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Microalgae comprise a biodiverse group of photosynthetic organisms that reside in water sources and sediments. The green microalgae Chlamydomonas reinhardtii was adopted as a useful model organism for studying various physiological systems. Its ability to grow under both photosynthetic and heterotrophic conditions allows efficient growth of non-photosynthetic mutants, making Chlamydomonas a useful genetic tool to study photosynthesis. In addition, this green alga can grow as haploid or diploid cells, similar to yeast, providing a powerful genetic system. As a result, easy and efficient transformation systems have been developed for Chlamydomonas, targeting both the chloroplast and nuclear genomes. Since microalgae comprise a rich repertoire of species that offer variable advantages for biotech and biomed industries, gene transfer technologies were further developed for many microalgae to allow for the expression of foreign proteins of interest. Expressing foreign genes in the chloroplast enables the targeting of foreign DNA to specific sites by homologous recombination. Chloroplast transformation also allows for the introduction of genes encoding several enzymes from a complex pathway, possibly as an operon. Expressing foreign proteins in the chloroplast can also be achieved by introducing the target gene into the nuclear genome, with the protein product bearing a targeting signal that directs import of the transgene-product into the chloroplast, like other endogenous chloroplast proteins. Integration of foreign genes into the nuclear genome is mostly random, resulting in large variability between different clones, such that extensive screening is required. The use of different selection modalities is also described, with special emphasis on the use of herbicides and metabolic markers which are considered to be friendly to the environment, as compared to drug-resistance genes that are commonly used. Finally, despite the development of a wide range of transformation tools and approaches, expression of foreign genes in microalgae suffers from low efficiency. Thus, novel tools have appeared in recent years to deal with this problem. Finally, while C. reinhardtii was traditionally used as a model organism for the development of transformation systems and their subsequent improvement, similar technologies can be adapted for other microalgae that may have higher biotechnological value.

Metal–polysaccharides have recently raised significant interest due to their multifunctional bioactivities. The antimicrobial activity of a complex of Cu2O with the sulfated polysaccharide (PS) of the marine red microalga Porphyridium sp. was previously attributed to spikes formed on the complex surface (roughness). This hypothesis was further examined here using other Cu–PS complexes (i.e., monovalent-Cu2O, CuCl and divalent-CuO, CuCl2). The nanostructure parameters of the monovalent complexes, namely, longer spikes (1000 nm) and greater density (2000–5000 spikes/µm2) were found to be related to the superior inhibition of microbial growth and viability and biofilm formation. When Escherichia coli TV1061, used as a bioluminescent test organism, was exposed to the monovalent Cu–PS complexes, enhanced bioluminescence accumulation was observed, probably due to membrane perforation by the spikes on the surface of the complexes and consequent cytoplasmic leakage. In addition, differences were found in the surface chemistry of the monovalent and divalent Cu–PS complexes, with the monovalent Cu–PS complexes exhibiting greater stability (ζ-potential, FTIR spectra, and leaching out), which could be related to spike formation. This study thus supports our hypothesis that the spikes protruding from the monovalent Cu–PS surfaces, as characterized by their aspect ratio, are responsible for the antimicrobial and antibiofilm activities of the complexes.

Red microalgae represent a natural reservoir of beneficial substances with applications in different industrial sectors. They are rich in natural biomolecules known for their antihypertensive, antioxidant, antimicrobial, antiviral, anti-inflammatory, antitumor, and anticoagulant activities. Many red microalgae are a source of vitamins, minerals, photochemicals, polyunsaturated fatty acids, and a wide spectrum of polysaccharides. The content of their valuable compounds and their activities have turned red microalgae into cellular factories of special interest in food, nutraceutical, and pharmaceutical industries. Like all microalgae, the red ones are superior to traditional crops for the aims of biotechnology as they are renewable sources widely available in great quantities and are easy to culture. Moreover, some of the most studied red microalgae are generally recognized as safe. This review summarizes the valuable biochemicals from red microalgae and highlights their health and nutritional benefits.

