Preferential utilization of intracellular nutrients supports microalgal growth under nutrient starvation: multi-nutrient mechanistic model and experimental validation

Amphidinium carterae growth in hydroponic wastewater. A sustainable approach to a microalgae-based process promoting a circular bioeconomy

Hydroponic cultivation is being increasingly used worldwide for horticultural production. However, this technique consumes large quantities of freshwater and produces significant amounts of wastewater. Effluent wastewater from hydroponic cultures may contain high nitrogen (N) and phosphorus (P) concentrations, thus contributing to soil, surface, and subsurface water pollution if directly discharged into the environment; it also potentially leads to ecosystem degradation. In the present work, a synthetic hydroponic effluent wastewater was formulated to evaluate the potential of a marine microalga to remove the main nutrients (N and P) and to test its suitability for sustainable, large-scale cultivation. The marine dinoflagellate microalga Amphidinium carterae successfully removed 100 % of the N and P from the hydroponic wastewater. The formulation yielded comparable biomass yields (0.5 g L-1) to those of the same culture grown in a control medium but considerably increased the production of carotenoids (40 %), polyunsaturated fatty acids (17 %), and, significantly, amphidinols (56 %). Hence, the use of A. carterae to treat and valorise hydroponic effluents shows significant promise, supporting further investigation into utilizing hydroponic wastewater from different origins to cultivate marine microalgae that can then be used to produce agricultural bio-based fungicides and other bioproducts in line with the principles of the circular bioeconomy.

Unveiling potential of promising filamentous microalga Klebsormidium cf. nitens: Shear stress resilience and carotenoid-fatty acid dynamics in tubular photobioreactor

In this study, the effects of shear stress and different culture media on the growth of the filamentous microalga Klebsormidium cf. nitens were studied. The microalga's growth, carotenoids and fatty acids were further evaluated in a pump-driven tubular photobioreactor. The results show that this microalga had the ability to withstand high shear stress and the adaptability to grow in a culture medium that lacks certain trace elements. K. cf. nitens grew consistently in the tubular photobioreactor at different average light intensities although it did not grow well in a tall bubble column. The carotenoid analysis revealed that the xanthophyll cycle was activated to protect the cell photosynthetic system. The fatty acids increased with irradiance, with linoleic acid (C18:2n6) making up over 50 % of the total fatty acids. This study supports the potential of employing pump-driven tubular photobioreactors to produce the filamentous microalga K. cf nitens at the large scale.

A reliable multi-nutrient model for the rapid production of high-density microalgal biomass over a broad spectrum of mixotrophic conditions

The superior microalgal biomass productivities obtained under mixotrophic conditions have been widely demonstrated. However, to attain the full potential of the method, optimal conditions for biomass production and resource utilization need to be determined and successfully exploited throughout the process operation. Detailed kinetic mathematical models have often proved most efficient tools for predicting process behavior and governing its overall operation. This paper presents an extensive study for obtaining a highly reliable model for mixotrophic production of microalgae covering a wide set and range of nutritional conditions (10-fold the concentration range of Bold's Basal Medium) and biomass yields up to 6.68 g.L^-1 after only 6 days. The final reduced model includes a total of five state variables and nine parameters: model calibration resulted in very small 95% confidence intervals and relative errors below 5% for all parameters. Model validation showed high reliability with R² correlation values between 0.77 and 0.99.

Effect of Nitrogen, Phosphorous, and Light Colimitation on Amphidinol Production and Growth in the Marine Dinoflagellate Microalga Amphidinium carterae

The marine dinoflagellate microalga Amphidinium carterae is a source of amphidinols, a fascinating group of polyketide metabolites potentially useful in drug design. However, Amphidinium carterae grows slowly and produces these toxins in tiny amounts, representing a hurdle for large-scale production. Understanding dinoflagellate growth kinetics under different photobioreactor conditions is imperative for promoting the successful implementation of a full-scale integrated bioproduct production system. This study evaluates the feasibility of growing Amphidinium carterae under different ranges of nitrogen concentration (NO₃⁻ = 882–2646 µM), phosphorus concentration (PO₃³⁻ = 181–529 µM), and light intensity (Y₀ = 286–573 µE m⁻² s⁻¹) to produce amphidinols. A mathematical colimitation kinetic model based on the “cell quota” concept is developed to predict both algal growth and nutrient drawdown, assuming that all three variables (nitrogen, phosphorous and light) can simultaneously colimit microalgal growth. The model was applied to the semicontinuous culture of the marine microalgae Amphidinium carterae in an indoor LED-lit raceway photobioreactor. The results show that both growth and amphidinol production strongly depend on nutrient concentrations and light intensity. Nonetheless, it was possible to increase Amphidinium carterae growth while simultaneously promoting the overproduction of amphidinols. The proposed model adequately describes Amphidinium carterae growth, nitrate and phosphate concentrations, and intracellular nitrogen and phosphorus storage, and has therefore the potential to be extended to other systems used in dinoflagellate cultivation and the production of bioproducts obtained therein.

