Climate change affects the photosynthesis of carbon-storing mosses

Scientists find that peatland mosses are affected by temperature, so weather conditions could significantly reduce their ability to store carbon

Peatlands cover only 3% of the global land surface, however, they store a third of the global soil carbon. The uptake of CO2 through peat mosses is vital, but there is little knowledge about how their physiology is affected by rising CO2 levels.

As CO2 emissions continue to rise with human activity, it is increasingly important to capture CO2 to mitigate the associated climate change. As peatlands are the largest soil carbon stores globally, scientists are looking to find the impact of climate change on peatlands to mitigate the climate change risk.

Peat mosses’ responses to increased atmospheric CO2?

Global atmospheric CO2 concentrations have increased by nearly 50% during the 20th century. With further increases being inevitable according to the Intergovernmental Panel on Climate Change (IPCC), severe consequences for humanity are inevitable. Currently, the uptake of CO2 by the land biosphere has dampened the CO2 rise and prevented even more severe effects.

Therefore, scientists have begun developing ways to decipher effects of the CO2 rise during the past 100 years on metabolic fluxes of the key plant species in peatlands, mosses.

Taken from analyses of cellulose in peat cores gathered by collaborating scientists working across five continents, their findings reveal that a CO2-driven increase in photosynthesis of mosses is strongly dependent on the water table, which may change the species composition of peat moss communities.

For the study, scientists collected peat cores from ten locations worldwide. In a novel use of nuclear magnetic resonance pectroscopy, distributions of the stable hydrogen isotope deuterium in cellulose of modern and century-old peat mosses were then compared.

They reconstructed changes in photosynthetic efficiency during the 20th century, by estimating the impact of photorespiration, a side reaction of photosynthesis.

Jürgen Schleucher, Professor at Department of Medical Biochemistry and Biophysics at Umeå University, said: “Photorespiration is critical for the carbon balance of plants because it reduces the efficiency of photosynthesis by up to 35%, and it is suppressed by increasing CO2 but accelerated by increasing temperature.”

The vital impact of photorespiration

The analysis discovered that during the last 100 years, increasing CO2 has reduced photorespiration, leading to a boosting carbon storage in peatlands, which has dampened climate change. Nevertheless, increasing atmospheric CO2 has only reduced photorespiration in peatlands when water levels were intermediate, not when conditions were too wet or too dry.

Unlike other, higher plants, mosses cannot transport water. The water table level controls their moisture content, which affects their photosynthetic performance, so models based on higher plants’ physiological responses cannot therefore be applied.

Only mosses that grow at an intermediate distance from the water table level benefit from the higher atmospheric CO2 concentration, so the effect of CO2 depends on the water table level, potentially having major consequences for the composition of peatland species.

“The changes have already had devastating effects”

Changes in the peatlands’ water balance can also strongly affect their future carbon balance as too wet or too dry conditions reduce peat mosses’ ability to scavenge carbon.

Although peatlands have dampened CO2-driven climate change, the changes have already had devastating effects. If humans cannot reduce CO2 emissions, the atmospheric CO2 concentration will further increase by hundreds of ppm by 2100, and average global temperatures will rise several degrees Celsius above pre-industrial levels.

However, it remains unclear how peatlands will be affected by this.

Jürgen Schleucher adds: “To get a clearer picture of photorespiration’s importance for peat mosses and peat carbon accumulation, the next step is to transfer our data into tailored photosynthesis models to estimate global peatland carbon fluxes.

“Future CO2 levels, temperature rises, changes in precipitation and water table levels will all need to be considered to forecast peatlands’ fate in a changing climate.”