Interactions Between the Diatom Thalassiosira pseudonana and Heterotrophic Bacterium Pelagibacter ubique Examined in Co-Culture

Recent studies employing network theory have focused attention on the importance of interactions between taxa in microbial plankton community ecology. In this study, the centric diatom Thalassiosira pseudonana and the heterotrophic bacterium Pelagibacter ubique HTCC1062, both coastal organisms, were cultured independently and together in co-culture to investigate phytoplankton-bacteria interactions at the molecular level. A basis for predicting synergistic interactions between these plankton taxa can be found in published descriptions of specific vitamin, nutrient, and carbon compound requirements for their growth, but no experiments have been done that measure their physiological and molecular responses to co-culturing. When grown in co-culture with T. pseudonana, HTCC1062 grew slightly, but significantly faster than in monoculture. The generation time of HTCC1062 grown in co-culture was consistently 2.5 to 6.3 hours faster than when grown in monoculture (p-value <0.05). There was no significant difference in the growth rate of T. pseudonana between co- and monoculture.  These results indicate that HTCC1062 is benefiting from the presence of the diatom by an unknown mechanism. Messenger RNA sequencing is being utilized to examine transcriptional responses that may provide insight into specific interactions that contribute to the observed shift in Pelagibacter growth rates.