Role of Carbon Transport Proteins in Fueling Open Ocean Nitrogen Fixing Diatom-Cyanobacteria Symbioses

Mercedes Nieves1, Enrique Flores2, Martin Hagemann3 and Rachel Foster1, (1)Stockholm University, Department of Ecology, Environment and Plant Sciences, Stockholm, Sweden, (2)CSIC and Universidad de Sevilla, Instituto de Bioquímica Vegetal y Fotosíntesis, Seville, Spain, (3)University of Rostock. Institute of Biological Sciences, Deparment of Plant Physiology, Germany
Abstract:
In the sunlit zone of the open ocean, N2-fixing cyanobacteria dominate and some form partnerships (symbioses) with a few genera of diatoms. The benefit to the host diatoms is nitrogen supply and has been shown on the cellular level, however the role of carbon and the mechanism for metabolite exchange remain unknown. We are focusing on filamentous symbionts that form heterocysts, cells specialized for the fixation of N2. Draft genomes are available for various symbionts where genome size and content are reflected in the symbiont cellular location: internal (RintHH01, 3.24 Mbp), partial integration residing between the host frustule and cell membrane (RintRC01, 5.49 Mbp), and externally attached (CalSC01, 5.97 Mbp). Equally unexplored are the numbers and types of transporters involved in metabolite exchange between the partners. Using comparative genomics, we have identified 12, 17 and 21 homologues of carbon uptake transporters and related proteins in RintHH01, RintRC01, and CalSC01, respectively. To investigate carbon assimilation in the symbionts, we have cloned a SulP-like protein from RintRC01 that is a putative bicarbonate transporter. To test the functionality of this protein, we have expressed it heterologously in wild-type Synechococcus sp. PCC 7942 and in a carbon uptake mutant of Synechocystis sp. PCC 6803, and we are performing growth tests and 14C-bicarbonate transport assays. We have also cloned an InvB (invertase) protein from RintHH01 to confirm its function in sucrose cleavage by complementation of a double invertase mutant of Anabaena sp. PCC 7120, since sucrose could be transferred from the diatom to the endosymbiont. Our goal is to understand nutrient assimilation and exchange in these relevant marine N2-fixing diatom-cyanobacteria symbioses. Using gene complementation strategies, we have found an invaluable circumvention to the currently uncultivable nature of our symbiotic system that still allows insight into the role of membrane transporters.
Back to: Microbial Interactions at the Single-Cell Level and Their Relevance in Oceanic Biogeochemical Cycles I Posters