1 Introduction Annually, marine phytoplankton fix up to 50 billion tons of carbon (Falkowski 1994), with roughly half of this carbon being metabolised by marine bacteria (Azam et al. 1983). Freshly synthesised small biomolecules, known as metabolites, are among the most readily used compounds, playing an important role in fostering taxa interdependencies that structure microbial communities (Poretsky et al. 2010; Vorobev et al. 2018; Boysen et al. 2022; Moran et al. 2022).