As smart as humans: how does amoeba obtain vitamin B12?

Despite the fact that Homo sapiens is the most intelligent species living, we are not the only ones who can come up with smart diet solutions! 

As humans, we are able to get B12 by balancing our diets and supplementing our food, when some species of microalgae are able to transform B12 bioinactive analogue pseudocobalamin into cobalamin. (Helliwell et al., 2016)

It is a well-known fact that other eukaryotes need vitamins, recent studies showed that microalgae and protozoa can be very dependent on B12 vitamin also known as cobalamin (Helliwell et al., 2016).

An interesting case of getting much needed compound has been recently found among protozoa. Amoeba is a single cell eukaryote that feeds by phagocytosis. Some species can be grown solely on cyanobacteria like Synechococcus elongatus or Leptolyngbya, but an exception was found: phylogenetically distant species of amoeba LPG3 grows with the heterotrophic bacteria being always present on plates. (Ma et al., 2016)

Ma, Beld and Brahamsha, (2017) had a closer look at this relationship. Amoeba isolate was grown with Synechococcus elongates as a food source and the amoeba's bacterial co-isolate was isolated and identified as Pseudomonas species.

By making an antibiotic-resistant mutant from Pseudomonas sp., and growing it in a co-culture with amoeba and Synechococcus in the presence and absence of antibiotics, authors revealed that Pseudomonas is required to maintain the amoeba’s growth. However, its concentration in liquid culture was not changing significantly during the course of experiment, unlike Synechococcus, which concentrations were decreasing. Conclusion drawn was that cyanobacteria remains as a main food source for the amoeba.

To further investigate why amoeba needs another bacterium to survive, other heterotrophic species were then tested (Pseudomonas fluorescens, Vibrio cholera). Both of them were able to support amoeba’s growth.

The transposon library was then made for V.cholera and mutants that could not support protozoan growth were identified. After the analysis of disrupted genes, Pseudomonas co-isolate mutant with the deletion of corrinoid dependent pathway was made and tested. It could not support amoebas growth and therefore it was concluded that it was cobalamin that amoeba requires from its coisolate.

Result fitted in the picture very well: cobalamin is produced by Pseudomonas species and pseudocobalamin is produced by Synechococcus. Considering the previous research on the eukaryotic preference for cobalamin, results make a very good sense and explain why this particular species of amoeba could not grow solely on cyanobacteria. By the combination of growth experiments and genomics, it was revealed that amoeba can balance they diet by supplementing their meal with cobalamin producing bacteria.

In my opinion, this is a very neat and elegant study that shows a great combination of biological assays, use of mutants and transposon library constraction that was of a particular interest to me. Even though it is quite time-consuming method, it clearly gave neat results, identifying the phenotype based on the genomics.

Methods flow in a very nice way, showing step by step procedures and good experiment design.

For further research, it would be interesting to look at this case from the bacterial perspective and see what exactly bacteria gains from this relationship. That might be achieved by the whole genome sequencing and looking at the metabolic pathways involved in the presence and absence of the amoeba. Another interesting route is to closely look at what is happening with other amoeba species  (Ma et al., 2016) that were able to grow solely on bacteria that cannot produce cobalamin. Possibly, those are the species that do not require cobalamin or have other ways of getting it.

Paper reviewed:

Ma, A., Beld, J. and Brahamsha, B. (2017). An Amoebal Grazer of Cyanobacteria Requires Cobalamin Produced by Heterotrophic Bacteria. Applied and Environmental Microbiology, 83(10), pp.e00035-17.

Further reading:

Ma, A., Daniels, E., Gulizia, N. and Brahamsha, B. (2016). Isolation of diverse amoebal grazers of freshwater cyanobacteria for the development of model systems to study predator-prey interactions. Algal Research, 13, pp.85-93.

Helliwell, K., Lawrence, A., Holzer, A., Kudahl, U., Sasso, S., Kräutler, B., Scanlan, D., Warren, M. and Smith, A. (2016). Cyanobacteria and Eukaryotic Algae Use Different Chemical Variants of Vitamin B12. Current Biology, 26(8), pp.999-1008.