rRNA internal transcribed spacer phylogeny showed that Chesapeake Bay is populated with diverse Synechococcus strains, including members of the poorly studied marine cluster B. Marine cluster B prevailed in the upper bay, while marine cluster A was common in the lower bay. Interestingly, marine cluster B Synechococcus included phycocyanin-and phycoerythrin-rich strains.

We monitored bacterioplankton communities from Chesapeake Bay over 2 years (2002)(2003)(2004) by use of denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene. Chesapeake Bay bacterioplankton exhibited a repeatable annual pattern and strong seasonal shifts. In winter, the bacterial communities were dominated by Alphaproteobacteria and Actinobacteria, whereas in summer, the predominant bacteria were members of Alphaproteobacteria, Gammaproteobacteria, Cyanobacteria, Actinobacteria, Planctomycetes, and Bacteroidetes. Phylotypes of Alphaproteobacteria and Actinobacteria present in warm seasons were different from those in cold seasons. Relatively stable communities were present in summer-fall across the sampling years, whereas winter communities were highly variable interannually. Temporal variations in bacterial communities were best explained by changes of chlorophyll a (Chl a) and water temperature, but dissolved oxygen, ammonia, nitrite and nitrate, and viral abundance also contributed significantly to the bacterial seasonal variations.

Marine sponges are hosts to diverse and dense bacterial communities and thus provide a potential environment for quorum sensing. Quorum sensing, a key factor in cell-cell communication and bacterial colonization of higher animals, might be involved in the symbiotic interactions between bacteria and their sponge hosts. Given that marine Proteobacteria are known to produce N-acyl homoserine lactone (AHL) signal molecules, we tested the production of AHLs by Alpha- and Gammaproteobacteria isolated from marine sponges Mycale laxissima and Ircinia strobilina and the surrounding water column. We used three different AHL biodetection systems in diffusion assays: Chromobacterium violaceum, Agrobacterium tumefaciens and Sinorhizobium meliloti with optimal sensitivity to short-chain (C4-C6), moderate-chain (C8-C12) and long-chain (>or= C14) AHLs respectively. Thirteen of 23 isolates from M. laxissima and five of 25 isolates from I. strobilina were found to produce AHLs. Signals were detected from two of eight proteobacterial strains from the water column. Thin-layer chromatographic assays based on the A. tumefaciens reporter system were utilized to determine the AHL profiles of the positive isolates. The types and amounts of AHLs synthesized varied considerably among the strains. Small ribosomal rRNA gene sequencing revealed that the AHL-producing alphaproteobacterial isolates were mainly from the Silicibacter-Ruegeria subgroup of the Roseobacter clade. Two-dimensional gel electrophoresis (2DGE)-based proteomic analyses were congruent with phylogenetic relationships but provided higher resolution to differentiate these closely related AHL-producing strains.