Kartika Nur Azizah 1, Delianis Pringgenies 2* and Sri Sedjati 2
1Master’s Programme in Marine Sciences, Faculty of Fisheries and Marine Science, Universitas Diponegoro, Semarang 50275, Indonesia; ²Department of Marine Science, Faculty of Fisheries and Marine Science, Universitas Diponegoro, Semarang 50275, Indonesia
*Corresponding author: delianispringgenies@lecturer.undip.ac.id
Horseshoe crabs (Tachypleus gigas) inhabit dynamic coastal ecosystems, where gut-associated symbiotic bacteria play essential roles in host physiology and represent a promising source of marine bioactive compounds. This study investigated the enzymatic and antibacterial activities of gut symbionts isolated from T. gigas and examined their potential functional association with the amino acid composition of the eggs as a supporting physiological aspect. Gut bacteria were isolated using Marine Agar and characterized based on colony morphology, hydrolytic enzyme production (amylase, lipase, protease, and cellulase), and antibacterial activity against Escherichia coli and Staphylococcus aureus. The most bioactive isolates were identified through 16S rRNA gene sequencing, while amino acid profiling of the eggs was conducted to assess their nutritional and bioactive potential. The results revealed high morphological and functional diversity among gut symbionts. Isolates H.A.1.2 and H.A.1.5 exhibited broad-spectrum antibacterial activity against both Gram-negative and Gram-positive bacteria. Distinct enzymatic activities were observed, including amylase production by H.B.2.3, lipase by H.C.4.3, cellulase by H.A.1.5, and protease by H.A.1.2, indicating their roles in macromolecule degradation and nutrient recycling. Amino acid analysis showed a total amino acid content of 59.32%, dominated by essential amino acids such as leucine (5.72%), isoleucine (4.72%), lysine (3.98%), and valine (3.97%), as well as non-essential amino acids, including glutamate (7.91%) and aspartate (7.27%). The enzymatic activity of gut symbionts is likely linked to the availability of free amino acids necessary for embryonic development and vitellogenesis. Molecular identification revealed that H.A.1.2 was closely related to Pseudoalteromonas piscicida, while H.A.1.5 showed close similarity to Vibrio alginolyticus. Overall, this study highlights the functional interplay between gut symbiotic bacteria and egg biochemical composition in T. gigas, underscoring its potential as a sustainable source of antibacterial agents, industrial enzymes, and marine-derived bioactive metabolites.