2.7: Bioengineering on land

So far we’ve considered only “natural” organisms. Bioengineering on land is another possibility.

Root to shoot ratio

If engineering trees sounds completely fanciful, read this paper and listen to the second half of this podcast episode, the latter in which Gabriel Licina mentions combining the Miyawaki Method of tree planting which leads to fast growth, and genetic engineering.Dyson in his essays estimates best natural carbon eaters permanently sequester only 1/10 of the carbon they absorb but that this could be engineered to increase. Dyson proposes as a conceptually illustrative example the possibility of genetically engineering plants to have increased “root to shoot ratio”, a parameter that seems to be tunable by evolution if not by epigenetic regulation.

Dyson proposed genetically engineered trees that would sequester more carbon; he calls them “carbon eating” trees, and proposes replacing ¼ of the existing land vegetation with carbon eating tree varieties of same species, arguing that this should not impact on valuable agricultural vegetation.

An eminent team of Salk Institute scientists are working on a trick not unlike the increased root to shoot idea, by selectively breeding legumes to sequester carbon in underground cork:

“While plants store this carbon in the form of numerous biomolecules, almost all of these materials are degraded by animals, fungi and bacteria and the stored CO2 thereby released. The Salk team has identified one particular plant made molecule, called suberin, that is highly resistant to this degradation and can thereby remain in the soil. Suberin, better known to wine aficionados as cork, is a waxy, water-repellant and carbon-rich substance ‘We realized that a crop with a larger, suberin-dense root would capture more carbon in the ground… Roots last longer than other parts of plants, particularly in perennial plants that live multiple years. Even in dead roots, suberin decays very slowly.’”

So far, this seems to involve conventional selective breeding, and scientific study, not genetic engineering per se. Plant associated microorganisms are an interesting aspect here as well.

Relatedly, converting annual food crops such as wheats into perennials can both improve the plants and increase the amount of carbon they store in roots. Also, you generally get a ~50% production gain by perennialization of crops, just due to the extended growing season.

More generally, this strategy is to increase recalcitrance of biomass so that it decays slower. A common version of this is to increase the Lignin to Cellulose ratio. There’s a lively argument about whether this is a long-term path to both high quality soil and increased soil carbon. There is also a part of the community that believes that long-term soil carbon, being microbial in nature, is best increased by increasing root sugar exudates into the soil.

The idea of increasing root mass has been taken up by an ARPA-E program called ROOTS (“Rhizosphere Observations Optimizing Terrestrial Sequestration”) where it has been studied in more detail. A key potential of this approach would be to modify agricultural crops to increase their root mass, so that one does not compete for space, water or fertilizer with agriculture for one’s carbon sequestration:

“There are numerous land management practices that can be adopted to increase soil carbon storage in agricultural soils (e.g., changes in crop rotations, tillage, fertilizer management, organic amendments, etc.) which have been extensively reviewed and assessed in the scientific literature. One of the most effective means for increasing soil C sequestration is through changing land cover, such as converting annual cropland to forest or perennial grasses, which generally contribute much more plant residue to soils. However, if widely applied, such land use conversions would have negative consequences for food and fiber production from the crops that are displaced. An option that has not yet been widely explored is to modify, through targeted breeding and plant selection, crop plants to produce more roots, deeper in the soil profile where decomposition rates are slower compared to surface horizons, as an analogous strategy to increase soil C storage.”

How much carbon can this approach sequester? The ARPA-E report states,

“We found that around 87% of total US cropland (major annual crops plus hay/pasture land) had soils of sufficient depth and lacking major root-restricting soil layers to allow for crops with enhanced phenotypes… Based on this calculation, average annual (averaged over the initial 30 yr period) soil C accrual rates (assuming 100% adoption of improved phenotypes) ranged up to 280 Tg C per year (1026 Tg CO2eq) for the most optimistic scenario of a doubling of root C inputs and an extreme downward shift in root distributions. This is equivalent to an average rate of increase of almost 1.8 Mg C per hectare per year, similar to rates of soil C increase that have been observed with conversion of annual cropland to high productivity perennial grasses.”

That’s about a quarter of a gigatonne of carbon per year, or about 1 gigatonne CO2, for US farmland alone. Now, total world farm-land is about 8x higher, so optimistically this approach could get up to around 8 gigatonnes CO2 per year globally, using existing land, at very low cost in principle, and with other potential benefits like improved soil quality or decreased fertilizer or water use if the overall characteristics of the crop plants improved along those dimensions as well. Perhaps not a complete solution in itself, but could be a very powerful component.

Meanwhile, a paper on a nominal “theoretical upper limit to plant productivity" stated:

“…theoretical maximum NPP approached 200 tC ha–1 yr–1 at point locations, roughly 2 orders of magnitude higher than most current managed or natural ecosystems. Recalculating the upper envelope estimate of NPP limited by available water reduced it by half or more in 91% of the land area globally.”

So, say the theoretical upper limit of any kind of bioengineering/plant approach would be 50 tC ha–1 yr–1, call it 20 even, and then compare that to the ROOTS analysis which had an optimistic figure of “equivalent to an average rate of increase of almost 1.8 Mg C ha-1 yr-1, similar to rates of soil C increase that have been observed with conversion of annual cropland to high productivity perennial grasses”. Since a Mg C is the same as a tC, the nominal theoretical upper limit is way higher even than ROOTS is contemplating.

Photosynthetic efficiency

Plant bioengineering is also aiming to increase the efficiency of photosynthesis itself.James Webber has a nice post on how one might go about improving cyanobacterial photosynthesis.The Rubisco enzyme, which carries out the initial steps of photosynthesis, is strangely inefficient, and sometimes burns its substrate with oxygen rather than affixing carbon to it. Although some studies suggest Rubisco may be nearly perfectly optimized for its difficult job:

“…optimizing an unusual compromise: an advanced product-like transition state for CO2 addition aids discrimination between CO2 and O2, but its strong resemblance to the subsequent six-carbon carboxyketone intermediate causes that intermediate to bind so tightly that it restricts maximum catalytic throughput”

Even still, not all crop plants use the most efficient form of photosynthesis, so this leads to room for improvement, e.g., C4 rice, and one can also work on the temporal regulation of photosynthesis pathways in the plant, or increase the CO2 retention of other parts of the pathway.

Cyanobacteria concentrate CO2 in nanoscale organelles optimized for photosynthesis, called Carboxysomes, making it more available to the Rubisco enzyme complex, and people are working to put Carboxysomes into land plants!

One company that is actually pushing these ideas forward is living carbon who are creating photosynthesis-enhanced trees. They recently published a post citing a "a 53% increase in the production of above ground biomass". This was in lab conditions but it is an exciting first step towards real world bioengineering.

In general, there is certainly large head-room for innovation and novel bioengineering approaches in biological carbon capture, e.g., Y Combinator discusses cell-free photosynthetic bioreactors. People have done it in the lab to some extent, though not in a self-sustaining way. This seems to be one of the less explored routes in the carbon removal space and one that has a lot of potential.