Phosphorus Enhances Photosynthetic Storage Starch Production in a Green Microalga (Chlorophyta) Tetraselmis subcordiformis in Nitrogen Starvation Conditions

A comprehensive review on versatile microalga Tetraselmis: Potentials applications in wastewater remediation and bulk chemical production

Microalgae are considered sustainable resources for the production of biofuel, feed, and bioactive compounds. Among various microalgal genera, the Tetraselmis genus, containing predominantly marine microalgal species with wide tolerance to salinity and temperature, has a high potential for large-scale commercialization. Until now, Tetraselmis sp. are exploited at smaller levels for aquaculture hatcheries and bivalve production. However, its prolific growth rate leads to promising areal productivity and energy-dense biomass, so it is considered a viable source of third-generation biofuel. Also, microbial pathogens and contaminants are not generally associated with Tetraselmis sp. in outdoor conditions due to faster growth as well as dominance in the culture. Numerous studies revealed that the metabolite compositions of Tetraselmis could be altered favorably by changing the growth conditions, taking advantage of its acclimatization or adaptation ability in different conditions. Furthermore, the biorefinery approach produces multiple fractions that can be successfully upgraded into various value-added products along with biofuel. Overall, Tetraselmis sp. could be considered a potential strain for further algal biorefinery development under the circular bioeconomy framework. In this aspect, this review discusses the recent advancements in the cultivation and harvesting of Tetraselmis sp. for wider application in different sectors. Furthermore, this review highlights the key challenges associated with large-scale cultivation, biomass harvesting, and commercial applications for Tetraselmis sp.

Tailoring nitrogen and phosphorus levels for tunable glycogen and protein production in halophilic Cyanobacterium aponinum PCC10605

Cyanobacteria hold promise for simultaneous carbon capture and chemicals production, but the regulation and effect of nitrogen (N) and phosphorus (P) remains unclear. This study investigates major productions of glycogen, protein, and C-phycocyanin (C-PC) in Cyanobacterium aponinum PCC10605 under different N/P levels, alongside changes in light and CO2. Increasing nitrate (NO3-) from 2 to 6 mM resulted in a 9.7-fold increase in C-PC and reduced glycogen to 8.9 %. On the other hand, elevating phosphorus from 0.1 to 2 mM under limited nitrogen enhanced biomass and glycogen through the upregulation of carbonic anhydrase, ADP-glucose pyrophosphorylase, and glycogen phosphorylase. Changes in phosphorus levels and CO2 inlet concentrations affected metabolites accumulation and carbon capture efficiency, leading to the best condition of 76 % uptake capacity in direct air capture (DAC). All findings underscore the trade-off between glycogen and protein, representing the importance of N/P levels in nutrient modulation of PCC10605.

Tetraselmis species for environmental sustainability: biology, water bioremediation, and biofuel production

With increasing demand of fossil fuels and water pollution and their environmental impacts, marine green microalgae have gained special attention in both scientific and industrial  fields. This is due to their fast growth in non-arable lands with high photosynthetic activity, their metabolic plasticity, as well as their high CO2 capture capacity. Tetraselmis species, green and eukaryotic microalgae, are not only considered as a valuable source of biomolecules including pigments, lipids, and starch but also widely used in biotechnological applications. Tetraselmis cultivation for high-value biomolecules and industrial use was demonstrated to be a non-cost-effective strategy because of its low demand in nutrients, such as phosphorus and nitrogen. Recently, phycoremediation of wastewater rich in nutrients, chemicals, and heavy metals has become an efficient and economic-alternative that allows the detoxification of waters and induces mechanisms in algal cells for biomolecules rich-energy synthesis to regulate their metabolic pathways. This review aims to shed light on Tetraselmis species for their different culture conditions and metabolites bioaccumulation, as well as their human health and environmental applications. Additionally, phycoremediation of contaminants associated to biofuel production in Tetraselmis cells and their different intracellular and extracellular mechanisms have also been investigated.

