# Secondary Metabolites and Their Applications
###### Abstract  
The identification of secondary metabolites present in both terrestrial and marine species continues to be a fundamental and privileged path for the emergence of new and fundamental natural products available on the market with very different applications. For example, apilidine is a new natural anticancer agent, and it was approved in Australia in 2018 to treat multiple myeloma [1] and was isolated from the first time from tunicate _Aplidium albicans_ [PERSON]. (+)-Nootkatone is a natural sesquiterpene compound, found in very small amounts in several species such as _Chamacapparis nootkatensis_ ([PERSON], _Chrysopogon zizanioides_ [PERSON][PERSON] and _Citrus paradisi_ [PERSON], which exhibits highly appreciated organoleptic properties and is, therefore, highly demanded as a flavoring agent or adjuvant in the food, pharmaceutical and perfumery industries [2, 3]. This same natural secondary metabolite is applied as insecticide and acaricide, and it was very recently authorized by United Sates Environmental Protection Agency to be include on formulations to control the spread of mosquitoes that transmit infectious diseases as dengue and zika [4]. And who does not know the application of ascorbic acid, found for example in acerola and lemon fruits, as an antioxidant agent [5], widely used by the food industry?  
The successful application of secondary metabolites in diverse requests often involves the use of, more or less, complex mixtures of metabolites, extracted by different methods and from different natural sources, but whose chemical composition and active principles are perfectly established. For example, there are the defined botanical mixtures, called natural product botanicals, which are recognized by the FDA as medicinal entities, successfully used in clinical therapy for the treatment of various diseases. This is the case of the mixture of the secondary metabolites of _Solanum sodomeamum L._, solamarine, solosamine, and mono- and di-glycos derivatives of solasodine, approved by the European Medicine Agency (trade name Curaderm) for the treatment of basal cell carcinoma of the skin [6].  
One area in which the application of secondary metabolites mixtures rather than of pure compounds is common is in the formulation of biopesticides. For example, the insecticide Grandevo(r) is a mixture of metabolites produced in the fermentation of _Chroobacterium subtsugae_, which includes pigments from the violacein family and proteins that are repellent and antifeeding [7].  
Despite the great successes already achieved regarding the secondary metabolites' identification and the development of new applications for these metabolites, this is an area of research that should not slow down. Research must continue identifying and isolating secondary metabolites in unexplored natural sources; new methodologies for extracting secondary metabolites should be tested and optimized to develop greener and moreefficient processes; pure secondary metabolites and chemically characterized mixtures must be tested in different biological activities in order to enhance new applications responding to the growing needs of humanity.  
The Special Issue of _Applied Sciences_, \"Advances in Applications of Natural Bioactive Compounds\", aims to contribute to the desired continuous advance in this scientific field, bringing together publications focused on the most recent advances in the identification of secondary metabolites from terrestrial and marine sources, in new extractive methodologies, and in proposals for applications that add value to natural resources and contributes to a more sustainable development.