New Use for Well-known Algae

Shock! Horror! Some treasured  molecular paleoenvironmental tools of organic biogeochemists—namely the alkenones, an esoteric group of long-chained compounds made by aquatic microalgae—are  being put forward  by O’Neil et al.¹ as candidates for conversion to jet fuel on an industrial scale! (See Jet Fuel from Algae?)

To burn them? It seems like sacrilege! But hold on—we still get to extract fossil alkenones from  buried ocean sediments and use them  to suss out  past climates^2-4. This  latest research on these unusual  algal lipids, really opens up a new  line in the search for abundant biofuels. The researchers use some pretty snazzy metathetical organic chemistry with butene, and a ruthenium complex as catalyst, to snip these long-chained, C₃₅-to-C₄₀ alkenones into  convenient C₁₀-to-C₁₉, kerosene–size bits.

Ever since satellites first scanned the oceans, we’ve known of the enormous scale of biosynthesis of microscopic marine algae such as the calcareous alga Emiliana huxleyi. Their blooms appear every year in their quintillions as giant swirls of milky surface water, hundreds of kilometers across, particularly in the North Atlantic off Iceland, Greenland, and the British Isles. Indeed, the waters of the North Sea are notorious for clogging fishing nets with thick oily layers blamed on spills from oil drilling platforms. We now know that frequent culprits are alkenones from algal blooms;  these lipids can make up to 20 percent of the algae’s cellular carbon. A related algae, Isochrysis, is also a source of these alkenones.

Chemists first realized more than 30 years ago that the relative individual abundances of these unusual, environmentally very persistent biomolecules, when  extracted from ancient buried marine sediments, could be used as a proxy for the paleo-sea surface temperatures of those past times. The algae obligingly produce different ratios of the various alkenone molecules in proportionate response to changes in sea surface temperatures! Later on, with compound–specific carbon isotopic data, their use was even extended to estimate paleo-atmospheric carbon dioxide concentrations (pCO₂).

These molecular proxies have now been applied worldwide to estimate sea surface temperatures and atmospheric carbon dioxide concentrations for ancient time horizons spanning millions of years, but especially for the  last few hundred thousands of years of the recent ice ages. These records of ancient ocean temperatures are now an essential part of the enormous modeling efforts currently being made by the scientific community to understand what drove past climate change and, presumably, drives it now. We will need this knowledge if we are to take action aimed at avoiding future global disaster.

As a  cost–effective fuel feedstock, whether from algae harvested direct from the oceans or  from tank farms, alkenones may be a long way off, but, in principle, they would meet one requirement of tomorrow’s energy-hungry economies—a fully bio-renewable, carbon-based fuel, effectively capable of annually recycling CO₂ on a truly global scale.

¹ O’Neil et al., Energy & Fuels, January 2015

² The WHOI Workshop on Alkenone–Based Paleoceanographic Indicators, 1999. Geochemistry, Geophysics, Geosystems, 2001 DOI:10.1029/2000GC000122

³ Eglinton & Eglinton, Earth and Planetary Science Letters, 2008, DOI:10.1016/j.epsl.2008.07.012

⁴ From Mud to Molecules: WHOI podcast with Geoff Eglinton