Biofuels represent a significant potential sustainable energy source, yet land-based biofuel crops pose land use challenges, using large stretches of arable land that could otherwise be utilized to produce food. Scientists are now turning to an ocean-based alternative: seaweed. Unlike other raw plant materials, brown seaweed does not contain a lignin structure. Biodegradation of lignin is a requirement for processing biofuel from raw plant materials. The sugars in brown seaweed can be released by simply milling or crushing the macroalgae, eliminating the need for a pretreatment and hydrolysis process before fermentation and lowering the potnential cost of bioconversion of the raw plant material into ethanol. Brown seaweed contains four types of sugars—alginate, laminarin, mannitol, and cellulose. Of these, alginate is most abundant. Previous attempts to convert macroalgae into biofuels were limited by the availability of microorganisms that could metabolize alginate polysaccharides. Wargacki et al. (2012) have engineered a microbial platform for direct ethanol synthesis of brown seaweed. The newly designed microbial platform can simultaneously degrade, uptake, and metabolize alginate to produce ethanol. The strain could potentially be engineered to produce a wide range of chemical and fuels. This project is a significant step in realizing production of renewable fuels from sustainable and scalable biomass sources. —Meredith Reisfield
Wargacki, A., Leonard, E., Win, M., Regitsky, D., Santos, C., Kim, P., Cooper, S., Raisner, R., Herman, A., Sivitz, A., Lakshmanaswamy, A., Kashiyama, Y., Baker, D., Yoshikuni, Y., 2012. An Engineered Microbial Platform for Direct Biofuel Production from Brown Macroalgae. Science 20, 308–313.
Wargacki et al. Engineered the bacterium Escherichia coli to digest brown seaweed and ferment ethanol. They isolated the genes of a marine microbe, Vibrio splendidus, that were responsible for breaking down alginate into simple sugars like pyruvate, and inserted the genes into an E. coli strain. This modification of E. coli allowed the bacterium to take up alginate oligomers which carried a stretch of DNA bearing genes for alginate-degrading enzymes. The strain was further engineered to convert sugars into ethanol and enable a single-step manufacture of ethanol from brown seaweed. The researchers added a chemical pathway borrowed from Zymomonas mobilis, a bacterium isolated from cane juice, to turn the pyruvate into ethanol.
When the new strain was fed crushed kombu (a common brown seaweed), the cells fermented a concentration of 5% ethanol, which is comparable to the benchmark for bioconversion of woody biomass. The U.S. Department of Energy reported a macroalgae productivity of 59 dry metric tons/ha/year, and an ideal ethanol yield from macroalgae of 0.254 weight ethanol / weight dry macroalgae. These numbers estimate an peak bio-ethanol productivity of 19,000 liters/ha/year, approximately twice the ethanol productivity from sugarcane and 5 times the ethanol productivity from corn. Initial evaluations of the E. coli platform in ethanol production achieved over 80% of the maximum theoretical yield of ethanol from sugars in seaweed. The engineered strain successfully processed alginate and glucose in addition to increasing the rate of mannitol fermentation. These results support the team’s hypothesis that alginate pathways play a significant role in balancing intracellular redox reactions by consuming excess-reducing equivalents generation in the fermentation of mannitol.
Brown seaweed seems to be an ideal source for the production of biofuels since it avoids the economic concerns associated with land management and the adverse effect on global food supplies, eliminating the “food versus fuel” concerns presented by the production of ethanol using sugar cane and corn. A few barriers remain to be conquered before we can take advantage of biofuels from beneath the waves. Seaweeds are currently harvested for use in human consumption, animal feeds, and agricultural fertilizers and are not currently grown at a sufficient scale for wide use as fuel. Although brown seaweed holds several advantages over currently existing methods to produce ethanol via microbial fermentation of biomass, the question remains whether seaweed can be produced at a scale that would have a significant impact on the global energy economy. The platform engineered by Wargacki, et al. is a significant step toward scalable and diverse feedstocks that would enable sustainable use of biomass technologies.
Ocean Thermal Energy Conversion (OTEC) is recognized as a significant potential renewable energy resource for coastal communities, yet OTEC plants can only be constructed on sites that meet key logistical and technical requirements. Finding a suitable candidate site requires extensive study of the atmosphere, ocean and seabed environment surrounding the location where the plant will be installed. McCallister et al. (2010) studied the specific aspects of a candidate site for installation of an OTEC pilot plant off Pearl Harbor, O’ahu, Hawai’i. Using geophysical information and oceanographic data, the authors were able to conduct a preliminary survey of the installation area and identify potential risks and hazards at the installation site. The Pearl Harbor site was found to meet the key characteristics of a preliminary study, including a sufficient temperature differential (more than 20° C difference from deep water to cool water) and a favorable seafloor environment for anchoring the physical system. The site also presents low seismic and tsunami risk, moderate risk of mass sediment movement, and a significant long-term risk presented by tropical cyclones. Additionally, the authors explored the data collection and physical characteristics required to undertake installation of an OTEC system. –Meredith Reisfield
McCallister, M., Switzer, T., Arnold, F., Ericksen, T., 2010 .Geophysical and oceanographic site survey
OTEC works by utilizing the difference in temperature between deep water and surface water to drive turbines and generate electricity. A fluid with a low boiling point, typically ammonia, is circulated through a network of pipes. The fluid is vaporized by a heat exchanger warmed by surface water, and the resulting gas is used to spin a turbine and generate electricity. The gas is then cooled by a second heat exchanger and cooled by seawater pumped up from lower depths. It condenses into a liquid and the process is repeated. Pearl Harbor is the site of a ten-megawatt OTEC pilot plant that should be operational by 2015.
McCallister et al. obtained the geophysical data required to install an OTEC system through high-resolution sonar and video imaging. The O’ahu site was found to share common characteristics of favorable OTEC locations, including a steep slope, irregular bathymetry (depth of ocean floors), and uncertain sediment thickness. Sediment sampling revealed significant sediment thickness, suggesting that the O’ahu site could support the multiple anchoring locations required to secure the physical system in place. Additionally, an examination of the primary risks to facility found the highest potential risks to be from major caldera collapse slides, which would cause the anchors to destabilize. There is also the prospect of tropical cyclones damaging equipment.
McCallister et al. also discuss the need for extensive knowledge of physical and chemical seawater properties, recorded in situ over time to observe seasonal and local variations in temperature and current structure. Nutrient levels must be monitored at a variety of depths. The introduction of nutrient-rich deep waters into the nutrient-poor surface waters by the operation of the plant could stimulate plankton blooms and adversely affect the local ecological balance. Additional ecological challenges arise from the potential presence of endangered of threatened species, biological nursery areas, and other sensitive habitat. Since OTEC facilities must be located near islands or within several miles of coastal zones due to cabling constraints, near-shore circulation patterns play a crucial role in determining the thermal structure of the water column. Upwelling, bringing deep water closer to the surface, can benefit an OTEC facility by reducing the required length of a cold-water intake pipe. Downwelling, by contrast, can push colder waters out of range of the cold-water pipe. Thus, the construction of an OTEC plant requires a detailed model of the water column’s temperature structure across myriad conditions.