Activity and abundance of bacterial groups in the Sargasso Sea and Mid-Atlantic Bight

Date
2011
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University of Delaware
Abstract
Microbial communities are essential to the processing of dissolved organic matter and nutrients in the ocean. Molecular methods are used to assess phylogenetic structure and activity within marine bacterial populations. However, types of activity and varying abundances can affect bacterial contributions to biogeochemical and organic matter fluxes. The goals of this study were to evaluate activity levels of rare and abundant bacterial groups in the Mid-Atlantic Bight and Sargasso Sea, and to compare shifts in the active and total bacterial populations on a short temporal scale with changing activity levels. Bacteria that are replicating their DNA are active members of the community. Thus, a metric for bacterial activity is to monitor DNA synthesis. I assessed bacterial activity via DNA replication using incorporation of a thymidine analog, 5-bromo-2’-deoxyuridine, into bacterial DNA. Denaturing gradient gel electrophoresis and sequencing were used to generate profiles of the total and active populations in the Sargasso Sea and Mid-Atlantic Bight in spring and summer. Total and active communities were 65-95% similar within seasons, indicating that most bacteria are actively dividing. Bacterial groups varied in activity within each sample. SAR11 (Alphaproteobacteria) was active and abundant throughout communities. Another alphaproteobacterial member, a Rhodobacter sp., was enriched only in active communities. Additionally, representatives of Bacteroidetes and Gammaproteobacteria were more active than their abundances suggested. In marine systems, abundances of bacterial members may not be indicative of their activity levels. I used tag pyrosequencing of the 16S rDNA V1-V2 region to compare the active and total communities from spring and summer 2009 and 2010 in the Mid-Atlantic Bight and Sargasso Sea. Activity levels of a majority of bacterial taxa were proportional to their abundances in the total community. However, I observed differential activity in bacteria of varying abundances and activity. Flavobacteria (Bacteroidetes) were between 5 and 20% of the total community in both environments, but were consistently more active than suggested by their abundances. Rhodobacterial (Alphaproteobacteria) ribotypes were active, rare members of the Mid-Atlantic Bight bacterial community. In the Sargasso Sea, SAR86 (Gammaproteobacteria) was more active when rare. SAR11 was consistently abundant in both locations and seasons, but had discrepancy in activity levels relative to abundance. Changes in the active bacterial community also influenced the composition of the total community in spring 2009 of the Mid-Atlantic Bight and spring 2010 of the Sargasso Sea. Rare and active bacteria, such as unclassified bacteria in the Sargasso Sea and rhodobacters in the Mid-Atlantic Bight, become more abundant members of the microbial population within 12-96 hours of sampling. The research in this thesis investigated differential activity of bacterial groups in the Sargasso Sea and Mid-Atlantic Bight. I analyzed bacterial activity at fine phylogenetic and temporal scales. I identified rare and abundant bacterial taxa with higher or lower activity levels than expected. Furthermore, I observed shifts in community structure with changes in bacterial activity and environmental conditions. By profiling changing activity levels of bacterial taxa and rapid responses in community structure, this study examined the interplay of diversity and activity in microbial assemblages, and contributed to knowledge of the ecology of marine microbes.
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