Over the past five years or so, there has been a huge upsurge in interest in producing microbial oils that could be used for the production of biodiesel. The production of biodiesel itself is now well established using a variety of plant oils, principal of which are palm, rapeseed, and sunflower oil. The oils are transesterified to give the methyl esters of the fatty acids which are then directly usable in motor engines. There is no intrinsic difficulty in producing biodiesel from these sources at competitive prices. But, in spite of all the current interest in microbial and especially algal oils for this purpose, are these economically realistic as alternative sources of fatty acids for biodiesel manufacture? From about 2006, there have been major developments, mainly in the USA, in using photosynthetically-grown algae as potential sources of biodiesel oil with over 200 start-up companies being launched 1.
The requirement for making a satisfactory biodiesel is that the TAG oil should be rich in saturated and MUFA, such as palmitate (16:0), stearate (18:0), and oleate (18:1). Unsaturated fatty acids, particularly the polyununsaturated fatty acids with three or more double bonds, are undesirable as they readily auto-oxidize making the final biodiesel technically unsatisfactory as well as giving it an unpleasant smell. They should therefore be avoided. Also, and obviously, any source of fatty acids that seeks to be an alternative to the currently used plant oils should be as cheap as them, if not cheaper. On the face of it, algae look very promising sources of oils from which fatty acids can be derived. Species are known that produce over 50% of their biomass as lipid and some will even produce up to 70% under the best growth conditions. Being photosynthetic, they should be cheap to grow as they do not require a fixed source of carbon but instead use CO2 with sunlight; photosynthesis then provides the necessary energy to convert the CO2 into biomass and lipid. They do though need to grow in water and, because of their photosynthetic nature, they need to grow on the surface of the water so that the maximum photosynthetic efficiency can be achieved. However, because they grow more rapidly than plants, they can be harvested numerous times throughout the year and not just the once as with plants.
Unfortunately, as exciting as the prospects of achieving cheap oils for biodiesel may appear using algae, there are massive problems which, as yet, appear insuperable.
Firstly, algae, like all other oleaginous microorganisms, need carbon to be in excess for lipid accumulation to occur. Lipid accumulation only begins when the organism is faced with a surplus of carbon (i.e., CO2). When the accumulation of oils by various algae has been recorded, these algae have invariably been grown in the laboratory in photobioreactors with an enrichment of CO2 in the inlet air. As it is prohibitively expensive to consider using photobioreactors of any design on a scale large enough to produce significant quantities of algal biomass, one has to consider how is the necessary CO2 going to be bubbled into large ponds and lagoons, where will it come from, and at what cost? Unless CO2 is provided, the alga of choice will never become lipogenic as the normal growth condition is for the alga to be carbon-limited. Under such conditions, the lipid content of the cells will probably not exceed 10% of biomass and, moreover, the majority of lipids will be complex ones being involved with the photosynthetic apparatus and therefore be unsuitable for conversion into biodiesel.
Secondly, there is the cost. As already indicated, photobioreactors of whatever design are too expensive to be used to produce a cheap product. Tubular photobioreactors, though, are used on a commercial scale but only to produce high-value products such as astaxanthin. Costs of such systems are probably in the region of US$40/kg biomass: if an oleaginous alga were then grown similarly giving an oil content of, say, 50% this would mean that the cost per tonne of oil would be in the region of US$80 000 2. This compares to the current prices of most plant oils of less than $900/tonne and the price of crude petroleum oil of about $500/tonne (=∼$70/barrel) 3.
But even if by some miracle, algae could be grown to produce cheap oils, the oil itself would be primarily composed of PUFA. These would therefore need partial hydrogenation before they would be suitable for conversion into biodiesel. In addition, the chain length of the fatty acids would be too long as most are of C20 or C22 in length. Thus the characteristics of the final biodiesel product would not be those required by the market.
Estimations of the production of algal oil being able to reach over 100 000 L per hectare per year (compared with just 450 L for soybean oil and 6000 L for palm oil) seem attractive 3 but these are not realistic. They are extrapolated from laboratory data. Moreover, these extrapolations ignore what will happen to algae during cultivation outdoors in ponds and lagoons: even if lipid accumulation could be promoted in some way (either by genetic manipulation of the alga or by forced CO2 input), then there is the “dark reaction” to consider. Every night, algae respire. They cease to fix CO2 and consequently metabolize intracellular materials, including any accumulated lipid, to maintain viability. Some of the lipid that is accumulated during daylight hours is then lost during the night. There is also the winter to consider when the productivity of any outdoor algal system will be much less than during summer months. Thus there are many innate metabolic reasons why the annual productivity of algae will be much less than the predictions.
Finally, if it was that easy to find an alga that could accumulate lipid with the right specifications of fatty acyl residues when grown outdoors why should a multinational company, such as Exxon, decide to invest $10 million with the Craig Venter Institute in California to achieve this goal? The answer is clear: it is not an easy proposition. It is going to take a massive genetic manipulation programme to develop an alga that has improved CO2 capabilities (and therefore will not need CO2 enrichment), will be able to accumulate lipid to the levels needed (probably 50% of the biomass) and will produce the fatty acids of the right degree of unsaturation and chain length. The alga will also have to be robust to survive outdoors. At the moment, none of these are achievable; hence the need for major investment. These difficulties then explain why a company, Solazyme, founded in 2003 with the purpose of producing algal lipids for biodiesel, now considers that it will be much more sensible to aim at producing those lipids that algae are already good at producing – that is the very long chain PUFA – and develop these as high value products for the human nutritional market. Their current association with Unilever 3 indicates this change in strategy: Unilever is a food company, not an energy company. Biodiesel is a low value product and algae are best suited to producing high value products simply because the costs of production are so very high.
Regretfully, I fear a lot of investors appear not to have comprehended the simple basics of algal metabolism. What is feasible in the laboratory is not achievable outdoors on the very large scale. The only way that algal lipids could be produced using current technology would result in prices far in excess of plant oils and crude petroleum oil. Without a major genetic redesign of algal metabolic processes, it is therefore my opinion that the prospects for producing biodiesel in this way will simply not be feasible within the foreseeable future.