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-mol ecular weight (HMW; >1000 Da) and low-molecular weight (LMW; <1000 Da) frac tions of dissolved organic carbon from a southeastern US estuary was determ ined by sequencing and terminal restriction fragment length polymorphism (T -RFLP) analysis of 16S rRNA gene amplicons in 2 enrichment studies. 16S rDN A sequence analysis indicated that the bacterial communities growing on the LMW fraction were dominated by <gamma>- and epsilon -Proteobacteria relate d to Pseudomonas fluorescens and Arcobacter nitrofigilis (accounting for 90 % of the clones) while the communities using the HMW fraction were dominat ed by alpha-, beta-, and gamma -Proteobacteria and Cytophaga-Flexibacter-Ba cteroides related to Rhizobium-Agrobacterium, Janthinobacterium lividum, Ps eudomonas fluorescens, Marinobacterium georgiense, Pseudoalteromonas, and S phingobacterium comitans (accounting for 98 % of the clones). Methylotrophi c bacteria were present in the inoculum community but not found in either L MW 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 an d LMW carbon to support several bacterial generations. Although the composi tion 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 refl ecting temporal variations in the chemical composition of the dissolved org anic carbon.