Nutrient removal and biomass production of marine microalgae cultured in recirculating aquaculture systems (RAS) water with low phosphate concentration.

This study was conducted to investigate the feasibility of microalgal biomass production and nutrient removal from recirculating aquaculture systems (RAS) water (RASW) with low phosphate concentration. For this purpose, Nannochloropsis oculata, Pavlova gyrans, Tetraselmis suecica, Phaeodactylum tricornutum, and their consortium were cultivated in RASW and RASW supplemented with vitamins (+V). Among them, N. oculata showed the maximum biomass production of 0.4 g/L in RASW. Vitamins supplementation significantly increased the growth of T. suecica from 0.16 g/L in RASW to 0.33 g/L in RASW + V. Additionally, T. suecica showed the highest nitrate (NO3-N) removal efficiency of 80.88 ± 2.08 % in RASW and 83.82 ± 2.08 % in RASW + V. Accordingly, T. suecica was selected for scaling up study of microalgal cultivation in RASW and RASW supplemented with nitrate (RASW + N) in 4-L airlift photobioreactors. Nitrate supplementation enhanced the growth of T. suecica up to 2.2-fold (day 15). The fatty acid nutritional indices in T. suecica cultivated in RASW and RASW + N showed optimal polyunsaturated fatty acids (PUFAs)/saturated fatty acid (SFAs), omega-6 fatty acid (n-6)/omega-3 fatty acid (n-3), indices of atherogenicity (IA), and thrombogenicity (IT)). Overall, the findings of this study revealed that despite low phosphate concentration, marine microalgae can grow in RASW and relatively reduce the concentration of nitrate. Furthermore, the microalgal biomass cultivated in RASW consisting of pigments and optimal fatty acid nutritional profile can be used as fish feed, thus contributing to a circular bioeconomy.

Removal and recovery of nutrients from septic tank wastewater using microalgae: Key factors and practical implications.

Treatment of septic tank wastewater (STWW) with high concentrations of ammonium (NH4+) and total phosphorus (TP), is challenging in decentralized areas. Utilizing microalgae for STWW treatment can simultaneously recover nutrients in the form of high-value microalgal biomass. However, despite the potential benefits, microalgal treatment of STWW is rarely reported. Therefore, this work utilized bench-scale photobioreactors (PBR) to investigate different factors that could affect microalgal cultivation in STWW and treatment efficiency. Accordingly, it was observed that suspended solids present in STWW did not significantly affect the microalgae growth and nutrient removal efficiencies in bubble column PBR. On the other hand, the effect of endemic microorganism could not be verified in this study due to observed fungal contamination and change in nutrient profile of STWW after autoclave. Nevertheless, the highest microalgal growth and nutrient removal efficiencies of NH4+-N = 79.14% and TP = 41.11% were observed within 14 days of photoautotrophic cultivation in raw STWW. Further, 25 days of upscaled photoautotrophic cultivation in 4-L bubble column PBR was performed to study biomass yield, nutrient removal kinetics, and nutrient removal efficiency. Consequently, 0.75 g‧L-1 dry biomass was produced with improved removal efficiency of NH4+-N (96.16%), and TP (69.57%). Elemental analysis of biomass revealed that 62.99 ± 1.46 mg‧L-1 TN and 11.41 ± 1.42 mg‧L-1 TP were recovered. Further, 1.02 geq carbon dioxide (CO2) was bio-fixed with every liter of STWW treated. The findings of this study revealed that microalgae can be successfully utilized for the removal and recovery of nutrients from STWW.

Enhanced production of bioactive compounds from marine microalgae Tetraselmis tetrathele under salinity and light stresses: A two-stage cultivation strategy.

This study leveraged the salinity and light intensity stresses during the stationary phase for enhancing the pigment contents and antioxidant capacity of Tetraselmis tetrathele. The highest pigments content was obtained in cultures under salinity stress (40 g L-1) illuminated using fluorescent light. Furthermore, the best inhibitory concentration (IC50) for scavenging the 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radicals was found as 79.53 µg mL-1 in ethanol extract and cultures under red LED light stress (300 µmol m-2 s-1). The highest antioxidant capacity in a ferric-reducing antioxidant power (FRAP) assay (1,778.6 µM Fe+2) was found in ethanol extract and cultures under salinity stress illuminated using fluorescent light. Maximum scavenging of the 2.2-diphenyl-1-picrylhydrazyl (DPPH) radical was found in ethyl acetate extracts under light and salinity stresses. These results indicated that abiotic stresses could enhance the pigment and antioxidant components of T. tetrathele, which are value-added compounds in the pharmaceutical, cosmetic, and food industries.

The effects of light regime on carbon cycling, nutrient removal, biomass yield, and polyhydroxybutyrate (PHB) production by a constructed photosynthetic consortium.

Microalgae can add value to biological wastewater treatment processes by capturing carbon and nutrients and producing valuable biomass. Harvesting small cells from liquid media is a challenge easily addressed with biofilm cultivation. Three experimental photobioreactors were constructed from inexpensive materials (e.g. plexiglass, silicone) for hybrid liquid/biofilm cultivation of a microalgal-bacterial consortia in aquaculture effluent. Three light regimes (full-spectrum, blue-white, and red) were implemented to test light spectra as a process control. High-intensity full-spectrum light caused photoinhibition and low biomass yield, but produced the most polyhydroxybutyrate (PHB) (0.14 mg g-1); a renewable bioplastic polymer. Medium-intensity blue-white light was less effective for carbon capture, but removed up to 82 % of phosphorus. Low-intensity red light was the only net carbon-negative regime, but increased phosphorus (+4.98 mg/L) in the culture medium. Light spectra and intensity have potential as easily-implemented process controls for targeted wastewater treatment, biomass production, and PHB synthesis using photosynthetic consortia.