Bacteria Normobiosis and Gracilaria edulis Growth; Metagenomic and Culture Studies Unfold New Insights on the Associated Bacterial Diversity

In this study, agarophyte Gracilaria edulis was successfully cultivated in laboratory condition. An attempt to grow G. edulis by removing associated bacteria was made to identify their role in the host growth. But antibiotic treatment did not remove the associated bacteria completely rather reduced the bacterial load. Antibiotic untreated (ABUT) sample grew well in the beginning of culture but degraded gradually in prolonged cultivation. This ascertained by the dominance of surface-associated harmful bacteria. The antibiotic treatment for 48 h (ABT-T48) showed the best growth but antibiotic treatment for 48 h weekly once for 4 weeks (ABT-W48) resulted comparatively less growth. The metagenomic analysis revealed variation in the bacterial diversity and population between the samples of ABUT and ABT-W48. The ABUT sample possessed abundant agar-degrading and algae-lytic bacteria. In contrast, numerous beneficial bacteria like Lactobacillus, Prevotella, Clostridium and Swaminathania are dominant in ABT-W48 sample. It was observed that weekly antibiotic treatment inhibited some beneficial bacteria like Acetobacter. This show necessity of moderate antibiotic treatments to reduce harmful bacteria and encourage beneficial bacteria association. Removing the harmful bacteria promote the alga growth by keeping a balance in bacterial diversity (normobiosis) within the host. Six culturable bacteria associated with G. edulis isolated and identified are Vibrio brasiliensis, two strains of Paracoccus zeaxanthinifaciens, Alteromonas sp., Pseudoalteromonas sp.(a putative novel), and Bacillus licheniformis.