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A special issue of Plants (ISSN 2223-7747). This special issue belongs to the section "Plant Physiology and Metabolism".

Deadline for manuscript submissions: closed (31 May 2021) | Viewed by 38733

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Dr. Sousuke Imamura

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Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Interests: plant molecular biology; stress response; nitrogen regulation; transcriptional regulation; signal transduction; chloroplast
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Dr. Imran Pancha

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Department of Industrail Biotechnology, Gujarat Biotechnology University, Gandhinagar 382355, Gujarat, India
Interests: microalgae biofuels; biorefinery; biomass production
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Special Issue Information

Dear Colleagues,

Algae are photosynthetic organisms which utilize atmospheric CO₂ as the carbon source and accumulate various valuable compounds such as pigments, triacylglycerols (TAGs) and starch in the cells. Compared to other photosynthetic organisms, algae have suitable characteristics for biomass productions such as high growth rate, simple growth requirement, applicable wastewater/seawater for the medium, no need for arable land, etc. Thus, algae have been thought to be excellent biofactories for the production of various industrial relevant compounds. However, several fundamental and technical issues still need to be addressed in order to establish a commercial production of algal biomass, including (i) the molecular mechanism behind the accumulation of valuable compounds, (ii) downstream processing for harvesting and extraction of the desired compounds from the cells, and (iii) biorefinery to produce a broad range of compounds from the algal biomass. This Special Issue invites submissions on topics related to algal bioprospecting, biochemical and metabolic engineering of algae, algal stress physiology, algal biorefinery, etc. The Special Issue also welcomes review articles that summarize and discuss comprehensive recent findings and future direction in the field of algal biomass.

Dr. Sousuke Imamura
Dr. Imran Pancha
Guest Editors

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Keywords

Microalgae
Cyanobacteria
Diatom
Algal biomass
Biofuels/bioenergy
Valuable products
Biochemical and metabolic engineering
Stress physiology
Downstream processing of algal biomass
Algal biorefinery

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Published Papers (9 papers)

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Research

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13 pages, 2969 KiB

Open AccessArticle

Green Microalgae Strain Improvement for the Production of Sterols and Squalene

by Supakorn Potijun, Suparat Jaingam, Nuttha Sanevas, Srunya Vajrodaya and Anchalee Sirikhachornkit

Plants 2021, 10(8), 1673; https://doi.org/10.3390/plants10081673 - 13 Aug 2021

Cited by 9 | Viewed by 3892

Abstract

Sterols and squalene are essential biomolecules required for the homeostasis of eukaryotic membrane permeability and fluidity. Both compounds have beneficial effects on human health. As the current sources of sterols and squalene are plant and shark oils, microalgae are suggested as more sustainable sources. Nonetheless, the high costs of production and processing still hinder the commercialization of algal cultivation. Strain improvement for higher product yield and tolerance to harsh environments is an attractive way to reduce costs. Being an intermediate in sterol synthesis, squalene is converted to squalene epoxide by squalene epoxidase. This step is inhibited by terbinafine, a commonly used antifungal drug. In yeasts, some terbinafine-resistant strains overproduced sterols, but similar microalgae strains have not been reported. Mutants that exhibit greater tolerance to terbinafine might accumulate increased sterols and squalene content, along with the ability to tolerate the drug and other stresses, which are beneficial for outdoor cultivation. To explore this possibility, terbinafine-resistant mutants were isolated in the model green microalga Chlamydomonas reinhardtii using UV mutagenesis. Three mutants were identified and all of them exhibited approximately 50 percent overproduction of sterols. Under terbinafine treatment, one of the mutants also accumulated around 50 percent higher levels of squalene. The higher accumulation of pigments and triacylglycerol were also observed. Along with resistance to terbinafine, this mutant also exhibited higher resistance to oxidative stress. Altogether, resistance to terbinafine can be used to screen for strains with increased levels of sterols or squalene in green microalgae without growth compromise. Full article

(This article belongs to the Special Issue Algal Biomass)

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12 pages, 1809 KiB

Open AccessFeature PaperArticle

Phosphite Reduces the Predation Impact of Poterioochromonas malhamensis on Cyanobacterial Culture

