Responses in marine bacteria to labile dissolved organic matter compounds of different complexity

Bacteria play a key role in the turnover of dissolved organic matter (DOM), the main biologically available carbon source in the ocean because they assimilate various organic compounds. However, little is known about the functional contribution of substrate specialists and generalists to organic matter transformations, and whether ecological niche theory accurately applies to the functional gene expression of bacterioplankton. Thus, we conducted seawater culture experiments enriched with a set of ecologically relevant organic model compounds in the form of polymers (polysaccharides, DNA, protein) or their corresponding monomers (monosaccharides, nucleotides, amino acids). Bacterioplankton abundances ranged between 1.5 to 4.6 million cells/mL in the cultures and were similar between mono- and polymers in carbohydrate and nucleotide but not in protein enrichment. However, pronounced changes in functional community composition and associated gene expression occurred in a manner systematically related to DOM composition. More than 1200 genes differed significantly in expression between substrates, including sugar and amino acid Ton/Tol and ABC transporters, glycoside hydrolases, peptidases, motility and chemotaxis-related genes. Our results question that DOM complexity per se favours a response of generalist species. These findings rather suggest a high degree of resource partitioning among marine bacteria under competitive environmental conditions. Thus, copiotroph substrate specialists may disproportionately contribute to the turnover of carbohydrates and proteins in seawater.