Lipid-accumulating microorganisms

Figure 3. Stylistic presentation of the course of lipid accumulation by an oleaginous microorganism. The concentration of nitrogen (NH3) in the medium is adjusted so that it becomes exhausted after the first 24 hours growth after this point, the cells enter the lipid accumulation phase in which the excess carbon (e.g., glucose) continues to be assimilated by the cells, and because there is no new cell synthesis because of the lack of nitrogen, the surplus carbon is converted into lipid, which functions as a reserve of carbon and energy for the cells.

Microorganisms lacking ATP citrate lyase do not accumulate lipid much above 10-15%, although the corollary, that organisms with ATP citrate lyase activity will accumulate lipid may not always hold because other enzymes, such as AMP deaminase, AMP-dependent isocitrate dehydrogenase or malic enzyme, may be lacking or may not be under sufficiently stringent control to allow extensive lipid accumulation. There is clearly much still to be learnt about lipid accumulation and its attendant biochemistry to explain all these variations at the fundamental level. 

Wei A, Zhang X, Wei D, (Then G, Wu Q, Yang ST (2009) Effects of cassava starch hydrolysate on cell growth and lipid accumulation of the heterotrophic microalgae Chlorella protothecoides. J Ind Microbiol Biotechnol 36(11) 1383-1389. doi 10.1007/sl0295-(X)9-0624-x Wynn JP, Ratledge C (2005) Oils from microorganisms. In Shahidi F (ed) Bailey s industrial oil and fat products, vol 3, 6th edn, Edible oil and fat products specially oils and oil products. Wiley, Hoboken, pp 121-153... 

Botham, P.A., Ratledge, C., 1979. A biochemical explanation for lipid accumulation in Candida 107 and other oleaginous microorganisms. Journal of General Microbiology 114, 361-375. 

We can now consider some typical nutrient solutes like amino acids and phosphate. Such molecules are ionized, which means that they would not readily cross the permeability barrier of a lipid bilayer. Permeability coefficients of liposome membranes to phosphate and amino acids have been determined [46] and were found to be in the range of 10 11 -10 12 cm/s, similar to ionic solutes such as sodium and chloride ions. From these figures one can estimate that if a primitive microorganism depended on passive transport of phosphate across a lipid bilayer composed of a typical phospholipid, it would require several years to accumulate phosphate sufficient to double its DNA content or pass through one cell cycle. In contrast, a modern bacterial cell can reproduce in as short a time as 20 min. 

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