Team:Amsterdam/test/max/Carbon

Traditional land-based crops are often seen as humanity’s greatest hope for self-sustained energy and commodity production. However, there is another organism which might have something to say about that. Cyanobacteria have been photosynthesizing and compounds for billions of years, and in this time, they have become quite good at it.

CO ₂

With the advent of human-induced climate change, the idea of a low-carbon, bio-based economy has come to the forefront. This type of economy has many benefits, primarily: reduced CO ₂ emissions, sustainable primary production, a robust food chain, and new business opportunities with high-skilled jobs [1]. Towards this end, agriculture has been the dominant sector so far, with biofuels and other commodities being primarily derived from carbon sources (e.g. glucose) produced from crops such as sugarcane or maize. However, this process requires multiple harvesting, extracting, and purifying steps with each step begetting its own inefficiencies [2,3].

A more straightforward approach involves using photoautotrophic microorganisms, such as our cyanobacterium Synechocystis , to directly convert CO ₂ to product [4]. Some advantages of using cyanobacteria over land-based crops, in addition to eschewing the need for inefficient intermediary steps, includes greater photosynthetic efficiency and a lack of competition for arable land with other food and feed crops [5]. Furthermore, Borak et al. [2] calculated the annual carbon flux from atmospheric CO ₂ to product for commonly used biofuel crops, with sugarcane having the highest carbon flux of 4 Mg _C ha ^-1 yr ^-1 (tonnes of carbon per hectare per year).

In order to compare this carbon flux with the production rates of Synechocystis , we first calculate what the minimum required production rate would need to be in order for Synechocystis to have the same carbon flux:

\[\frac{4\,Mg_C}{ha\cdot y}\,\frac{10^6\,g_C}{1\,Mg_C}\,\frac{1000\,mmol_C}{12\,g_C}\,\frac{1\,y}{8760\,hr}\,\frac{1\,ha}{10000\,m^2}\,\frac{600\,m^2}{35\,m^3}\,\frac{1\,m^3}{1000\,L}\,\frac{1\,L}{0.5\,gDW}=\frac{0.130\,mmol_C}{gDW\cdot hr}\]

For this calculation, we used a ground surface area-to-volume ratio of 600 m ² :35 m ³ , which is the same ratio as the large “algae farm” in Klotze, Germany [6]. Additionally, we also assume a biomass concentration of 0.5 g L ^-1 , which is easily attainable by Synechocystis [7].

We have compared (Table ??) this minimum production rate to that of other producing strains, namely our ∆fumC strain (strain 1) and the strain with the highest reported production rate (strain 2) [8]. We can see that Synechocystis has already been engineered to have production rates that are much higher than what is necessary in order to have the same carbon flux as sugarcane (the crop with the highest carbon flux), based on this calculation.

Strain	Product	Production rate (mmol C gDW -1 hr -1 )
Calculation	-	0.130
Strain 1	Fumarate	0.280
Strain 2	Ethanol	1.984

Table ??. Comparison of the minimum calculated production rate Synechocystis needs to have in order to have the same carbon flux as sugarcane to the production rates of other Synechocystis producing strains. Production rates are shown in mmol of carbon per gram dry weight per hour.

It should be noted that this calculation is rudimentary at best, and should not be taken as an absolute threshold. Production rates for both crops and cyanobacteria will vary due to numerous additional variables (e.g. light availability, culturing apparatus, temperature, CO ₂ concentration, etc.) that are wholly ignored in this calculation. However, in an attempt to compensate for this lack of detail, we chose conservative values for our assumptions, namely the ground surface area-to-volume ratio and biomass concentration. Other facilities, such as the one owned by Mera Pharmaceuticals (formerly Aquasearch, Inc) in Hawaii have a ground surface area-to-volume ratio of 100 m ² :25 m ³ , which is more than four times smaller than the ratio used in the calculation [2]. Furthermore, Synechocystis is capable of growing to concentrations ?? times higher than 0.5 g L ^-1 [7]. Therefore, rather than comparing exact production rates requiring great detail, we aimed to highlight the productivity disparity between land based crops and cyanobacteria even when the culturing capabilities of cyanobacteria are underestimated.
It is important to note as well that while the cultivation of crops has been refined for at least 23,000 years [9], the usage of cyanobacteria to produce commodity chemicals is around for less than 20 years [10]. If one looks at the progress in this field during the last decade; considers the remarkable cyanobacterial bloom in the scientific literature; and compares the current production rates reported [11] - it is easy to imagine the prominent role that cyanobacteria will play in the biobased economy and in ensuring the sustainability of our production processes.