Abstract
The marine green macroalga Ulva (Chlorophyta) lives in a mutualistic symbiosis with bacteria that influence growth, development, and morphogenesis. We surveyed changes in Ulva’s chemosphere, which was defined as a space where organisms interact with each other via compounds, such as infochemicals, nutrients, morphogens, and defense compounds. Thereby, Ulva mutabilis cooperates with bacteria, in particular, Roseovarius sp. strain MS2 and Maribacter sp. strain MS6 (formerly identified as Roseobacter sp. strain MS2 and Cytophaga sp. strain MS6). Without this accompanying microbial flora, U. mutabilis forms only callus-like colonies. However, upon addition of the two bacteria species, in effect forming a tripartite community, morphogenesis can be completely restored. Under this strictly standardized condition, bioactive and eco-physiologically-relevant marine natural products can be discovered. Solid phase extracted waterborne metabolites were analyzed using a metabolomics platform, facilitating gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) analysis, combined with the necessary acquisition of biological metadata. Multivariate statistics of the GC-MS and LC-MS data revealed strong differences between Ulva’s growth phases, as well as between the axenic Ulva cultures and the tripartite community. Waterborne biomarkers, including glycerol, were identified as potential indicators for algal carbon source and bacterial-algal interactions. Furthermore, it was demonstrated that U. mutabilis releases glycerol that can be utilized for growth by Roseovarius sp. MS2.
Highlights
Interactions between the marine macroalga Ulva and its associated bacteria impact directly on the algal morphology and physiology of both partners
We investigated the composition of the bacterial and algal metabolites released during algal growth under standardized conditions, and compared the chemosphere between the axenic algal culture and the tripartite community using an exo-metabolomic approach (Figure 1C)
The following experimental metadata were recorded from Ulva cultures in bioreactors: (i) the growth of U. mutabilis; (ii) the depletion of nitrate and phosphate in growth media; (iii) the changes in the algal life cycle; (iv) the status of the inducibility of Ulva’s gametogenesis; and (v) the axenicity of the cultures, as well as the associated microbiome of the tripartite community
Summary
Interactions between the marine macroalga Ulva and its associated bacteria impact directly on the algal morphology and physiology of both partners. Those interactions supposedly change the chemical composition of their environment, and, shape the bacterial diversity of the biocoenose [1,2]. Metabolomics can help in providing a functional understanding of the physiological state of an organism or symbiotic interactions. This is intriguing in the light of the exo-metabolome, which is defined by metabolites secreted into the environment by one or more organisms.
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