The brown macroalgal genus Lobophora plays important ecological roles in many marine ecosystems. This group has received much attention over the past decade, and a considerable number of new species have been identified globally. However, our knowledge of the genus diversity and ecology along south-east Asian coasts are still limited. Given the growing body of research that uses a combination of molecular and morphological data to identify cryptic species, this study investigates the diversity of Lobophora in the western Gulf of Thailand using morphological and molecular data, as well as their interactions with scleractinian corals. A total of 36 Lobophora specimens were collected from 15 sites in the western Gulf of Thailand and used for molecular and morphological analyses. One mitochondrial (cox3) and two chloroplast (psbA and rbcL) genes were amplified and sequenced for molecular phylogenetic analyses. Based primarily on phylogenetic evidence, two new species were formally described, L. chumphonensis sp. nov. and L. thailandensis sp. nov. Additionally, L. lamourouxii was newly recorded from Thailand. Two new lineages of Lobophora obscura were identified, L. obscura12 and L. obscura13. Among the Lobophora species identified, three were found in interaction with corals, the most notable of which was the massive coral Porites. Lobophora chumphonensis sp. nov. only interacted with Porites by growing on bare coral skeleton between Porites colonies. Furthermore, L. obscura13 was observed under the branching coral Pocillopora. Our findings revealed that Lobophora presented both effects and absence of effects on coral. A thorough understanding of Lobophora diversity and ecology is essential for ongoing and future research on coral–macroalgal ecological relationships.

Melanogenesis involves a synthesis of melanin pigment and is regulated by tyrosinase. The addition of whitening agents with tyrosinase-inhibiting properties in cosmetics is becoming increasingly important. In this study, the ethanolic extracts from twelve seaweeds were assessed for tyrosinase-inhibiting activity using mushroom tyrosinase and melanin synthesis in B16F10 melanoma cells. The highest mushroom tyrosinase inhibition (IC50) was observed with Lobophora challengeriae (0.15 ± 0.01 mg mL−1); treatment was more effective than kojic acid (IC50 = 0.35 ± 0.05 mg mL−1), a well-known tyrosinase inhibitor. Three seaweeds, Caulerpa racemosa, Ulva intestinalis, and L. challengeriae, were further investigated for their ability to reduce melanogenesis in B16F10 cells. The ethanolic extracts of C. racemosa, U. intestinalis, and L. challengeriae showed inhibitory effects by reducing melanin and intracellular tyrosinase levels in B16F10 cells treated with α-melanocyte stimulating hormone in a dose-dependent manner. C. racemosa (33.71%) and L. challengeriae (36.14%) at 25 µg mL−1 reduced melanin production comparable to that of kojic acid (36.18%). L. challengeriae showed a stronger inhibition of intracellular tyrosinase (decreased from 165.23% to 46.30%) than kojic acid (to 72.50%). Thus, ethanolic extracts from C. racemosa, U. intestinalis, and L. challengeriae can be good sources of natural tyrosinase inhibitors and therapeutic or cosmetic agents in the future.