Molecular characterization of estuarine bacterial communities that use high- and low-molecular weight fractions of dissolved organic carbon

The composition of bacterial communities growing at the expense of high-molecular weight (HMW; >1000 Da) and low-molecular weight (LMW; <1000 Da) fractions of dissolved organic carbon from a southeastern US estuary was determined by sequencing and terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene amplicons in 2 enrichment studies. 16S rDNA sequence analysis indicated that the bacterial communities growing on the LMW fraction were dominated by γand ε-Proteobacteria related to Pseudomonas fluorescens and Arcobacter nitrofigilis (accounting for 90% of the clones) while the communities using the HMW fraction were dominated by α-, β-, and γ-Proteobacteria and Cytophaga-Flexibacter-Bacteroides related to Rhizobium-Agrobacterium, Janthinobacterium lividum, Pseudomonas fluorescens, Marinobacterium georgiense, Pseudoalteromonas, and Sphingobacterium comitans (accounting for 98% of the clones). Methylotrophic bacteria were present in the inoculum community but not found in either LMW or HMW enrichments. T-RFLP analysis of the enrichment communities showed measurable changes in community composition during the enrichment period, and companion respiration assays confirmed utilization of sufficient HMW and LMW carbon to support several bacterial generations. Although the composition of the estuarine inoculum used for the 2 enrichment studies (conducted in April 1997 and May 1999) was quite similar, the communities developing on the HMW and LMW fractions differed between experiments, potentially reflecting temporal variations in the chemical composition of the dissolved organic carbon.