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

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

doi:10.1111/nph.14832

Katherine E Helliwell, David A Russo + Show 10 more

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https://doi.org/10.1111/nph.14832

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Abstract

Summary 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.

Highlights

Microalgae are a polyphyletic set of photosynthetic eukaryotes found across the eukaryotic tree of life which are estimated to be responsible for c. 50% of global CO2 fixation (Falkowski, 1998)
It was important to consider the possibility that shared peptides between the two organisms might influence the results, namely that peptides from M. loti might be assumed to be from L. rostrata, and interfere with protein quantifications
The result was that 0.31% of tryptic peptides within this size range were shared between C. reinhardtii and M. loti

Summary

Introduction

Microalgae are a polyphyletic set of photosynthetic eukaryotes found across the eukaryotic tree of life which are estimated to be responsible for c. 50% of global CO2 fixation (Falkowski, 1998). 50% of global CO2 fixation (Falkowski, 1998) In both soil and aquatic (marine and freshwater) habitats, algae exist alongside a wide spectrum of other microbes, including bacteria, archaea, fungi and cyanobacteria. This has led to millions of years of coevolution between contemporaneous species. Underpinning many mutualistic interactions is nutrient exchange (Cooper & Smith, 2015), where algal photosynthate is exchanged for micronutrients provided by bacteria, for example by facilitation of iron uptake (Amin et al, 2009), or provision of the vitamins thiamine (vitamin B1) (Paerl et al, 2015) or cobalamin (vitamin B12) (Croft et al, 2005; Wagner-D€obler et al, 2010; Kazamia et al, 2012; Durham et al, 2015). Analysis of environmental samples has detected the co-occurrence of bacterial producers and algal requirers of cobalamin in a variety of marine environments (Koch et al, 2013; Bertrand et al, 2015), and fertilization experiments have demonstrated that these compounds are limiting for algal productivity (Koch et al, 2012), providing evidence that algal– bacterial interactions are likely to be widespread and of considerable significance for global net primary production

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