Trends in Biotechnology
OpinionDiatoms: a fossil fuel of the future
Section snippets
The need for carbon-neutral fuels
The first major oil well, drilled in 1859 by Edwin Drake, supplied cheap fuel for kerosene lamps, and led to a dramatic reduction in the demand for whale blubber. Although the use of kerosene as a fuel for lighting can be claimed as saving whales from being hunted to extinction, there were other unintended consequences to follow. The subsequent invention of internal combustion engines provided a huge demand for gasoline, which previously had been a worthless byproduct of kerosene distillation.
Diatoms are major sources of fossil fuels
Diatoms (see Glossary) are unicellular eukaryotic algae that entered the fossil record ∼150 million years ago [3]. They are secondary symbionts, distinguished from most other algal forms by possessing a siliceous shell, or frustule. They rose to ecological prominence ∼34 million years ago in the Oligocene, with the opening of the Drake Passage and subsequent global cooling [4]. Their ecological success introduced a major source of organic carbon for marine food webs, leading to the formation of
Targeting diatom lipids for biofuel production
To date, biofuels have been clustered into four generations of innovation. The first and second generations are based on higher plant oils, which can satisfy only a small fraction of the existing demand for transportation fuels, without competing with crops for food and other resources 12, 13. The third and fourth generation biofuels are based on algae, and have received a great deal of attention in the past ∼50 years.
Diatoms are extremely successful in the contemporary oceans. They often
The scaling problem in displacing petroleum
The physical and chemical properties of algal biodiesel are similar to petroleum-based diesel fuels (the latter being derived from the former), and thus require little or no modifications for use in conventional engines 24, 25, 26. Yet, to be economically competitive with fossil petroleum, there are major hurdles to be overcome, starting with identifying the best strains through optimizing cultivation, harvesting, extracting, and refining. Engineering solutions to these problems are required to
Boosting lipid production in diatoms using environmental manipulations
The production of diatoms in photobioreactors, as well as indoor and outdoor ponds, has been demonstrated for >50 years 31, 32, 33, and large-scale culture of diatoms is used for feeding shrimp and mollusks in commercial aquaculture 34, 35, 36, but diatoms have been largly neglected as a biofuel feedstock. In the laboratory, many researchers have searched for the ‘sweet spot’ of controlling the ‘carbon decision tree’ for switching between biomass accumulation and lipid production in algae in
Diatoms as fourth-generation biofuels
Although third-generation strategies may prevail, genetic manipulation of cells is also possible. The aim of the so-called fourth-generation biofuel is to co-opt basic biochemical pathways by using molecular genetic tools to generate photoautotrophic algal strains with high lipid yield. To date, there is abundant literature on genetic manipulation of algae that focuses on green algae in general, and on Chlamydomonas reinhardtii in particular. However, it is unlikely that this freshwater green
Optimizing the efficiency of lipid production
An objective metric of a production system is based on the concept of EROI, which is the ratio of the energy obtained to the amount of energy invested. Although there are large uncertainties at virtually all steps of the biofuel production process, an EROI higher than the break-even value of 1 requires an algal biomass productivity above ∼17 g/m/day, corresponding to ∼1.2 g C/m/day, in purely photosynthetic systems [62]. Energy costs could be influenced by managing nutrient supply, lipid
Concluding remarks and future perspectives
A major portion of current high-quality petroleum is derived from fossilized diatoms. The rise of diatoms to ecological prominence over the past ∼34 million years is due to their high photosynthetic energy conversion efficiency and rapid uptake and assimilation of nutrients. It would be worthwhile to take advantage of the natural selection of these organisms that have evolved such high photosynthetic energy conversation with lipids as their primary storage product. Growing diatoms as feedstock
Acknowledgments
Our research is supported by the United States Department of Energy (DOE) Consortium of Algal Biofuels Commercialization (CAB-Comm) program, a gift from James G. Gibson to PGF, the Bennett L. Smith Endowment, and the Rutgers Energy Institute. We thank Benjamin Van Mooy for analyzing the TAG amounts and composition of our Phaeodactylum tricornutum cultures. We thank Robert Kopp for comments about life cycle assessments. Confocal microscopy to view lipid bodies was done at Rutgers University SEBS
Glossary
- Aquatic Species Program (ASP)
- the first comprehensive project to estimate the potential of algae as biofuel feedstock, run by the US Department of Energy (DOE) from 1978 to 1996 with an overall investment of more than $25 million USD. Most studies focused on induction of lipid production in the tested strains under different environmental conditions.
- Biologically based renewable fuels (biofuels)
- alternative fuel sources based on converting living organisms to fuel within a short to intermediate
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