Filed under: Earth history, Geoengineering, Global change, Interventions in the carbon/climate crisis, Plant physiology
Here’s the video of a conversation I had with Jim at the Nature offices a few weeks ago. Also available in its full glory at the Nature site. And indeed apparently at The Guardian too. I’d post excerpts from a transcript if I had the time, or a transcript…
Cheryl made me aware of the excellent idea of a synchronised posting about women in technology in honour of Ada Lovelace (my image of whom, for what its worth, was set irevocably and doubtless unreliabley by Bruce Sterling and Bill Gibson in The Difference Engine). I said I’d join in, and my subject is Constance Hartt, about whom I know very little, but whose work is of fundamental importance to people trying to understand the evolution of photosynthesis over the past 30 million years or so, and also to opening up the possibility of radical improvements to various crops.
Hartt was a laboratory researcher at the Hawaiian Sugar Planters Association Experiment Station, and her assiduous work on the biochemistry of sugar cane in the 1930s and 1940s convinced her that, for that plant at least, the primary product of photosynthesis is malate, a four carbon sugar. Later carbon-14 studies showed that she was right — and led to an interesting conundrum. Why did some plants — most plants, indeed, and almost all algae — make a three carbon sugar, phophoglycerate, while sugar cane and, it later became clear, various other grasses made a four-carbon sugar?
The answer lies in the process of photorespiration. The enzyme which fixes carbon into phosophglycerate, rubisco, is very ancient and rather easily confused — left to itself it will sometimes grab oxygen molecules rather than carbon dioxide molecules, and instead of making phosphoglycerate makes phosphoglycolate. This is no good to man nor beast nor, most tellingly, plant: recycling the phosphoglycolate made accidentally in this process of photorespiration into a form of carbon that can be used for further photosynthesis takes energy, and thus making less phosphoglycolate in the first place is a good thing. The malate-initiated photosynthesis that Hartt was instrumental in discovering is an evolutionary response to that problem: malate is part of a clever biochemical/physiological supercharger that concentrates a great deal more carbon dioxide into the cells where rubisco is doing its thing, thus making it less likely to commit that costly error with the oxygen.This supercharging system is known as C4 photosynthesis, the 4 denoting the number of carbons in malate; the regular sort of photsynthesis is called C3 in contrast.
C4 photsynthesis confers various advantages: in particular, it makes plants more efficient in their use of water. The mechanisms that concentrate carbon dioxide mean that the pores through which it is taken up, the plant’s stomata, don’t have to be as wide open as they would be otherwise, and thus less water is lost. C4 plants resist various sorts of stress better, including direct sunlight and salty ground. The mechanism has evolved independently many, many times over the past 30 million years or so, mostly but not entirely in the grasses, which either have a propensity for the sorts of physiological re-design that is required or are particularly prone to finding themselves in the sort of niches where this approach helps, or both. Sugar cane is not the only domesticated or agriculturally relevant example — there’s also maize and sorghum, and for energy crops switch grass and miscanthus, among others. There is now considerable interest in building the pathway into some grasses that have not learned it naturally — most importantly rice. C4 rice, with higher water use efficiency and other extra hardiness, might have considerably higher yields than traditional varieties while needing less water (my colleague Emma wrote a little about this not so long ago, though her words are behind the Nature paywall).
This knowledge and potential all flows from the work of Hartt and her colleagues in Hawai’i. It was small scale stuff, and more or less by defintition the team was isolated form the mainstream. Their work was for some time almost forgotten, and may still not be as well remembered as it should be; the elucidation of the C4 pathways took place in Australia a decade or so later, and that work tended, afterwards, to eclipse the discovery work done in Hawai’i. The secondary sources that I have say little about Hartt, other than noting the devoted careful work she invested in the subject, and giving the impression that the team she worked in, led by a sweet sounding Quaker called Hugo Kortschak, was a friendly and happy one.
Do I think she is a great unsung scientist? Well unsung, yes. Great, probably not. But whenever one looks into the history of science — or indeed into the way it goes today — one sees that you do not need to be great to matter, to discover, to move the story on, or to fulfill yourself through it. She and her colleagues, tucked away far from the mainstream, trying to do some good, discovered something of profound importance for science, and perhaps, in time, for technology and humanity. What more is needed?
Update: Gary has some wise words on the subtleties of C3 and C4. His point that C4 plants tend to be protein poor is a good one (though in a higher CO2 world that might even out a bit, as the rubisco content in C3 plants will drop whereas in C4 you’d expect it to stay the same, ceteris paribus) and reminds me of Arnold Bloom‘s idea that photorespiration might help with nitrate assimilation. His bigger point is that ceteris paribus is a poor way to see the world, and that to concentrate on any single factor, such as C3 v C4, is to overlook a great deal that you should probably be paying attention to. And that’s true.
Image from Flickr user _Wiedz, used under a creative commons licence
Filed under: Farming, Geoengineering, Interventions in the carbon/climate crisis, Plant physiology, Published stuff
I wrote a little piece for Nature today today about a paper by Andy Ridgwell at Bristol and some of his colleagues on changing the albedo of crops. The gist as published:
Manipulating the waxiness of crops through traditional breeding techniques or genetic modification should raise their albedo by about 20%, from 0.2 to 0.24. On the basis of climate modelling they calculate that the planet would cool by a modest 0.11 ºC. “It’s very small on the global average,” says Ridgwell. But “what is more important is the summertime effect in specific regions”. The mid-latitudes of North America and Eurasia could cool by as much as 1 °C in June, July and August, according to the models. Ridgwell and his colleagues report their results in Current Biology.
