Storage Compound Accumulation in Diatoms as Response to Elevated CO2 Concentration

Mixotrophic growth of a ubiquitous marine diatom

Diatoms are major players in the global carbon cycle, and their metabolism is affected by ocean conditions. Understanding the impact of changing inorganic nutrients in the oceans on diatoms is crucial, given the changes in global carbon dioxide levels. Here, we present a genome-scale metabolic model (iMK1961) for Cylindrotheca closterium, an in silico resource to understand uncharacterized metabolic functions in this ubiquitous diatom. iMK1961 represents the largest diatom metabolic model to date, comprising 1961 open reading frames and 6718 reactions. With iMK1961, we identified the metabolic response signature to cope with drastic changes in growth conditions. Comparing model predictions with Tara Oceans transcriptomics data unraveled C. closterium's metabolism in situ. Unexpectedly, the diatom only grows photoautotrophically in 21% of the sunlit ocean samples, while the majority of the samples indicate a mixotrophic (71%) or, in some cases, even a heterotrophic (8%) lifestyle in the light. Our findings highlight C. closterium's metabolic flexibility and its potential role in global carbon cycling.

Response of varying combined nutrients on biomass and biochemical composition of marine diatoms Chaetoceros gracilis and Thalassiosira weissflogii

Marine diatoms have high adaptability and are known to accumulate lipids under nutrient stress conditions. The present study involves determining the effect of varying macro and micronutrients on growth kinetics and metabolite production of oleaginous marine diatoms, Thalassiosira weissflogii and Chaetoceros gracilis. The results highlighted that C. gracilis and T. weissflogii showed maximum biomass yield of 0.86 ± 0.06 g/L and 0.76 ± 0.01 g/L in the 2f and f supplemented medium respectively. A 2.5-fold increase in cellular lipid content was recorded in the 2f culture setup of both strains ranging from 20 % to 26.7 % (w/w). The study also reveals that high eutrophic nutrient media (f, 2f and 4f) triggered biomass productivity as well as total protein and carbohydrate content in both strains. Thus, providing a reproducible insight of trophic flexibility of diatoms, concomitant with the increment in multiple commercially valuable products.

Location of the photosynthetic carbon metabolism in microcompartments and separated phases in microalgal cells

Carbon acquisition, assimilation and storage in eukaryotic microalgae and cyanobacteria occur in multiple compartments that have been characterised by the location of the enzymes involved in these functions. These compartments can be delimited by bilayer membranes, such as the chloroplast, the lumen, the peroxisome, the mitochondria or monolayer membranes, such as lipid droplets or plastoglobules. They can also originate from liquid-liquid phase separation such as the pyrenoid. Multiple exchanges exist between the intracellular microcompartments, and these are reviewed for the CO2 concentration mechanism, the Calvin-Benson-Bassham cycle, the lipid metabolism and the cellular energetic balance. Progress in microscopy and spectroscopic methods opens new perspectives to characterise the molecular consequences of the location of the proteins involved, including intrinsically disordered proteins.

Nanoparticles from Microalgae and Their Biomedical Applications

Over the years, microalgae have been a source of useful compounds mainly used as food and dietary supplements. Recently, microalgae have been used as a source of metabolites that can participate in the synthesis of several nanoparticles through inexpensive and environmentally friendly routes alternative to chemical synthesis. Notably, the occurrence of global health threats focused attention on the microalgae application in the medicinal field. In this review, we report the influence of secondary metabolites from marine and freshwater microalgae and cyanobacteria on the synthesis of nanoparticles that were applied as therapeutics. In addition, the use of isolated compounds on the surface of nanoparticles to combat diseases has also been addressed. Although studies have proven the beneficial effect of high-value bioproducts on microalgae and their potential in medicine, there is still room for understanding their exact role in the human body and translating lab-based research into clinical trials.

