One of the big constraints on photosynthetic land plants today is the sheer difficulty of fixing CO2 into organic molecules in the presence of oxygen, plus the resultant water stress.
Each of the double bonds of CO2 has an electron configuration remarkably similar to the electron configuration of the O2 double bond. Even though the RuBisCo enzyme that cracks CO2 and sticks it on an organic molecule to enlarge it is optimized to grab CO2, the ambient oxygen level is >400 times the ambient CO2 level. At that concentration difference, even a very selective enzyme is going to misincorporate oxygen a reasonable fraction of the time, producing a toxic peroxide that requires energy to square away and deal with. The faster the enzyme runs the less selective it can be. End result is that the RuBisCo of land plants today fixes perhaps one CO2 per enzyme per second, fantastically slow, so as to only misincorporate O2 something like 1/4 of the time. Slower and it would be a drag on growth, faster and it would poison the plant.
This also produces water stress on plants because of their need to keep their stomata pores open while photosynthesizing because they can't deal with internal depletion of CO2, they need lots of airflow into their tissues to bring in the very thin gas. This airflow carries away lots of water from their tissue into the air via transpiration in all but the most humid of climates, increasing their water requirements. This is why C4 and CAM photosynthesis plants have evolved. These plants via various mechanisms concentrate CO2 from the air into their photosynthetic tissues. CAM plants like pineapples leave their stomata open only at night, sucking up CO2 in the cool air and sequestering it as organic acids using very little energy, then close up their pores during the day decompose the acids back to a high local CO2 concentration and then run their RuBisCo faster. C4 plants use various mechanisms to do something similar in surface tissues during the day and then transport the acids to specialized photosynthesizing cells where the CO2 is released at high concentration and a faster RuBisCo can run. These plants appear to have originated over 100 million years ago in marginal dry environments, saving water, and become much more common something like 30-40 million years ago as atmospheric carbon levels dropped precipitously. Modern C4 plants we all know and love include corn.
Both of those described systems involve very complicated anatomical structures and regulatory mechanisms. There are people who have tried to put faster RuBisCo into crop plants by itself and it's totally pointless. Marine RuBisCo runs something like 8 times as fast because there's oodles of carbonate and bicarbonate ions in the water which marine algae can enzymatically convert to CO2 in the cell for basically no cost. Someone in a tour de force of genetic engineering recently managed to get a marine RuBisCo working in tobacco plants as a demonstration with the justification that increasing carbon fixation rates in plants was good. And their tobacco plants did indeed fix carbon much faster per unit of RuBisCo protein. But it was pointless since they died horribly if exposed to light in air due to all the peroxide damage. They had to be kept in 10x enriched CO2 growth chambers and they still grew slower than normal.
The RuBisCo of land plants is pretty much at the optimum. C4 and CAM plants have a faster one since their anatomical structures and regulatory mechanisms allow it. Anything you do to mess with carbon fixation in open air runs into hard physical limits and requires much more complicated changes than messing with a few enzymes and stomata.
Now messing with NITROGEN fixation on the other hand... just look at Kudzu for an example of what good nitrogen fixation can do for a species.
(EDIT: just a note, plants can get a lot more energy from light than they can ultimately fix into a long-lasting form, so the further above mentioned idea of increasing light capture via new pigments is not hitting the process at the bottleneck)
Marine RuBisCo runs something like 8 times as fast because there's oodles of carbonate and bicarbonate ions in the water which marine algae can enzymatically convert to CO2 in the cell for basically no cost
So if we could grow crops underwater we could get a lot more energy?
Does anyone know something about this alteration of Klebsiella planticola? Paywalled paper here. (If someone has got access please PM me, I would like to read the paper to write a more fleshed out article.)
While I am not convinced that it would really have spread to every terrestrial ecosystem, or even every wheat field and I am not even sure if it could compete successfully with the wild type, I certainly would not bet the world on that. Even if it might only have become a nasty crop bug instead of an ecosystem killer, I think this may be the closest encounter with a true existential risk we have had so far. This suggests, that even our current low end biotech may be the greatest existential risk we face at the moment. Or is this just hyped bullshit for some reason I do not see right now (without reading the paper)?
Edit: Upon reading the original paper I am quite sure Cracked.com greatly exagerated the potential threat. 10^8 cfu (colony formin units) K. planticolata per gram soil (dry weight) was added on day 0, but after 8 weeks only 10^2 cfu survived (this is true for both wild type and modified K. planticolata). This suggests, that K. planticolata in the wild has typical densities more like 10^2 cfu per g than 10^8 cfu per g. 10^2 cfu per g is nowhere near enough to produce lethal ethanol concentrations in the soil, even if the modified strain could compete in the wild. Furthermore the concentration of the modified K. planticolata decreased faster than the concentration of the wild type suggesting reduced fitness of the GMO. On the other hand after 8 weeks both K. planticolata strains arrived at the same density of 100 cfu per g indicating comparable medium term survivability in unsterilized soil (I am not sure if indigenous K. planticolata which could compete with the GMO was present in the soil sample used). Yes, they did avoid the obvious failure mode of not differentiating between wild type and modified K. planticolata during recovery of K. planticola strains from the samples.