Orily Depraetere

Harvesting carbohydrate-rich Arthrospira platensis by spontaneous settling

The filamentous cyanobacterium Arthrospira platensis is an attractive feedstock for carbohydrate-based biofuels because it accumulated up to 74% of carbohydrates when nitrogen stressed. Nitrogen stressed A. platensis also settled spontaneously, and this occurred simultaneously with carbohydrates accumulation, suggesting a link between both phenomena. The increased settling velocity was neither due to production of extracellular carbohydrates, nor due to degradation of gas vacuoles, but was caused by an increase in the specific density of the filaments as a result of accumulation of carbohydrates under the form of glycogen. Settling velocities of carbohydrate-rich A. platensis reached 0.64mh(-1), which allowed the biomass to be harvested using a lamella separator. The biomass could be concentrated at least 15 times, allowing removal of 94% of the water using gravity settling, thus offering a potential application as a low-cost and high-throughput method for primary dewatering of carbohydrate-rich A. platensis.

Trade-Off between Growth and Carbohydrate Accumulation in Nutrient-Limited Arthrospira sp. PCC 8005 Studied by Integrating Transcriptomic and Proteomic Approaches

Cyanobacteria have a strong potential for biofuel production due to their ability to accumulate large amounts of carbohydrates. Nitrogen (N) stress can be used to increase the content of carbohydrates in the biomass, but it is expected to reduce biomass productivity. To study this trade-off between carbohydrate accumulation and biomass productivity, we characterized the biomass productivity, biomass composition as well as the transcriptome and proteome of the cyanobacterium Arthrospira sp. PCC 8005 cultured under N-limiting and N-replete conditions. N limitation resulted in a large increase in the carbohydrate content of the biomass (from 14 to 74%) and a decrease in the protein content (from 37 to 10%). Analyses of fatty acids indicated that no lipids were accumulated under N-limited conditions. Nevertheless, it did not affect the biomass productivity of the culture up to five days after N was depleted from the culture medium. Transcriptomic and proteomic analysis indicated that de novo protein synthesis was down-regulated in the N-limited culture. Proteins were degraded and partly converted into carbohydrates through gluconeogenesis. Cellular N derived from protein degradation was recycled through the TCA and GS-GOGAT cycles. In addition, photosynthetic energy production and carbon fixation were both down-regulated, while glycogen synthesis was up-regulated. Our results suggested that N limitation resulted in a redirection of photosynthetic energy from protein synthesis to glycogen synthesis. The fact that glycogen synthesis has a lower energy demand than protein synthesis might explain why Arthrospira 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.

Cultivation of Chlorella vulgaris and Arthrospira platensis with Recovered Phosphorus from Wastewater by Means of Zeolite Sorption

In this study, zeolite was employed for the separation and recovery of P from synthetic wastewater and its use as phosphorus (P) source for the cultivation of the green microalga Chlorella vulgaris and the cyanobacterium Arthrospira (Spirulina) platensis. At P-loaded zeolite concentration of 0.15–1 g/L, in which P was limited, the two species displayed quite different behavior regarding their growth and biomass composition. C. vulgaris preferred to increase the intracellular P and did not synthesize biomass, while A. platensis synthesized biomass keeping the intracellular P as low as possible. In addition under P limitation, C. vulgaris did display some little alteration of the biomass composition, while A. platensis did it significantly, accumulating carbohydrates around 70% from about 15%–20% (control). Both species could desorb P from zeolite biologically. A. platensis could recover over 65% and C. vulgaris 25% of the P bounded onto zeolite. When P-loaded zeolite concentration increased to 5 g/L, P was adequate to support growth for both species. Especially in the case of C. vulgaris, growth was stimulated from the presence of P-loaded zeolite and produced more biomass compared to the control.

Decolorisation of piggery wastewater to stimulate the production of Arthrospira platensis

The aim of this study was to evaluate the potential of color removal methods for enhancing the growth rate and biomass yield of Arthrospira produced using piggery wastewater as a nutrient source. Color could be removed from the piggery wastewater by means of oxidation (H2O2-UV) or by means of positively charged flocculants (e.g., ferric chloride, magnesium hydroxide), biopolymers (chitosan, cationic starch) or adsorbents (hydrotalcite). Some methods remove not only color but also phosphate (e.g., hydrotalcite) while other do not affect phosphate concentrations (e.g., chitosan). Color removal using chitosan resulted in a doubling of initial growth rate and a 50% increase in final biomass yield of Arthrospira produced on piggery wastewater. Color removal using hydrotalcite resulted in a low biomass yield of Arthrospira due to phosphate limitation.

Two-stage cultivation of Nannochloropsis oculata for lipid production using reversible alkaline flocculation

Two-stage cultivation for microalgae biomass is a promising strategy to boost lipid accumulation and productivity. Most of the currently described processes use energy-intensive centrifugation for cell separation after the first cultivation stage. This laboratory study evaluated alkaline flocculation as low-cost alternative separation method to harvest Nannochloropsis oculata prior to cultivation in the second nutrient-depleted cultivation stage. Biomass concentration over time and the maximum quantum yield of photosystem II expressed as Fv:Fm ratio showed identical patterns for both harvesting methods in both stages. The composition of total lipids, carbohydrates, and protein was similar for biomass harvested via alkaline flocculation or centrifugation. Likewise, both harvest methods yielded the same increase in total lipid content, to 40% within the first 2days of the nutrient-depleted stage, with an enrichment in C16 fatty acid methyl esters. Centrifugation can therefore be replaced with alkaline flocculation to harvest Nannochloropsis oculata after the first cultivation stage.

Impact of different nitrogen sources on the growth of Arthrospira sp. PCC 8005 under batch and continuous cultivation – A biochemical, transcriptomic and proteomic profile

The aim of the present study was to evaluate the effects of varying concentrations of different nitrogen sources (individually or in combination) on the biochemical, transcriptomic and proteomic profiles of Arthrospira sp. PCC 8005 under batch and continuous modes. In batch mode, while ammonium showed a repressive effect on nitrate-assimilation pathway of the cyanobacteria; better growth and nutrient uptake rate were observed in presence of urea than nitrate. The inhibitory effect of ammonium was further confirmed by the continuous photobioreactor study wherein the nutrient feed was transiently replaced from nitrate to ammonium (28mM turbiostat regime). The changes in lipid, exopolysaccharide, transcriptomic and proteomic profiles of cyanobacteria on transition from nitrate to ammonium indicated at an onset of nutrient stress.

Wastewater as a Source of Nutrients for Microalgae Biomass Production

Production of microalgal biomass requires large amounts of nitrogen (N) and phosphorus (P). The sustainability and economic viability of microalgae production could be significantly improved if N and P are not supplied by synthetic fertilizers but with wastewater. Microalgae already play an important role in wastewater treatment, yet several challenges remain to optimally convert wastewater nutrients into microalgal biomass. This book chapter aims to give an overview of the potential of using wastewater for microalgae production, as well some challenges that should be taken into account. We also review the benefits of combining microalgal biomass production with wastewater treatment.