Figure 1: Workflow. Selected bacteria IH2, IH18, IH25, and G8 were collected from the surface of Ulva ohnoi, phenocopying the activity of Roseovarius sp. MS2 and forming a tripartite community with Maribacter sp. MS6 and the gametophyte of Ulva mutabilis (morphotype "slender"; strain FSU-UM5-1). Ulva mutabilis (25 mg dry weight) was cultivated with the two bacterial strains (OD620 = 0.001) under standard conditions (Wichard and Oertel 2010). Propagules of equal length were stressed by a temperature shift from 18 °C to 30 °C using continuous light (80 µmol photon m−2 s−1) to avoid chronobiological effects. Axenic cultures and tripartite communities were prepared according to Spoerner et al. (2012). exo-Metabolomics and multivariate analysis of the metabolite profiling of the supernatant (150 mL) of four tripartite communities were performed according to Alsufyani et al. (2017) and Ghaderiardakani et al. (2022). Drawings of Ulva were taken from Wichard (2023) under the terms of CC BY 4.0. Created with BioRender.com.