Differential Incorporation of Carbon Substrates among Microbial Populations Identified by Field-Based, DNA Stable-Isotope Probing in South China Sea

To determine the adapted microbial populations to variant dissolved organic carbon (DOC) sources in the marine environment and improve the understanding of the interaction between microorganisms and marine DOC pool, field-based incubation experiments were carried out using supplemental C-13-labeled typical substrates D-glucose and D-glucosamine (D-Glc and D-GlcN, respectively), which are two important components in marine DOC pool in the South China Sea. C-13- and C-12-DNA were then fractionated by ultracentrifugation and the microbial community was analyzed by terminal-restriction fragment length polymorphism and 454 pyrosequencing of 16S rRNA gene. C-12-DNA-based communities showed relatively high similarities with their corresponding in situ communities, and their bacterial diversities were generally higher than C-13-DNA-based counterparts. Distinct differences in community composition were found between C-13- and C-12-DNA-based communities and between two substrate-supplemented C-13-DNA-based communities; these differences distinctly varied with depth and site. In most cases, there were more genera with relative abundances of >0.1% in D-Glc-incorporating communities than in D-GlcN-incorporating communities. The Roseobacter clade was one of the prominent actively substrate-incorporating bacterial populations in all C-13-DNA-based communities. Vibrio was another prominent actively D-GlcN-incorporating bacterial population in most incubations. However notably, different OTUs dominated this clade or genus in different treatments at different depths. Altogether, these results suggested that there were taxa-specific differences in DOC assimilations and, moreover, their differences varied among the typical water masses, which could have been caused by the variant compositions of original bacterial communities from different hydrological environments. This implies that ecologically, the levels of labile or recalcitrance of DOC can be maintained only in a specific environmental context with specific bacterial community composition.