Symbiotic CO2 Sequestration

Bioengineered microbial community working together to store carbon

Photosynthesis is a valuable natural system for sequestering carbon
dioxide. However, simply forming biomass does not fully exploit this
system. A Chinese team of researchers, whose study is published in the
journal Angewandte Chemie, has now genetically engineered a
microbial community which could serve as a living carbon sink. In this
community, carbon dioxide is first converted into sugar by
photosynthesis, then the sugar is converted into useful chemicals.

Symbiotic CO<sub>2</sub> Sequestration - Bioengineered microbial community working together to store carbon

© Wiley-VCH, re-use with credit to ‘Angewandte Chemie’ and a link to the original article.

Various bacterial strains are used in biotechnology to produce
specific chemicals. For example, some genetically modified strains
produce lactic acid, which in turn is used to produce the biodegradable
plastic, polylactic acid (PLA). Other strains are used to enrich
precursors for biofuels or pharmaceuticals. However, because the
bacteria require energy and nutrients, bacterial production of chemicals
is often inefficient.

In contrast, phototrophic organisms naturally produce sugar from
carbon dioxide, water, and sunlight. In a symbiotic community,
therefore, chemical-producing bacteria could theoretically use this
sugar as food, thus making them a potential carbon sink and
simultaneously producing useful chemicals. However, many
photoautotrophic organisms produce sucrose as their stored sugar, the
exact sugar which bioengineered bacteria struggle to consume and
utilize.

With this in mind, the research group of Jun Ni at Shanghai Jiao Tong
University in Shanghai (China) carried out a systematic search for
candidate bacterial strains that could be bioengineered but which could
also grow naturally on sucrose. They found what they were looking for in
a marine bacterium known as Vibrio natriegens: “Luckily, V. natriegens naturally harbors the complete sucrose transport and metabolism pathway,” reveal the authors. In addition, V. natriegens
can be genetically manipulated and tolerates salt stress. This is
important because salt stimulates photosynthetic cyanobacteria to
produce sucrose, thereby creating mutually reinforcing processes.

The research team then used this knowledge to produce an integrated modular system for CO2 sequestration from V. natriegens and the known cyanobacterium Synechococcus elongatus. They improved sugar production in the cyanobacteria using genetic engineering, as well as adding genes to V. natriegens,
which increased sugar uptake and conversion into chemicals. In an
unexpectedly efficient process, the team observed that the cyanobacteria
may package up the nutrients in vesicles which were then excreted. The
marine bacteria were then readily able to ingest these vesicles.

The team produced four variants of V. natriegens in order to
produce either lactic acid, butanediol for biofuel synthesis, or
coumarin and melanin as precursors for chemicals and pharmaceuticals.
The bacteria, in symbiosis with the cyanobacteria, produced the
chemicals with a negative carbon balance. “This system could absorb more
than 20 tons of carbon dioxide per ton of product,” the team report.
The authors consider their results to be proof that symbiotic microbial
communities can be used as effective carbon sinks.

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