California Sea Grant College Program

Most phytoplankton, including the toxic species of interest, cannot be grown axenically and require cohabitabing,associated, or symbiotic bacterial partners. However we have been able to prepare stable binary cultures of phytoplankton of interest with just a single species of bacteria (most of which are species of Marinobacter). While we recognize the culturable bacteria that we have isolated and are now studying are not necessarily qualitatively orquantitatively representative of the bacterial population associated with the phytoplankton in the wild, the ability to produce viable binary phytoplankton/bacterial cultures represents a critically important experimental tool. While other non-culturable bacteria could be quantitatively more important inhabitants of the "phycosphere" in the wild, our ability to ascertain the specific chemical determinants supplied by these bacteria and necessary tophytoplankton survival would be severely limited or impossible with such a heterogeneous and constantly changing assemblage. The ability to form stable, viable binary cultures indicates that the bacteria in question, irrespective of their numerical importance in a natural state, must provide some important factor for phytoplankton growth and greatly enhances our ability to define the nature of that substance and to test hypotheses based on it. Our hypothesis is that the critical element provided by the bacteria that control algal growth and survival involves siderophores, either directly through some form of iron trafficking between the species or indirectly possibly as boron-mediated interspecies cell-cell signaling molecules.