Ten compounds (1~10) were successfully isolated from green algae Ulva prolifera through the combination of silica gel column chromatography, Sephadex LH-20 column chromatography and repeated preparative thin-layer chromatography. These ten compounds showed antialgal activity against red tide microalgae. Among them, compounds 3, 6, and 7 showed stronger antialgal activity against red tide microalgae. Furthermore, their structure was identified on the basis of spectroscopic data. There are three glycoglycerolipids: 1-O-octadecanoic acid-3-O-β-D-galactopyranosyl glycerol (2), 1-O-palmitoyl-3-O-β-D-galactopyranosyl glycerol (4), and 1-O-palmitoyl-2-O-oleoyl-3-O-β-D-galactopyranosyl glycerol (5); two monoglycerides: glycerol monopalmitate (1), 9-hexadecenoic acid, 2,3-dihydroxypropyl ester (3); two terpenoids: loliolide (6) and lsololiolide (7); one lipid-soluble pigments: zeaxanthin (8); one sterol: cholest-5-en-3-ol (9); and one alkaloid: pyrrolopiperazine-2,5-dione (10). These compounds were isolated from U. prolifera for the first time, and compounds 2, 3, 5, and 8 were isolated from marine macroalgae for the first time.

Ichthyotoxic red tide is a problem that the world is facing and needs to solve. The use of antialgal compounds from marine macroalgae to suppress ichthyotoxic red tide is considered a promising biological control method. Antialgal substances were screened and isolated from Bangia fusco-purpurea, Gelidium amansii, Gloiopeltis furcate, Hizikia fusifarme, Laminaria japonica, Palmaria palmata, and Sargassum sp. to obtain new materials for the development of algaecides against ichthyotoxic red tide microalgae using bioactivity-guided isolation methods. The fractions of seven macroalgae exhibited selective inhibitory activities against Amphidinium carterae and Karenia mikimotoi, of which the ethyl acetate fractions had the strongest and broadest antialgal activities for the two tested red tide microalgae. Their inhibitory effects on A. carterae and K. mikimotoi were even stronger than that of potassium dichromate, such as ethyl acetate fractions of B. purpurea, H. fusifarme, and Sargassum sp. Thin-layer chromatography and ultraviolet spectroscopy were further carried out to screen the ethyl acetate fraction of Sargassum sp. Finally, a new glycolipid derivative, 2-O-eicosanoyl-3-O-(6-amino-6-deoxy)-β-D-glucopyranosyl-glycerol, was isolated and identified from Sargassum sp., and it was isolated for the first time from marine macroalgae. The significant antialgal effects of 2-O-eicosanoyl-3-O-(6-amino-6-deoxy)-β-D-glucopyranosyl-glycerol on A. carterae and K. mikimotoi were determined.

Floridoside is a compatible solute synthesized by red algae that has attracted considerable attention due to its promising antifouling and therapeutic properties. However, research on industrial applications of floridoside is hampered by limited compound availability and the development of a production process yielding high amounts of this glycoside has not been explored yet. In the present work, floridoside accumulation by the red microalgae Galdieria sulphuraria under different conditions was investigated in order to optimize the production of this glycoside in this microalgae. G. sulphuraria shows consider advantages over other red algae as potential industrial producer of floridoside due to its unicellular nature, its ability to grow heterotrophically in complete darkness and its acidophilic lifestyle. The main compatible solute accumulated by G. sulphuraria under salt stress was purified, identified as floridoside by 1 H-NMR and used as standard for quantification. Our results showed that applying the osmotic stress after the cells had grown first in medium with no salt resulted in higher floridoside yields compared to those obtained in cells growing under osmotic stress from the beginning. Among several parameters tested, the use of glycerol as carbon source for cell growth showed the most significant impact on floridoside accumulation, which reached a maximum of 56.8 mg/g dry biomass.

