Utilization and degradation of laminarin-based substrates by marine yeasts suggests their niche-specific role in microbial loop dynamics.

In the oceans, the diversity of phytoplankton primary products supports a wide range of microbial heterotrophs, including bacteria and fungi. The organic substrate dynamics within pelagic microbial communities are strongly controlled by microorganismal interactions, resulting in a dense interactome. While the role of bacteria in the microbial loop is well documented, the degradation capacity and substrate specificity of marine fungi, as well as their role and function in metabolic guilds with bacteria, is comparatively less understood. We chose the polysaccharide laminarin, a major product of marine primary production, as well as oligomeric laminarin subunits and monomeric glucose, to study the degradation capacity of eleven marine yeast isolates from the pelagic microbial community of Helgoland Roads. Our aim was to measure yeast growth and correlate degradation yields and putative intermediate degradation products with the size of laminarin-based organic precursor substrates. We developed a reproducible, temporally resolved, high-throughput growth protocol to measure resource-specific yeast growth. Measurement of temporally fine-scaled growth kinetic models of isolates were accompanied with qualitative and quantitative chemical analyses of substrates and degradation intermediates. Our data showed that yeast growth was negatively correlated with oligomer length. Fluorophore-assisted carbohydrate electrophoresis suggested the lack of enzymatic endo-activity for laminarin in yeasts under investigation, suggesting they may occupy a niche in the microbial loop, benefitting from extracellular hydrolysis of carbohydrates by other microorganisms. In terrestrial environments, namely forest soil ecosystems, yeasts have been assigned a similar niche, supporting a prominent role of yeasts in microbial interactomes.