Nitrogen deprivation of microalgae: effect on cell size, cell wall thickness, cell strength, and resistance to mechanical disruption

Abstract

Nitrogen deprivation (N-deprivation) is a proven strategy for inducing triacylglyceride accumulation in microalgae. However, its effect on the physical properties of cells and subsequently on product recovery processes is relatively unknown. In this study, the effect of N-deprivation on the cell size, cell wall thickness, and mechanical strength of three microalgae was investigated. As determined by analysis of micrographs from transmission electron microscopy, the average cell size and cell wall thickness for N-deprived Nannochloropsis sp. and Chlorococcum sp. were ca. 25% greater than the N-replete cells, and 20 and 70% greater, respectively, for N-deprived Chlorella sp. The average Young's modulus of N-deprived Chlorococcum sp. cells was estimated using atomic force microscopy to be 775 kPa; 30% greater than the N-replete population. Although statistically significant, these microstructural changes did not appear to affect the overall susceptibility of cells to mechanical rupture by high pressure homogenisation. This is important as it suggests that subjecting these microalgae to nitrogen starvation to accumulate lipids does not adversely affect the recovery of intracellular lipids.

Transcriptional Regulation of Cellulose Biosynthesis during the Early Phase of Nitrogen Deprivation in Nannochloropsis salina

“…41.2–60.4%) and TAGs (23.8% vs . 30.9–58.4%) 1 , and it was robust to changes in salinity 11 . High biomass production would be advantageous for commercial applications using two-stage N deprivation cultivation 41 .…”

“…Nitrogen deficiency (N−) is the most effective external factor for triggering TAG production 14, 15 ; it causes a series of physiological changes in some Nannochloropsis species including growth retardation, enhanced cell size, pigment alterations and lipid accumulation, and it reduces photosynthesis, carbon fixation, and protein synthesis 11, 15, 16, 17 . We were interested in the response of Nannochloropsis cell wall metabolism to N−, where photoassimilates are known to be partitioned primarily into the synthesis of storage carbohydrate and lipid.…”

“…Thus, metabolic engineering combined with varying culture conditions such as light intensity, temperature, salinity, and nutrients is a potential target for developing microalgal-based biofuel production. Nitrogen deficiency induces cell wall thickening in some algae such as Nannochloropsis sp., Chlorella sp., Chlorococcum sp., and Chlamydomonas 10, 11 . In the present study, we tested whether N− caused changes in cell wall biosynthesis and resulted in cell wall thickening in N. salina .…”

“…Cell wall thickness in Nannochloropsis species varies from 63 to 119 nm due to the distinct genetic traits in each strain 9 . Changes in culture conditions, such as reductions in the concentrations of salt, nitrogen, phosphate, and sulphur induce cell wall thickening in Nannochloropsis , Chlorella , Chlorococcum , and Chlamydomonas 9–11 . Algal cell wall properties are important for biotechnological applications.…”

“…Algal cell wall properties are important for biotechnological applications. However, few studies 11 have investigated the effects of cultural stress conditions and nitrogen deficiency (N−) on the biosynthesis of cell wall components and TAG production in the majority of algal species.…”