Author: Saritas, Sevim (Euro Fed Lipid)

Effect of Environmental Conditions on Various Enzyme Activities and
Triacylglycerol Contents in Cultures of the Freshwater Diatom,
Asterionella formosa (Bacillariophyceae)
Malika Mekhalfi a, Sawsan Amara b, Sylvie Robert b, Frédéric Carrière b, Brigitte Gontero a,*
a Aix Marseille Université, CNRS, UMR 7281 Laboratoire de Bioénergétique et Ingénierie des Protéines, 31 Chemin Joseph Aiguier,
13402 Marseille Cedex 20 France
bUMR 7282 Enzymology at Interfaces and Physiology of Lipolysis, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20 France

A detailed analysis of triacylglycerols (TAGs) contents, fatty acid patterns and key enzyme activities in the freshwater diatom Asterionella formosa was performed under various conditions, including nitrate, iron and silicon limitation (stress conditions), or bicarbonate and phytohormones supplementation (stimulation conditions). Of all the conditions tested, the addition of bicarbonate produced the greatest increase (5-fold) in TAGs contents compared to the control while the biomass increased. The addition of phytohormones also allowed a significant increase in TAGs of about 3-fold while the biomass increased. Silicon, unlike iron and nitrate limitation, also triggered a significant increase in TAGs contents of 3.5-fold but 
negatively affected the biomass. Analysis of fatty acid profiles showed that the mono-unsaturated C16:1 fatty acid was the most abundant in A. formosa, followed by C16:0, C14:0 and eicosapentaenoic acid (EPA; C20:5 n-3). EPA levels were found to increase under nitrate and iron limitation. Glyceraldehyde-3- phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), phosphofructokinase (PFK), glucose- 6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase (MDH) activities differed with growth 
conditions. Most enzymes were up-regulated in stimulated cells while in the case of stressed cells, the pattern of activities was more variable. Detailed analysis of all enzyme activities showed that the most important enzyme among those tested was GAPDH which could be a good candidate for genetic engineering of high lipid-producing algae. This study provides a better understanding of key enzymes and biochemical pathways involved in lipid accumulation processes in diatoms.