Biomass valorization of agriculture wastewater grown freshwater diatom Nitzschia sp. for metabolites, antibacterial activity, and biofertilizer

The sustainable utilization of agricultural wastewater is a major global challenge. This study evaluated the impact of agricultural fertilizer on the biomass potential of Nitzschia sp. for metabolite production, antibacterial activity, and slow release biofertilizer. Cultivation of Nitzschia sp. in agriculture wastewater (0.5 mg ml^-1) exhibited maximum cell density (12×10⁵ cells ml^-1), protein content (10.0 mg g^-1), and lipid content (14.96%). Carbohydrate and phenol content increases in a dose-dependent manner with 8.27 mg g^-1 and 2.05 mg g^-1 at a concentration of 2 mg ml^-1 respectively. There was a 2.1-fold increment in chrysolaminarin content. Both gram-negative and gram-positive bacteria were susceptible to the antibacterial activity of the biomass. The effects of using diatom biomass as a biofertilizer were evaluated on the growth of periwinkle plants, which showed significant improvements in leaf development, branching at an early stage, flowering, and a marked increase in shoot length. Diatom biorefinery holds immense potential in addressing agriculture wastewater recycling and sustainable generation of high-value compounds.

Testing for terrestrial and freshwater microalgae productivity under elevated CO2 conditions and nutrient limitation

BACKGROUND

Microalgae CO₂ fixation results in the production of biomass rich in high-valuable products, such as fatty acids and carotenoids. Enhanced productivity of valuable compounds can be achieved through the microalgae's ability to capture CO₂ efficiently from sources of high CO₂ contents, but it depends on the species. Culture collections of microalgae offer a wide variety of defined strains. However, an inadequate understanding of which groups of microalgae and from which habitats they originate offer high productivity under increased CO₂ concentrations hampers exploiting microalgae as a sustainable source in the bioeconomy.

RESULTS

A large variety of 81 defined algal strains, including new green algal isolates from various terrestrial environments, were studied for their growth under atmospheres with CO₂ levels of 5-25% in air. They were from a pool of 200 strains that had been pre-selected for phylogenetic diversity and high productivity under ambient CO₂. Green algae from terrestrial environments exhibited enhanced growth up to 25% CO₂. In contrast, in unicellular red algae and stramenopile algae, which originated through the endosymbiotic uptake of a red algal cell, growth at CO₂ concentrations above 5% was suppressed. While terrestrial stramenopile algae generally tolerated such CO₂ concentrations, their counterparts from marine phytoplankton did not. The tests of four new strains in liquid culture revealed enhanced biomass and chlorophyll production under elevated CO₂ levels. The 15% CO₂ aeration increased their total carotenoid and fatty acid contents, which were further stimulated when combined with the starvation of macronutrients, i.e., less with phosphate and more with nitrogen-depleted culture media.

CONCLUSION

Green algae originating from terrestrial environments, Chlorophyceae and Trebouxiophyceae, exhibit enhanced productivity of carotenoids and fatty acids under elevated CO₂ concentrations. This ability supports the economic and sustainable production of valuable compounds from these microalgae using inexpensive sources of high CO₂ concentrations, such as industrial exhaust fumes.

Nutrient acclimation in benthic diatoms with adaptive laboratory evolution

The growth of marine diatom algaeChaetoceros gracilisandThalassiosira weissflogiiin agricultural fertilizers and additional carbon sources were evaluated. The main objective behind the study was to increase the growth and productivity of the diatom acclimatized under adaptive laboratory culture conditions. In optimized conditions,C.gracilisshowed the highest cell density in NPK (202.5 ± 2.6 × 10⁵ cells mL^-1), maximum carbohydrate (212.8 ± 4.0 mg g^-1) and protein (133.9 ± 1.5 mg g^-1) in urea. In contrast,T.weissflogiishowed the highest cell density in glycerol (148.2 ± 2.5x10⁵ cells mL^-1), maximum carbohydrate in glycerol (273.7 ± 3.3 mg g^-1), and protein in sucrose (126.2 ± 0.7 mg g^-1). Lipid content was maximum in glycerol (73.4 ± 0.6%) and glucose (39.7 ± 0.2%) in C. gracilisand T. weissflogii respectively. Increased pigment production and chrysolaminarin concentration were obtained in both marine species. The study highlights the importance of adaptive laboratory evolution as an promising tool in enhancing productivity in diatom algae.

