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The aim of this study was to evaluate the antioxidant activity of ethyl acetate and methanolic extracts of two marine algae, Nannochloropsis oculata and Gracilaria gracilis. The extracts were assayed for total phenol and flavonoid content, DPPH free radical scavenging capacity, nitric oxide activity, iron chelation activity, and reducing power activity. Total phenol and flavonoid content were found to be high in both algae. Ethyl acetate extracts of both algae were found to exhibit significant antioxidant activity. Ethyl acetate extract of N. oculata exhibited a good capacity for iron chelation, nitrate oxide, and scavenging DPPH free radicals (72.95±2.30, 73.73±1.76, and 39.03±0.97% inhibition at 400 µg mL-1 respectively).

Algae are natural sources of nutrients, but the presence of anti-nutritional factors often compromises nutrient apparent digestibility coefficients (ADCs) in several fish species. In this study, physical-mechanical and enzymatic technological processing was applied to two seaweeds (Gracilaria gracilis and Ulva rigida) and three microalgae (Nannochloropsis oceanica, Chlorella vulgaris, and Tetraselmis sp.) in order to evaluate its effectiveness in improving nutrient ADC values in diets for European seabass. A practical commercial-based diet was used as reference (REF) and experimental diets were prepared by replacing 30% of REF diet with each test alga used either intact or after processing. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and fast performance liquid chromatography (FPLC) analyses revealed that enzymatic processing was more effective than the physical one in changing the protein and peptides composition, increasing the amount of low-molecular-weight compounds in seaweeds and N. oceanica microalgae. Protein digestibility was significantly affected by algae species and in the case of the microalgae by the technological process. Gracilaria gracilis is better digested than U. rigida and physical processing enhanced protein and energy ADC values. Nannochloropsis oceanica and C. vulgaris are better digested than Tetraselmis sp.; the highest protein and energy ADCs were observed in diets containing enzymatically processed N. oceanica (NAN-ENZ) and physically processed C. vulgaris (CHLO-PHY), followed by the diet with physically processed Tetraselmis sp. (TETR-PHY). Results clearly showed that it is possible to increase nutrient accessibility and digestibility of algae by fish, by selecting the most adequate method to disrupt the cell wall. Moreover, the physical-mechanical and enzymatic technological processes used in this study are scalable to the industrial level.

Gracilaria gracilis and Nannochloropsis oceanica, single or blended, were tested in European seabass (Dicentrarchus labrax) diets. A control (CTRL) diet was compared with experimental diets including either 8% G. gracilis (GRA8), 8% N. oceanica (NAN8), or a blend of 4% of each alga (NAN4GRA4). After 106 days of feeding, growth, nutrient utilization, antioxidant defense, immunological status, and end-product quality were evaluated. All fish exhibited similar feed intake (1.4–1.5%), body weight, growth, and feed conversion ratio (1.6). Dietary inclusion N. oceanica did not affect digestible N intake and gain. Fish fed GRA8 had the lowest digestible N and energy intake (P < 0.05), and simultaneously the highest nitrogen retention efficiency and energy retention efficiency, resulting in a N and energy gain similar to all other treatments. All fish had well-preserved intestinal morphology; feeding NAN8 resulted in a significant increase in neutral goblet cells compared with GRA8. Fish fed the algal diets had significantly lower (P < 0.05) hepatosomatic index (1.7–1.8 vs 2.1) and plasma triglyceride levels than CTRL, but whole body composition remained similar among treatments. The liver total antioxidant capacity of fish fed NAN8 was significantly higher than that of fish fed GRA8 but did not differ significantly from the CTRL group. NAN4GRA4 resulted in lower values of total glutathione, glutathione peroxidase, and alternative complement. N. oceanica decreased fillet springiness; however, with NAN4GRA4, the muscle fillet became less resilient. G. gracilis and N. oceanica biomass, either used single (8%) or blended (4% each), can be valuable natural ingredients for partial replacement of fish meal in European seabass diets.