by Narumi Toda, Hiroki Murakami, Akihiro Kanbara, Akio Kuroda and Ryuichi Hirota

Plants 2021, 10(7), 1361; https://doi.org/10.3390/plants10071361 - 2 Jul 2021

Cited by 6 | Viewed by 3305

Abstract

Contamination by the predatory zooplankton Poterioochromonas malhamensis is one of the major threats that causes catastrophic damage to commercial-scale microalgal cultivation. However, knowledge of how to manage predator contamination is limited. Previously, we established a phosphite (Pt)-based culture system by engineering Synechococcus elongatus, which exerted a competitive growth advantage against microbial contaminants that compete with phosphate source. Here, we examined whether Pt is effective in suppressing predator-type contamination. Co-culture experiment of Synechococcus with isolated P. malhamensis revealed that, although an addition of Pt at low concentrations up to 2.0 mM was not effective, increased dosage of Pt (~20 mM) resulted in the reduced grazing impact of P. malhamensis. By using unsterilized raw environmental water collected from rivers or ponds, we found that the suppression effect of Pt was dependent on the type of environmental water used. Eukaryotic microbial community analysis of the cultures using environmental water samples revealed that Paraphysomonas, a colorless Chrysophyceae, emerged and dominated under high-Pt conditions, suggesting that Paraphysomonas is insensitive to Pt compared to P. malhamensis. These findings may provide a clue for developing a strategy to reduce the impact of grazer contamination in commercial-scale microalgal cultivation. Full article

(This article belongs to the Special Issue Algal Biomass)

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23 pages, 8837 KiB

Open AccessArticle

Tetraselmis jejuensis sp. nov. (Chlorodendrophyceae), a Euryhaline Microalga Found in Supralittoral Tide Pools at Jeju Island, Korea

by Jun-Ho Hyung, Eun-Joo Kim, Seung-Joo Moon, Nam Seon Kang and Jaeyeon Park

Plants 2021, 10(7), 1289; https://doi.org/10.3390/plants10071289 - 24 Jun 2021

Cited by 6 | Viewed by 5514

Abstract

We found the euryhaline microalga, Tetraselmis jejuensis sp. nov., which was adapted to supralittoral tide pools with salinities varying from 0.3–3.1%. Fifteen strains of T. jejuensis were isolated from Daejeong (DJ) and Yongduam (YO), and clonal cultures were established in the laboratory. Morphological characterization revealed that the cells have a compressed shape, four flagella emerging from a depression near the apex in two opposite pairs, a cup-shaped chloroplast containing one pyrenoid surrounded by starch, and eyespot regions not located near the flagellar base. T. jejuensis cells showed distinct characteristics compared to other Tetraselmis species. First, a regular subunit pattern with honeycomb-like structures was predominantly displayed on the surface in the middle of the cell body. Second, the pyrenoid was invaded by both cytoplasmic channels comprising electron-dense material separated from the cytoplasm, and two branches of small cytoplasmic channels (canaliculi) in various directions, which characterize the subgenus Tetrathele. Eyespot regions containing a large number of osmiophilic globules, packed closely together and arranged in subcircular close packing of diverse sizes, were dispersed throughout the chloroplast. In the phylogenetic analysis of small subunit (SSU) rDNA sequences, the 15 strains isolated from DJ and YO separated a newly branched clade in the Chlorodendrophyceae at the base of a clade comprising the T. carteriiformi/subcordiformis clade, T. chuii/suecica clade, and T. striata/convolutae clade. The strains in the diverging clade were considered to belong to the same species. The SSU rDNA sequences of the DJ and YO strains showed a maximum difference of 1.53% and 1.19% compared to Tetraselmis suecica (MK541745), the closest species of the family based on the phylogenetic analysis, respectively. Based on morphological, molecular, and physiological features, we suggest a new species in the genus Tetraselmis named Tetraselmis jejuensis, with the species name “jejuensis” referring to the collection site, Jeju Island, Korea. Full article

(This article belongs to the Special Issue Algal Biomass)

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14 pages, 1836 KiB

Open AccessFeature PaperArticle

Identification of Transcription Factors and the Regulatory Genes Involved in Triacylglycerol Accumulation in the Unicellular Red Alga Cyanidioschyzon merolae

by Sota Takahashi, Riho Okubo, Yu Kanesaki, Baifeng Zhou, Kazuhiro Takaya, Satoru Watanabe, Kan Tanaka and Sousuke Imamura

Plants 2021, 10(5), 971; https://doi.org/10.3390/plants10050971 - 13 May 2021

Cited by 11 | Viewed by 3277

Abstract

Microalgal triacylglycerols (TAGs) are a good feedstock for liquid biofuel production. Improving the expression and/or function of transcription factors (TFs) involved in TAG accumulation may increase TAG content; however, information on microalgae is still lacking. In this study, 14 TFs in the unicellular red alga Cyanidioschyzon merolae were identified as candidate TFs regulating TAG accumulation using available transcriptome and phosphoproteome data under conditions driving TAG accumulation. To investigate the roles of these TFs, we constructed TF-overexpression strains and analyzed lipid droplet (LD) formation and TAG contents in the cells grown under standard conditions. Based on the results, we identified four TFs involved in LD and TAG accumulation. RNA-Seq analyses were performed to identify genes regulated by the four TFs using each overexpression strain. Among the TAG biosynthesis-related genes, only the gene encoding the endoplasmic reticulum-localized lysophosphatidic acid acyltransferase 1 (LPAT1) was notably increased among the overexpression strains. In the LPAT1 overexpression strain, TAG accumulation was significantly increased compared with the control strain under normal growth conditions. These results indicate that the four TFs positively regulate TAG accumulation by changing their target gene expression in C. merolae. Full article

