Influence of nitrogen stress on Isochrysis galbana strain U4, a candidate for biodiesel production

Abstract

Lipid accumulation has been investigated in numerous microalgal species to assess their potential with respect to biodiesel production. The present work determines the effect of nitrogen stress on physiological and ultrastructural changes in Isochrysis galbana U4. This study is unique in showing the correlations between growth, lipid production, pigmentation and ultrastructural changes in Isochrysis cells undergoing nitrogen starvation. The continuation of algal growth after the complete depletion of external nitrogen was shown to be supported by internal nitrogen stores, possibly in the pyrenoid. Cell growth ceased and lipid accumulation was initiated after the internal store of nitrogen had become exhausted. The depletion of intracellular nitrogen reservoirs to critical thresholds initiated the onset of the stationary phase, a decline in chlorophyll content and the initiation of lipid and carotenoid accumulation. The most notable ultrastructural changes, upon nitrogen stress, were the accumulation of plastidial and cytoplasmic lipid bodies and the dismantling of the chloroplast. The size of the pyrenoid when external nitrogen became depleted was found to decrease significantly, up to four-fold. This was attributed to the remobilization of nitrogen from Rubisco. The level of expression of heterochromatin was found to increase when cells were nitrogen starved. This is thought to favor long-term dormancy in this species because aging cells have been noted to recover rapidly when returned to conditions favorable for growth. The observations of this study are consistent with the hypothesis that the responses of Isochrysis cells to nitrogen starvation are regulated by the internal reserves of nitrogen, and the depletion of these reserves is an important trigger for lipid accumulation in this species. The findings of this study also indicate that Isochrysis galbana U4 is a promising candidate for biodiesel lipid production.

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