This study evaluated the growth and biochemical composition of farming Gracilaria gracilis (Stackhouse) M. Steentoft, L. M. Irvine & W. F. Farnham in the Bizerte Lagoon (BL) and Bizerte Bay (BB) in the North Coast of Tunisia, using lantern nets. Effects of site and depth on alga daily growth rate (DGR) and biochemical composition were investigated. The DGR was affected by culture site (1.42 ± 0.65% day −1 and 1.19 ± 0.34% day −1 for the BL and the BB respectively). Agar yield, was higher ( p  < 0.05) in the BB than the BL (23.31 ± 2.64% vs. 19.19 ± 2.32%) with a higher ( p  < 0.05) 3,6-anhydrogalactose (3,6-AG) contents (41.37 ± 3.68% vs 23.30 ± 5.40%) and a lower ( p  < 0.05) sulphate degree (6 ± 2.00% vs 8.80 ± 0.86%). The proteins contents were independent of the site and depth of culture (20.74 ± 7.22% and 22.02 ± 6.34% for the BL and the BB respectively). R-phycoerythrin (R-PE) contents were significantly higher ( p  < 0.05) in the BB (0.86 ± 0.31 mg g −1 ) than those obtained in the BL (0.33 ± 0.12 mg g −1 ). The salinity, transparency, nitrate and ammonium were monitored in both sites, and their influences were discussed. Our results suggest that G. gracilis cultured in Bizerte Bay can be used in a cascading biorefinery approach.

Seaweeds are an important source of nutrients and bioactive compounds and have a high potential as health boosters in aquaculture. This study evaluated the effect of dietary inclusion of Gracilaria gracilis biomass or its extract on the European seabass ( Dicentrarchus labrax ) gut microbial community. Juvenile fish were fed a commercial-like diet with 2.5% or 5% seaweed biomass or 0.35% seaweed extract for 47 days. The gut microbiome was assessed by 16S rRNA amplicon sequencing, and its diversity was not altered by the seaweed supplementation. However, a reduction in Proteobacteria abundance was observed. Random forest analysis highlighted the genera Photobacterium , Staphylococcus , Acinetobacter , Micrococcus and Sphingomonas, and their abundances were reduced when fish were fed diets with algae. SparCC correlation network analysis suggested several mutualistic and other antagonistic relationships that could be related to the predicted altered functions. These pathways were mainly related to the metabolism and biosynthesis of protective compounds such as ectoine and were upregulated in fish fed diets supplemented with algae. This study shows the beneficial potential of Gracilaria as a functional ingredient through the modulation of the complex microbial network towards fish health improvement.

Enormous marine biodiversity offers an endless reservoir of chemicals for many applications. In this scenario, the extraction of seaweeds represents an interesting source of compounds displaying antimicrobial activity. In particular, among the different red algae, Gracilaria gracilis plays an important role due to the presence of important bioactives in its composition. In spite of these features, an efficient culture system is still absent. In the present study, a novel algal culture method was developed and compared to another more common cultural practice, widely reported in literature. A higher efficiency of the new method, both for daily growth rate and biomass, was assessed. Furthermore, the growth inhibitory activity of five extracts, obtained using ethanol, methanol, acetone, chloroform or diethyl ether as a solvent, from the cultured G. gracilis was tested against Gram-positive and Gram-negative pathogens. Algal extracts exhibited a considerable inhibitory activity against B. subtilis strains, while a slight inhibition was observed against V. fischeri. The different extracts showed significant differences in bacterial growth inhibition, with the highest activity that was recorded for the ethanol extract, followed by that of methanol. Based on the chemical characterization, these findings could be related to the antimicrobial activity played by the combination of total carbohydrates and polyphenols, which were determined at high levels in ethanol and methanol extracts, as well as by the highest number and levels of single polyphenols. Conversely, the lower growth inhibitory activities found in chloroform and diethyl ether extracts could be related to the isolation of minor lipid classes (e.g., neutral and medium polar lipids) composed by fatty acids, such as stearic, oleic and arachidonic acids, typically characterized by antimicrobial activity. In consideration of the results obtained, the present study has a double implication, involving both the field of cultural practices and the exploitation of natural sources for the isolation of antimicrobial agents useful both in pharmaceutical and food applications.

The use of mesenchymal stem cells is a promising strategy for treating bone-related diseases. Therefore, the development of agents capable of inducing the differentiation of stem cells into osteoblasts is beneficial for such treatments. In this study, the effects of aqueous and organic extracts of Arthrospira platensis and Gracilaria gracilis on the induction of bone differentiation in human amniotic mesenchymal stem cells (hAMSCs) were evaluated. First, the cytotoxicity of aqueous and organic extracts at concentrations of 60, 100 and 140 μg mL-1 was measured using the MTT method. Then, the osteogenic differentiation effects of the cells were evaluated through Alizarin Red staining, alkaline phosphatase enzyme activity measurement and the expression level of Runx2, OCN and β-catenin genes. The results showed that 60 μg mL-1of aqueous and methanol extracts of the algae did not exhibit cytotoxicity. Treatment of cells with these extracts resulted in the accumulation of calcium deposits and increased alkaline phosphatase enzyme activity. Additionally, the expression of Runx2 and OCN genes increased with the influence of A. platensis aqueous and G. gracilis methanol extracts, while the β-catenin gene expression increased with the influence of A. platensis aqueous extract. In conclusion, the study suggests that A. platensis aqueous and G. gracilis methanol extracts are capable of inducing bone differentiation in hAMSCs. These findings provide new insights into osteogenesis mediated by biocompounds from algae and may contribute to the development of therapeutic agents for bone-related diseases.

