# _Arthrospira platensis_ as a Feasible Feedstock for Bioethanol Production
## 3 Environmental Stresses for Glycogen Accumulation
### Nitrogen Depletion  
The deficiency of nitrogen in the culture media affects biomass production but significantly enhances the production of energy-rich components like lipids and carbohydrates as a response to adverse conditions like starvation or environmental stress [69]. The proportion of carbohydrates in _Arthospira platensis_ cells can reach up to 50% dry weight (DW) by optimizing the cultivation conditions [17]; some studies have even reached 60% DW of glycogen under the N-limitation condition [31]. Under nitrogen starvation, the flow of the photosynthetically fixed carbon is diverted from the protein synthesis metabolic pathway to carbohydrates or lipids [57]. Interestingly, [PERSON] et al. [61] demonstrated that complete nitrogen starvation is not required, as glycogen accumulation was stimulated by using 3 mM of nitrate. This finding is of interest for industrial scale production, as a limitation is sufficient to promote the synthesis, thus opening the possibility of producing glycogen in continuous mode.  
In 2013, [PERSON] et al. [64] compared the glycogen and protein content of _Arthospira_ in replete and depleted nitrate conditions. In the presence of nitrate, the glycogen and protein contents remained nearly constant at \(\sim\)15% DW and \(\sim\)50% DW, respectively. In contrast, in the absence of nitrate, the glycogen content increased to 63.2% DW while the protein content decreased to 15.4%. Analyzing the metabolic intermediates, they observed that shifting the cyanobacterial cells into nitrogen depletion and cultivating them for 72 h resulted in global metabolic modifications that involved reductions in the levels of several free amino acids (aspartate, glutamate, and glutamine), and increased in the levels of organic acids (2-ketoglutate, malate, and succinate). Time-course profiling of metabolites demonstrated transient increases in the levels of other amino acids (glycine, histidine, isoleucine, leucine, methionine, phenylalanine, proline, threonine, and tyrosine). Such transient increases might be caused by protein hydrolysis and subsequent assimilation of these amino acids into glycogen via gluconeogenesis. In another study, [PERSON] et al. [29] observed an increase in total carbohydrates (from 14% DW to 74% DW) and a decrease in protein content (from 37% DW to 10% DW). Transcriptomic and proteomic analysis indicated that the N-limited condition induced a downregulation of _de novo_ protein synthesis. In addition, photosynthetic energy production and carbon fixation were both downregulated, while glycogen synthesis was upregulated. The fact that glycogen synthesis has a lower energy demand than protein synthesis might explain why _Arthospira_ is able to achieve a similar biomass productivity under N-limited as under N-replete conditions, despite the fact that photosynthetic energy production was impaired by N-limitation.  
As demonstrated for other cyanobacteria [70], the proteins first degraded by _A. platensis_ seem to be their light-harvesting apparatus, phycobilisome, to provide nitrogen when exogenous nitrogen is limited. Although the precise role that glycogen plays in cyanobacteria remains unclear, it has been suggested that the accumulation of glycogen by cyanobacteria may be advantageous during starvation periods, providing both a stored source of energy and a carbon surplus [69].