Propagule pressure and native macrophyte biomass mediate the success of an invasive alga: the role of below-ground microbial communities

Biological invasions pose a significant threat to local ecosystems and native species worldwide. For macrophytes, the interaction of factors such as native species biomass loss and propagule pressure of invasive species have important consequences for invasion success. How these factors are mediated by below-ground processes is, however, poorly understood. Through a field survey and manipulative experiment, we simultaneously investigated the influences of native seagrass (Zostera muelleri) biomass, invasive propagule pressure and below-ground bacterial communities (evaluated through 16S amplicon sequencing) in mediating the establishment of the invasive green alga, Caulerpa taxifolia. Results from the field survey showed that rhizosphere bacterial communities and predicted metabolism were clearly distinct between Z. muelleri and C. taxifolia, but bulk sediment microbes and sediment characteristics (i.e. proportion of fine sediments and organic carbon) did not differ. Increasing seagrass or C. taxifolia biomass only affected rhizosphere bacterial richness, and this was apparent on specific bacterial groups involved in sulphur and nitrogen metabolism. Comparatively, the experimental manipulation showed that seagrass biomass significantly decreased C. taxifolia biomass, but this effect depended on invasive propagule levels. More specifically, C. taxifolia biomass was lower in treatments with higher Z. muelleri biomass although the magnitude of this effect was stronger in plots with high propagule pressure. Overall, increasing C. taxifolia propagule pressure may intensify negative interspecific competition among invading individuals, while higher seagrass biomass also reduces overall invasion success. These effects could potentially be mediated by root-associated microbial communities and directly influence invasion success.