Previous studies showed that methanol extracts from Porphyra yezoensis significantly inhibited Karenia mikimitoi and Skeletonema costatum . Five sesquiterpenoids ( 1 – 5 ) were successfully isolated from this marine macroalga through a combination of silica gel column chromatography and repeated preparative thin-layer chromatography in this paper. Their structure was identified as gossonorol ( 1 ), 7,10-epoxy-ar-bisabol-11-ol ( 2 ), cyclonerodiol ( 3 ), cadinol, ( 4 ) and 4-cadinen-1-ol ( 5 ) on the basis of spectroscopic data. These sesquiterpenoids were isolated from Porphyra yezoensis for the first time, and cyclonerodiol ( 3 ) and cadinol ( 4 ) isolated from marine macroalgae for the first time. Further, a quantitative relationship between the inhibition of algal growth and the concentration of each antialgal sesquiterpenoid (gossonorol, 7,10-epoxy-ar-bisabol-11-ol and cyclonerodiol) was determined and important parameters, e.g., EC 50-96h for future practical HAB control are to be obtained. Results showed that three sesquiterpenoids ( 1 – 3 ) had selective antialgal activity against the growth of red tide microalgae ( Amphidinium carterae , Heterosigma akashiwo , Karenia mikimitoi , Phaeocystis globosa , Prorocentrum donghaiense , and Skeletonema costatum ). More than two test red tide microalgae were significantly inhibited by these three sesquiterpenoids ( 1 – 3 ). Their antialgal activity against red tide microalgae has not been previously reported. Furthermore, EC 50-96h of gossonorol ( 1 ) and 7,10-epoxy-ar-bisabol-11-ol ( 2 ) for specific test red microalgae were not only significantly less than 10 μg/mL, but also were smaller than/or very close to those of potassium dichromate. Gossonorol ( 1 ) and 7,10-epoxy-ar-bisabol-11-ol ( 2 ) possessed good application potential than potassium dichromate as a characteristic antialgal agent against the specific harmful red tide microalgae ( Heterosigma akashiwo , Phaeocystis globosa , and Prorocentrum donghaiense ) (or Heterosigma akashiwo and Karenia mikimitoi ).

The ethanol extracts of Gracilaria lemaneiformis that have inhibitory effects on Karenia mikimotoi and Skeletonema costatum were separated by liquid-liquid extraction using different polar solvents into five fractions with antialgal activities (petroleum ether, chloroform, ethyl acetate, n -butanol, and water-soluble fractions). These fractions were chromatographed on silica gel to give, after repeated preparative thin-layer chromatography (PTLC) purification processes, 1-β- d -ribofuranosyluracil ( 1 ), 3-hydroxymethyl-pyrrolopiperazine-2,5-dione ( 2 ), benzene-1,2-propanoic acid ( 3 ), 1- O -palmitoyl-2- O -palmitoleoyl-3- O -β- d -galactopyranosyl glycerol ( 4 ), 7-oxabicyclo[4.1.0]-heptan-3-ol ( 5 ), linoleic acid ( 6 ), 3,4-dimethoxy-6-(methoxymethyl)-tetrahydro-2H-pyran-2,5-diol ( 7 ), and 3,7,11,16-tetramethyl -2-heptadecen-1-ol ( 8 ). Five of them, natural products 1 , 2 , 5 , 7 , and 8 , were isolated from Gracilaria lemaneiformis for the first time, and three natural products ( 3 , 5 , and 8 ) were isolated from marine macroalgae for the first time. Among them, natural products ( 1 , 2 , 3 , 4 , and 6 ) showed the most obvious inhibition activities to the growth of Karenia mikimotoi and Skeletonema costatum at the concentration of 80 μg/mL. Therefore, antialgal activities of these five natural products against Amphidinium carterae , Heterosigma akashiwo , Karenia mikimotoi , Phaeocystis globosa , Prorocentrum donghaiense , and Skeletonema costatum were further tested at different concentrations (0.4, 2, 10, and 50 μg/mL). This was the first report of antialgal activities of five natural products ( 1 , 2 , 3 , 4 , and 6 ) to these six red tide microalgae. They showed significantly selective antialgal activities against all tested red tide microalgae. At the concentration of 50 μg/mL, the growth of Amphidinium carterae , Heterosigma akashiwo , Karenia mikimotoi , and Phaeocystis globosa was obviously inhibited; for Karenia mikimotoi , natural products 1 , 2 , and 6 have significant antialgal activities; the growth inhibition of Skeletonema costatum that was exposed to natural products 1 , 3 , and 4 was remarkable. Furthermore, by analyzing and comparing EC 50–96 h values, it has been determined that natural product 3 (natural product 4 ) showed the superior application potential than potassium dichromate and some reported natural products (such as gossonorol isolated from Porphyra yezoensis , trehalose purified from Ulva pertusa ) as a characteristic antialgal agent against Amphidinium carterae ( Phaeocystis globosa ). In addition, natural products 1 and 3 also showed good superiority than some reported natural products in inhibiting Skeletonema costatum ; however, it was a pity that they were inferior to potassium dichromate in the inhibiting this red tide microalgae. Taken together, it is not hard to conclude that Gracilaria lemaneiformis was a good source of natural products with antialgal activities against some red tide microalgae.

