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LSSS 2017-2018

2017LSSS2018

Life Sciences Seminar Series

 

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Alison G. Smith

University of Cambridge

Vitamins as drivers of evolution of algal-bacterial mutualisms

Talk abstract

Vitamins are organic micronutrients that are required by organisms because they are the precursors to enzyme cofactors. Many eukaryotic algae, despite their photosynthetic lifestyle, require an exogenous supply of certain vitamins to allow growth, so in that respect they are like animals. More than half of microalgal species surveyed require cobalamin (vitamin B12), over 20% require thiamine (vitamin B1), and ~5% require biotin (vitamin B7). There is no phylogenetic relationship between those that require the vitamin and those that do not, suggesting that this has evolved multiple times throughout the algal lineages. Levels of these compounds free in solution in the aquatic environment are frequently very low or undetectable, and there is evidence that at least in some cases, algae obtain the vitamins they need from bacteria in the environment; this is particularly relevant for B12, since this is not made by eukaryotes. Moreover, stable co-cultures of algae and bacteria have been established in the laboratory, where algae receive the vitamins they need directly from bacteria in exchange for some form of fixed carbon. The question arises therefore can vitamin exchange be the means to initiate mutualism? Using genome sequence information we have established the underlying genetic basis for cobalamin auxotrophy in algae, and tested the hypothesis with an experimental evolution approach that resulted in a mutant strain of the B12-independent alga Chlamydomonas reinhardtii, which now needs the vitamin; it now forms a stable mutualism with a B12-producing bacterium. At the same time we have screened the genome sequences of over 8000 bacteria to identify those that might be predisposed to forming mutualisms with algae.

Selected Publications

Singlet oxygen initiates a plastid signal controlling photosynthetic gene expression.Page MT, McCormac AC, Smith AG, Terry MJ
New Phytol 2017 Feb; 213(3):1168-1180

Abstract

Retrograde signals from the plastid regulate photosynthesis-associated nuclear genes and are essential to successful chloroplast biogenesis. One model is that a positive haem-related signal promotes photosynthetic gene expression in a pathway that is abolished by the herbicide norflurazon. Far-red light (FR) pretreatment and transfer to white light also results in plastid damage and loss of photosynthetic gene expression. Here, we investigated whether norflurazon and FR pretreatment affect the same retrograde signal. We used transcriptome analysis and real-time reverse transcription-polymerase chain reaction (RT-PCR) to analyse the effects of these treatments on nuclear gene expression in various Arabidopsis (Arabidopsis thaliana) retrograde signalling mutants. Results showed that the two treatments inhibited largely different nuclear gene sets, suggesting that they affected different retrograde signals. Moreover, FR pretreatment resulted in singlet oxygen ((1) O2 ) production and a rapid inhibition of photosynthetic gene expression. This inhibition was partially blocked in the executer1executer2 mutant, which is impaired in (1) O2 signalling. Our data support a new model in which a (1) O2 retrograde signal, generated by chlorophyll precursors, inhibits expression of key photosynthetic and chlorophyll synthesis genes to prevent photo-oxidative damage during de-etiolation. Such a signal would provide a counterbalance to the positive haem-related signal to fine tune regulation of chloroplast biogenesis.

Cyanobacteria and Eukaryotic Algae Use Different Chemical Variants of Vitamin B12.Helliwell KE, Lawrence AD, Holzer A, Kudahl UJ, Sasso S, Kräutler B, Scanlan DJ, Warren MJ, Smith AG
Curr Biol 2016 Apr 25; 26(8):999-1008

Abstract

Eukaryotic microalgae and prokaryotic cyanobacteria are the major components of the phytoplankton. Determining factors that govern growth of these primary producers, and how they interact, is therefore essential to understanding aquatic ecosystem productivity. Over half of microalgal species representing marine and freshwater habitats require for growth the corrinoid cofactor B12, which is synthesized de novo only by certain prokaryotes, including the majority of cyanobacteria. There are several chemical variants of B12, which are not necessarily functionally interchangeable. Cobalamin, the form bioavailable to humans, has as its lower axial ligand 5,6-dimethylbenzimidazole (DMB). Here, we show that the abundant marine cyanobacterium Synechococcus synthesizes only pseudocobalamin, in which the lower axial ligand is adenine. Moreover, bioinformatic searches of over 100 sequenced cyanobacterial genomes for B12 biosynthesis genes, including those involved in nucleotide loop assembly, suggest this is the form synthesized by cyanobacteria more broadly. We further demonstrate that pseudocobalamin is several orders of magnitude less bioavailable than cobalamin to several B12-dependent microalgae representing diverse lineages. This indicates that the two major phytoplankton groups use a different B12 currency. However, in an intriguing twist, some microalgal species can use pseudocobalamin if DMB is provided, suggesting that they are able to remodel the cofactor, whereas Synechococcus cannot. This species-specific attribute implicates algal remodelers as novel and keystone players of the B12 cycle, transforming our perception of the dynamics and complexity of the flux of this nutrient in aquatic ecosystems.

