producing renewable fuels and valued products is a sustainability goal for the 21^st century.  This Emerging Frontiers in Research and Innovation (EFRI) project showed that diatom algae sustainably produce three valuable products:  lipids for liquid transportation fuels, nanostructured silica for advanced materials applications, and biopolymer nanofibers for medical and nutraceutical applications. Diatoms are photosynthetic algae found in fresh and ocean waters across the planet, and are responsible approximately 20% of global carbon cycling.  What makes diatoms special among the algae is that they possess a cell wall composed of silica, and take up dissolved silicon from their surrounding environment to make a new cell wall prior to cell division. 

We found that diatoms within the genus Cyclotella accumulate large amounts of triacylglyceride lipids within the cell for biodiesel production, and extrude long fibers composed of the polymer N-acetyl glucosamine (NAGlc) from the cell.  This biopolymer is special because its fibers are in a pure form, have nanoscale diameter (50 nm), and possess unique beta-crystalline and biocompatible properties ideal for use in biomedical materials or conversion to glucosamine, a widely used nutraceutical.  We learned that the formation of both lipid and biopolymer product was triggered when the cell perceived it was limited by dissolved silicon (silicic acid).  We exploited this attribute of the cell's metabolism to overproduce lipids and the biopolymer.  Towards this end, a scalable photobioreactor cultivation process was developed to control the input of light, carbon dioxide (CO₂), dissolved silicon, and nutrients (nitrate, phosphate, trace metals) to the diatom cell suspension.  We first learned that controlling the cell division rate by controlling the input of dissolved silicon enabled the simultaneous production of cell biomass, biofuel lipids, and NAGlc nanofibers.  With this knowledge in hand, the other inputs were programmed to rationally manipulate the co-production of all three product streams by the cell.  Ultimately, we demonstrated sustained production to high biomass density (greater than 5 g dry mass per liter).  Depending upon the specific light, CO₂, and nutrient input strategy used, the diatom cell biomass contained up to 60% of its weight in biofuel lipids, 15% chitin, and 7% nanostructured silica.  Nearly 70% of the CO₂ captured by the cell was incorporated into these products. 

Based on the process information described above, we performed a techno-economic (TEA) and life cycle analysis (LCA) on the diatom-based photosynthetic biorefinery, and learned that if glucosamine is the targeted end product, then biomass production rate and chitin content are the most sensitive variables for lowering cost, with lipid biofuel co-production being relatively unimportant. Finally, to improve sustainability aspects of the TEA-LCA, we learned that residual marine diatom biomass was readily converted to biogas by anaerobic digestion.  This process released about 50% of the nitrogen in biomass for recovery and recycle, but did not release phosphorus due to salts in the biomass.

This project also provided for workforce training, undergraduate course development, and opportunities for broadening participation in science and engineering.   Over the course of this project, 1 post-doctoral research associate, 8 graduate students, and 16 undergraduate students received research training.  Models developed by the TEA-LCA analysis described above were incorporated into learning module on uncertainty analysis for the bioengineering capstone design course at OSU.  During the summer, the laboratories used in the research hosted 8 high school (HS) students for one-week team projects on algae for sustainability topics, with logistics provided through the SESEY program at OSU. The HS students were mentored by undergraduate students on the research team.  Most of the HS participants were from under-represented groups in engineering (women and under-represented minorities).  Over the first 4 years of the project, 32 HS students participated.

Last Modified: 01/02/2019
Modified by: Bettye L Maddux