Cell size, density, and nutrient dependency of unicellular algal gravitational sinking velocities

Eukaryotic phytoplankton, also known as algae, form the basis of marine food webs and drive marine carbon sequestration. Algae must regulate their motility and gravitational sinking to balance access to light at the surface and nutrients in deeper layers. However, the regulation of gravitational sinking remains largely unknown, especially in motile species. Here, we quantify gravitational sinking velocities according to Stokes' law in diverse clades of unicellular marine microalgae to reveal the cell size, density, and nutrient dependency of sinking velocities. We identify a motile algal species, Tetraselmis sp., that sinks faster when starved due to a photosynthesis-driven accumulation of carbohydrates and a loss of intracellular water, both of which increase cell density. Moreover, the regulation of cell sinking velocities is connected to proliferation and can respond to multiple nutrients. Overall, our work elucidates how cell size and density respond to environmental conditions to drive the vertical migration of motile algae.

Circadian rhythm promotes the biomass and amylose hyperaccumulation by mixotrophic cultivation of marine microalga Platymonas helgolandica

BACKGROUND

Microalgal starch can be exploited for bioenergy, food, and bioplastics. Production of starch by green algae has been concerned for many years. Currently commonly used methods such as nutrient stress will affect cell growth, thereby inhibiting the production efficiency and quality of starch production. Simpler and more efficient control strategies need to be developed.

RESULT

We proposed a novel regulation method to promote the growth and starch accumulation by a newly isolated Chlorophyta Platymonas helgolandica. By adding exogenous glucose and controlling the appropriate circadian light and dark time, the highest dry weight accumulation 6.53 g L-1 (Light:Dark = 12:12) can be achieved, and the highest starch concentration could reach 3.88 g L-1 (Light:Dark = 6:18). The highest production rate was 0.40 g L-1 d-1 after 9 days of production. And this method helps to improve the ability to produce amylose, with the highest accumulation of 39.79% DW amylose. We also discussed the possible mechanism of this phenomenon through revealing changes in the mRNA levels of key genes.

CONCLUSION

This study provides a new idea to regulate the production of amylose by green algae. For the first time, it is proposed to combine organic carbon source addition and circadian rhythm regulation to increase the starch production from marine green alga. A new starch-producing microalga has been isolated that can efficiently utilize organic matter and grow with or without photosynthesis.

Metabolic pathways for biosynthesis and degradation of starch in Tetraselmis chui during nitrogen deprivation and recovery

Tetraselmis chui is known to accumulate starch when subjected to stress. This phenomenon is widely studied for the purpose of industrial production and process development. Yet, knowledge about the metabolic pathways involved is still immature. Hence, in this study, transcription of 27 starch-related genes was monitored under nitrogen deprivation and resupply in 25 L tubular photobioreactors. T. chui proved to be an efficient starch producer under nitrogen deprivation, accumulating starch up to 56% of relative biomass content. The prolonged absence of nitrogen led to an overall down-regulation of the tested genes, in most instances maintained even after nitrogen replenishment when starch was actively degraded. These gene expression patterns suggest post-transcriptional regulatory mechanisms play a key role in T. chui under nutrient stress. Finally, the high productivity combined with an efficient recovery after nitrogen restitution makes this species a suitable candidate for industrial production of high-starch biomass.

Fine-tuned regulation of photosynthetic performance via γ-aminobutyric acid (GABA) supply coupled with high initial cell density culture for economic starch production in microalgae