(This article belongs to the Special Issue Algal Biomass)

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17 pages, 2532 KiB

Open AccessArticle

Effects of Monochromatic Illumination with LEDs Lights on the Growth and Photosynthetic Performance of Auxenochlorella protothecoides in Photo- and Mixotrophic Conditions

by Giorgos Markou, Alexandros Diamantis, Evagelina Korozi, Vasiliki Tsagou, Io Kefalogianni and Iordanis Chatzipavlidis

Plants 2021, 10(4), 799; https://doi.org/10.3390/plants10040799 - 19 Apr 2021

Cited by 9 | Viewed by 2854

Abstract

This study examined the effects of monochromatic illumination (blue, red, green and yellow) employing light-emitting diodes (LEDs), trophic conditions (photoautotrophic and mixotrophic), and nitrogen availability (high and low peptone concentration) on the growth and biochemical composition of Auxenochlorella protothecoides. The results revealed that mixotrophic conditions did not favor A. protothecoides, giving lower growth rates compared to heterotrophy (dark conditions). However, mixotrophy gave significantly higher growth rates compared to photoautotrophy. The best light wavelengths for mixotrophic cultivation were that of white and red. In all cases investigated in this study, high peptone concentration (4 g/L) resulted in decreased growth rates. Regarding the biochemical composition of A. protothecoides, the strongest effect, irrespective of trophic conditions, was caused by nitrogen availability (peptone concentration). Specifically, at nitrogen replete conditions (4 g/L peptone), biomass was rich in proteins (32–67%), whereas under deplete conditions (0.5 g/L peptone), A. protothecoides accumulated mainly carbohydrates (up to 56%). Mixotrophic conditions generally favored higher carbohydrate content, whereas photoautotrophic conditions favored higher protein content. The different illumination spectra did not have any clear effect on the biochemical composition (metabolites content), except that, in all trophic conditions, the use of the green spectrum resulted in higher chlorophyll b content. Chlorophyll a fluorescence studies revealed that the trophic conditions and the high peptone concentrations impacted the photosystem II (PSII) performance, and also affected plastoquinone re-oxidation kinetics and the heterogeneity of the PSII reaction centers. Full article

(This article belongs to the Special Issue Algal Biomass)

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16 pages, 1522 KiB

Open AccessArticle

Assessment of Chlorella sorokiniana Growth in Anaerobic Digester Effluent

by Elvira E. Ziganshina, Svetlana S. Bulynina and Ayrat M. Ziganshin

Plants 2021, 10(3), 478; https://doi.org/10.3390/plants10030478 - 3 Mar 2021

Cited by 23 | Viewed by 3019

Abstract

Microalgae are considered a potential source of valuable compounds for multiple purposes and are potential agents for bioremediation of aquatic environments contaminated with different pollutants. This work evaluates the use of agricultural waste, unsterilized and anaerobically digested, to produce biomass from a strain of Chlorella sorokiniana. Furthermore, the presence of bacteria in these wastes was investigated based on the bacterial 16S rRNA gene sequencing. The results showed a specific growth rate ranging between 0.82 and 1.45 day⁻¹, while the final biomass yield in different digestate-containing treatments (bacterial-contaminated cultures) ranged between 0.33 and 0.50 g L⁻¹ day⁻¹. Besides, substantial amounts of ammonium, phosphate, and sulfate were consumed by C. sorokiniana during the experimental period. The predominant bacteria that grew in the presence of C. sorokiniana in the effluent-containing treatments belonged to the genera Chryseobacterium, Flavobacterium, Sphingomonas, Brevundimonas, Hydrogenophaga, Sphingobacterium, and Pseudomonas. Therefore, this microalga can tolerate and grow in the presence of other microorganisms. Finally, these results show that anaerobically digested agricultural waste materials are a good substitute for growth media for green microalgae; however, phosphate and sulfate levels must also be controlled in the media to maintain adequate growth of microalgae. Full article

(This article belongs to the Special Issue Algal Biomass)

