Hyun Woo Kim and Raveender Vannela, researchers at the Biodesign Institute at Arizona State University are perfecting the means to culture cyanobacteria, a potentially rich source of biofuels and biomaterials in greater abundance. Cyanobacteria are among the oldest organisms in nature, responsible for generating the atmospheric oxygen we breathe today.
The pair’s work is meant to provide a vital foundation for optimizing a device known as a photo bio reactor (PBR), in which these energy-packed photosynthetic organisms can proliferate.
Dr. Kim explains, “Cyanobacteria are much easier to re-engineer because we have a lot of knowledge about them. We can control their growth so that we can produce large amounts of biofuel or biomaterial.
Culturing is akin to ‘farming’ the cyanobacteria, thus the new research indicates that the optimization of cyanobacterial growth requires a delicate interplay of CO2, phosphorus and sufficient light irradiation within the PBR vessel containing the microbial crop. The group’s foundational study provides quantitative tools for evaluating factors limiting production of cyanobacteria within PBRs – a critical step along the path to large-scale biofuel production. Results appeared recently in the journal Biotechnology and Bioengineering.
Photosynthetic cyanobacteria are amazingly productive – able to produce roughly 100 times the amount of clean fuel per acre compared with other biofuel crops. Because their survival needs are simple – sunlight, water, CO2 and a few nutrients – they do not require arable land to be taken out of food production. Rather, cyanobacteria can be grown in rooftop PBRs or wherever sufficient quantities of sunlight and CO2 can be provided.
Maybe . . .
Vannela notes, “The PBR uses solar photons as an energy source to convert CO2 to reduced forms such as biomass, proteins, lipids, and carbohydrates. It’s a biological reactor, fixing solar energy into very useful forms of energy for human society.”
Cyanobacteria reproduction achieves a high biomass yield and they are tolerant of a wide range of temperatures, salinities and pH conditions. In addition to biofuels, which are extracted from fat-containing lipids in the cyanobacteria, the organisms can also produce many chemically based materials useful for industrial applications, like biopolymers or isoprenes. Photosynthetic microbes are also valuable for the growing field of neutriceuticals, permitting the manufacture of anti-cancer agents from fatty acids or antioxidants like beta carotene.
The pair used wild type Synechocystis PC6803, cultured in a bench top PBR, and supplied with the customary growth medium, known as BG-11. A series of semi-continuous experiments were conducted, in which three principle variables were manipulated and the resulting growth of cyanobacteria, observed. These were CO2, light irradiance and phosphorus.
Kim explains, “In this study we found that phosphorus is really important.” The cyanobacteria were unable to make efficient use of carbon dioxide in their growth cycle until the BG-11 medium was supplemented with phosphorus. Augmenting the medium with additional phosphorus allowed higher biomass productivity in the bioreactor. Once the phosphorus limitation was overcome, light irradiance and CO2 became the limiting factors for growth. That’s the ‘maybe’ coming up.
Organically ready phosphorus isn’t a low cost fertility additive. Many believe its in short and expensive supply now.
In a series of experiments, the team simulated the natural pattern of light irradiance produced by sunlight, while carefully controlling the levels of CO2 (applied at 2.5, 5.0 and 7.5 percent) and phosphorus. Results showed that when all essential nutrients are supplied, light irradiance becomes the limiting factor, as the crowding of biomass within the containment vessel increasingly blocks available light to the cyanobacteria. This condition is overcome through periodic harvesting of biomass from the reactor. The advance of the team’s research was in quantifying these factors, in order to obtain optimal values for nutrients, CO2 and light irradiance. Now it’s known for the Synechocystis PC6803 species.
Vannela and Kim point out that while they supplied CO2 and nutrients including phosphorus to the PBR’s cyanobacteria in their experimental design, ultimately, the nutrient source could come from waste streams or be recycled from the harvested biomass, while the excess CO2 produced by power plants could fulfill the microbe’s respiratory requirements. Thus, a closed loop could be formed, generating useful energy from water contaminants and the CO2 currently contributing to greenhouse warming.
All very politically correct. The ‘maybe’ rises again as the effort to reduce power plant emissions is going to limit access to concentrated CO2. Coming up with a CO2 scrubber from the atmosphere is a matter of some concern. Add that to the phosphorus matter and ‘maybe’ has considerable impact.
People in the media and the politically active like to talk of CO2 sequestration. They might want to wise up and focus on CO2 concentration so worthy research has real prospects for contributing to fuel and chemical production.
Vannela and Kim prove that cyanobacteria could be very productive in a PBR. PBRs could be fed a medium with the rich phosphorus needed for cyanobacteria production. But coming up with cheap a CO2 stream and phosphorus enriched medium looks like quite a challenge. Cyanobacteria in a PBR offer a huge opportunity with huge challenges as well.
On the other hand, Kim is right that cyanobacteria could be highly engineered. Perhaps the next steps might be finding and designing a species that can grow abundantly in today’s low CO2 atmosphere with a lower phosphorus requirement.