University of New Haven
As humanity seeks new forms of energy, one promising avenue is biofuels. Biofuels are renewable, liquid fuels, and they can be extracted from biomass such as algae [1]. Algae includes both microalgae as well as macroalgae. Seaweed is a well-known macroalgae that includes a collection of multiple species of marine plants [2]. Seaweed could provide a valuable source of biofuel that can be of great use to the world as society moves to cleaner, more renewable sources of energy.
The purpose of the experiment conducted was to cultivate seaweed collected from Long Island Sound to extract oil that could be converted into a useable biofuel. There are many different types of seaweed, and this experiment attempted to determine which contained more oil after a cultivation period.
The process began with collecting three different types of seaweed from Silver Sands beach in Milford CT. Red, green, and brown were collected with some seawater to help preserve the samples. Each type of seaweed was then divided into two separate containers: a container with carbon dioxide (CO2) and oxygen (O2) gas sensors as well as another container with a pH and conductivity sensor. After 3 weeks, the samples were degraded by mold and could not be used for oil extractions. A second trial was restarted after collecting fresh green and brown seaweed and passing freshly collected seawater used for seaweed cultivation through a UV sterilizer. After 2 weeks, there was significant mold growth, but the samples were usable for extractions. A Soxhlet extraction with hexane was used to remove the oil. This process works by boiling a solvent (hexanes) and using the heated solvent vapor to leach oil from the dried seaweed before the vapors condense back down into the boiling flask containing the liquid hexanes. A rotary evaporator was then used to separate the oil from the hexanes.
It was observed that mold growth is very common after a short period of time on seaweed samples. It reduced the structural integrity of each seaweed sample over the weeks of cultivation. During the first week of the first trial, CO2 concentration had a sharp increase while O2 decreased in each sample. In the green seaweed sample, after the initial rise, CO2 concentration started to drop and level off with the O2 levels. The first brown seaweed sample continued to see CO2 steadily drop and O2 rise slightly. The red seaweed sample had CO2 concentration drop after an initial jump, but then it rose dramatically again despite the O2 concentration remaining at a steady level. In each sample, conductivity showed a cyclical correlation with the 12-hour light cycle. The pH of both the brown and red seaweed samples appeared to increase slightly over time reaching 7-8. In the green seaweed sample, the pH slowly declined for about a week and a half until reaching an unexplainable gap in data that would then see it increase after the gap about a day and half later. This was likely due to sensor malfunction. The second round of samples collected shows similar results to the first round. There is an initial increase of CO2 and drop in O2. This is followed by the O2 rising slightly at about 120 hours and leveling off, occasionally dropping and rising again in both green and brown seaweed samples. The conductivity showed the same patterns as the first trial, maintaining the cyclical nature. The pH in both samples dropped until about 50 hours reaching levels as low as 5. Afterwards the pH steadily rose in both samples reaching levels of 7-8. For the extractions performed with the second trial of samples, 0.53 grams of oil from 6.0 grams of dried green seaweed were extracted. From the 5.5 grams of dried brown seaweed, 0.56 grams of oil were extracted. This resulted in oils yields of 8.8% and 10.2% for green and brown seaweed respectively.
The CO2 spikes and O2 drops appear to be due to the mold growth. The first week of cultivation is when the mold began to have noticeable development, which appears to correlate to the CO2 spike in the first 100 hours along with the large O2 drop. This makes sense as the mold consumes O2 and releases CO [3]. It has been observed in other studies that mold can increase pH in other substances such as tomato juice [4]. It is possible that it has the same effect on seawater as nutrients are used and the mold grows, causing the pH spike seen in the data after the initial ~50 hours.
The results show that very small amounts of oil can be extracted from seaweed samples and that mold growth is common. Further testing to get rid of mold growth is necessary to determine how it impacts oil content. Once mold growth can be controlled or eliminated, methods of better cultivating seaweed are needed to produce higher oil contents.
Sources:
[1] Biofuel Basics | Department of Energy. (n.d.-a). https://www.energy.gov/eere/bioenergy/biofuel-basics
[2] US Department of Commerce, N. O. and A. A. (2009a, September 24). What is seaweed?. NOAA’s National Ocean Service. https://oceanservice.noaa.gov/facts/seaweed.html
[3] Miller, D. D., & Golding, N. S. (2010, May 23). The gas requirements of molds. v. the minimum oxygen requirements for normal growth and for germination of six mold cultures,. Journal of Dairy Science. https://www.sciencedirect.com/science/article/pii/S0022030249920172?ref…
[4] Mundt, J. O. (1978, April 1). Effect of mold growth on the ph of tomato juice. Journal of Food Protection. https://www.sciencedirect.com/science/article/pii/S0362028X23023578?ref…