Biomolecules from extremophile microalgae: From genetics to bioprocessing of a new candidate for large-scale production

Harnessing genetic engineering to drive economic bioproduct production in algae

Our reliance on agriculture for sustenance, healthcare, and resources has been essential since the dawn of civilization. However, traditional agricultural practices are no longer adequate to meet the demands of a burgeoning population amidst climate-driven agricultural challenges. Microalgae emerge as a beacon of hope, offering a sustainable and renewable source of food, animal feed, and energy. Their rapid growth rates, adaptability to non-arable land and non-potable water, and diverse bioproduct range, encompassing biofuels and nutraceuticals, position them as a cornerstone of future resource management. Furthermore, microalgae's ability to capture carbon aligns with environmental conservation goals. While microalgae offers significant benefits, obstacles in cost-effective biomass production persist, which curtails broader application. This review examines microalgae compared to other host platforms, highlighting current innovative approaches aimed at overcoming existing barriers. These approaches include a range of techniques, from gene editing, synthetic promoters, and mutagenesis to selective breeding and metabolic engineering through transcription factors.

Chemical Triggering Cyanobacterial Glycogen Accumulation: Methyl Viologen Treatment Increases Synechocystis sp. PCC 6803 Glycogen Storage by Enhancing Levels of Gene Transcript and Substrates in Glycogen Synthesis

Two-stage cultivation is effective for glycogen production by cyanobacteria. Cells were first grown under adequate nitrate supply (BG11) to increase biomass and subsequently transferred to nitrogen deprivation (-N) to stimulate glycogen accumulation. However, the two-stage method is time-consuming and requires extensive energy. Thus, one-stage cultivation that enables both cell growth and glycogen accumulation is advantageous. Such one-stage method could be achieved using a chemical triggering glycogen storage. However, there is a limited study on such chemicals. Here, nine compounds previously reported to affect cyanobacterial cellular functions were examined in Synechocystis sp. PCC 6803. 2-Phenylethanol, phenoxyethanol, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and methyl viologen can stimulate glycogen accumulation. The oxidative stress agent, methyl viologen significantly increased glycogen levels up to 57% and 69% [w/w dry weight (DW)] under BG11 and -N cultivation, respectively. One-stage cultivation where methyl viologen was directly added to the pre-grown culture enhanced glycogen storage to 53% (w/w DW), compared to the 10% (w/w DW) glycogen level of the control cells without methyl viologen. Methyl viologen treatment reduced the contents of total proteins (including phycobiliproteins) but caused increased transcript levels of glycogen synthetic genes and elevated levels of metabolite substrates for glycogen synthesis. Metabolomic results suggested that upon methyl viologen treatment, proteins degraded to amino acids, some of which could be used as a carbon source for glycogen synthesis. Results of oxygen evolution and metabolomic analysis suggested that photosynthesis and carbon fixation were not completely inhibited upon methyl viologen treatment, and these two processes may partially generate upstream metabolites required for glycogen synthesis.

Acid Tolerant and Acidophilic Microalgae: An Underexplored World of Biotechnological Opportunities

Despite their large number and diversity, microalgae from only four genera are currently cultivated at large-scale. Three of those share common characteristics: they are cultivated mainly autotrophically and are extremophiles or tolerate “extreme conditions.” Extreme growth conditions aid in preventing contamination and predation of microalgae, therefore facilitating outdoor cultivation. In search for new extremophilic algae suitable for large-scale production, we investigated six microalgal strains able to grow at pH below 3 and belonging to four genera; Stichococcus bacillaris ACUF158, Chlamydomonas acidophila SAG 2045, and Chlamydomonas pitschmannii ACUF238, Viridiella fridericiana ACUF035 and Galdieria sulphuraria ACUF064 and ACUF074. All strains were cultivated autotrophically at light intensity of 100 and 300 μmol m−2 s−1 and pH between 1.9 and 2.9. The autotrophic biomass productivities were compared with one of the most productive microalgae, Chlorella sorokiniana SAG 211-8K, grown at pH 6.8. The acid tolerant strains have their autotrophic biomass productivities reported for the first time. Mixotrophic and heterotrophic properties were investigated when possible. Five of the tested strains displayed autotrophic biomass productivities 10–39% lower than Chlorella sorokiniana but comparable with other commercially relevant neutrophilic microalgae, indicating the potential of these microalgae for autotrophic biomass production under acidic growth conditions. Two acid tolerant species, S. bacillaris and C. acidophila were able to grow mixotrophically with glucose. Chlamydomonas acidophila and the two Galdieria strains were also cultivated heterotrophically with glucose at various temperatures. Chlamydomonas acidophila failed to grow at 37°C, while G. sulphuraria ACUF64 showed a temperature optimum of 37°C and G. sulphuraria ACUF74 of 42°C. For each strain, the biomass yield on glucose decreased when cultivated above their optimal temperature. The possible biotechnological applications of our findings will be addressed.

