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.