title: Mutualistic interactions between vitamin B12 -dependent algae and heterotrophic bacteria exhibit regulation.
abstract: Many algae are auxotrophs for vitamin B(12)  (cobalamin), which they need as a cofactor for B(12) -dependent methionine synthase (METH). Because only prokaryotes can synthesize the cobalamin, they must be the ultimate source of the vitamin. In the laboratory, a direct interaction between algae and heterotrophic bacteria has been shown, with bacteria supplying cobalamin in exchange for fixed carbon. Here we establish a system to study this interaction at the molecular level. In a culture of a B(12) -dependent green alga Chlamydomonas nivalis, we found a contaminating bacterium, identified by 16S rRNA analysis as Mesorhizobium sp. Using the sequenced strain of M. loti (MAFF303099), we found that it was able to support the growth of B(12) -dependent Lobomonas rostrata, another green alga, in return for fixed carbon. The two organisms form a stable equilibrium in terms of population numbers, which is maintained over many generations in semi-continuous culture, indicating a degree of regulation. However, addition of either vitamin B(12)  or a carbon source for the bacteria perturbs the equilibrium, demonstrating that the symbiosis is mutualistic and facultative. Chlamydomonas reinhardtii does not require B(12)  for growth because it encodes a B(12) -independent methionine synthase, METE, the gene for which is suppressed by addition of exogenous B(12) . Co-culturing C. reinhardtii with M. loti also results in reduction of METE expression, demonstrating that the bacterium can deliver the vitamin to this B(12) -independent alga. We discuss the implications of this for the widespread distribution of cobalamin auxotrophy in the algal kingdom.
authors.affiliation: Department of Plant Sciences, University of Cambridge, Cambridge, UK.
authors.forename: Elena
authors.initials: E
authors.lastname: Kazamia
authors.forename: Hjördis
authors.initials: H
authors.lastname: Czesnick
authors.forename: Thi Thanh Van
authors.initials: TT
authors.lastname: Nguyen
authors.forename: Martin Tom
authors.initials: MT
authors.lastname: Croft
authors.forename: Emma
authors.initials: E
authors.lastname: Sherwood
authors.forename: Severin
authors.initials: S
authors.lastname: Sasso
authors.forename: Sarah James
authors.initials: SJ
authors.lastname: Hodson
authors.forename: Martin James
authors.initials: MJ
authors.lastname: Warren
authors.forename: Alison Gail
authors.initials: AG
authors.lastname: Smith
mesh_terms: D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase / Q000378:metabolism; D001419:Bacteria / Q000235:genetics / Q000378:metabolism*; D000460:Chlorophyta / Q000502:physiology*; D052836:Heterotrophic Processes; D012988:Soil Microbiology; D013559:Symbiosis / Q000502:physiology*; D014805:Vitamin B 12 / Q000378:metabolism*
chemical_list: D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase; D014805:Vitamin B 12