The seaweed microbiome of Ulva australis changes nitrogen metabolism under eutrophic conditions

The microbiome of macroalgae supports their hosts by promoting recruitment, development and growth, especially by nutrient cycling. Green algae in the genus Ulva have a close relationship with their microbiomes and are greatly valued for aquaculture, particularly to bioremediate eutrophication. Here, we explore the Ulva australis surface microbiome responses to nutrient enrichment to investigate their role in phosphorus and nitrogen cycling. We exposed the seaweed to increasing nutrient concentrations in a 7-day tank experiment and analyzed changes to their microbiomes by using shotgun metagenomics. Our results showed that eutrophic conditions selected for specific microbial taxa and gene functions, but it did not change the relative abundance of phosphorus metabolism genes. By contrast, changes in the microbial nitrogen metabolism genes were pronounced, especially related to denitrification pathways. Specifically, Rhizobiales and nitrogen fixation genes decreased under high nutrient conditions as seaweed biomass increased. Thus, the U. australis microbiome was sensitive to increased nitrogen but not to increased phosphorus. The decrease in relative abundance of Rhizobiales and ammonia assimilation genes could indicate that U. australis was outcompeting some bacteria in their associated microbiome for nitrogen use. This study has discovered potential mechanisms of nutrient cycling in the seaweed microbiome.