Microalgal starch is considered as renewable and sustainable feedstock for biofuels and biorefinery. High cell density culture is favourable for photoautotrophic starch production in microalgae in the aspects of productivity and economy, but it often encounters low starch content or extra stress exposure that limits the production. This study aimed to economically enhance photosynthetic starch production from CO2 fixation in a green microalga Tetraselmis subcordiformis by regulating photosynthetic stress status with a signalling molecule γ-aminobutyric acid (GABA) combined with the application of high initial cell density culture. By increasing initial cell density (ICD) from the normal of 1.1 g L-1 (NICD) to as high as 2.8 g L-1 (HICD), the starch content, yield, and theoretical productivity were improved by 7%, 63%, and 42%, respectively. The addition of GABA under HICD resulted in 14%, 19%, and 26% of further enhancement in starch content, yield, and theoretical productivity, respectively. GABA exhibited distinct regulatory mechanisms on photosynthesis and stress status under HICD relative to NICD. GABA augmented excessive light energy absorption and electron transfer through photosystem II that reinforced the photoinhibition under NICD, while alleviated the stress reversely under HICD, both of which facilitated starch production by enabling a suitable stress status while simultaneously maintaining a sufficient photosynthetic activity. The increase of ICD and/or GABA supply particularly boosted amylopectin accumulation, leading to the changes in starch composition and was more favourable for fermentation-based biofuels production. Preliminary techno-economic analysis showed that the highest net extra benefit of 9.64 $ m-3 culture could be obtained under HICD with 2.5 mM GABA supply where high starch content (62%DW) and yield (2.5 g L-1) were achieved. The combined HICD-GABA regulation was a promising strategy for economic starch production from CO2 by microalgae for sustainable biomanufacturing.

Phosphate-Dependent Regulation of Growth and Stresses Management in Plants

The importance of phosphorus in the regulation of plant growth function is well studied. However, the role of the inorganic phosphate (Pi) molecule in the mitigation of abiotic stresses such as drought, salinity, heavy metal, heat, and acid stresses are poorly understood. We revisited peer-reviewed articles on plant growth characteristics that are phosphorus (P)-dependently regulated under the sufficient-P and low/no-P starvation alone or either combined with one of the mentioned stress. We found that the photosynthesis rate and stomatal conductance decreased under Pi-starved conditions. The total chlorophyll contents were increased in the P-deficient plants, owing to the lack of Pi molecules to sustain the photosynthesis functioning, particularly, the Rubisco and fructose-1,6-bisphosphatase function. The dry biomass of shoots, roots, and P concentrations were significantly reduced under Pi starvation with marketable effects in the cereal than in the legumes. To mitigate P stress, plants activate alternative regulatory pathways, the Pi-dependent glycolysis, and mitochondrial respiration in the cytoplasm. Plants grown under well-Pi supplementation of drought stress exhibited higher dry biomass of shoots than the no-P treated ones. The Pi supply to plants grown under heavy metals stress reduced the metal concentrations in the leaves for the cadmium (Cd) and lead (Pb), but could not prevent them from absorbing heavy metals from soils. To detoxify from heavy metal stress, plants enhance the catalase and ascorbate peroxidase activity that prevents lipid peroxidation in the leaves. The HvPIP and PHO1 genes were over-expressed under both Pi starvation alone and Pi plus drought, or Pi plus salinity stress combination, implying their key roles to mediate the stress mitigations. Agronomy Pi-based interventions to increase Pi at the on-farm levels were discussed. Revisiting the roles of P in growth and its better management in agricultural lands or where P is supplemented as fertilizer could help the plants to survive under abiotic stresses.

Acclimation to a broad range of nitrate strength on a euryhaline marine microalga Tetraselmis subcordiformis for photosynthetic nitrate removal and high-quality biomass production

