Variations in bacterial community structure during a dinoflagellate bloom analyzed by DGGE and 16S rDNA sequencing

The relationship between bacterial 16S rRNA gene composition and carbon metabolism was analyzed during an intense dinoflagellate bloom off the Southern California coast during the spring of 1997. Bacterial numbers and rate processes, chlorophyll a, and the dissolved and particulate organic matter pools were measured during the bloom to provide a framework within which to assess bacterial community composition. Free bacteria were numerically dominant, generally comprising >90% of the total, and were responsible for >70% of bacterial production. Attached bacteria had higher cell-specific growth rates than free bacteria (range = 0.5 to 15.1 and 0.7 to 2.5 d(-1), respectively) and had hydrolytic ectoenzyme activities at times more than an order of magnitude higher on a per cell basis. Denaturing gradient gel electrophoresis analysis of bacterial community composition indicated that: (1) the free and attached communities were distinct, and (2) marked shifts in bacterial community structure occurred concomitant with the peaks in attached enzyme activities, specific growth rates and DOC concentration. Of the 24 16S rDNA clones analyzed, 7 were related to the Cytophaga-like bacteria (CLB), 6 to the alpha -subclass and 5 to the gamma -subclass of the Proteobacteria; 3 were related to oxygenic phototrophs, 2 were heteroduplexes and 1 was a possible chimera. While the alpha- and gamma -Proteobacteria predominated in the <1.0 m fraction, CLB were identified in both the free and attached fractions as well as among bacteria cultured from the same water, without overlap among these groups. The observation that distinct Cytophaga group sequences were present in the free versus attached fractions is counter to the current understanding that these organisms occupy a principally 'particle-specialist' niche. Our results suggest that some CLB are also important in the decomposition of polymeric organic matter in the dissolved phase with implications for the accumulation of dissolved organic matter and pathways of carbon flow during phytoplankton blooms.