Agar is a sulfated polysaccharide extracted from certain marine red algae, and its gel properties depend on the seaweed source and extraction conditions. In the present study, the seaweed Gracilaria gracilis (Gracilariales, Rhodophyta) from Dakhla (Moroccan Atlantic Coast) was investigated for its agar content, structure, and gel properties. The agar yields of G. gracilis were 20.5% and 15.6% from alkaline pretreatment and native extraction, respectively. Agar with alkaline pretreatment showed a better gelling property supported by higher gel strength (377 g·cm−2), gelling (35.4 °C), and melting (82.1 °C) temperatures with a notable increase in 3,6-anhydro-galactose (11.85%) and decrease in sulphate (0.32%) contents. The sulfate falling subsequent to alkaline pretreatment was verified through FT-IR spectroscopy. The 13C NMR spectroscopy showed that alkaline-pretreated agar has a typical unsubstituted agar pattern. However, native agar had a partially methylated agarose structure. Overall, this study suggested the possibility of the exploitation of G. gracilis to produce a fine-quality agar. Yet, further investigation may need to determine the seasonal variability of this biopolymer according to the life cycle of G. gracilis.

Different light qualities, i.e. ultraviolet (280–400 nm), blue (400–450 nm), green (500–550 nm) and red (620–670 nm) were combined with the actinic yellow light (SOX) (590 nm) under saturation of photosynthetic activity conditions, in order to consider the specific effects of each waveband on the red macroalga Gracilaria gracilis physiology. In this context, the fluctuation of biomass production, photosynthetic activity estimated as in vivo chlorophyll a fluorescence, protein and pigment contents, polyphenol concentration, antioxidant activity and the content and composition of the mycosporine-like amino acids (MAAs)-UV-absorbing compounds were studied. After 3 days of culture, the supplemented light had not induced any variation in photosynthetic pattern compared to the control (only SOX light). However, after 2 weeks of culture, supplemented blue light stimulated the accumulation of phenolic compounds which reached a value of 2.92 ± 0.22 mg g−1 DW, 68% higher than the control. Moreover, supplemented green light induced the accumulation of proteins and R-phycoerythrin (R-PE), i.e. 29 ± 4.32 mg g−1 DW and 0.91 ± 0.34 mg g−1 DW, respectively, being an increase of 56% and 78%, respectively, compared to the control. Red light boosted the growth rate, i.e. 7.24% ± 1.06 compared to the control which was 2.17% ± 0.2. Finally, UV radiation induced a marked increase of MAAs reaching 133.03 ± 41.54 mg g−1 DW, 162% higher content than the one in the control. The use of supplemented light of specific wavelengths to a saturated light irradiance for photosynthesis as an effective approach to increase the content of bioactive compound for biotechnology application is concretely discussed.

Gracilaria gracilis, a red macroalga, represents an exploited important biomass. One of the potential uses of this red seaweed is the production of valuable molecules such as R-phycoerythrin. This compound has recently found applications in food and cosmetic industries as a natural colorant, in conducting medical diagnosis and in biology. However, after R-phycoerythrin extraction the protein extract has to be concentrated and pre-purified, which is commonly expensive and requires many steps. In this paper, R-phycoerythrin from the crude extract obtained by phosphate buffer 20 mM, pH 7.1, was purified by one-step chromatographic method. Native R-phycoerythrin was achieved with a high purity index (A565/A280 ratio) of 3.25 (3.2 = standard R-phycoerythrin purity) after purification on DEAE Sepharose fast flow chromatography with the obtained fraction at 200 mM NaCl. This fraction presented the absorption and emission spectra of R-phycoerythrin with major absorbance at 498, 540, and 565 nm. Native form of this molecule was presented about 260 kDa. Besides, the purified pigment R-phycoerythrin included four subunits: α subunit (18 kDa), β subunit (21 kDa), γ subunit (29 kDa), and γ’ subunit (27 kDa) by gel electrophoresis.