Microalgae and cyanobacteria modeling in water resource recovery facilities: A critical review

Microalgal and cyanobacterial resource recovery systems could significantly advance nutrient recovery from wastewater by achieving effluent nitrogen (N) and phosphorus (P) levels below the current limit of technology. The successful implementation of phytoplankton, however, requires the formulation of process models that balance fidelity and simplicity to accurately simulate dynamic performance in response to environmental conditions. This work synthesizes the range of model structures that have been leveraged for algae and cyanobacteria modeling and core model features that are required to enable reliable process modeling in the context of water resource recovery facilities. Results from an extensive literature review of over 300 published phytoplankton models are presented, with particular attention to similarities with and differences from existing strategies to model chemotrophic wastewater treatment processes (e.g., via the Activated Sludge Models, ASMs). Building on published process models, the core requirements of a model structure for algal and cyanobacterial processes are presented, including detailed recommendations for the prediction of growth (under phototrophic, heterotrophic, and mixotrophic conditions), nutrient uptake, carbon uptake and storage, and respiration.

Real-time iTRAQ-based proteome profiling revealed the central metabolism involved in nitrogen starvation induced lipid accumulation in microalgae

To understand the post-transcriptional molecular mechanisms attributing to oleaginousness in microalgae challenged with nitrogen starvation (N-starvation), the longitudinal proteome dynamics of Chlorella sp. FC2 IITG was investigated using multipronged quantitative proteomics and multiple reaction monitoring assays. Physiological data suggested a remarkably enhanced lipid accumulation with concomitant reduction in carbon flux towards carbohydrate, protein and chlorophyll biosynthesis. The proteomics-based investigations identified the down-regulation of enzymes involved in chlorophyll biosynthesis (porphobilinogen deaminase) and photosynthetic carbon fixation (sedoheptulose-1,7 bisphosphate and phosphoribulokinase). Profound up-regulation of hydroxyacyl-ACP dehydrogenase and enoyl-ACP reductase ascertained lipid accumulation. The carbon skeletons to be integrated into lipid precursors were regenerated by glycolysis, β-oxidation and TCA cycle. The enhanced expression of glycolysis and pentose phosphate pathway enzymes indicates heightened energy needs of FC2 cells for the sustenance of N-starvation. FC2 cells strategically reserved nitrogen by incorporating it into the TCA-cycle intermediates to form amino acids; particularly the enzymes involved in the biosynthesis of glutamate, aspartate and arginine were up-regulated. Regulation of arginine, superoxide dismutase, thioredoxin-peroxiredoxin, lipocalin, serine-hydroxymethyltransferase, cysteine synthase, and octanoyltransferase play a critical role in maintaining cellular homeostasis during N-starvation. These findings may provide a rationale for genetic engineering of microalgae, which may enable synchronized biomass and lipid synthesis.

Continuous cultivation of lipid rich microalga Chlorella sp. FC2 IITG for improved biodiesel productivity via control variable optimization and substrate driven pH control

A novel two-stage continuous heterotrophic cultivation of Chlorella sp. FC2 IITG was demonstrated for enhanced lipid productivity. Initially, effect of control variable e.g. dilution rate and feed stream substrate concentrations on biomass productivity was evaluated. This showed significant variation in biomass productivity from 2.4gL^-1day^-1 to 11.2gL^-1day^-1. Further, these control variables were optimized by using multi-nutrient mechanistic model for maximizing the biomass productivity. Finally, continuous production of lipid rich algal biomass was demonstrated in two sequential bioreactors for enhanced lipid productivity. The biomass productivity of 92.7gL^-1day^-1 was observed in the first reactor which was operated at model predicted optimal substrate concentrations of feed stream. The intracellular neutral lipid enrichment by acetate addition resulted in lipid productivity of 9.76gL^-1day^-1 in the second reactor. Both the biomass and lipid productivities obtained from current study are significantly high amongst similarly reported literatures.

Synchronized growth and neutral lipid accumulation in Chlorella sorokiniana FC6 IITG under continuous mode of operation

Synchronized growth and neutral lipid accumulation with high lipid productivity under mixotrophic growth of the strain Chlorella sorokiniana FC6 IITG was achieved via manipulation of substrates feeding mode and supplementation of lipid elicitors in the growth medium. Screening and optimization of lipid elicitors resulted in lipid productivity of 110.59mgL(-1)day(-1) under the combined effect of lipid inducers sodium acetate and sodium chloride. Fed-batch cultivation of the strain in bioreactor with intermittent feeding of limiting nutrients and lipid inducer resulted in maximum biomass and lipid productivity of 2.08 and 0.97gL(-1)day(-1) respectively. Further, continuous production of biomass with concomitant lipid accumulation was demonstrated via continuous feeding of BG11 media supplemented with lipid inducers sodium acetate and sodium chloride. The improved biomass and lipid productivity in chemostat was found to be 2.81 and 1.27gL(-1)day(-1) respectively operated at a dilution rate of 0.54day(-1).