Discovery of a SAR 11 growth requirement for thiamin ’ s pyrimidine precursor and its distribution in the Sargasso Sea Running title : SAR 11 requires HMP

1 Vitamin traffic, the production of organic growth factors by some microbial 2 community members and their use by other taxa, is being scrutinized as a potential 3 explanation for variation and highly connected behavior observed in ocean plankton by 4 community network analysis. Thiamin (vitamin B1), a cofactor in many essential 5 biochemical reactions that modify carbon-carbon bonds of organic compounds, is 6 distributed in complex patterns at sub-picomolar concentrations in the marine surface 7 layer (0-300 m). Sequenced genomes from organisms belonging to the abundant and 8 ubiquitous SAR11 clade of marine chemoheterotrophic bacteria contain genes coding for 9 a complete thiamin biosynthetic pathway, except for thiC, encoding the 4-amino-510 hydroxymethyl-2-methylpyrimidine (HMP) synthase, which is required for de novo 11 synthesis of thiamin’s pyrimidine moiety. Here we demonstrate that the SAR11 isolate 12 ‘Candidatus Pelagibacter ubique’, strain HTCC1062, is auxotrophic for the thiamin 13 precursor HMP, and cannot use exogenous thiamin for growth. In culture, strain 14 HTCC1062 required 0.7 zeptomoles cell (ca. 400 HMP molecules cell). Measurements 15 of dissolved HMP in the Sargasso Sea surface layer, showed HMP ranged from 16 undetectable (detection limit: 2.4 pM) to 35.7 pM, with maximum concentrations 17 coincident with the deep chlorophyll maximum. In culture, some marine cyanobacteria, 18 microalgae and bacteria exuded HMP, and in the Western Sargasso Sea, HMP profiles 19 changed between the morning and evening, suggesting a dynamic biological flux from 20 producers to consumers. 21