Industrial wastewaters usually possess a wide range of nitrate strength. Microalgae-based nitrate-rich wastewater treatment could realize nitrate recovery along with CO₂ sequestration for sustainable biomass production, but the low tolerance of the microalgal strains to high-strength nitrate restricted the treatment process. The present study comprehensively evaluated a euryhaline marine microalga Tetraselmis subcordiformis for photosynthetic nitrate removal and biomass production in synthetic wastewater with a broad range of nitrate strength (0.24-7.0 g NO₃^--N/L). This alga could acclimate to high nitrate strength up to 3.5 g NO₃^--N/L (HN) without compromising biomass production. Nitrate could be completely removed within four days when low nitrate (0.24 g NO₃^--N/L, LN) was loaded. The maximum nitrate removal rate of 331 mg N/L/day and specific nitrate removal rate of 360 mg N/day/g cell was obtained under medium nitrate condition (1.8 g NO₃^--N/L, MN). High-nitrate stress under 7.0 g NO₃^--N/L (SHN) caused an increased light energy dissipation while decreased the density of photosystem II active reaction center, which partially protect the cells from photodamage and contributed to their acclimation to SHN. The algae also enhanced amino acid/fatty acid proportions essential for maintaining intracellular redox states to cope with the stress caused by LN or SHN. HN and SHN was in favor of protein accumulation and maintenance with enhanced proportion of essential amino acids, which entitled the algal biomass to be of high quality for animal feed applied in livestock graziery and aquaculture. LN facilitated productive starch and lipid accumulation with good quality for biofuels production. The nitrate removal rate and biomass productivity exceeded most of the microalgae reported in literature under similar conditions, which highlighted Tetraselmis subcordiformis as a potent strain for flexible nitrate-rich wastewater remediation coupled with fast CO₂ bio-mitigation and high-quality biomass production for sustainable algal biorefinery.

Stresses as First-Line Tools for Enhancing Lipid and Carotenoid Production in Microalgae

Microalgae can produce high-value-added products such as lipids and carotenoids using light or sugars, and their biosynthesis mechanism can be triggered by various stress conditions. Under nutrient deprivation or environmental stresses, microalgal cells accumulate lipids as an energy-rich carbon storage battery and generate additional amounts of carotenoids to alleviate the oxidative damage induced by stress conditions. Though stressful conditions are unfavorable for biomass accumulation and can induce oxidative damage, stress-based strategies are widely used in this field due to their effectiveness and economy. For the overproduction of different target products, it is required and meaningful to deeply understand the effects and mechanisms of various stress conditions so as to provide guidance on choosing the appropriate stress conditions. Moreover, the underlying molecular mechanisms under stress conditions can be clarified by omics technologies, which exhibit enormous potential in guiding rational genetic engineering for improving lipid and carotenoid biosynthesis.

The Influence of Dissolved Organic Carbon on the Microbial Community Associated with Tetraselmis striata for Bio-Diesel Production

The green alga Tetraselmis striata is regarded as a suitable candidate microalga for bio-diesel production. Recently, T. striata was cultured near Yeonghueung Island, Korea, in a “marine culturing field”; however, its environmental impacts are not yet studied. We estimated the amount of dissolved organic carbon (DOC) released from T. striata cultivation in the marine culturing field, and we investigated the changes in bacterial composition. Then, we designed and installed a mesocosm for further understanding. From the mesocosm results, the DOC released from the cultivation of T. striata led to changes in bacterial communities, disturbance of the microbial food web structure, rapid depletion of nutrients, and a decrease in dissolved oxygen (DO) and pH. Our novel work demonstrates that large amounts of DOC secreted by large-scale microalgal cultures such as that of T. striata can potentially have a significant impact on the structure and function of the surrounding microbial ecosystem.

Storage of starch and lipids in microalgae: Biosynthesis and manipulation by nutrients

Microalgae accumulate starch and lipid as storage metabolites under nutrient depletion, which can be used as sustainable feedstock for biorefinery. Omics analysis coupled with enzymatic and genetic verifications uncovered a partial picture of pathways and important enzymes or regulators related to starch and lipid biosynthesis as well as the carbon partitioning between them under nutrient depletion conditions. Depletion of macronutrients (N, P, and S) resulted in considerable enhancement of starch and/or lipid content in microalgae, but the accompanying declined photosynthesis hampered the achievements of high concentrations. This review summarized the current knowledge on the pathways and the committed steps as well as their carbon allocation involved in starch and lipid biosynthesis, and focused on the manipulation of different nutrients and the alleviation of oxidative stress for enhanced storage metabolites production. The biological and engineering approaches to cope with the conflict between biomass production and storage metabolites accumulation are proposed.

