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This study examined the effect of dissolved organic matter (DOM) on ectoenzymatic activity, bacterial growth and community structure in the Hudson River. Our main approach was to mix bacterial communities and water from various locations in the Hudson River and its tributaries, and then to monitor bacterial activity and community structure determined by fluorescence in situ hybridization with oligonucleotide probes. The locations differed significantly in DOM composition and concentrations, ectoenzyme activity and bacterial community structure. We found that water source and, to a lesser extent, source of the inoculum significantly affected nearly all aspects of bacterial activity and community structure. A common inoculum grown in different waters often led to as much as a 2-fold difference in enzyme activities. When 2 different bacterial communities were inoculated in the same water, community structure and the activity of some ectoenzymes remained different after several days. Other data also pointed to a dependence of ectoenzyme activity on community structure. Activity of several ectoenzymes covaried with the relative abundances of the 4 bacterial groups we examined (alpha-, beta- and gamma-proteobacteria, and Cytophaga-like bacteria); the highest correlation was between beta-proteobacteria and phosphatase activity. In multi-variate regression analyses, community structure explained a significant amount of the variation in rates of all ectoenzymes except 2 proteases. The abundance of Cytophaga-like bacteria was the dominant variable in the regression models for the activity of 3 ectoenzymes. These data suggest that DOM can affect the relative abundance of the major heterotrophic bacterial groups, and that the relative abundance of these groups could have an impact on DOM hydrolysis.

Changes in bacterial activity and community structure in response to dissolved organic matter in the Hudson River, New York