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 met abolism was analyzed during an intense dinoflagellate bloom off the Souther n California coast during the spring of 1997. Bacterial numbers and rate pr ocesses, chlorophyll a, and the dissolved and particulate organic matter po ols were measured during the bloom to provide a framework within which to a ssess bacterial community composition. Free bacteria were numerically domin ant, 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), respect ively) and had hydrolytic ectoenzyme activities at times more than an order of magnitude higher on a per cell basis. Denaturing gradient gel electroph oresis analysis of bacterial community composition indicated that: (1) the free and attached communities were distinct, and (2) marked shifts in bacte rial community structure occurred concomitant with the peaks in attached en zyme 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 Proteobacteri a; 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 <mu>m fraction, CLB were identified in both the free and attac hed fractions as well as among bacteria cultured from the same water, witho ut overlap among these groups. The observation that distinct Cytophaga grou p sequences were present in the free versus attached fractions is counter t o the current understanding that these organisms occupy a principally 'part icle-specialist' niche. Our results suggest that some CLB are also importan t in the decomposition of polymeric organic matter in the dissolved phase w ith implications for the accumulation of dissolved organic matter and pathw ays of carbon flow during phytoplankton blooms.