The important role of vitamins in sustaining marine life: a new study identifies complex vitamin cycling between marine phytoplankton
Research shines a new light on how nature’s most complex metabolite, vitamin B12, is cycled in microbial communities.
In aquatic habitats, microalgae live in the sunlit ‘photic zone’, alongside other photosynthetic microbes such as the cyanobacteria. Known collectively as the phytoplankton, these microbes form the foundations of aquatic food chains. Understanding what governs the growth of these important microbes is therefore crucial to understanding how marine ecosystems function.
Just like humans, many algae require vitamins in order to grow. An example of this is vitamin B12, which over half of all algal species require. Only bacteria and archaea possess the genes necessary to synthesise this complex molecule from scratch. Since algae cannot make their own supply of this vitamin, a key question is where do they get their B12?
Cyanobacteria are extremely abundant in aquatic ecosystems, and these bacteria appear to have the genes necessary to produce B12. Could cyanobacteria be important B12 producers for algae? Katherine Helliwell, Johan Kudahl and Alison Smith, together with colleagues at the Universities of Warwick and Kent, have shown the story is not so simple. The new study, published this week in Current Biology, demonstrates that cyanobacteria actually produce a form of the vitamin called pseudocobalamin (Fig. 1), and that pseudocobalamin is not bioavailable to microalgae. This suggests the two major phytoplankton classes use different chemical currencies of B12, implicating competition between these photosynthetic microbes for the vitamin. However, in an interesting twist, certain algae appear to be able to ‘remodel’ pseudocobalamin to a functional form with precursor molecule, DMB. This species-specific attribute implicates algal remodellers as important components of the B12 cycle.
Together this work provides new insight of how B12 is cycled in microbial communities, challenging the conventional and oversimplified assumption of a linear flux of vitamin B12 from producers to consumers. Instead, a much more complex network of B12 production, uptake and cycling is likely to take place, implicating both competition and cooperation between marine microbes.
- Helliwell KE, Lawrence AD, Holzer A, Kudahl UJ, Sasso S, Kräutler B, Scanlan DJ, Warren MJ, Smith AG. (2016) Cyanobacteria and Eukaryotic Algae Use Different Chemical Variants of Vitamin B12. Current Biology. http://dx.doi.org/10.1016/j.cub.2016.02.041