Dynamics of Microbial Abundance in Unvegetated and Seagrass Habitats: A Case Study

Seagrass meadows are recognized for their ecological importance, yet their influence on microbial community structure remains insufficiently characterized. This study examined the effects of seagrass presence on microbial assemblages in a subtropical coastal environment by comparing seagrass habitats to adjacent unvegetated sediments. Microbial abundances, including viruses, bacteria, picophytoplankton (Synechococcus spp. and picoeukaryotes), and heterotrophic nanoflagellates, were quantified using flow cytometry. Viral concentrations were significantly higher in seagrass treatments (2.4–9.2 × 106 viruses mL−1) than in controls (0.6–2.0 × 106 viruses mL−1), while bacterial abundances were slightly lower in seagrass treatments (5.1–16.0 × 105 cells mL−1) than in controls (7.9–16.6 × 105 cells mL−1). As a result, the virus-to-bacteria ratio (VBR) was significantly elevated in seagrass habitats, suggesting enhanced viral regulation of bacterial populations. Additionally, picophytoplankton and heterotrophic nanoflagellates increased in seagrass incubations, with strong correlations indicating that nanoflagellates are likely major grazers of picophytoplankton. These results highlight the role of seagrass habitats in modulating microbial interactions and emphasize the need to consider habitat-specific characteristics when evaluating microbial dynamics and biogeochemical processes in coastal systems.