Humans have been pumping carbon dioxide into the air at an exponential rate ever since the coal-powered steam engine was invented in the 1750s. The increased rate of CO2 entering our atmosphere (ranging in the gigatons per year) has lead to a number of environmental effects that have direct impacts on human well-being.
Normally, there’s a certain percentage of radiation from the sun that’s absorbed by the Earth and turned into heat. The rest of the radiation is reflected back into space. Carbon dioxide, like water vapor, is a gas that absorbs the radiation that is normally reflected out of the atmosphere. Therefore, more of the sun’s energy is turned into heat. That extra heat is used to power more intense weather events such as hurricanes and can raise the overall temperature of the planet, leading to ice reserves melting and rising sea levels.
Unfortunately, carbon dioxide stays in the atmosphere much longer than water vapor. Carbon dioxide does not liquify or solidify like water at the temperatures that are normally found on Earth. The rate at which carbon dioxide is pumped into the atmosphere also far exceeds the rate that it is absorbed by plants and other carbon-sinks. As a result, according to many climate models, climate change is already irreversible. A large percentage of the CO2 that we are pumping into the atmosphere will likely remain for millenia, continuously trapping heat, unless we figure out a way to capture emissions faster than we produce them.
If you leave a glass of pure water out overnight, you can watch as the pH of the water changes from neutral to slightly acidic. The reason this happens is because carbon dioxide from the air gets absorbed into water and turned into carbonic acid. This effect also occurs in the Earth’s oceans on a much larger scale.
Before the industrial revolution, the Earth’s oceans were at an equilibrium. The amount of carbonic acid formed was stable because the amount of CO2 in the atmosphere was stable. As humans started pumping more CO2 into the atmosphere, the oceans started absorbing a greater amount of CO2. Roughly the same amount of CO2 absorbed by plants since the industrial revolution has been absorbed by the ocean. As a result, the ocean has become 30% more acidic. This increase in acidity has caused a significant decline in the amount of coral reefs as their calcified structures dissolve in acidic environments.
Even if humankind stops emitting CO2 today and uses only renewables, the effects of climate change will still exist for thousands of years.
The best carbon capture technology costs $94 to $232 per ton of CO2 captured [11]. In order for the world to go carbon neutral (not even carbon sink), ~37 billion tons of CO2 would need to be sunk each year [12]. The cost of sinking that much carbon (at a modest estimate) would cost ~3.5 trillion dollars per year. Unless a major and highly coordinated global government action is taken, it will never be economically feasible to stop, much less reverse the effects of global warming.
Unless the cost per ton of CO2 captured is reduced by orders of magnitude or becomes economically profitable, carbon capture on a mass scale will likely never be adopted.
Luckily for us, nearly 2.35 billion years ago, nature came up with the solution to high atmospheric CO2 levels. This solution is known as the “Great Oxidation Event.” Cyanobacteria, the first oxygen-releasing photosynthetic organisms, changed the composition of the atmosphere from containing large amounts of CO2 and virtually no oxygen, to being nearly as oxygen rich as our atmosphere is today.
Cyanobacteria are one of the only organisms that have a historical precedent for changing the entire world’s climate and atmospheric composition. When trying to solve our CO2 problems today, it makes sense to look back on nature’s example. Therefore, for our project, we chose to genetically engineer a well-studied and moderately fast-growing strain of cyanobacteria for the purpose of carbon sequestration. Specifically, we will engineer our cyanobacteria to produce sucrose, which can serve as a feedstock for industrial fermentation, used to produce PLA plastic and ethanol biofuel. By using cyanobacteria-derived sucrose as opposed to corn-derived sucrose, food prices for developing nations are not driven up, and unsustainable amounts of fertilizers and fossil fuels are not required [1]. By using cyanobacteria-derived sucrose over sugarcane-derived sucrose, deforestation and disruption of the natural ecosystems in the Amazon rainforest are reduced [2].
Photosynthetic cyanobacteria strains such as Synechococcus elongatus PCC 7942 can produce sucrose more efficiently than both corn and sugarcane. S. elongatus fixes CO2 and secretes roughly 80% of fixed carbon as sucrose without the need for cell-harvesting [3]. Crops such as corn and sugarcane only allocate 20% of their fixed carbon as sucrose, and large amounts of land and fertilizer are required for their production. Because of the high efficiencies involved, sucrose production from cyanobacteria may be an effective method for carbon sequestration by converting CO2 into sugars that can be used to make stable, high-value products such as bio-plastics.
Ducat et al. and Xuefeng et al. both successfully engineered S. elongatus to produce and secrete sucrose in large quantities; however, the process has not been commercialized because of harvesting costs and inefficient photobioreactor design [4].
Our goal to make cyanobacteria carbon sequestration industrially viable. We characterized cscB - a sucrose symporter protein - as well as the ferric-ion repressible promoter associated with the idiA gene [7,8], the high-light inducible promoter of psbA2 [9], and strong constitutive promoters of cpcB native to S. elongatus and Synechocystis sp. PCC 6803 [10]. We also constructed novel gene circuits, making sucrose production and secretion easy to toggle.