Quantitative proteomics of a B12-dependent alga grown in coculture with bacteria reveals metabolic tradeoffs required for mutualism

Katherine E. Helliwell, Jagroop Pandhal*, Matthew B. Cooper, Joseph Longworth, Ulrich Johan Kudahl, David A. Russo, Eleanor V. Tomsett, Freddy Bunbury, Deborah L. Salmon, Nicholas Smirnoff, Phillip C. Wright, Alison G. Smith

*Corresponding author for this work

Research output: Contribution to journalArticleResearchpeer-review

23 Citations (Scopus)


The unicellular green alga Lobomonas rostrata requires an external supply of vitamin B12 (cobalamin) for growth, which it can obtain in stable laboratory cultures from the soil bacterium Mesorhizobium loti in exchange for photosynthate. We investigated changes in protein expression in the alga that allow it to engage in this mutualism. We used quantitative isobaric tagging (iTRAQ) proteomics to determine the L. rostrata proteome grown axenically with B12 supplementation or in coculture with M. loti. Data are available via ProteomeXchange (PXD005046). Using the related Chlamydomonas reinhardtii as a reference genome, 588 algal proteins could be identified. Enzymes of amino acid biosynthesis were higher in coculture than in axenic culture, and this was reflected in increased amounts of total cellular protein and several free amino acids. A number of heat shock proteins were also elevated. Conversely, photosynthetic proteins and those of chloroplast protein synthesis were significantly lower in L. rostrata cells in coculture. These observations were confirmed by measurement of electron transfer rates in cells grown under the two conditions. The results indicate that, despite the stability of the mutualism, L. rostrata experiences stress in coculture with M. loti, and must adjust its metabolism accordingly.

Original languageEnglish
Pages (from-to)599-612
Number of pages14
JournalNew Phytologist
Issue number2
Publication statusPublished - Jan 2018
Externally publishedYes


  • Lobomonas rostrata
  • Mesorhizobium loti
  • iTRAQ proteomics
  • mutualism
  • photosynthesis
  • vitamin B


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