The effect of thiamine on the growth and fatty acid content of Aurantiochytrium sp

Abstract

Aurantiochytrium sp. is a heterotrophic microorganism that accumulates high amounts of fatty acids (FAs), including odd-chain FAs and an essential FA, docosahexaenoic acid (DHA). Since complex nutrient sources such as tryptone and yeast extract are conventionally utilized in the growth medium for Aurantiochytrium, it is difficult to investigate which individual vitamins affect growth and FA production. Thus, in the present study, we simplified the nitrogen source and defined the vitamins required for Aurantiochytrium sp. cultivation. Glutamate was found to be the most effective simple nitrogen source in cultivation media and thiamine (vitamin B1) was identified as a unique essential vitamin for this organism. Media supplemented with thiamine, as the only member of the vitamin B complex added, allowed for propagation of four strains of Aurantiochytrium. The ratio of odd-chain FAs significantly decreased in the medium containing glucose, glutamate, and thiamine when compared with the conventional medium containing glucose, tryptone, and yeast extract. This is likely caused by a decreased availability of branched-chain amino acids. Under thiamine starvation, FA content per dry cell weight gradually decreased throughout cultivation, and the specific activity of pyruvate dehydrogenase (PDH), a thiamine-dependent enzyme, was severely repressed. These results indicate that when thiamine is absent, the cells compensate some amount of acetyl-CoA through β-oxidation of FAs in vivo, instead of from the reaction by pyruvate, as occurs under normal growth conditions. Providing experimental evidence that the activity of vitamin-dependent enzymes affects biosynthesis of FAs is critical to a better understanding of the effect of vitamin supplementation in Aurantiochytrium sp. media. In the present study, we demonstrated the close relationship between PDH activity, FA content and the importance of thiamine supplementation in the culture medium in Aurantiochytrium sp.