Mass cultivation of marine diatoms using local salts and its impact on growth and productivity

Diatoms are of great interest for many biotechnological applications. The present study highlights the comparative analysis for mass cultivation under the effect of seawater made from table salt (TS), rock salt (RS), and synthetic seawater in the presence of normal silica and induction coupled plasma (ICP) nanosilica (Nano Si) for inducing diatom growth. Out of all the test formulations, RS-f/2 Nano Si showed the best results with maximum cell density (3.16x107±0.04 and 3.24x107±0.05 cells mL^-1), carbohydrate (403.0±3.4 and 398.0±8.1 mg g^-1), and chrysolaminarin yield (66.2±5.5 and 49.3±5.1 mg g^-1) in both Chaetoceros gracilis and Thalassiosira weissflogii respectively. The presence of a rich pigment profile and lipids further highlights the importance of TS and RS for cost-effective mass culturing. Results reveal that mass cultivation of marine diatoms with TS and RS in the presence of nanosilica not only reduces costs but also enhances metabolite production.

Multiplexed CRISPR/Cas9 editing of the long-chain acyl-CoA synthetase family in the diatom Phaeodactylum tricornutum reveals that mitochondrial ptACSL3 is involved in the synthesis of storage lipids

Long-chain acyl-CoA synthetases (LACS) play diverse and fundamentally important roles in lipid metabolism. While their functions have been well established in bacteria, yeast and plants, the mechanisms by which LACS isozymes regulate lipid metabolism in unicellular oil-producing microalgae, including the diatom Phaeodactylum tricornutum, remain largely unknown. In P. tricornutum, a family of five genes (ptACSL1-ptACSL5) encodes LACS activities. We generated single lacs knockout/knockdown mutants using multiplexed CRISPR/Cas9 method, and determined their substrate specificities towards different fatty acids (FAs) and subcellular localisations. ptACSL3 is localised in the mitochondria and its disruption led to compromised growth and reduced triacylglycerol (TAG) content when cells were bubbled with air. The ptACSL3 mutants showed altered FA profiles in two galactoglycerolipids and phosphatidylcholine (PC) with significantly reduced distribution of 16:0 and 16:1. ptACSL5 is localised in the peroxisome and its knockdown resulted in reduced growth rate and altered molecular species of PC and TAG, indicating a role in controlling the composition of acyl-CoAs for lipid synthesis. Our work demonstrates the potential of generating gene knockout mutants with the mutation of large fragment deletion using multiplexed CRISPR/Cas9 and provides insight into the functions of LACS isozymes in lipid metabolism in the oleaginous microalgae.

Laminarin, a Major Polysaccharide in Stramenopiles

During the processes of primary and secondary endosymbiosis, different microalgae evolved to synthesis different storage polysaccharides. In stramenopiles, the main storage polysaccharides are β-1,3-glucan, or laminarin, in vacuoles. Currently, laminarin is gaining considerable attention due to its application in the food, cosmetic and pharmaceuticals industries, and also its importance in global biogeochemical cycles (especially in the ocean carbon cycle). In this review, the structures, composition, contents, and bioactivity of laminarin were summarized in different algae. It was shown that the general features of laminarin are species-dependence. Furthermore, the proposed biosynthesis and catabolism pathways of laminarin, functions of key genes, and diel regulation of laminarin were also depicted and comprehensively discussed for the first time. However, the complete pathways, functions of genes, and diel regulatory mechanisms of laminarin require more biomolecular studies. This review provides more useful information and identifies the knowledge gap regarding the future studies of laminarin and its applications.

