Anastasiia Barilo ¹ Aschwin H. Engelen ² Susanne Wilken ¹ Harro Bouwmeester ³ Gerard Muyzer ^1*

• ¹ Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, Netherlands
• ² Center of Marine Sciences, Faculty of Sciences and Technology, University of Algarve, Faro, Portugal
• ³ Swammerdam Institute for Life Sciences, University of Amsterdam, Amsterdam, Netherlands

Caulerpa is a genus of green macroalgae that lives in tropical and subtropical coastal waters. It is an intriguing organism because, despite having plant-like structures, it is one giant cellwhich, next to multiple nuclei, chloroplasts, and mitochondria, also contains endo-and epiphytic bacteria. The role of these bacteria is unknown, but they might impact the growth and development of the host, adaptation to environmental parameters, and, hence, the ecological success of these algae. We hypothesized that increased sulfide concentrations would trigger a significant shift in the microbial community composition associated with C. prolifera rhizoids, favoring sulfide-oxidizing bacteria. To test this hypothesis, we conducted a mesocosm experiment incubating C. prolifera in sediments with different sulfide concentrations and analysed the algal photosynthesis, growth, and microbiome composition. While photosynthesis was not affected, the Caulerpa weight-based growth rate decreased linearly with increasing sulfide concentration. To analyse the microbiome, we extracted DNA and RNA from the fronds, rhizoids, and the accompanying sediments and performed 16S amplicon sequencing. The microbiome of the fronds was unaffected in both the DNA and RNA samples. However, an increase in sulfide concentration coincided with a decrease in the relative abundance of sulfate-reducing bacteria associated with Caulerpa rhizoids, particularly from the family Desulfocapsaceae. In the RNA samples, potential sulfide oxidisers of the rhizoid-associated members of the Beggiatoaceae were detected. Our results suggest that the rhizobiome of Caulerpa plays a significant role in its adaptation to sulfide-rich environments, offering new insights into the complex interactions within marine holobionts.