Establishing Chlamydomonas reinhardtii as an industrial biotechnology host.Scaife MA, Nguyen GT, Rico J, Lambert D, Helliwell KE, Smith AG
Plant J 2015 May; 82(3):532-46

Abstract

Microalgae constitute a diverse group of eukaryotic unicellular organisms that are of interest for pure and applied research. Owing to their natural synthesis of value-added natural products microalgae are emerging as a source of sustainable chemical compounds, proteins and metabolites, including but not limited to those that could replace compounds currently made from fossil fuels. For the model microalga, Chlamydomonas reinhardtii, this has prompted a period of rapid development so that this organism is poised for exploitation as an industrial biotechnology platform. The question now is how best to achieve this? Highly advanced industrial biotechnology systems using bacteria and yeasts were established in a classical metabolic engineering manner over several decades. However, the advent of advanced molecular tools and the rise of synthetic biology provide an opportunity to expedite the development of C. reinhardtii as an industrial biotechnology platform, avoiding the process of incremental improvement. In this review we describe the current status of genetic manipulation of C. reinhardtii for metabolic engineering. We then introduce several concepts that underpin synthetic biology, and show how generic parts are identified and used in a standard manner to achieve predictable outputs. Based on this we suggest that the development of C. reinhardtii as an industrial biotechnology platform can be achieved more efficiently through adoption of a synthetic biology approach.

Green genes: bioinformatics and systems-biology innovations drive algal biotechnology.Reijnders MJ, van Heck RG, Lam CM, Scaife MA, dos Santos VA, Smith AG, Schaap PJ
Trends Biotechnol 2014 Dec; 32(12):617-26

Abstract

Many species of microalgae produce hydrocarbons, polysaccharides, and other valuable products in significant amounts. However, large-scale production of algal products is not yet competitive against non-renewable alternatives from fossil fuel. Metabolic engineering approaches will help to improve productivity, but the exact metabolic pathways and the identities of the majority of the genes involved remain unknown. Recent advances in bioinformatics and systems-biology modeling coupled with increasing numbers of algal genome-sequencing projects are providing the means to address this. A multidisciplinary integration of methods will provide synergy for a systems-level understanding of microalgae, and thereby accelerate the improvement of industrially valuable strains. In this review we highlight recent advances and challenges to microalgal research and discuss future potential.

Triacylglyceride production and autophagous responses in Chlamydomonas reinhardtii depend on resource allocation and carbon source.Davey MP, Horst I, Duong GH, Tomsett EV, Litvinenko AC, Howe CJ, Smith AG
Eukaryot Cell 2014 Mar; 13(3):392-400

Abstract

To improve the economic viability of microalgal biodiesel, it will be essential to optimize the productivity of fuel molecules such as triacylglyceride (TAG) within the microalgal cell. To understand some of the triggers required for the metabolic switch to TAG production, we studied the effect of the carbon supply (acetate or CO₂) in Chlamydomonas reinhardtii (wild type and the starchless sta6 mutant) grown under low N availability. As expected, initial rates of TAG production were much higher when acetate was present than under strictly photosynthetic conditions, particularly for the sta6 mutant, which cannot allocate resources to starch. However, in both strains, TAG production plateaued after a few days in mixotrophic cultures, whereas under autotrophic conditions, TAG levels continued to rise. Moreover, the reduced growth of the sta6 mutant meant that the greatest productivity (measured as mg TAG liter⁻¹ day⁻¹) was found in the wild type growing autotrophically. Wild-type cells responded to low N by autophagy, as shown by degradation of polar (membrane) lipids and loss of photosynthetic pigments, and this was less in cells supplied with acetate. In contrast, little or no autophagy was observed in sta6 mutant cells, regardless of the carbon supply. Instead, very high levels of free fatty acids were observed in the sta6 mutant, suggesting considerable alteration in metabolism. These measurements show the importance of carbon supply and strain selection for lipid productivity. Our findings will be of use for industrial cultivation, where it will be preferable to use fast-growing wild-type strains supplied with gaseous CO₂ under autotrophic conditions rather than require an exogenous supply of organic carbon.