Indoor mass cultivation of marine diatoms for biodiesel production using induction plasma synthesized nanosilica

In this work, two benthic marine diatoms Chaetoceros sp. and Thalassiosira sp. were grown in modified f/2 medium in which normal silica was replaced with inductively coupled plasma (ICP) nanosilica for indoor mass cultivation and its impact on growth, lipid content, lipid quality and metabolite production were monitored. Results indicate thatunder mass cultivation using ICP nano silica medium, Thalassiosirasp. reached 3.6 and Chaetoceros sp. reached 3.2-fold higher cell density compared to normal Si medium. The primary metabolite production and total lipid content was higher in Chaetoceros sp. (44.33 ± 2.51% DCW) compared to Thalassiosira sp. (29.66 ± 1.52% DCW). In mass cultivation, ICP synthesized nanosilica powder was effective in enhancing the cell density, production of metabolites, pigments, and lipids in the marine diatoms studied. This is the first report on the use of ICP nanosilica in carrying out indoor mass cultivation of marine diatom isolates as potential biodiesel and biomolecule feedstocks.

Diatoms for Carbon Sequestration and Bio-Based Manufacturing

Carbon dioxide (CO2) is a major greenhouse gas responsible for climate change. Diatoms, a natural sink of atmospheric CO2, can be cultivated industrially in autotrophic and mixotrophic modes for the purpose of CO2 sequestration. In addition, the metabolic diversity exhibited by this group of photosynthetic organisms provides avenues to redirect the captured carbon into products of value. These include lipids, omega-3 fatty acids, pigments, antioxidants, exopolysaccharides, sulphated polysaccharides, and other valuable metabolites that can be produced in environmentally sustainable bio-manufacturing processes. To realize the potential of diatoms, expansion of our knowledge of carbon supply, CO2 uptake and fixation by these organisms, in conjunction with ways to enhance metabolic routing of the fixed carbon to products of value is required. In this review, current knowledge is explored, with an evaluation of the potential of diatoms for carbon capture and bio-based manufacturing.

Regulation of Carbon Metabolism by Environmental Conditions: A Perspective From Diatoms and Other Chromalveolates

Diatoms belong to a major, diverse and species-rich eukaryotic clade, the Heterokonta, within the polyphyletic chromalveolates. They evolved as a result of secondary endosymbiosis with one or more Plantae ancestors, but their precise evolutionary history is enigmatic. Nevertheless, this has conferred them with unique structural and biochemical properties that have allowed them to flourish in a wide range of different environments and cope with highly variable conditions. We review the effect of pH, light and dark, and CO2 concentration on the regulation of carbon uptake and assimilation. We discuss the regulation of the Calvin-Benson-Bassham cycle, glycolysis, lipid synthesis, and carbohydrate synthesis at the level of gene transcripts (transcriptomics), proteins (proteomics) and enzyme activity. In contrast to Viridiplantae where redox regulation of metabolic enzymes is important, it appears to be less common in diatoms, based on the current evidence, but regulation at the transcriptional level seems to be widespread. The role of post-translational modifications such as phosphorylation, glutathionylation, etc., and of protein-protein interactions, has been overlooked and should be investigated further. Diatoms and other chromalveolates are understudied compared to the Viridiplantae, especially given their ecological importance, but we believe that the ever-growing number of sequenced genomes combined with proteomics, metabolomics, enzyme measurements, and the application of novel techniques will provide a better understanding of how this important group of algae maintain their productivity under changing conditions.

Wealth from waste: Diatoms as tools for phycoremediation of wastewater and for obtaining value from the biomass

Diatoms are a type of microalgae with diverse capabilities which make them useful for multiple applications. The abundance of diatoms in water bodies facilitates the removal of pollutants from wastewater originating from different industries, such as agriculture and other anthropogenic sources. The unique photosynthetic, cellular and metabolic characteristics of diatoms allows them to utilize pollutants like nitrate, iron, phosphate, molybdenum, silica, and heavy metals, such as copper, cadmium, chromium, lead, etc., which make diatoms a good option for wastewater treatment. In addition, the biomass produced by diatoms growth on wastewaters has diverse applications and can, therefore, be valuable. This review focusses on the unique capabilities of diatoms for wastewater remediation and the capture of carbon dioxide, concomitant with the generation of valuable products. Diatom biorefinery can be a sustainable solution to wastewater management, and the biomass obtained from treatment can be turned into biofuels, biofertilizers, nutritional supplements for animal production, and used for pharmaceutical applications containing bioactive compounds like EPA, DHA and pigments such as fucoxanthin.