Application of an in situ CO2-bicarbonate system under nitrogen depletion to improve photosynthetic biomass and starch production and regulate amylose accumulation in a marine green microalga Tetraselmis subcordiformis

BACKGROUND

Microalgal starch is regarded as a promising alternative to crop-based starch for biorefinery such as the production of biofuels and bio-based chemicals. The single or separate use of inorganic carbon source, e.g., CO₂ and NaHCO₃, caused aberrant pH, which restricts the biomass and starch production. The present study applied an in situ CO₂-NaHCO₃ system to regulate photosynthetic biomass and starch production along with starch quality in a marine green microalga Tetraselmis subcordiformis under nitrogen-depletion (-N) and nitrogen-limitation (±N) conditions.

RESULTS

The CO₂ (2%)-NaHCO₃ (1 g L^-1) system stabilized the pH at 7.7 in the -N cultivation, under which the optimal biomass and starch accumulation were achieved. The biomass and starch productivity under -N were improved by 2.1-fold and 1.7-fold, respectively, with 1 g L^-1 NaHCO₃ addition compared with the one without NaHCO₃ addition. NaHCO₃ addition alleviated the high-dCO₂ inhibition caused by the single CO₂ aeration, and provided sufficient effective carbon source HCO₃ ^- for the maintenance of adequate photosynthetic efficiency and increase in photoprotection to facilitate the biomass and starch production. The amylose content was also increased by 44% under this CO₂-bicarbonate system compared to the single use of CO₂. The highest starch productivity of 0.73 g L^-1 day^-1 under -N cultivation and highest starch concentration of 4.14 g L^-1 under ±N cultivation were both achieved with the addition of 1 g L^-1 NaHCO₃. These levels were comparable to or exceeded the current achievements reported in studies. The addition of 5 g L^-1 NaHCO₃ under ±N cultivation led to a production of high-amylose starch (59.3% of total starch), which could be used as a source of functional food.

CONCLUSIONS

The in situ CO₂-NaHCO₃ system significantly improved the biomass and starch production in T. subcordiformis. It could also regulate the starch quality with varied relative amylose content under different cultivation modes for diverse downstream applications that could promote the economic feasibility of microalgal starch-based biofuel production. Adoption of this system in T. subcordiformis would facilitate the CO₂ mitigation couple with its starch-based biorefinery.

Production and structural characterization of a new type of polysaccharide from nitrogen-limited Arthrospira platensis cultivated in outdoor industrial-scale open raceway ponds

BACKGROUND

Carbohydrates are major biomass source in fuel-targeted biorefinery. Arthrospira platensis is the largest commercialized microalgae with good environmental tolerance and high biomass production. However, the traditional target of A. platensis cultivation is the protein, which is the downstream product of carbohydrates. Aiming to provide the alternative non-food carbohydrates source, the feasible manipulation technology on the cultivation is needed, as well as new separation methodology to achieve maximum utilization of overall biomass.

RESULTS

The present study aimed to demonstrate the feasibility of industrially producing carbohydrate-enriched A. platensis and characterize the structure of the polysaccharide involved. Cultivated in industrial-scale outdoor open raceway ponds under nitrogen limitation, A. platensis accumulated maximally 64.3%DW of carbohydrate. The maximum biomass and carbohydrate productivity reached 27.5 g m^-2 day^-1 and 26.2 g m^-2 day^-1, respectively. The efficient extraction and purification of the polysaccharides include a high-pressure homogenization-assisted hot water extraction followed by flocculation with a non-toxic flocculant ZTC1 + 1, with the polysaccharide purity and total recovery reaching 81% and 75%, respectively. The purified polysaccharide was mainly composed of (1→3)(1→4)- or (1→3)(1→2)-α-glucan with a molecular weight of 300-700 kDa, which differed from the commonly acknowledged glycogen.

CONCLUSIONS

By the way of controlled nitrogen limitation, the high carbohydrate production of A. platensis in the industrial scale was achieved. The α-glucan from A. platensis could be a potential glucose source for industrial applications. A non-toxic separation method of carbohydrate was applied to maintain the possibility of utilization of residue in high-value field.