Fig. 1

Experimental design to elucidate the transcriptional mechanism of microalgal TAG accumulation, triggered by nitrogen starvation, in M. neglectum. In a first experiment, the dynamic of cellular starch and lipid accumulation in response to −N treatment by M. neglectum was investigated in a long-term −N experiment. The individual sampling time points for starch and lipid determination are depicted; the corresponding metabolic data are shown in Fig. 2. In order to elucidate the transcriptional mechanisms correlating with starch and lipid accumulation, a second −N experiment was performed. In this experiment, an additional N resupply treatment was included, in order to induce the end of cellular quiescence and to consequently trigger the reversal of storage compound accumulation. By this combined treatment of alternating phases of N availability, the transcriptional program for both accumulation and degradation of TAGs was monitored, which facilitated the identification of central transcriptional responses underlying microalgal TAG accumulation. As starch accumulation precedes TAG accumulation under −N conditions, the −N phase was subdivided into two stages: e−N = starch accumulation stage and l−N = TAG and lipid hyperaccumulation stage. Upon N resupply, storage compounds were degraded, i.e., r+N = TAG degradation stage. The individual sampling time points for RNA and lipid isolation are depicted; the corresponding metabolic data are shown in Additional file 1: Figure S1