**Abstract**

The ocean harbors a wide diversity of beneficial fauna offering an enormous resource for novel compounds, and it is classified as the largest remaining reservoir of natural molecules to be evaluated for biological activity. The metabolites obtained from marine invertebrate-associated actinobacteria have different characteristics compared to terrestrial actinobacteria as marine environments are exigent and competitive. Actinobacteria produce a wide range of secondary metabolites, such as enzymes, antibiotics, antioxidative, and cytotoxic compounds. These allelochemicals not only protect the host from other surrounding pelagic microorganisms but also ensure their association with the host. The harnessing of such metabolites from marine actinobacteria assures biotechnological, agricultural, and pharmaceutical applications.

**Keywords:** actinobacteria, marine invertebrates, diversity, biological activity

## **1. Introduction**

Actinobacteria are gram-positive bacteria with high G + C DNA content [1] that can live in different habitats including the marine environment. These bacteria enclose significant biotechnological potential as they produce complex biopolymers, such as polysaccharides, extracellular and intracellular enzymes, antibiotics, inhibitors, and various metabolic products [2]. Since the discovery of streptomycin from **Streptomyces griseus***,* actinobacteria are recognized for antibiotic production. Approximately two-thirds of world bioactive compounds are isolated from these phyla. Therefore, actinobacteria are considered as a potential source for the development of new drugs [3]. It has been emphasized that actinobacteria from marine habitats may be valuable for the isolation of novel strains that can potentially produce secondary metabolites like enzymes, cosmetics, antibiotics, anti-parasitic, vitamins, nutritional material, and immunosuppressive agents having great economical and biotechnological importance [4]. Nowadays, research has been diverted toward marine niches to screen that actinobacteria can produce bioactive compounds with different metabolic characteristics as the frequency of novel bioactive compounds from terrestrial actinobacteria decreases with time [5]. Marine flora (mangroves, seaweed, seagrasses, and algae) and fauna (Porifera, coelenterates, ascidians, crustaceans, and mollusks) are part of highly productive ecosystems and are habitats of numerous bioactive compounds producing microorganisms. Bioactive compounds obtained from associated microorganisms are known for a broad range

of biological effects, such as antimicrobial, antifungal, insecticides, antiprotozoal, antiparasitic, anti-inflammatory, and antitumor [6–8]. Sponges (phylum: Porifera) and corals (phylum: Cnidaria) are the most predominant source of marine natural products. Ascidians (phylum: Chordata) have been classified as the second predominant source of natural products [9, 10]. Seaweeds-associated bacterium produces a diverse range of biologically active compounds that specifically target fouling organisms and also show antifungal and antibacterial effects [11]. The rare actinobacterial species are explored from marine environments based on conventional and molecular approaches. The changes in salinity along with the nutrient value within the ocean make the marine environment a good source of novel actinobacteria. Due to the nutrient diversity present in the oceans, one can expect that the organisms that inhabit the marine environment would be very diverse. The different zones of the ocean contain different nutrients and minerals. Therefore, the microorganisms that inhabit various zones generally have metabolites as well as functions that are conducive to their specific zone. It indicates a high probability of microbial diversity that leads to the production of novel antibiotics and enzymes in the marine environment [12, 13]. The present investigation has emphasized invertebrate-associated actinobacteria as a source for novel natural products. The rationale behind this is because invertebrate-associated microorganisms are capable of producing various secondary metabolites and enzymes that can be used in predator defense, antifouling, inhibition of overgrowth, protection from ultraviolet radiation as well as acting as mediators in the competition for settling space [14].
