Abstract

B vitamins are a large group of water soluble molecules that act as vital co-factors in many cellular metabolic processes. In the marine environment B vitamins are present in low concentrations and have been shown to be important limiting factors for algal growth. With an estimated half of all microalgal species requiring an exogenous source of cobalamin (vitamin B12 ), one quarter requiring thiamine (B1) and a tenth requiring biotin (B7), most algal species are auxotrophic for at least one member of the B vitamin family. Previous work in our laboratory has shown that algae can form a mutualistic interaction with bacteria for one of these limiting B vitamins. We have demonstrated an algal-bacterial symbiosis between the B12 -dependent green alga Lobomonas rostrata and the soil bacteria Mesorhizobium loti, in which bacteria provide B12 in return for fixed carbon from the alga. One aim of my PhD project was to identify a marine model for a similar mutualistic interaction between two species of genome sequenced and ecologically relevant algae and bacteria. I have been able to establish this model symbiosis between the cosmopolitan marine alga Ostreococcus tauri, which requires both B12 and B1 for growth, and the bacterium Dinoroseobacter shibae.

The main aim of this talk will be to describe this mutualistic interaction between D. shibae and O. tauri in both B12 and thiamine limited conditions. This mutualism is stable over multiple generations and sub-cultures and even persists in media containing no B vitamin supplements, which is notable as D. shibae itself requires an exogenous source of biotin, niacin (B3) and p-aminobenzoic acid (B10) indicating that vitamin traffic is not all one way. I will also describe currently on-going work to screen 16 ecotypes of O. tauri for naturally occurring mutualistic bacteria and future work into the search for communication molecules that may mediate the symbiosis between the two species. This mutualism shows that there is a complex interaction for B vitamins in certain marine microorganisms, which may have important implications for marine phytoplankton ecology as well as the widespread evolution of vitamin auxotrophy in the algal lineage.