Photosynthetic Biorefinery In Diatoms

Interview conducted by G.P. ThomasOct 16 2012

Diatoms are single-celled algae that possess a rigid, transparent shell made of silica that is patterned at the nanoscale. This diatom shell looks strikingly similar to a photonic crystal. Prof. Greg Rorrer talks about the potential uses of diatoms.

Could you give a brief introduction to your work on diatoms?

Our research is exploiting the unique biosynthetic machinery of living diatom algae to make novel and valuable products.

What are diatoms?

Diatoms are single-celled algae that possess a rigid, transparent shell made of silica that is patterned at the nanoscale. This diatom shell looks strikingly similar to a photonic crystal.

What is meant by a biofuel? Why are these important to develop?

A biofuel is a carbon-based fuel which is obtained from a renewable resource. Examples include ethanol from cellulosic biomass, and biodiesel from algae. At some point, society is going to run out of fossil fuels, and even before that time comes, increasing scarcity will make them more expensive. Biofuels have the potential to serve as sustainable replacement for petroleum-based liquid transportation fuels.

Why hasn’t commercial biofuel production taken off so far?

The fermentation of corn to bioethanol is already a commercial reality, with current US production at about 10 billion gallons per year. However, the production of biofuel from more abundant and sustainable feedstocks such as cellulosic biomass and photosynthetic algae is much more challenging - the manufacturing costs are high, and so they are not cost competitive at the present time.

How is this process unique compared to other forms of biofuel production from algae?

In our process, the living diatom cell serves as a “photosynethetic biorefinery” that uses light, carbon dioxide, and earth-abundant minerals to produce high-value “co-products” in addition to lipids for biofuel, all within the same organism. The co-products improve the economic viability of the process as a whole even though they are produced in much lower volumes, because they are so much more valuable than the biofuel.

What further by-products can diatoms produce?

Valuable co-products produced by diatom algae which are not produced by other types of algae include polyglucosamine-based biopolymers for nutraceutical & biomedical applications, and nanostructured silica for advanced material applications.

Could you briefly explain the theory behind how diatoms produce biofuels from minerals such as silicon?

Diatoms require dissolved silicon to make their cell walls; without a source of silicon, the cells cannot divide and ultimately die. But once the cell has sufficient silicon to make a new cell wall and divide, its photosynthetic machinery takes over to make energy-dense lipids which can be converted into biodiesel or liquid hydrocarbon transportation fuels. A typical diatom cell is about 10% silica, 50% lipids, and 40% other stuff by weight.

Diatoms can produce semiconductor materials as a by-product - what specific materials are produced and how is this achieved?

By feeding the living diatom cells with water-soluble forms of germanium or titanium, under certain conditions the diatom cell can deposit these alterative silicon substrates into its cell wall. After removal of organic materials, and thermal annealing, the diatom cell wall, which contains Ge-oxide or TiO2 nanoclusters dispersed within a nanopatterned matrix, has optoelectronic and photonic properties.

What impact may this have on the solar energy sector?

Solar photovoltaic device developers are always looking for low-cost, high-performance materials that manage light, either through wavelength selective reflection or improved light capture. We have created TiO2-based diatoms with high dielectric contrast and photonic properties which have the potential for improving the performance of TiO2-based dye sensitized solar cells.

National Science Foundation just provided a four-year, $2 million grant-how important is this to the research?

Support through NSF will uniquely enable this project. This project was funded by the Emerging Frontiers for Research and Innovation (EFRI) program under the Photosynthetic Biorefineries topic area – a perfect fit for our goals.

How far off is this process from being realised?

We have already demonstrated that the diatom cells can produce three products. But in order to realize this process for future applications, we need to learn how to control it, so that the product mix can be tailored.

Could this process one day be commercially viable?

We hope so. A significant component of the NSF award is to perform techno-economic and life-cycle analysis studies to answer this question.

About Greg Rorrer

Greg is professor and head of chemical engineering at Oregon State University. Current research is focused on two main areas: algal biotechnology, and biomass conversion.