Microalgae are ubiquitous unicellular eukaryotic photosynthetic organisms distributed in quite diverse and extreme environments. They are especially valuable due to their high content of compounds with different biological activities; including both complex organic compounds and primary and secondary metabolites; such as phytopigments (xanthophylls and carotenoids), polyunsaturated fatty acids (PUFAs); phenolic substances, docosahexaenoic acid (DHA), vitamins, carbohydrates, tannins, terpenoids, and peptides, among others.

Peptides derived from microalgae, mainly obtained by enzymatic digestion using proteolytic enzymes. Mostly obtained from protein hydrolyzates of Chlorella vulgaris; Chlorella ellipsoidea, Tetradesmus obliquus, Navicula incerta, and Nannochloropsis oculata. The resulting bioactive peptides have shown to possess different biological functionalities, such as antioxidant, anticancer; antihypertensive, and antimicrobial activities, with beneficial health effects and potential therapeutic applications.

Herein, they report the characterization and chemical synthesis of three T. suecica AMPs active against Gram-positive; and Gram-negative bacteria. However, the peptide with the highest activity is subjected to alanine scanning that allow identifying the contribution of each amino acid residue; to the peptide’s functionality, stability, and structure. In this report, the antibacterial activity of T. suecica determine, and for the first time the antimicrobial peptides (AMPs) identified.

Microalga underwent acid extract

Microalga subjected to acid extract, and purified by C-18 reverse-phase column chromatography; to obtain fractions with differential hydrophobicity, using an elution gradient of 5%, 40%, 80%; and 100% acetonitrile (ACN) in 0.01% trifluoroacetic acid (TFA) in water. However, this procedure modified from Mitta used by our group in the search of peptides from other natural products, with good result.

They report here the existence of AMPs from the microalgae T. suecica; with bactericidal effect against Gram− and Gram+ human pathogenic bacteria. The antibacterial properties of protein hydrolysates described for the microalgae Dunaliella salina and Chlorella sorokiniana; but the specific peptides responsible for this activity not determined.

Thus, to our knowledge, this is the first report of antimicrobial activity associate with peptides in microalgae. The antibacterial activity of microalgal extracts previously report; but not attributed to peptides, reported that microalgal peptides may exhibit different kinds of bioactivities such as antioxidant; antihypertensive, anticancer, and liver protection effects.

Protein hydrolysis of cell extracts

Nevertheless, no AMPs from microalgae have describe so far. Bioactive peptides from microalgae  commonly obtained by protein hydrolysis of cell extracts. These peptides are inactive and encrypted until liberation by enzymatic hydrolysis. In  Mar. Drugs 2019, 17, 453 12 of 18 this work; peptides naturally present in the microalgae T. suecica obtained using a purification procedure that minimized hydrolysis.

Furthermore, the eluted fraction after Sep-Pak chromatography displaying antibacterial activity; was the 40% ACN, similar to several reports where AMPs were purified. De novo sequencing by mass spectrometry allowed us to define the sequences of 24 peptides that chemically synthesize; whose antimicrobial properties evaluated.

Three of these synthetic peptides showed significant antibacterial activity. Among them, the peptide named AQ1766 showed the highest effect on Gram− and Gram+ bacterial strains. But the peptides had no cytotoxic effect on human cell line HEK293 at the concentrations used in the antibacterial assays.

Characterization of antibacterial peptides

However, the identification and functional characterization of antibacterial peptides; from the microalgae T. suecica is a contribution to the development of new alternatives for the control of infectious diseases. One advantage of antibacterial peptides from microalgae is that they could be administered as diet supplements for human consumption; or as medicinal feed in animal production, considering that microalgae and its derivatives are currently used for such purposes.

In this report, de novo sequences of three peptides with antibacterial activity; from the microalga T. suecica determined. One of these peptides, AQ-1766, which exhibited the higher activity; subjected to an alanine scan and subsequently to lysine replacement of some residues.