Abundance, size, and single-cell activity of bacterial groups in polar and temperate waters

Microbial communities dominate the fluxes of organic material in the ocean, in part due to their high abundance. To determine the amount of carbon processed by bacteria, bulk properties, such as production, abundance, biomass, and respiration, are measured for the total community. Phylogenetic analyses of bacteria are used to describe the structure within microbial communities. However, neither bulk activity measurements nor phylogenetic identification alone can determine which bacterial groups respond to certain environmental conditions or which bacterial groups use certain organic compounds. The goal of this dissertation was to assess the responses of different bacterial taxa to environmental conditions and available substrates.;A basic characteristic of microbes is cell size. The size of microbial cells affects ecological interactions with other organisms and may be related to rates of biomass production. Using a protein stain, I analyzed the biovolume of microbial communities in Arctic, Antarctic, and temperate waters. Microbes in higher latitudes were on average 30% larger than cells from temperate waters. The abundance of bacterial taxa varied among geographic regions, and the size of some bacterial groups also differed among regions. Gammaproteobacteria and members of the Sphingobacteria-Flavobacteria (SF) group were larger in high latitude waters. In each environment, SF cells were larger than other bacteria by about 15%, while Gammaproteobacteria were intermediate in size and Alphaproteobacteria did not differ in size from the average bacterial cell.;In addition to varying in size, bacterial taxa differ in the use of organic material. I used microautoradiography and fluorescent in situ hybridization to identify bacteria incorporating organic compounds. In the Delaware estuary and mid-Atlantic bight, about 30% of all cells incorporated leucine and other amino acids, while only 10% incorporated protein. Using light and dark treatments, I found that light affected single-cell activity in about 20% of cases, but there was no net effect of light on bulk bacterial production. Light did not affect Gamma- and Alphaproteobacteria differently. However, 25% more bacteria in the SAR11 clade used leucine in the light than the total community. Other environmental conditions besides light also correlated with the abundance and activity of bacterial groups. Gammaproteobacterial abundance correlated with bacterial production and concentrations of dissolved organic carbon and nitrogen, and a higher fraction of Gammaproteobacteria used leucine in the summer than in the fall.;There is also geographic variation in abundance and activity of specific bacterial taxa. I examined the abundance and single-cell activity of dominant bacterial clades in waters off the west Antarctic peninsula. More bacteria used leucine (40%) than used a mixture of amino acids or protein (12-22%). Gammaproteobacteria were a large fraction (20%) of the community in this region, and using a new probe I assessed the ecological role of the Ant4D3 gammaproteobacterial clade. The Ant4D3 clade constituted 10% of the total community, and while the active fraction of this clade did not differ among various compounds, Ant4D3 dominated the incorporation of amino acids. The use of organic material varied among the Polaribacter, SAR11, and Ant4D3 clades. Polaribacter contributed the most to protein uptake. Though dominated by different bacterial taxa, the activity of this Antarctic microbial community was comparable to that of temperate communities. The research presented in Chapter 4 is the first description of the single-cell activity of bacterial groups in coastal Antarctic waters.;The research described in this dissertation details the abundance of specific bacterial groups along with bacterial cell size (Chapter 2), light effects on bacterial activity (Chapter 3), and bacterial activity in polar waters (Chapter 4). Generally the approach taken was to divide the "black box" of all microbes into broad phylogenetic groups, which display characteristic differences yet are abundant as cohesive units in the microbial community. Assessing microbial communities at this scale, I found variation of broad bacterial taxa in size, activity, and response to environmental factors. The combination of single-cell methods with genomic approaches will enable us to move toward quantifying bacterial contribution to global processes and predicting the response of bacterial groups to environmental change.