Mutualistic interaction between vitamin B-12 dependent algae and heterotrophic bacteria exhibit regulation

Environmental Microbiology (Impact Factor: 6.2). 03/2012; 14(6):1466 - 1476. DOI: 10.1111/j.1462-2920.2012.02733.x


Many algae are auxotrophs for vitamin B12 (cobalamin), which they need as a cofactor for B12-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 B12-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 B12-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 B12 or a carbon source for the bacteria perturbs the equilibrium, demonstrating that the symbiosis is mutualistic and facultative. Chlamydomonas reinhardtii does not require B12 for growth because it encodes a B12-independent methionine synthase, METE, the gene for which is suppressed by addition of exogenous B12. Co-culturing C. reinhardtii with M. loti also results in reduction of METE expression, demonstrating that the bacterium can deliver the vitamin to this B12-independent alga. We discuss the implications of this for the widespread distribution of cobalamin auxotrophy in the algal kingdom.

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    • "Experimental evidence supports complementation of lost capabilities by non-specific interaction partners: a range of reduced sulfur sources can be used by SAR11 (Tripp et al., 2008) and many bacteria can provide B 12 for auxotrophs (Croft et al., 2005). Additionally, some obligate relationships, at least in artificial laboratory conditions, do not involve regulation or signaling (Durham et al., 2015), while others are regulated (Kazamia et al., 2012), suggesting a number of "
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    • "In some cases vitamin B 12 is known to originate from an epibiontic member of the Rhizobiales such as Rhizobium , Bradyrhizobium, or Mesorhizobium. For example, it has been demonstrated that the B 12 -dependent green flagellate Lobomonas rostrata obtains B 12 from a species of Mesorhizobium, and the alga provides the bacterium with fixed carbon in this mutualistic association (Kazamia et al. 2012). Likewise, the macroalga C. glomerata is known to require vitamin B 12 (Hoffman 1990), so it is reasonable to hypothesize that this host's microbiome would include taxa and genes associated with production of this vitamin. "
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    • "Evolutionary adaptations enabling B 12 auxotrophs to be successful competitors in B 12 -deprived regions could include becoming specialized at nutrient patch exploitation—being able to migrate rapidly to new nutrient sources upon a temporal change in the nutrient landscape for instance. Or alternatively, these organisms may meet their vitamin demands though the establishment and maintenance of direct symbiotic interactions with other microbes (Croft et al., 2005; Wagner-Dö bler et al., 2010; Kazamia et al., 2012). As algae in possession of both METE and METH may use B 12 if it is available, loss of METE could be a plausible mechanism to cause sympatric populations to embark on different evolutionary trajectories, driving the evolution of symbiotic interactions and/or other specialist nutrient-acquisition strategies. "
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