Project description:Algae and bacteria are frequently observed to form mutualisms for the exchange of a range of nutrients. We have developed a stable co-culture in which the cobalamin (vitamin B12)-requiring unicellular green alga Lobomonas rostrata can grow stably in the absence of this organic micronutrient with the -proteobacterium Mesorhizobium loti, which supplies B12 in exchange for fixed carbon. In this study, we report the results of a comparative quantitative proteomics experiment to evaluate the protein expression profile of L. rostrata grown in co-culture vs. axenic treatments. For this, we have successfully developed and applied an isobaric tagging proteomics platform (iTRAQ) comparing the proteome of L. rostrata (an organism without a sequenced genome) grown axenically with B12 supplementation or in co-culture with M. loti. A total of 588 proteins could be identified using the related Chlamydomonas reinhardtii as a reference genome, and there was little interference from bacterial proteins. Comparisons of relative protein abundance in the two growth conditions highlighted proteins that were higher in co-cultures, including a number of heat-shock proteins, suggesting that the alga was experiencing stress in the presence of the bacterium. Several enzymes of amino acid biosynthesis were elevated, but total cellular protein and free amino acids were substantially lower in cells in co-culture relative those grown axenically. The most striking observation was that many proteins of photosynthesis, both the light-dependent reactions and the Calvin cycle, and those of chloroplast protein synthesis, were significantly lower in L. rostrata cells in co-culture. These observations were confirmed by measurement of electron transfer rates in cells grown under the two conditions, and strongly suggest that the alga is experiencing nutrient limitation when B12 is supplied directly from a bacterial source.
