Lipids are a general set of cellular components that are grouped together by the common trait that they are soluble in non-polar solvents. Throughout living organisms, there are sev­eral sources of lipids that play various roles in biochemical processes including energy stor­age and water insoluble nutrient transport across cell membranes that include neutral lipids, phospholipids, steroids, waxes, and carotenoids. Since lipids have a generally low oxygen and high carbon and hydrogen content, they are very energy dense molecules. This charac­teristic, along with their natural abundance and similarities with petroleum based fuels, make them ready targets for processing and use as a blend or replacement to traditional fuels.

Neutral lipids (commonly referred to as "fats"), which are widely regarded as one of the most common sources of lipids, and which has the highest potential for use as an alternative fuel, can be found in various forms throughout different organisms, and will be the primary topic of focus for this discussion. Most marine and aquatic biomass can store lipids within the cell that can range from a small fraction to upwards of 80% of the cellular weight. Due to this trait, research and production scale operations have been centered on utilizing aquatic biomass for lipid production and conversion to fuel with the remaining cellular components being recycled for mineral content or discarded.

Scheme 1. Transesterification reaction schematic

Figure 2 shows an example of a marine diatom Niztchia sp. stained with Nile Red fluores­cence stain (red color shows chlorophyll and yellow shows lipid fluorescence).

Neutral lipids consist of a glycerol molecule (a three carbon alcohol) and one to three fatty acids (referred to as mono-, di-, or tri — acylglycerols depending on number of fatty acids present) with the fatty acids being various carbon chain lengths and having various levels of unsaturation (unsaturated, mono-unsaturated, poly-unsaturated, etc.). Fatty tissues in ani­mals serve as both an energy storage mechanism as well as a means of insulation against temperature extremes. Algae primarily store fats in the cell membrane to serve as an energy storage medium as well as a nutrient transport system to shuttle metabolites into and out of the cell. Several studies have been conducted to attempt to identify the distribution of fatty acids in algae and other aquatic biomass [13—15].