**2. Results**

#### *2.1. Strain Isolation and Phylogenetic Identification*

A total of 521 actinomycetial strains were isolated by 12 different agar media (Table S1). Partial 16S rRNA gene sequence analysis identified these strains as the following genera: *Micromonospora* (121 strains), *Streptomyces* (116), *Microbacterium* (36), *Rhodococcus* (35), *Brachybacterium* (28), *Isoptericola* (25), *Cellulosimicrobium* (21), *Brevibacterium* (17), *Serinibacter* (13), *Mycolicibacterium* (10), *Micrococcus* (10), *Agromyces* (10), *Kocuria* (9), *Mycobacterium* (6), *Gordonia* (6), *Aeromicrobium* (5), *Arthrobacter* (5), *Citricoccus* (5), *Janibacter* (5), *Nocardia* (5), *Corynebacterium* (4), *Glutamicibacter* (4), *Agrococcus* (3), *Intrasporangium* (2), *Kineococcus* (2), *Phycicoccus* (2), *Sinomonas* (2), *Serinicoccus* (2), *Actinomadura* (1), *Actinopolymorpha* (1), *Blastococcus* (1), *Demequina* (1), *Georgenia* (1), *Gulosibacter* (1), *Jonesia* (1), *Leucobacter* (1), *Motilibacter* (1), *Mumia* (1), *Salinibacterium* (1), and *Streptacidiphilus* (1). These strains were affiliated to 40 different genera in 23 families (Figure 1a, Tables S2 and S3). The genera *Micromonospora* (23.2%) and *Streptomyces* (22.3%) were dominant, followed by *Microbacterium* (6.9%), *Rhodococcus* (6.7%), *Brachybacterium* (5.4%), and *Isoptericola* (4.8%). Additionally, 12 genera were represented by only one isolate. The distribution of the 521 actinomycetial strains from 13 different mangrove soil samples is shown in Figure 1b and Table S2. Sample 1 gave the highest diversity and abundance (74 isolates in 17 genera). Samples 3, 8, and 9 shared the second-highest diversity (15 genera), followed closely by sample 13 (14 genera) and sample 2 (13 genera). Two samples collected from Dongsong island showed the lowest diversity (sample 6, 4 genera; sample 7, 5 genera). With respect to the medium composition, M1 yielded the highest recovery rate (12.5%), with 65 isolates representing 18 different genera (Figure 1c and Table S3). A similar result showing higher diversity (63 isolates in 23 genera) was obtained from the modified M1 medium (M11) by the addition of kelp (Table S1). M5 displayed the third-best recovery rates (10.9%) with 57 isolates in 15 genera and showed the best recovery rate of *Streptomyces* (21 isolates). M4 yielded the lowest number and diversity of isolates (13 isolates in 6 genera). Interestingly, M12 (18 isolates in 12 genera), the amended M4 medium with the addition of coconut juice (Table S1), showed a higher number and diversity of isolates than M4 (13 isolates in 6 genera).
