Angiotensin I-converting enzyme (ACE) inhibitory peptides derived from seaweed represent a potential source of new antihypertensive. The aim of this study was to isolate and purify ACE inhibitory peptides (ACEIPs) from the protein hydrolysate of the marine macroalga Ulva intestinalis. U. intestinalis protein was hydrolyzed by five different proteases (trypsin, pepsin, papain, α-chymotrypsin, alcalase) to prepare peptides; compared with other hydrolysates, the trypsin hydrolysates exhibited the highest ACE inhibitory activity. The hydrolysis conditions were further optimized by response surface methodology (RSM), and the optimum conditions were as follows: pH 8.4, temperature 28.5 °C, enzyme/protein ratio (E/S) 4.0%, substrate concentration 15 mg/mL, and enzymolysis time 5.0 h. After fractionation and purification by ultrafiltration, gel exclusion chromatography and reverse-phase high-performance liquid chromatography, two novel purified ACE inhibitors with IC₅₀ values of 219.35 μM (0.183 mg/mL) and 236.85 μM (0.179 mg/mL) were obtained. The molecular mass and amino acid sequence of the ACE inhibitory peptides were identified as Phe-Gly-Met-Pro-Leu-Asp-Arg (FGMPLDR; MW 834.41 Da) and Met-Glu-Leu-Val-Leu-Arg (MELVLR; MW 759.43 Da) by ultra-performance liquid chromatography-tandem mass spectrometry. A molecular docking study revealed that the ACE inhibitory activities of the peptides were mainly attributable to the hydrogen bond and Zn(II) interactions between the peptides and ACE. The results of this study provide a theoretical basis for the high-valued application of U. intestinalis and the development of food-derived ACE inhibitory peptides.

Substances with valuable antitumor properties have been identified in many marine algae, including an edible polysaccharide from the marine alga Gracilariopsis lemaneiformis (PGL). We previously reported transcriptome profiling data showing that PGL induced transcriptional alterations generate anti-lung cancer activity. To identify how PGL is detrimental to tumors, we purified PGL to characterize its chemical composition, molecular weight, and sugar and protein content and investigated its antitumor activity. We demonstrated that PGL exerted its antitumor activities by modulating cell viability, morphology, apoptosis, and the apoptosis-related Fas/FasL signaling pathway in the human lung cancer cell line A549, the gastric cancer cell line MKN28, and the mouse melanoma cell line B16. Our data provide the first evidence that PGL inhibits cell proliferation by inducing apoptosis, which is largely mediated by Fas/FasL in cancer cells, suggesting that PGL might be a novel therapeutic agent against cancer.

Ulva prolifera belongs to green macroalgae and is the dominant species of green tide. It is distributed worldwide and is therefore subject to high-temperature stress during the growth process. However, the adaptation mechanisms of the response of U. prolifera to high temperatures have not been clearly investigated yet. In this study, isobaric tags for relative and absolute quantitation (iTRAQ) labelling was applied in combination with the liquid chromatography-tandem mass spectrometry (LC-MS/MS) to conduct comparative proteomic analysis of the response of U. prolifera to high-temperature stress and to elucidate the involvement of this response in adaptation mechanisms. Differentially expressed proteins (DEPs) of U. prolifera under high temperature (denote UpHT) compared with the control (UpC) were identified. Bioinformatic analyses including GO analysis, pathway analysis, and pathway enrichment analysis was performed to analyse the key metabolic pathways that underlie the thermal tolerance mechanism through protein networks. Quantitative real-time PCR and western blot were performed to validate selected proteins. In the present study, 1223 DEPs were identified under high temperature compared with the control, which included 790 up-regulated and 433 down-regulated proteins. The high-temperature stimulus mainly induced the expression of glutathione S-transferase, heat shock protein, ascorbate peroxidase, manganese superoxide dismutase, ubiquitin-related protein, lhcSR, rubisco activase, serine/threonine protein kinase 2, adenylate kinase, Ca2+-dependent protein kinase (CDPK), disease resistance protein EDS1, metacaspase type II, NDPK2a, 26S proteasome regulatory subunit, ubiquinone oxidoreductase, ATP synthase subunit, SnRK2s, and cytochrome P450. The down-regulated proteins were photosynthesis-related proteins, glutathione reductase, catalase-peroxidase, thioredoxin, thioredoxin peroxidase, PP2C, and carbon fixation-related proteins. Furthermore, biological index analysis indicated that protein content and SOD activity decreased; the value of Fv/Fm dropped to the lowest point after culture for 96 h. However, APX activity and MDA content increased under high temperature. The present study implied an increase in proteins that were associated with the stress response, oxidative phosphorylation, the cytokinin signal transduction pathway, the abscisic acid signal transduction pathway, and the glutathione metabolism pathway. Proteins that were associated with photosynthesis, carbon fixation in photosynthesis organisms, and the photosynthesis antenna protein pathway were decreased. These pathways played a pivotal role in high temperature regulation. These novel proteins provide a good starting point for further research into their functions using genetic or other approaches. These findings significantly improve the understanding of the molecular mechanisms involved in the tolerance of algae to high-temperature stress.

