Better together: Microbial diversity might facilitate the invasion success of the seagrass <i>Halophila stipulacea</i> in mixed Mediterranean seagrass communities

Microorganisms are increasingly recognized as key facilitators of invasion success for introduced species into new environments. The globally invasive seagrass <i>Halophila stipulacea</i> flourishes in mixed environments with native seagrasses, where it exhibits enhanced growth, while, in contrast, native seagrasses in mixed environments experience reduced growth. Here, we hypothesize that microbes may support the success of invasive seagrass in mixed Mediterranean environments. We analyzed 16S rRNA genes to characterize the microbial diversity on the phyllosphere alongside biochemical, morphological, and sediment nutrient measurements of the Mediterranean-native seagrass <i>Cymodocea nodosa</i> and the invasive <i>H. stipulacea</i> from a controlled mesocosm experiment. Overall, <i>C. nodosa</i> in monoculture harbored a microbiome exhibiting higher ASV richness and a distinct community composition than <i>H. stipulacea</i>. Variation in bacterial diversity associated with hydrogen peroxide (H₂O₂) and internode length suggests that microbial communities of the native seagrass might be shaped by its stress. Conversely, <i>H. stipulacea's</i> microbiome was most abundant in mixed environments, with bacteria significantly reduced in monoculture, and bacterial diversity loosely associated with growth, suggesting that microbes are critical to assisting and possibly facilitating <i>H. stipulacea</i> in mixed environments. Overall, our findings suggest that invasive <i>H. stipulacea</i> in the Mediterranean Sea are capable of recruiting beneficial bacteria, creating microbial interactions that support its success, and undermining the resilience of native seagrasses in mixed beds. Future work should center on the mechanisms driving <i>H. stipulacea</i> bacterial communities and investigating whether <i>H. stipulacea</i> actively determines its own microbiome, or whether its microbiome is passively determined by environmental variables.