| Bacteria-phytoplankton interactions are critical for biogeochemical cycles and ecosystem dynamics in marine environments. Phytoplankton exudates can affect bacterial composition, which in turn may affect bacterial remineralization. It is, therefore, important to understand how these two groups of organisms interact in nutrient cycling. In this dissertation, the effect of phytoplankton photorespiration on bacterial composition was investigated. In marine environments photorespiration results in production of phytoplankton- and photorespiration-specific glycolate. A molecular marker specific to marine bacteria was developed to amplify glcD, the gene encoding the D-subunit of glycolate oxidase. Only a subset of laboratory cultures of marine bacteria could grow on glycolate alone and the ability to use glycolate correlated with possession of glcD. Hence, the presence of glcD could serve as a good indicator of the potential for glycolate utilization in marine bacteria. Amplification of glcD and 16S rDNA from two eutrophic sites in the Pacific NW and from two depths of an oligotrophic Atlantic Gulf Stream Ring indicated that glcD was present in only a subset of all four bacterial communities and that glcD diversity, though phylogenetically diverse, appeared to be different within and between the eutrophic and oligotrophic environments. To understand the factors affecting diversity of glycolate-utilizing bacteria, samples were collected during the spring phytoplankton bloom in Dabob Bay, WA in 2004. Sequencing of glcD from samples collected throughout the bloom period showed a decrease in glcD diversity as the bloom progressed. Specific glcD phylotypes displayed different patterns of abundance during the bloom. glcD RNA transcript abundance was higher during the day than at night, corresponding to known diel patterns in phytoplankton photorespiration and glycolate excretion, implying that these bacteria were using glycolate. Comparison of biological and environmental parameters suggests that the different abundance patterns among glycolate-utilizing bacteria may relate to resource partitioning of different photorespiratory exudates released by phytoplankton at different stages of the bloom. A model is proposed for different levels of reliance by marine bacteria on the various compounds associated with phytoplankton photorespiration. The application of this model to other phytoplankton processes can further understanding of the influence of phytoplankton on bacterial community dynamics. |