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12 pages, 4028 KiB

Open AccessArticle

Rapid Quantification of Microalgae Growth with Hyperspectral Camera and Vegetation Indices

by Pauliina Salmi, Matti A. Eskelinen, Matti T. Leppänen and Ilkka Pölönen

Plants 2021, 10(2), 341; https://doi.org/10.3390/plants10020341 - 10 Feb 2021

Cited by 10 | Viewed by 4068

Abstract

Spectral cameras are traditionally used in remote sensing of microalgae, but increasingly also in laboratory-scale applications, to study and monitor algae biomass in cultures. Practical and cost-efficient protocols for collecting and analyzing hyperspectral data are currently needed. The purpose of this study was to test a commercial, easy-to-use hyperspectral camera to monitor the growth of different algae strains in liquid samples. Indices calculated from wavebands from transmission imaging were compared against algae abundance and wet biomass obtained from an electronic cell counter, chlorophyll a concentration, and chlorophyll fluorescence. A ratio of selected wavebands containing near-infrared and red turned out to be a powerful index because it was simple to calculate and interpret, yet it yielded strong correlations to abundances strain-specifically (0.85 < r < 0.96, p < 0.001). When all the indices formulated as A/B, A/(A + B) or (A − B)/(A + B), where A and B were wavebands of the spectral camera, were scrutinized, good correlations were found amongst them for biomass of each strain (0.66 < r < 0.98, p < 0.001). Comparison of near-infrared/red index to chlorophyll a concentration demonstrated that small-celled strains had higher chlorophyll absorbance compared to strains with larger cells. The comparison of spectral imaging to chlorophyll fluorescence was done for one strain of green algae and yielded strong correlations (near-infrared/red, r = 0.97, p < 0.001). Consequently, we described a simple imaging setup and information extraction based on vegetation indices that could be used to monitor algae cultures. Full article

(This article belongs to the Special Issue Algal Biomass)

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14 pages, 1671 KiB

Open AccessReview

The Unicellular Red Alga Cyanidioschyzon merolae, an Excellent Model Organism for Elucidating Fundamental Molecular Mechanisms and Their Applications in Biofuel Production

by Imran Pancha, Kazuhiro Takaya, Kan Tanaka and Sousuke Imamura

Plants 2021, 10(6), 1218; https://doi.org/10.3390/plants10061218 - 15 Jun 2021

Cited by 17 | Viewed by 3995

Abstract

Microalgae are considered one of the best resources for the production of biofuels and industrially important compounds. Various models have been developed to understand the fundamental mechanism underlying the accumulation of triacylglycerols (TAGs)/starch and to enhance its content in cells. Among various algae, the red alga Cyanidioschyzonmerolae has been considered an excellent model system to understand the fundamental mechanisms behind the accumulation of TAG/starch in the microalga, as it has a smaller genome size and various biotechnological methods are available for it. Furthermore, C. merolae can grow and survive under high temperature (40 °C) and low pH (2–3) conditions, where most other organisms would die, thus making it a choice alga for large-scale production. Investigations using this alga has revealed that the target of rapamycin (TOR) kinase is involved in the accumulation of carbon-reserved molecules, TAGs, and starch. Furthermore, detailed molecular mechanisms of the role of TOR in controlling the accumulation of TAGs and starch were uncovered via omics analyses. Based on these findings, genetic engineering of the key gene and proteins resulted in a drastic increment of the amount of TAGs and starch. In addition to these studies, other trials that attempted to achieve the TAG increment in C. merolae have been summarized in this article. Full article

(This article belongs to the Special Issue Algal Biomass)

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31 pages, 1220 KiB

Open AccessReview

Microalgal Cell Biofactory—Therapeutic, Nutraceutical and Functional Food Applications

by Boda Ravi Kiran and S. Venkata Mohan

Plants 2021, 10(5), 836; https://doi.org/10.3390/plants10050836 - 21 Apr 2021

Cited by 72 | Viewed by 6660

Abstract

Microalgae are multifaceted photosynthetic microorganisms with emerging business potential. They are present ubiquitously in terrestrial and aquatic environments with rich species diversity and are capable of producing significant biomass. Traditionally, microalgal biomass is being used as food and feed in many countries around the globe. The production of microalgal-based bioactive compounds at an industrial scale through biotechnological interventions is gaining interest more recently. The present review provides a detailed overview of the key algal metabolites, which plays a crucial role in nutraceutical, functional foods, and animal/aquaculture feed industries. Bioactive compounds of microalgae known to exhibit antioxidant, antimicrobial, antitumor, and immunomodulatory effects were comprehensively reviewed. The potential microalgal species and biological extracts against human pathogens were also discussed. Further, current technologies involved in upstream and downstream bioprocessing including cultivation, harvesting, and cell disruption were documented. Establishing microalgae as an alternative supplement would complement the sustainable and environmental requirements in the framework of human health and well-being. Full article

(This article belongs to the Special Issue Algal Biomass)

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