Characterization of the interactive effects of labile and recalcitrant organic matter on microbial growth and metabolism

Abstract

15 16 keywords: interactive effects; terrestrially-derived, dissolved organic matter; Roseobacter; 17 species-specificity; community interactions 18 19 Running title: Organic matter interactive effects 20 2 Abstract 21Geochemical models typically represent organic matter (OM) as consisting of multiple, 22 independent pools of compounds, each accessed by microorganisms at different rates. However, 23 recent findings indicate that organic compounds can interact within microbial metabolisms. The 24 relevance of interactive effects within marine systems is debated and a mechanistic understanding 25 of its complexities, including microbe-substrate relationships, is lacking. As a first step toward 26 uncovering mediating processes, the interactive effects of distinct pools of OM on the growth and 27 respiration of marine bacteria, individual strains and a simple, constructed community of 28Roseobacter lineage members were tested. Isolates were provided with natural organic matter 29 (NOM) and different concentrations (1, 4, 40, 400 µM-C) and forms of labile organic matter 30 (acetate, casamino acids, tryptone, coumarate). The microbial response to the mixed substrate 31 regimes was assessed using viable counts and respiration in two separate experiments. Two marine 32 bacteria and a six-member constructed community were assayed with these experiments. Both 33 synergistic and antagonistic growth responses were evident for all strains, but all were transient. 34The specific substrate conditions promoting a response, and the direction of that response, varied 35 amongst species. These findings indicate that the substrate conditions that result in OM interactive 36 effects are both transient and species-specific and thus influenced by both the composition and 37