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Marine microorganisms are important sources for novel natural products. Hence, the aim of this study was to introduce marine microorganisms with the capability of producing antioxidant and cytotoxic metabolites. For this purpose, ten live sea cucumbers (Holothuria leucospilota) were collected from Larak Island, Persian Gulf. Then, their intestine contents were serially diluted and treated by heating (55°C). 100 µL of treated samples were inoculated on starch casein nitrate agar medium, which is supplemented with nalidixic acid and cycloheximide. The plates were incubated at 28 °C for 4 weeks and the colonies were purified. The antioxidant activity of extracted metabolites from the isolated actinobacteria was evaluated using DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging activity assay and the cytotoxic activity was screened by Brine-Shrimp micro well cytotoxicity method. In addition, the cytotoxic effect was evaluated against HCT 116 and SW 480 cell lines by MTT cell proliferation assay. A new strain of marine actinobacterium represented maximum antioxidant and cytotoxic activity among 17 actinobacterial isolates. The ethyl acetate culture extract of the isolate scavenged DPPH radicals with IC₅₀ value of 211.2 µg mL^-1. In addition, the extract exhibited high toxicity against HCT 116 and SW 480 tumor cell lines with IC₅₀ values of 26.48 and 18.53 µg mL^-1 respectively. The isolate was identified as Streptomyces sp. strain SC 156 and showed 98% similarity with type strains in NCBI database. These results suggested that Streptomyces sp. strain SC 156 could be considered as promising candidate for antioxidant and cytotoxic compound discovery. 

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The sea cucumbers of the genus Holothuria are important species in Asia. In present study in the Persian Gulf aimed to identify Holothuria species and investigate their relationships based on morphological and 16S rRNA molecular data. 30 specimens were collected from northern Persian Gulf coasts of Dayer (Bushehr Province) and Lengeh (Hormozgan Province). Based on ossicles morphology, three species were identified. DNA was isolated using CTAB method. Thereafter, 16S ribosomal RNA gene amplification was performed using universal primer and consequently PCR product was sequenced. Sequencing result was analyzed and identified in NCBI database using BLAST. 17 sequences of mitochondrial DNA of 16S rRNA gene (393bp length) were obtained after modification and alignment. The phylogenic tree of mentioned species showed a monophyletic group. The three identified species include Holothuria (Thymiosycia) arenicola, Holothuria leucospilota and Holothuria parva. Holothuria arenicola was found in high support in the sister clade of H. arenicola from China. Despite 7% genetic distance, no morphological differences were observed between H. arenicola with similar species.