Characteristics Of Heterotrophic Bacterioplankton Community In Typical Bays Of North Atlantic Ocean

Heterotrophic bacterioplankton plays an important role in marine biogeochemical cycles. Processes which mediate the transfer of carbon from dissolved pools to higher trophic levels are influenced by the growth and grazing losses of heterotrophic bacteria. The goals of this thesis are:to characterize seasonal and spatial variations in community structure of heterotrophic bacterioplankton in coastal (Logy Bay) and estuarine areas (Goose Bay and Lake Melville) of North Atlantic Ocean, including intensive sampling and experiments during the spring phytoplankton bloom in Logy Bay; to determine relationships among environmental factors, bacterial community structure, growth and grazing loss for bacterial population, as well as individual bacterial phylotypes; to provide insight into mechanism that lead to the observed variations at broad phylogenetic levels.1.Seasonal changes in heterotrophic bacterial standing stock, community structure, rates of growth and grazing mortality for bacterioplankton, as well as individual phylogenetic groups within the community, are examined in the surface layer of Logy Bay. Factors controlling seasonal changes of these parameters are investigated using Pearson correlation and regression analysis.(1) Heterotrophic bacterial abundance and biomass show high values in April and minimal in February, while cell volume exhibits maximum in May. Using fluorescence in situ hybridization (FISH), variations in composition of heterotrophic bacterial community were examined. Alpha-Proteobacteria and Cytophaga-Flavobacter cluster dominate the community overall, while Beta-Proteobacteria always comprise the lowest proportion of all detected phylogenetic groups. Relative abundances of all detected individual groups are low during winter (February and March), while higher values are detected during spring (April to June). (2) Using FISH combined with dilution assay, seasonal changes in rates of growth and grazing mortality for bacterial community, as well as individual phylogenetic groups within the community, are determined. Growth rates for bacterial community as well as individual groups are high during summer (July and August), whereas low values are found during winter (February and March), with the exception of Alpha-Proteobacteria. Grazing mortality for bacterial community shows maximum during summer (July and August) and minimum during winter (March). However, no seasonal patterns of grazing mortality are observed among individual phylogenetic groups(3) Results from Pearson correlation and regression analysis suggest that temperature is the main factor controlling rates of growth for bacteria, explained 40-60% variations in rates of growth for bacterial community as well as individual phylogenetic groups. Chlorophyll a concentration and dissolved organic carbon (DOC) are the controlling factors for bacterial abundance and cell volume, respectively. Correlations between rates of growth for bacterial community and those for individual groups implied bacterial phylogentic groups may share specific ecosystem function.2.Spatial distributions in heterotrophic bacterial standing stock, community structure, rates of growth and grazing mortality for bacterioplankton, as well as individual phylogenetic groups within bacterial community, are examined in Goose Bay and Lake Melville. Factors related to spatial changes above are investigated using Pearson correlation and regression analysis.(1) Bacterial abundance show distinct spatial distributions with minimal at low salinity waters and maximal at marine end of estuary. Vertical distribution of bacterial abundance show higher values at surface compared with those in the middle and at bottom. Cell volume and bacterial biomass show similar spatial patterns with each other, with no significant differences among stations. Cell volume increases with depth. Bacterial biomass show no significant differences in the middle and bottom layers at stations where salinity are low, while higher values are observed in the middle layer at stations in high salinity area.(2) Analyses of heterotrophic bacterial community structure are performed at two selected sampling stations located at the low-(No. 111 station) and high-salinity (No.131 station) stations, respectively. Results indicate that relative abundance of Alpha-Proteobacteria is high at 131 station, whereas Beta-Proteobacteria is the dominant group at 111 station. Relative abundance of Alpha-, Beta-and Gamma-Proteobacteria uniformly distributed throughout the water column.(3) Rates of growth and grazing mortality are determined using FISH combined with dilution assays at No. 111 station and No.131 station. No significant differences are observed between rates of growth for bacterial community at No. 111 station and No.131 station. Alpha-Proteobacteria growth rates are higher at No.131 station than those of other individual groups, while growth rates for Beta-Proteobacteria, as well as Gamma-Proteobacteria, are high at low-salinity station, No.111 station. No significant differences are found between grazing mortality for bacterial community at No.111 station and No. 131 station.(4) Results from correlation and regression analysis indicate that dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are controlling factors for heterotrophic bacterial abundance and biomass. Depth and salinity are primary factors regulating cell volume. Since rates of bacterial growth indicates uptake of DOM, the co-occurrence of high growth rates and high relative abundance of individual phylotypes implies the possible regulations of nutrients uptake on bacterial community structure in this area.3.Temporal and spatial variations in heterotrophic bacterial community structure are studied during spring phytoplankton bloom in Logy Bay to investigate possible mechanisms that lead to the observed variations in bacterial community composition during the bloom.(1) Abundance and biomass of heterotrophic bacterioplankton show high values during the developmental stage of the spring bloom, while the maximal of cell volume is found during the post-bloom stage. Alpha-Proteobacteria reach its maximal during the beginning of spring bloom, while Beta-, Gamma-Proteobacteria and CF cluster reach maximum during the post-bloom stage.(2) Using FISH combined with dilution assays, research indicate that significant differences in patterns of growth rates among bacterial community and detected phylogenetic groups, while patterns of grazing mortality are similar to each other during the study.(3) Results from correlation and regression analysis indicate that temperature is the controlling factor for the growth of heterotrophic bacteria during the spring bloom, while chlorophyll a and DOC concentration are responsible for the changes of bacterial biomass and cell volume, respectively. Nutrient supplies might be the main factor driving variations in bacterial community structure during the spring bloom, while predation might be a key mechanism for regulation of bacterial community composition during the senescent stage of the bloom.