ENVIRONMENTAL FACTORS AFFECTING GROWTH AND OMEGA 3 FATTY ACID COMPOSITION IN SKELETONEMA COSTATUM. THE INFLUENCES OF IRRADIANCE AND CARBON SOURCE
Abstract
The effects of carbon sources and three different irradiance levels on the growth and polyunsaturated fatty acid composition of the diatom Skeletonema costatum have been studied. Diatoms were grown in artificial seawater (ASW), which contained sodium bicarbonate (photoautotrophic growth) or sodium acetate (photoheterotrophic growth) at the same concentration (2.07 mM).
The optimum growth conditions were between 100–340 μmol photons m−2 s−1 in presence of sodium bicarbonate, both of which enhance photosynthesis. However, under limiting irradiance, results suggested that sodium acetate was more efficiently used than sodium bicarbonate during batch growth. As a consequence, in natural conditions and under low irradiance, availability of sodium acetate could be a limiting factor for cell metabolism. This diatom also seems to be able to support efficiently mixotrophic growth under low irradiance (20 μmol photons m−2 s−1).
The highest eicosapentaenoic acid (EPA) amounts were observed when cells were cultured under saturating irradiance (340 μmol photons m−2 s−1) during the late experimental and stationary phases. On the contrary, an increase in the EPA level was observed during the decline phase under limiting irradiance. Indeed, at low irradiance, an accumulation of omega 3 fatty acids in the thylakoid membranes could facilitate the photosynthetic reactions suggesting a peculiar photoadaptation of Skeletonema costatum.
Finally, during the exponential phase, a decreased of omega 3 fatty acid amount and specifically EPA with acetate as a carbon source has been observed under limiting and saturating irradiance. This could be explained by the decreased acetate availability that could be considered as responsible for the fall in EPA level.