*3.1. Isolation and Molecular Identification of the Strain*

Marine sediment from Gulf of Mannar was pre-treated with physical and chemical methods. Grown on SCA and ISP-2 medium, the cultural characteristics were identical on either of them. The aerial hyphae were white in color and substrate mycelium was colorless. SCA and ISP-2 culture plates did not show any pigment diffusion. Micromorphological studies of strain using SEM showed smooth spore ornamentation and rectiflexibilis spore morphology (Figure 1). The genomic DNA of the strain was isolated using the phenol-chloroform method and examined for 16S r-RNA sequence. The amplified sequences were subjected to BLAST analysis using the megablast tool of Genebank at NCBI under the accession number (KU921225). The BLAST search revealed that the strain belonged to *Streptomyces* sp. The highest similarity value index was found between the sequences of *Streptomyces* sp.S2A and *Streptomyces griesoruber* (100%). The neighbor-joining phylogenetic tree was drawn using MEGA 7.0 (Figure 2).

**Figure 1.** (**A**) Scanning electron micrograph showing spore ornamentation in *Streptomyces* sp.S2A; (**B**) Microscopic image of *Streptomyces* sp.S2A under 100×.

**Figure 2.** Phylogenetic tree of *Streptomyces* sp.S2A and the relationships with the closest species based on 16S rRNA gene sequencing using the neighbor-joining method.