The models also show pronounced cooling in the North Atlantic Ocean and the Barents Sea in the wintertime — which might have a positive effect on sea ice — but a drying out of the soil in some parts of the subtropics. Ridgwell points out that climate models do not predict future precipitation well on a regional basis and treats the latter results more as evidence that there might be effects far from the fields being changed than as a clear indication that there would be damaging consequences.
There are some interesting details and implications to this “bio-geoengineering” scheme. Though you might think that reflecting more light off the surfaces of leaves means less photosynthesis, according to the paper the evidence in the literature suggests not. This may be because more reflective leaves stay cooler and more efficient; another possibility is that the light is reflected mostly from leaves in direct sunlight (which are not constrained by a lack of light) and some of what is reflected ends up with leaves that are in shadow (which are constrained by lack of light). More detailed studies, of course, may show that in fact photosynthesis does go down.
Making the plants more reflective, if it proved a good idea at all, might well necessitate genetic engineering, which in some places is distrusted. That engineering might be more acceptable in energy crops than it is in food crops. It might make sense, if people are going to engineer energy crops for other purposes, to make them a little lighter too, all other things being equal.
Another point is that this is very small beer as geoengineering goes. A similar but more dramatic proposal along similar lines by Robert Hamwey (pdf) has a radiative forcing of about 0.6 Wm-2, which is smallish by the standards of the CO2 forcing; I would guess if they expressed it in the same way the forcing in the Ridgwell et al scheme would be a good bit less than that. But it might still have some marginal utility. This is a trend I suspect we will be seeing more and more of in geoengineering studies over the next few years, a shift away from totalising projects such as sunshades for the whole earth and layers of aersosol all through the stratosphere towards smaller regional and semi regional ideas.
Talking about this trend Tim Lenton has suggested that we may be moving towards a discussion of geoengineering that has some similarities to Socolow’s “wedge” approach to decarbonization: breaking the big problem down into smaller lumps that feasible technologies could bite off and chew; as I report in the Nature piece, Tim and some colleagues are looking at setting up a unit to compare geoengineering schemes and their potential payoffs on this basis. I’m not sure this is necessarily a good development. Every geoengineering scheme has strange knock-ons and side effects around the edges, and it seems reasonable to suspect that the more such schemes you have, the more chance there is for one of the side effects to be unexpectedly serious — or for two of them to interact with each other catastrophically. But that said, the fact that it is probably a lot easier to find little forcings than big ones suggests that the portfolio approach may be in the ascendant for a while.
Filed under: Plant physiology
Over at News@nature, Mike Hopkin reports from the Ecological Society of America’s meeting in San Jose on research into tropical forest growth rates. Looking at plots in Panama and Malaysia, the researchers found that increases in mean daily minimum temperature over a couple of decades correlated with decreases in growth rates. They associate this with lower net photosynthetic activity.
The team, led by Harvard’s Ken Feeley, suggests that if this sort of effect were repeated in bigger rainforests (most of which have only experienced marginal warming to date, as I understand it) then what are now stable stores of carbon would become net sources as theworld heats up. This is obviously a considerably less optimistic scenario than the possibility that carbon-dioxide fertilisation would make them sinks. It would presumably make the net effect of the increase in soil respiration that Peter Cox and others always stress (Nature paper from 2000) an even worse problem.
It’s not a dead cert that the change is due to temperature — the paper (published in Ecology Letters) seems to suggest that increased cloudiness could be playing a role. And there could be internal botanical changes too — maybe the lianas are doing more damage? But all in all it doesn’t sound good.
Mike is blogging the conference on the newsblog.
Cross posted at Climate Feedback
This week in Nature we have a news story on an attempt to follow up Frank Keppler’s work on methane produced aerobically by green plants which we published early last year (news story | paper). The Keppler piece, which suggested that methane emissions from green plants were a significant but previously unappreciated factor in global methane emissions, caused quit a lot of fuss, understandably, in the media — since methane is a greenhouse gas which, over short time horizons, is about 75 times more powerful than carbon dioxide — and quite a lot of befuddlement among plant scientists. If it were true, it would have significant implications for the way that people model methane production, and the levels of production that one might predict in a warming world. The debate rumbled on last year (another news report, this time by my colleague Quirin).
The new work that Tom Dueck and colleagues have published in New Phytologist (paper), though , finds no methane emissions from plants at all.
Obviously, not necessarily the last word. As Mike Hopkin reports:
Both groups have criticized the other’s choice of experimental method. Dueck says that Keppler’s group kept plants in sealed plastic containers instead of flow chambers, and exposed them to sources of stress such as bright sunlight and high temperature, which could have produced methane as an artefact. Keppler retorts that the use of 13C is an artificial piece of chemical trickery with unknown effects on plant metabolism, and also argues that methane production can vary by up to three orders of magnitude between species.
Keppler says other teams will be publishing results that back him up on the methane; but Mike reports that at least one other team is siding strongly with Dueck.
What Mike doesn’t mention, because an evil news editor (me) wouldn’t give him the space, is that various people in the community have pointed to an interesting contrast between the way plant scientists responded to the discovery of isoprene emissions and the Keppler work. With isoprene people said oh that’s interesting, replicated, and got on with it. This work has had a far frostier welcome.
On isoprene, this is as good a place as any to mention an interesting perspective by Manuel Lerdau in Science a few weeks ago on a possible isoprene-ozone positive feedback (paper). Isoprene within leaves protects the plants that produce it against ozone. But when isoprene gets out into the air, as it will, it can react with nitrogen oxides to make ozone. Only some species produce isoprene, and so these isoprene-producing plants both protect themselves against ozone and, in Nox-rich environments, increase the ozone stress on their non-isoprene-producing neighbours.
If this effect is real, it might have significant effects on forest composition over the next century.
One last thing to note on the Keppler story: it led to Carl Zimmer saying something nice about us, and that is always a good thing. As of course is Carl.