Soaking up the rays

Microalgae currently utilize sunlight purely to grow and produce carbon-based compounds like sugars and lipids, which can then be converted into biofuels. But scientists are beginning to realize that, with a bit of assistance, microalgae might be able to utilize sunlight to do other things as well, such as clean dirty water or produce electricity. This could help microalgae achieve something that, up to now, has eluded them: commercial viability.

To endow microalgae with the ability to clean dirty water with light, a team of scientists from Switzerland and Spain, led by Albert Serrà at Empa, the Swiss Federal Laboratories for Materials Science and Technology, covered them in a photocatalyst. Specifically, they covered a species of spiral-shaped cyanobacteria known as Spirulina platensis in a photocatalyst. They chose this species because it can produce high concentrations of glycogen, a storage form of glucose that can easily be converted into it, and because it has an extremely brittle membrane, for easy extraction of the glycogen.

They grew the cyanobacteria in a bioreactor, and then extracted the cells and fixed them in glutaraldehyde to maintain their spiral shape. Next, they covered the cells in a thin coating of nickel, onto which they deposited a layer of the semiconductor zinc oxide. Finally, they reacted the zinc oxide layer with a sulfur-based compound to convert some of the zinc oxide to zinc sulfide, which is also a semiconductor.

The nickel makes the cyanobacteria magnetic for easy manipulation, while the zinc-based semiconductors produce oxygen radicals when immersed in water and illuminated with light. These oxygen radicals can then break down the organic compounds in dirty water.

As reported in a paper in Advanced Science, when they tested their photocatalyst cyanobacteria on water containing the dye methylene blue, as a test organic compound, they found the cyanobacteria could completely break down the dye into harmless compounds. Furthermore, it could do this seven times without losing any of its efficiency, and only lost 10% of its efficiency after 25 times. At this point, the nickel and zinc coating can be removed with an acidic solution to leave behind the intact cyanobacteria, still with their load of glycogen.

Using a simultaneous saccharification/fermentation process, in which the enzymes glucosidase and amylase convert the glycogen to glucose for subsequent fermentation by yeast, Serrà and his team were able to produce ethanol from the cyanobacteria with a high yield of almost 0.4L per kg of dried biomass. Meanwhile, the cleaned water could simply be used for growing more algae, producing a closed-loop system that would help to reduce production costs.

Rather than use light in a different way, a team of Korean chemists, led by Jung-Yong Lee at the Korea Advanced Institute of Science and Technology (KAIST), have developed an algal system for using more of it. For like all photosynthesizing organisms, microalgae can only really utilize a portion of the spectrum of visible light, mainly longer, redder wavelengths, meaning that a lot of the energy in sunlight simply goes to waste. Solar cells, on the other hand, tend to work well with the shorter, bluer wavelengths that algae can’t make use of. So Lee and his team decided to try combining the two.

Their idea was to coat solar cells containing the carbon material known as fullerenes (C₆₀), which are especially efficient at generating electricity from blue wavelengths, onto a flat surface. They then placed this surface at an angle of 30° over a bioreactor containing the microalgae Chlorella vulgaris, which is commonly used for biofuel production. This set-up ensured that incoming sunlight would first hit the solar cells, which would absorb all the blue and green wavelengths, while the remaining red wavelengths would be deflected down to the bioreactor for use by the algae.

After confirming with computer models that this approach should work, Lee and his team built a working prototype. As they report in a paper in Scientific Reports, this prototype could produce 20.3g per m² of algal biomass each day, representing a reduction of just 15% on the amount produced in full sunlight, while also producing 220Wh per m² of electricity. This additional electricity could either help power the biofuel production process, thus reducing costs, or be sold to the power grid.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.