Last week I attended a PhD student symposium. It was a very interesting day filled with student research talks and poster sessions and networking. One of the talks was given by Matt Cooper (Alison Smith’s group University of Cambridge) about algal requirements for vitamin B12 and how these can be met by symbiotic (mutually beneficial) interactions with bacteria. One of the interactions mentioned was between the green alga Lobomonas rostrata and the gram negative bacteria Mesorhizobium loti.
Many algae require vitamin B12 (cobalamin) as a co-factor for methionine synthase (METH) to produce the amino acid methionine. However, algae cannot produce vitamin B12 so they are dependent on it being available in the environment. Only bacteria can produce vitamin B12 so they are the ultimate source of it.
In a paper published in 2012 in Environmental Microbiology Kazamia and colleagues (1) reported finding bacterial contamination in a culture of Chlamydomonous nivalis (a B12-dependent alga) that enabled the it to grow without the addition of vitamin B12 to the medium. By sequencing the 16S rRNA gene they identified the bacterium as a Mesorhizobium sp. A lab-cultured strain of M. loti could also support growth of C. nivalis and another B12-dependent alga L. rostrata in media where no vitamin B12 had been added.
The group focussed on the interaction between M.loti and L. rostrata. Neither species could grow on the culture medium in the absence of the other organism. When co-cultured the populations of the organisms would reach a stable equilibrium that was maintained over several weeks, suggesting there is some regulation of the populations. Addition of either a carbon source or vitamin B12 to the culture medium disturbed the equilibrium in favour of the bacteria or algae respectively.
Not all algae can form a symbiotic interaction with M. loti. The green alga Chlamydomonous reinhardtii has both the B12-dependent isoform of methionine synthase (METH) and a B12-independent form (METE) so can utilise B12 when it is available but can also grow in its absence. Co-culturing C. reinhardtii and M. loti resulted in suppression of METE expression, suggesting that C. reinhardtii was utilising vitamin B12 produced by M. loti. However, the bacteria would only grow with C. reinhardtii when a carbon source was added to the medium so C. reinhardtii does not appear to releasing sufficient carbon to support bacterial growth.
Other bacteria species can form close associations with algae and some have been shown to be able to provide B12 to the algae in return for receiving carbon (2,3). In low nutrient environments it could be very advantageous for algae and bacteria to form symbiotic interactions to “trade” resources. Since M. loti is also a nitrogen-fixing bacterium I would be very interested to know whether it can supply ammonia to the algae as well as vitamin B12.
1. Kazamia E, Czesnick H, Nguyen TT, Croft MT, Sherwood E, Sasso S, Hodson SJ, Warren MJ, Smith AG. (2012). Mutualistic interactions between vitamin B12 -dependent algae and heterotrophic bacteria exhibit regulation. Environmental Microbiology DOI: 10.1111/j.1462-2920.2012.02733.x
2. Croft et al (2005) Algae acquire vitamin B12 through a symbiotic relationship with bacteria Nature.
3. Wagner-Döbler et al (2010) The complete genome sequence of the algal symbiont Dinoseobacter shibae: a hitchiker’s guide to life in the sea. ISME.