The cost of sharing a B12 currency in a freshwater microalga

Recent studies have demonstrated an essential role of vitamins in microalgal growth: for instance, 50% of studied species requires vitamin B₁₂ for survival in culture (Helliwell, 2017). Vitamin auxotrophy (“auxotroph” being an organism that needs nutritional supplements, in the form of organic compounds that it cannot synthesise) calls into question the traditional view of members of the phytoplankton as self-sufficient autotrophs, being able to synthesise organic compounds purely from inorganic sources. Despite several ex situ experiments having confirmed the growth benefits of vitamin supplements to auxotroph algal cultures, the environmental vitamin sources remain unclear. As for vitamin B₁₂ (cobalamin), the only known natural producers are certain bacteria and archaea; therefore, the non-eukaryotic component of the plankton is a primary suspect (please refer to last week’s lecture). This has been confirmed in laboratory experiments performed on an “artificial” co-culture of the freshwater alga Lobomonas rostrata and the soil bacterium Mesorhizobium loti, with the former providing fixed carbon “in exchange” for cobalamin, produced by the latter (Kazamia et al., 2012). The next challenge will be to gain a mechanistic understanding of this “currency exchange” process at the molecular level: a very recent study aimed to do this through proteomics, and the results have been published very recently (Helliwell et al., 2017). Briefly, L. rostrata cultures were grown autotrophically either supplemented with B₁₂ or in co-culture with B₁₂-producing M. loti. By using a quantitative bottom-up proteomics approach, putative proteins were identified from database searches of the LC-MS spectra and the degree of differential expression was determined for all proteins. Several proteins involved in carbon metabolism, fatty acid synthesis and photosynthesis were significantly downregulated in co-cultured algae, whereas proteins involved in amino acid metabolism and, interestingly, a few putative heat shock proteins, were upregulated. Q-PCR analysis on a subset of transcripts coding for proteins involved in the Calvin cycle and measurement of photosynthetic rate through PAM (Pulse Amplified Modulated) fluorometry corroborated the results of the proteomic analysis: there was a depression in the photosynthetic capacity of L. rostrata grown in co-cultures at the transcriptional, proteomic and physiological levels. Similarly, the upregulation of proteins involved in amino acid metabolism was corroborated by a higher amino acid content in co-cultured microalgae.
Despite the mutualistic nature of the B₁₂-photosynthase “exchange”, additional nutrients might place the algae and the bacteria in competition for resources, as noted by the authors: thus, a nutritional balance involving physiological trade-offs could be achieved by diverting energy away from photosynthesis, as seems to be the case in L. rostrata. In this respect, it would be interesting to systematically add micronutrients to the cultures in follow-up experiments, to test whether the recorded depression in photosynthetic output was indeed caused by competition for nutrients, and if so, by which ones.
The study represents a significant leap forward in our understanding of the molecular mechanisms underpinning vitamin B₁₂ acquisition and use by a freshwater microalga and its symbiotic relationship with a B₁₂-producing bacterium. Future studies should aim to use ecologically realistic models, in order to gain an understanding of vitamin exchange dynamics in the plankton. This will become increasingly possible in the future, as more and more algal genomes are being sequenced, making transcriptomic and proteomic analyses easier in less characterised algal species.

Reviewed article:
Helliwell, K. E., Pandhal, J., Cooper, M. B., Longworth, J., Kudahl, U. J., Russo, D. A., Tomsett, E. V., Bunbury, F., Salmon, D. L., Smirnoff, N., Wright, P. C. & Smith, A. G. (2017) 'Quantitative proteomics of a B₁₂-dependent alga grown in coculture with bacteria reveals metabolic tradeoffs required for mutualism'. The New phytologist (early view).

References
Helliwell, K. E. (2017) 'The roles of B vitamins in phytoplankton nutrition: new perspectives and prospects'. New Phytologist, 216 (1), pp. 62-68.

Kazamia, E., Czesnick, H., Thi, T. V. N., Croft, M. T., Sherwood, E., Sasso, S., Hodson, S. J., Warren, M. J. & Smith, A. G. (2012) 'Mutualistic interactions between vitamin B₁₂-dependent algae and heterotrophic bacteria exhibit regulation'. Environmental Microbiology, 14 (6), pp. 1466-1476.