Engineering Solar Bacteria

Wouldn't life be so much easier if we didn't have to regularly eat food, absorbing the energy we need to live from sunlight instead? If we were genetically engineered to be phototrophs — able to use light as an energy source like plants and algae — this would be a normal part of life. Skeptical that this could ever happen outside of a science fiction novel? Barring any tricky ethical issues, it's probably closer than you might at first think.

As mentioned, plants and algae (photosynthetic protists) are the most well-known practitioners of phototrophy, but many species of marine microbes also use sunny conditions to their ecological advantage. However, these single-celled organisms aren't photosynthesising — using light to turn carbon dioxide into useful organic molecules — like their eukaryotic distant cousins but instead using a special protein called proteorhodopsin to generate ATP, the energy currency of the cell, in the presence of sunlight.

The basic mechanism for proteorhodopsin is well known and mimics what mitochondria do to generate ATP in our own cells: When photons of light hit the protein, which is embedded in the cell membrane, it pumps hydrogen ions from inside the cell to outside, generating ATP from a special protein complex as the positively charged atoms rush back in again to equalize the concentration. The microbes then use the ATP to power all their chemical reactions and cellular processes, in the same way we use the ATP generated from the glucose we consume in our food. Imagine if we had proteorhodopsin in our skin cells: We could simply sit outside to meet our energy needs! Recent research has brought this "solar human" reality closer than ever.

In 2007, UK researchers at MIT successfully genetically engineered E. coli bacteria to produce proteorhodopsin, as well as five crucial photopigment synthesis enzymes, via genes taken from marine bacteria known as Alphaproteobacteria, a major component of the ocean's phytoplankton.¹ E. coli are one of the workhorses of the genetic engineering community, having being made to produce everything from insulin for diabetics to vaccines, but in the end they are chemoorganotrophs like us — meaning they generate ATP from organic compounds they find in the environment. However, once the researchers' E. coli had accepted the recombinant DNA containing proteorhodopsin and the accessory enzymes, they were found to produce ATP in the presence of light, effectively changing their nutritional classification from chemoorganotrophs to phototrophs!

But this is where an important distinction between photosynthetic plants and phototrophic bacteria comes in. Plants are autotrophic and build their own organic compounds from basic nutrients in the soil and carbon dioxide in the air: They completely synthesise all of their own amino acids, nucleotides and other important biomolecules — like the ultimate subsistence farmers.

On the other hand, marine phototrophic bacteria, while they use light as a source of ATP, are heterotrophic and need to consume biomolecules from their environment, usually from their dead companions. Photoheterotrophs, the proper classification of the engineered E. coli, still need to eat to sustain their biomass: While they might not be getting energy from their food, they still need a source of molecular building blocks. If you were engineered to express proteorhodopsin in your skin cells, you would still need to eat food in order to grow and/or repair damaged tissue.

The machinery responsible for proper photosynthesis is immensely more complicated than the marine bacterial system and poses a significant, but not insurmountable, biotechnological challenge, which sadly means that we won't be living off of only sunlight and basic nutrients any time soon.

However, the MIT E. coli research has at the very least given us a taste of what is possible in the vast, uncharted waters of genetic engineering, especially when applied to trophisms. Exciting years in this exponentially expanding field of research lie ahead.

--Jack Scanlan

Image Credit: Wikimedia

References:

1. Martinez et al. Proteorhodopsin photosystem gene expression enables photophosphorylation in a heterologous host. Proceedings of the National Academy of Sciences USA 104, 5590 (2007).