Characterization of microbial diversity in the harbour and ballast water of ships in the Gulf of Mannar, India using environmental DNA

Marine ecosystems are rich in diverse microbial communities, which are vital to nutrient cycling and the overall health of the ecosystem. Preserving native microbial diversity is crucial for maintaining ecosystem stability; however, human activities, particularly maritime shipping, pose a significant risk of introducing non-native species through ballast water discharge. Despite the ecological significance of the Gulf of Mannar, data on the microbial diversity in its harbour and ballast waters remain sparse. This study aimed to characterize the microbial diversity in the Gulf of Mannar’s harbour and ballast waters using 16S rRNA amplicon sequencing. Analysis revealed the presence of dominant phyla, including Proteobacteria, Bacteroidetes, Planctomycetes, and Actinobacteria, in both water sources, underscoring their significant ecological roles. At the genus level, ballast water was dominated by species such as Alteromonas, Neptuniibacter, Aestuariibacter, Pirellula, Pseudoalteromonas, Thalassotalea, Rubripirellula, Vibrio, Algicola, and Blastopirellula, while harbour water exhibited species abundance from genera like Alteromonas, Oleibacter, Uncultured-bacterium, Pirellula, Blastopirellula, Tropicimonas, Marinobacter, Neptuniibacter, Illumatobacter, and Candidatus-actinomarina. Diversity indices showed higher species richness in harbour water but greater abundance in ballast water. Significant differences in species abundance were identified between the two environments, notably in Oleiphilus messimensis, a haloalkaliphilic bacterium (Acht6.2), various species within Marinobacter and Marinomonas, and the orders Verrucomicrobiales and Dadabacteriales. This study provides a baseline of microbial biodiversity in the Gulf of Mannar’s harbour and ballast waters, offering insights into microbial community dynamics and informing management strategies to mitigate ecological risks linked to ballast water discharge in the region.