Seven antialgal compounds ( 1 – 7 ) were successfully isolated from the red alga Gracilaria lemaneiformis through a combination of silica gel column chromatography and repeated preparative thin-layer chromatography. On the basis of the spectral data, the compounds were identified as gossonorol ( 1 ), 7,10-epoxy-ar-bisabol-11-ol ( 2 ), glycerol monopalmitate ( 3 ), stigmasterol ( 4 ), 15-hydroxymethyl-2, 6, 10, 18, 22, 26, 30-heptamethyl-14-methylene-17-hentriacontene ( 5 ), 4-hydroxyphenethyl alcohol ( 6 ), and margaric acid ( 7 ). These seven compounds were isolated from G. lemaneiformis for the first time, while the compounds 4 , 6 , and 7 were isolated from marine macroalgae for the first time. Furthermore, a quantitative relationship between the inhibition of algal growth and the concentration of each antialgal compound was determined and important parameters for future practical HAB control, e.g., EC 50-96h , were also obtained. The results indicated that isolated compounds 1–7 possess selective antialgal activity against the growth of several red tide microalgae (including Amphidinium carterae , Heterosigma akashiwo , Karenia mikimitoi , Phaeocystis globsa , Prorocentrum donghaiense , and Skeletonema costatum ). Their antialgal activity against test red tide microalgae has not been previously reported. Furthermore, the EC 50-96h of one or more of the compounds towards the tested red microalgae was not only significantly less than 10 μg/mL but also was smaller than that of the characteristic antialgal agent potassium dichromate. The study demonstrates that compounds 1–7 possess significant application potential as antialgal agents against several harmful red tide microalgae.

The cells of the red microalga Porphyridium UTEX 637 are encapsulated within a sulfated polysaccharide whose external part (i.e., the soluble fraction) dissolves into the medium. It is thought that the main function of the polysaccharide is to protect the algal cells from the extreme environmental conditions, such as drought and high light, prevailing in their native sea-sand habitat. In this study, we evaluated the antioxidant properties of the water-soluble polysaccharide of Porphyridium sp. by determining the ability of a polysaccharide solution to inhibit: (1) autooxidation of linoleic acid, as determined by the standard thiobarbituric acid (TBA) and ferrous oxidation (FOX) assays; and (2) oxidative damage to 3T3 cells as determined by the dichlorofluorescein (DCFH) assay. In all three assays, the polysaccharide inhibited oxidative damage in a dose-dependent manner. Antioxidant activity was also exhibited by fractions of the polysaccharide obtained by sonication followed by separation on a reverse-phase HPLC with a C sub(8) semi-preparative column. It is suggested that the antioxidant activity of the sulfated polysaccharide protects the alga against reactive oxygen species produced under high solar irradiation, possibly by scavenging the free radicals produced in the cell under stress conditions and transporting them from the cell to the medium.