Genetic Analysis of the Roseobacter-Dinoflagellate Interaction

University Of Maryland Biotechnology Institute, Baltimore MD

Investigators

Abstract

Members of the Roseobacter clade of the a-proteobacteria are among the most abundant and ecologically relevant marine bacterial groups. Roseobacter isolates and gene sequences derived from them have been retrieved from marine environments ranging from sea ice to open ocean mixed layer to tropical coral reefs, and in ecological niches ranging from free-living plankton to sponge symbiont to biofilm pioneer. Although Roseobacter are cosmopolitan in the marine environment, their numbers and activity significantly rise with increases in the population density of phytoplankton and dinoflagellates. However, little is known about the cellular factors and molecular mechanisms required for roseobacters to move towards and remain in the phycosphere surrounding phytoplankton and dinoflagellates. The long-term goal of this research is to understand the signals and molecular mechanisms used to initiate and maintain the interaction between the roseobacter and its eukaryotic host. Members of the Roseobacter clade are prevalent and often numerically dominant in laboratory cultures of dinoflagellates. They are found within the dinoflagellate's phycosphere or attached to its surface. Axenic cultures of dinoflagellates grow poorly compared to cultures with associated bacterial assemblages, and adding back either a single isolate (Silicibacter TM1040) or mixed bacterial assemblages restores normal growth. TM1040 actively metabolizes the dinoflagellate secondary metabolite dimethylsulfoniopropionate (DMSP), and is motile and chemotactically attracted to dinoflagellate homogenates, DMSP, amino acids, and other chemicals released by the dinoflagellate. The research will test the following hypothesis: Roseobacter clade bacteria establish the initial interaction with their eukaryotic partners through a response to dinoflagellate signals, e.g., DMSP, and chemotaxis-driven swimming. This behavior brings the bacteria into close physical association with their eukaryotic host, at which point other bacterial gene products are expressed in response to dinoflagellate-derived signals to maintain the interaction. Using TM1040 as a model, the specific objectives of this research are to (1) analyze flagellar arrangement, morphology, energetics, and chemotaxis of TM1040; (2) identify potential genes involved in the bacterial-dinoflagellate interaction through a comparative genomics analysis of Silicibacter TM1040, S. pomeroyi DSS-3, and Jannaschia sp. CCS-1; and, (3) use genetic techniques to identify and characterize the genes affecting the bacteria-dinoflagellate interactions, and analyze how each mutation affects the interaction between bacteria and dinoflagellates. The results of this research will be important to current research efforts in marine microbial ecology and will benefit society through a better understanding of the effect of Roseobacter/dinoflagellate interactions as they affect nutrient cycling, especially sulfur cycling, in the world's oceans, and may have implications to global climate control.