Solazyme, a San Francisco based algae biofuel company has received a lot of press and attention lately, including a large amount of funding and a mention in last week in the economist http://www.economist.com/business-finance/displaystory.cfm?story_id=15773820. As the economist says, Solazyme is an anomaly. Unlike other biofuels companies, they say they will be delivering large volumes of algae oils soon (20,000 L of algal based biofuel to the US Navy). This feels like progress for the algae industry and its nice to see that a company that is aiming to produce algae based fuel actually doing so . But when you look into the technology Solazyme is using to make this possible, you will see that they they are using algae in a different way. They are growing algae to make lipids but they are not harnessing the power of the sun and photosynthesis. They are growing algae in a dark fermentor - essentially using the algae to convert sugars to lipid. This may be a clever loophole in the system of making high energy fuel from lower energy biomass, but its difficult to imagine this system can be a sustainable.
The science behind what Solazyme is doing:
Solazyme is growing algae in the dark. This may seem counter-intuitive since most of us know algae as tiny photosynthetic organisms. Photosynthesis is one of the only things that links all the algal groups together - since they don't all share a common evolutionary history, only a common functionality. Typically these organism only get attention as phototrophs, but actually most of them are photoheterotrophs. This means that in the presence of light, these organisms will photosynthesize. In the dark, they can respire - using biosynthetic pathways similar to many other heterotrophic organisms. In general, "respiration" just refers to any metabolic process that produces carbon dioxide.
In algae, the dark respiratory processes appear to function in the light to some degree as well as in the dark, because the various carbon compounds needed for growth can be synthesized through different pathways. In the dark where oxygen is not present, the organisms will undergo fermentation and in the presence of oxygen, they will employ glycolysis for metabolism, just like you and me.
Why would an organism do both photosynthesis and respiration? Well, why wouldn't an organism do both if it could. Since we tend to be very animal centric when it comes to thinking about biology, we think its weird to be metabolically diverse since we are so metabolically constrained. We can only gain energy from cellular respiration. The organisms that have multiple modes of metabolism hedge their bets and can survive and even grow and reproduce under a range of conditions. It turns out that most algae grow just as well using external sugar as they do photosynthetically. Depending on the condition, algae can do a combination of these metabolisms.
Solazyme is growing their alage in fermentation reactors - presumably without oxygen and definitely in the dark. See the schematic below from their web site.
However, Solazyme's process is still dependent on photosynthesis, which is responsible for creating biomass that is then fed to the algae. They say their system can utilize any kind of biomass available to feed the alage, but they still need to convert it to sugar which can be one or more production steps. It is understandable why they took this route. They can consistently grow algae in high density, they don't need to worry about how to get light into a closed system, and they can use a tried an true reactor system. But how can it be sustainable?
Sustainability, economics, and energy mass balance:
It appears that Solazyme is collaborating with a BlueFire Energy, a next - generation sugar producer (http://www.greentechmedia.com/articles/read/from-hype-to-reality-not-all-algae-were-created-equally/ ) that can produce sugar from agricultural waste products, recycled paper, and other sources of biomass. If so, this seems like a reasonable source of sugar. Solazyme's dependance on this other system of producing sugar means that there are a lot of energy requiring steps involved. There is energy used to grow, harvest, or collect the initial biomass. There is energy used to move the biomass to the sugar production facility and then the sugar to the algae production facility in addition to the energy used in extracting the oil from the algae. It is hard to imagine all these energy inputs could equal the energy out. We know that biofuel made from conventional crops suffers from this problem. For the production of biofuel from any source to be a solution to diminishing energy resources, it needs to be as simple as possible
I feel that what really makes algae based biofuel a sustainable option (if not an economically viable one) is that it can remediate nutrients in waste water and can utilize carbon dioxide that would be emitted to the atmosphere. In this situation, we envision a closed loop. If the energy provided for photosynthesis comes from the sun, then there is very little energy in so that any energy out is a bonus to the benefits of remediation. In working toward sustainability, Solazymes fermentation reactors could be co-located with its sugar production to limit the energy in. Because all algae companies technologies are top secret, I can't claim to understand everything Solazyme is doing, but it is important to keep sustainability at the forefront of our technological development.
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