Re-engineering Photosynthesis

New Scientist has an article today titled "A blue-green revolution: Upgrading photosynthesis."  In it, the author describes various possible upgrades that could be made to the photosynthetic machinery in modern plants.

The main point is that the chloroplasts in modern plants can trace their ancestry back to a photosynthetic bacteria which was engulfed by another cell some 1.5 billion years ago or so.  Since that time, this original chloroplast has been passed down to all photosynthetic algae and plants.  Of course it has evolved over time, and there are differences amongst the chloroplasts in various species of plants, but the divergence between these chloroplasts is very small compared to the changes that have occurred in single cell photosynthetic bacteria.

Why wouldn't we expect to see an equal diversification in the chloroplasts in algae and plants vs. photosynthetic bacteria? "Think of the countless cyanobacteria living in the sea. If a mutation enables one cyanobacterium to photosynthesise more efficiently, it will grow and reproduce faster, and its descendants could come to dominate a population within weeks. The rampant gene-swapping among simple cells means other kinds of bacteria could acquire this trait too.  Now suppose that same mutation occurs in a chloroplast in a plant. It might not be beneficial in plants, as what is good for chloroplasts can be bad for the host cell. Even if the mutation is beneficial, the odds are the chloroplast is in a leaf that will fall to the ground and die. And even if the mutation occurs in a cell that eventually gives rise to a new plant, the much slower reproduction rate of plants means it will take many decades for the mutation to spread through a population."  In other words, the fact that the bacteria are single-celled with short-lifetimes and prolific gene swapping, translates to a faster rate of evolution when compared to multi-cellular, longer lived plants and algae.

So what are the trends we see in today's most fashionable photosynthetic bacteria?
Adjusting to developments in the atmosphere.  The modern atmosphere is composed of about 78% Nitrogen, 21% Oxygen, and only 0.039% Carbon Dioxide (although it seems like we're trying to change that balance as fast as we can...).  Of course 2 billion years ago when that original photosynthetic bacteria was engulfed and on its way to becoming the ancestor of chloroplasts, the atmosphere had almost no Oxygen, and significantly more Carbon Dioxide, Carbon Monoxide, and other gases.

Proteins like rubisco, which take the carbon from CO2 and use it to help form sugar molecules.  The ancient rubisco would take up Oxygen or Carbon Dioxide.  Since there was a negligible amount of Oxygen, this didn't matter that much.  Now however, with a lot of Oxygen present, rubisco frequently grabs an Oxygen instead of a Carbon Dioxide, which results in a destruction of food instead of creation.

Cyanobacteria on the otherhand have evolved a way to mitigate this.  These organisms create carboxysomes, small compartments that allow Carbon Dioxide but keep Oxygen out.  Rubisco in carboxysomes is therefore free to spend its time being productive.  Moreover, carboxysomes also have an enzyme called carbonic anhydrase, which will convert bicarbonate to CO2, further increasing the amount of CO2 present to form sugar.

A second optimization that is discussed is in Nitrogen fixation.  Many plants are unable to take molecular Nitrogen from the air to use as raw Nitrogen in cellular processes.  Instead they often rely on symbiotic bacteria near their roots to do this for them, or alternatively, a friendly farmer to throw down some Nitrogen containing fertilizer.  The enzyme nitrogenase is capable of fixing molecular Nitrogen for use in plants, but unfortunately, nitrogenase is destroyed by Oxygen.  If plants could be designed to produce Nitrogenase at night however, when photosynthesis is not occurring and consequently lower levels of Oxygen are present, this might allow plants to effectively produce their own fertilizer.  Or, if nitrogenase could be inserted into a compartment like carboxysome, that also might prove beneficial.