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Revision as of 05:19, 11 October 2016
Mechanisms of gas production
An important consideration for balloon production was the actual mechanism of flight. With Mars’ surface pressure being only 0.6% that of Earth’s, [1] we ideally wanted a gas less dense than air (1.293 kg/m3 at 0˚C [2]). Although helium gas would have been preferable to experiment with due to its low density and inflammability, there are no biological or chemical methods of creating helium. We instead opted for hydrogen gas, which has a density of 0.0899 kg/m3 at 0˚C [2] and is producible using the alga Chlamydomonas reinhardtii.
Generally, C. reinhardtii absorbs CO2 from the atmosphere to create various carbohydrates, releasing oxygen gas as a byproduct. [3] Green algae in particular can produce hydrogen gas instead of oxygen through a process called direct biophotolysis, [4] where a reversible hydrogenase enzyme catalyzes a reaction between photosystem II and ferredoxin. The water-splitting reaction of photosystem II creates electrons that are sent to ferredoxin, and the reversible hydrogenase in the stroma of the chloroplast combines these electrons with free-floating protons in the medium to create H2. [5] Our problem was that C. reinhardtii’s reversible hydrogenase is extremely sensitive to O2 pressure, and will irreversibly inactivate upon sensing oxygen. Thus the algae's photosynthetic production of H2 and O2 must be temporally separated, and can be done upon inducing two-stage direct biophotolysis. CO2 is fixed normally through oxygenic photosynthesis during Stage 1, and H2 is then generated under anaerobic conditions in Stage 2. This can be easily done by depleting sulfur from the culture medium. [6]
Prior research by Jo, et al. has shown that optimizing growth conditions for C. reinhardtii can result in a little over 2mL of H2 produced for a 10mL culture of alga after 96 hours. [7] This optimized process gave a hydrogen production rate approximately 1.55 times higher than typical cultivation in sulfur deprived TAP medium. Our process was not perfectly optimized, so we expected a gas production rate of approximately 1.33mL of H2 for a 10mL culture of alga after 96 hours.
References
1. http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html
2. http://www.engineeringtoolbox.com/gas-density-d_158.html
3. Melis, A. Photosystem-II damage and repair cycle in chloroplasts: what modulates the rate of photodamage in vivo? Trends Plant Sci. 1999, 4, 130-135.
4. Miura, Y. Hydrogen production by biophotolysis based on microalgal photosynthesis. Process Biochem. 1995, 30, 1-7.
5. Adams, M. W. W. The structure and mechanism of iron-hydrogenases Biochem. Biophys. Acta 1990, 1020, 115-145.
6. Ghirardi, M. L.; Zhang, L.; Lee, J. W.; Flynn, T.; Seibert, M.; Greenbaum, E.; Melis, A. Microalgae: a green source of renewable H2. Trends Biotechnol. 2000, 18, 506-511.
7. https://www.ncbi.nlm.nih.gov/pubmed/16599558
Prior research by Jo, et al. has shown that optimizing growth conditions for C. reinhardtii can result in a little over 2mL of H2 produced for a 10mL culture of alga after 96 hours. [7] This optimized process gave a hydrogen production rate approximately 1.55 times higher than typical cultivation in sulfur deprived TAP medium. Our process was not perfectly optimized, so we expected a gas production rate of approximately 1.33mL of H2 for a 10mL culture of alga after 96 hours.
References
1. http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html
2. http://www.engineeringtoolbox.com/gas-density-d_158.html
3. Melis, A. Photosystem-II damage and repair cycle in chloroplasts: what modulates the rate of photodamage in vivo? Trends Plant Sci. 1999, 4, 130-135.
4. Miura, Y. Hydrogen production by biophotolysis based on microalgal photosynthesis. Process Biochem. 1995, 30, 1-7.
5. Adams, M. W. W. The structure and mechanism of iron-hydrogenases Biochem. Biophys. Acta 1990, 1020, 115-145.
6. Ghirardi, M. L.; Zhang, L.; Lee, J. W.; Flynn, T.; Seibert, M.; Greenbaum, E.; Melis, A. Microalgae: a green source of renewable H2. Trends Biotechnol. 2000, 18, 506-511.
7. https://www.ncbi.nlm.nih.gov/pubmed/16599558
Experimental design & data
We grew 500uL of C. reinhardtii in 10mL sulfur-free BG-11 media, with exposure to light at 21 W/m2 in 12 hour cycles. After the algae had grown to a sufficient density, the flask was covered in aluminium foil to briefly deprive the algae of light, and a commercial latex water balloon was inserted over the mouth of the flask for gas collection.
