BIODIVERSITAS ISSN: 1412-033X Volume 24, Number 3, March 2023 E-ISSN: 2085-4722 Pages: 1346-1355 DOI: 10.13057/biodiv/d240303 Potential of prospective medicinal plants of Rhizophoraceae from North Kalimantan, Indonesia SAAT EGRA 1,2,3 , HARLINDA KUSPRADINI 4 , IRAWAN WIJAYA KUSUMA 4 , IRMANIDA BATUBARA 5 , IMRA 6 , NURJANNAH 2 , ETTY WAHYUNI 2 , KOSEI YAMAUCHI 3,♥ , TOHRU MITSUNAGA 3, 1 United Graduate School of Agriculture Sciences, Gifu University. Yanagido 501-1193 Gifu, Japan 2 Faculty of Agriculture, Universitas Borneo Tarakan. Jl. Amal Lama No. 1, Tarakan 77123, North Kalimantan, Indonesia 3 Faculty of Applied Biological Sciences, Gifu University. 1-1 Yanagido, 501-1193 Gifu, Japan. Tel.: +81-58-293-2835,  email: kosei_y@gifu-u.ac.jp,  email: mitunaga@gifu-u.ac.jp, saat.egra.shaumi@gmail.com 4 Faculty of Forestry, Universitas Mulawarman. Jl. Penajam, Kampus Gunung Kelua, Samarinda 75119, East Kalimantan, Indonesia 5 Department of Chemistry, Faculty of Mathematics and Natural Sciences, Tropical Biopharmaca Research Center, Institut Pertanian Bogor. Jl. Meranti, Kampus IPB Dramaga, Bogor 16680, West Java, Indonesia 6 Faculty of Fishery, Universitas Borneo Tarakan. Jl. Amal Lama No. 1, Tarakan 77123, North Kalimantan, Indonesia Manuscript received: 19 January 2023. Revision accepted: 1 February 2023. Abstract. Egra S, Kuspradini H, Kusuma IW, Batubara I, Imra, Nurjannah, Wahyuni E, Yamauchi K, Mitsunaga T. 2023. Potential of prospective medicinal plants of Rhizophoraceae from North Kalimantan, Indonesia. Biodiversitas 24: 1346-1355. The abundance of mangrove forests in the equatorial region necessitates proper management, including the sustainable use of plant parts for functional products such as food, medicine, etc. This study aimed to assess the medicinal potential of five mangrove plants through phytochemical analysis, antibacterial assays against Streptococcus sobrinus, DPPH free radicals, and toxicity assay using Artemia salina L., Bruguiera parviflora (Roxb.) Wight and Arn. ex Griff, Bruguiera cylindrica (L.) Blume, Ceriops tagal (Perr.). The plants used were Ceriops tagal (Perr.) C.B. Rob, Rhizophora mucronata Poir, and Rhizophora apiculata Blume. Plant samples were extracted with n-hexane, ethyl acetate, and methanol in that order, and then the obtained plant extracts were subjected to various assays. The results showed that B. cylindrica wood and C. tagal leaf extract had the highest antibacterial activity, with more than 50% relative inhibition. The C. tagal leaf methanolic extract had the highest antioxidant activity, by 91% relative inhibition. Followed by R. mucronata wood ethyl acetate extract and B. parviflora leaf methanolic extract, with 87% and 86%, respectively. The highest value in the cytotoxicity assay was discovered in the B. cylindrica in the very strong category with an LC50 value of 22.9 μg/mL. The present study revealed the potential of mangrove plant extracts to have strong antibacterial, cytotoxic, and antioxidant properties. Keywords: Antibacterial, antioxidant, mangrove, phytochemical, toxicity INTRODUCTION Tarakan City is a small island with an area of 657.33 km 2 . The island is located in the northern region of Kalimantan, Indonesia which has mangrove forest areas of 1,244.9 ha. The mangrove forest of Tarakan City is part of the coastal ecosystem. That provides a productive natural resource for fish (fish, shrimps, crabs, gastropods, shells), animals, snakes, monkeys, proboscis's monkeys, birds, and mangrove vegetation, namely mangrove plants from Rhizophora, Avicennia, Combretocarpus, Nypa, etc. Specifically, the most mangrove vegetation found on the side of coastal mangrove forest land in Tarakan City is Rhizophora plants (Sawitri et al. 2013). Rhizophora plants, commonly called bakau in Indonesia, are mangrove vegetation that mostly grows on the side of tropical coastal land. Five types of mangrove plants can commonly be found in the Indonesian mangrove forest, i.e., Rhizophora apiculata Blume, Rhizophora mucronata Lam., Rhizophora stylosa Griffith, Bruguiera gymnorrhiza (L.) Lam., and Ceriops tagal (Perr.) C.B.Rob. This vegetation can also be found mostly in the mangrove conservation area of Tarakan City, North Kalimantan, Indonesia. The Indonesian coastal peoples have long utilized mangroves as materials for paper, coal, and firewood. Mangroves contain a lot of tannins, especially in the stems. Tannins in plants play an important role in protecting plants from predation by herbivores and pests and regulating growth. In addition, tannins are a group of polyphenols with antibacterial activity (Nguyen et al. 2023). Tannins in mangroves can be used as a coating material. People living near the Indonesian coast also utilize mangroves as traditional medicinal for generations. However, the coastal peoples of Tarakan City have not optimally utilized the mangrove plants (Sawitri et al. 2013). Sough, Ambay and Mandori tribes from Papua near the coastal area have used mangroves as traditional medicinal plants for generations (Mahmud and Wahyudi 2014). The Sough tribe in Papua utilized Rhizophora bark for toothache and malaria, while Rhizophora root was utilized to cure diarrhea. The Ambay tribe utilized Rhizophora stem and twig coal for strengthening the toddler's bones. The Mandori tribe utilized Rhizophora root as a diarrhea curing following the Sough tribe. These tribes believed that using mangrove plants as traditional medicines had no side effects. The coastal peoples of Mamuya Village in East