As the concentration of CO 2 in the atmosphere increases, so does the amount of carbon that plants sequester in their own tissues through photosynthesis. This so-called “CO 2 fertilization effect” is well known in a general sense. But until now, scientists haven’t been able to quantify how much it might dampen the impact of our carbon emissions in the real world.

A group of Swedish researchers has solved this problem by looking to the past, comparing carbon metabolism in century-old herbarium specimens and modern plants. They say their study, the first to use historical specimens to document a shift in plant metabolism, will help scientists build more accurate global climate models in the future.

The key to the CO 2 fertilization effect is an enzyme that plays a central role in photosynthesis. This enzyme, nicknamed Rubisco, can interact with both carbon and oxygen – so it both captures or “fixes” CO 2 through photosynthesis, and un-fixes it through a side reaction known as photorespiration.

Using a method called nuclear magnetic resonance (NMR) spectroscopy, the researchers identified a biochemical signal that reflects the balance of photosynthesis and photorespiration. They calibrated this signal by analyzing tissue from sunflowers grown in experimental greenhouses at CO 2 concentrations ranging from 180 parts per million (ppm) to 1,500 ppm, they reported yesterday in the Proceedings of the National Academy of Sciences [1].

These results, as well as analysis of spinach and beans grown at experimentally increased CO 2 levels, confirmed that the ratio of photosynthesis to photorespiration increases along with the amount of CO 2 in the air, and by how much.

Then, they analyzed samples from sugar beets grown between 1890, when atmospheric CO 2 was just under 300 ppm, and 2012, when it reached 395 ppm. This showed that, indeed, increasing atmospheric CO 2 has altered the balance of photosynthesis and photorespiration in sugar beets over the past century.

Finally, they collected samples of spinach, fireweed, and peat moss, and compared them to herbarium specimens of the same species. These showed a similar shift in metabolic balance over the past century, suggesting that the magnitude of the CO 2 fertilization effect is similar across a broad variety of plant species.

So, as we pump carbon dioxide into the atmosphere, we are altering the biochemistry of plants. For the moment, that’s good news for us – the study results suggest that land plants are currently absorbing about one-third of human-caused CO 2 emissions, in line with previous estimates.

But as the climate changes, we’ll see those benefits less and less: photorespiration is known to increase at higher temperatures.

In fact, another paper published yesterday, this one in Nature Climate Change, paints a less rosy picture of the CO 2 fertilization effect [2].

In that study, researchers used satellite data to show that over the past 30 years, plant growth hasn’t increased as much as expected in response to increasing CO 2 . This may be because increased temperature means more water stress for plants, or because their growth is limited by the availability of nutrients like nitrogen and phosphorus.

“Unfortunately, our observation-based estimates of global vegetation growth indicate that plant growth may not buy us as much time as expected,” says study leader William Kolby Smith of the University of Minnesota. “[So] action to curb emissions is all the more urgent.” – Sarah DeWeerdt | December 8, 2015

Sources:

2

3

2

[1]: Ehlers I. et al. “Detecting long-term metabolic shifts using isotopomers: CO-driven suppression of photorespiration in Cplants over the 20th century.” Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1504493112 [2]: Kolby Smith W. et al. “Large divergence of satellite and Earth system model estimates of global terrestrial COfertilization.” Nature Climate Change DOI: 10.1038/nclimate2879

Header image: A tree in the process of sequestering carbon emissions. Credit: Andrew E. Larson via Flickr.