Unconventional high-value products from microalgae: A review

Microalgae have gained significant importance in biotechnology development, providing valuable goods and services in multiple applications. Although there is a rising market for most of these applications, the incorporation and introduction of microalgae into new venues will extend in the near future. These advances are due to the vast biodiversity of microalgal species, recent genetic engineering tools, and culture techniques. There are three main possible approaches for novel algal compounds from: (1) recently isolated yet less known microalgae; (2) selectively stressed conditions; and (3) enzymatically adjusted compounds from conventional molecules. All these approaches can be combined in a specific manner. This review discusses the opportunities, potential and limitations of introducing novel microalgae-based products, and how the recent technologies can be deployed to make these products financially viable. To give an outlook to the future, an analysis of the developments and predicted future market that further enlarge the promise of cultivating microalgae for commercial purposes are considered.

Cultivation of the Acidophilic Microalgae Galdieria phlegrea with Wastewater: Process Yields

Algal based wastewater treatment offers the opportunity to recover, in the form of biomass, the nutrients and internal chemical energy of wastewater. Recently, there has been a growing interest in the use of extremophilic microalgae, as they can easily adapt to difficult and often pollutant-rich environments. The thermo-acidophilic microalga Galdieria phlegrea is a species of recent discovery and great metabolic versatility, but it has still been poorly studied. Here, G. phlegrea was cultivated using raw municipal wastewater in 1 L Erlenmeyer flasks with 700 mL working volume at 37 °C for up to nine days. During the cultivation phase, biomass growth, phycocyanin content, ammonium and phosphate removal from the wastewater, lipid fraction, total carbon and nitrogen in the biomass, and variation in δ¹³C and δ¹⁵N isotopic ratios (a novel analytical contribution in these experiments) were monitored. Results indicated that G. phlegrea was able to grow in raw effluent, where it removed more than 50% ammonium and 20% phosphate in 24 h; total lipid content was in the range of 11–22%, while average C-N content was of 45% and 6%, respectively; isotopic analyses proved to be a useful support in identifying C and N metabolic pathways from effluent to biomass. Overall, G. phlegrea showed consistent performance with similar Cyanidiophyceae and is a potentially viable candidate for municipal wastewater valorization from a circular economy perspective.

Microalgae-Derived Pigments: A 10-Year Bibliometric Review and Industry and Market Trend Analysis

Microalgae productive chains are gaining importance as sustainable alternatives to obtain natural pigments. This work presents a review on the most promising pigments and microalgal sources by gathering trends from a 10-year bibliometric survey, a patents search, and an industrial and market analysis built from available market reports, projects and companies' webpages. The performed analysis pointed out chlorophylls, phycocyanin, astaxanthin, and β-carotene as the most relevant pigments, and Chlorella vulgaris, Spirulina platensis, Haematococcus pluvialis, and Dunaliella salina, respectively, as the most studied sources. Haematococcus is referred in the highest number of patents, corroborating a high technological interest in this microalga. The biorefinery concept, investment in projects and companies related to microalgae cultivation and/or pigment extraction is increasingly growing, particularly, for phycocyanin from Spirulina platensis. These pieces of evidence are a step forward to consolidate the microalgal pigments market, which is expected to grow in the coming years, increasing the prospects of replacing synthetic pigments by natural counterparts.

A Comprehensive Review of the Nutraceutical and Therapeutic Applications of Red Seaweeds (Rhodophyta)

The red seaweed group (Rhodophyta) is one of the phyla of macroalgae, among the groups Phaeophyceae and Chlorophyta, brown and green seaweeds, respectively. Nowadays, all groups of macroalgae are getting the attention of the scientific community due to the bioactive substances they produce. Several macroalgae products have exceptional properties with nutraceutical, pharmacological, and biomedical interest. The main compounds studied are the fatty acids, pigments, phenols, and polysaccharides. Polysaccharides are the most exploited molecules, which are already widely used in various industries and are, presently, entering into more advanced applications from the therapeutic point of view. The focuses of this review are the red seaweeds' compounds, its proprieties, and its uses. Moreover, this work discusses new possible applications of the compounds of the red seaweeds.