Red algae are economically valuable for food and in industry. However, their genomic information is limited, and the genomic data of only a few species of red algae have been sequenced and deposited recently. In this study, we annotated a draft genome of the macroalga Gracilariopsis lemaneiformis (Gracilariales, Rhodophyta). The entire 88.98 Mb genome of Gp. lemaneiformis 981 was generated from 13,825 scaffolds (≥500 bp) with an N50 length of 30,590 bp, accounting for approximately 91% of this algal genome. A total of 38.73 Mb of scaffold sequences were repetitive, and 9281 protein-coding genes were predicted. A phylogenomic analysis of 20 genomes revealed the relationship among the Chromalveolata, Rhodophyta, Chlorophyta and higher plants. Homology analysis indicated phylogenetic proximity between Gp. lemaneiformis and Chondrus crispus. The number of enzymes related to the metabolism of carbohydrates, including agar, glycoside hydrolases, glycosyltransferases, was abundant. In addition, signaling pathways associated with phytohormones such as auxin, salicylic acid and jasmonates are reported for the first time for this alga. We sequenced and analyzed a draft genome of the red alga Gp. lemaneiformis, and revealed its carbohydrate metabolism and phytohormone signaling characteristics. This work will be helpful in research on the functional and comparative genomics of the order Gracilariales and will enrich the genomic information on marine algae.

Gracilariopsis lemaneiformis polysaccharides (GLP) were degraded using pectinase, glucoamylase, cellulase, xylanase, and β-dextranase into low-molecular-weight polysaccharides, namely, GPP, GGP, GCP, GXP, and GDP, respectively, and their antioxidant capacities were investigated. The degraded GLP showed higher antioxidant activities than natural GLP, and GDP exhibited the highest antioxidant activity. After the optimization of degradation conditions through single-factor and orthogonal optimization experiments, four polysaccharide fractions (GDP1, GDP2, GDP3, and GDP4) with high antioxidant abilities (hydroxyl radical scavenging activity, DPPH radical scavenging activity, reduction capacity, and total antioxidant capacity) were obtained. Their cytoprotective activities against H₂O₂-induced oxidative damage in human fetal lung fibroblast 1 (HFL1) cells were examined. Results suggested that GDP pretreatment can significantly improve cell viability, reduce reactive oxygen species and malonaldehyde levels, improve antioxidant enzyme activity and mitochondria membrane potential, and alleviate oxidative damage in HFL1 cells. Thus, the enzyme degradation of GLP with β-dextranase can significantly improve its antioxidant activity, and GDP might be a suitable source of natural antioxidants.

Culturing the economically important macroalga Gracilariopsis lemaneiformis (Rhodophyta) is limited due to the high temperatures in the summertime on the southern Chinese coast. Previous studies have demonstrated that two phytohormones, salicylic acid (SA) and methyl jasmonate (MJ), can alleviate the adverse effects of high-temperature stress on Gp. lemaneiformis. To elucidate the molecular mechanisms underlying SA- and MJ-mediated heat tolerance, we performed comprehensive analyses of transcriptome-wide gene expression profiles using RNA sequencing (RNA-seq) technology. A total of 14,644 unigenes were assembled, and 10,501 unigenes (71.71%) were annotated to the reference databases. In the SA, MJ and SA/MJ treatment groups, 519, 830, and 974 differentially expressed unigenes were detected, respectively. Unigenes related to photosynthesis and glycometabolism were enriched by SA, while unigenes associated with glycometabolism, protein synthesis, heat shock and signal transduction were increased by MJ. A crosstalk analysis revealed that 216 genes were synergistically regulated, while 18 genes were antagonistically regulated by SA and MJ. The results indicated that the two phytohormones could mitigate the adverse effects of heat on multiple pathways, and they predominantly acted synergistically to resist heat stress. These results will provide new insights into how SA and MJ modulate the molecular mechanisms that counteract heat stress in algae.