Mirror image molecules reveal drought stress in the Amazon rainforest

In 2023, the Amazon rainforest experienced its worst recorded drought since records began. River levels dropped dramatically and vegetation at all levels deteriorated due to intense heat and water shortages. In such conditions, plants release increased amounts of monoterpenes—small, volatile organic compounds that act as a defense mechanism and help communication with their environment. Some molecules, such as α-pinene, which smells like pine, occur as mirror-image pairs, known as enantiomers. 

The ratio of these two forms changes measurably when plants are under stress, for example due to heat or water shortage. Researchers at the Max Planck Institute for Chemistry investigated how this ratio changed in the Amazon before, during, and after the drought period. The results show that under normal conditions a clear ratio was consistently measured, but with increasing drought stress it shifted to ever higher values. In the most extreme phase of the drought, the usual ratio of the two α-pinene variants even reversed. Thus, the mirror molecules of α-pinene can tell us how much stress an ecosystem is currently under. 

Giovanni Pugliese, a scientist from the Max Planck Institute for Chemistry who was on site during the measurement campaign, recalls: “The heat was unbearable when collecting the samples. The forest was clearly suffering; its leaves were yellowing and the dry clay soil was cracking”. The problem in 2023 was that the September to October dry season coincided with an El Niño event. This is part of the global climate oscillation ENSO, and in El Niño mode, it brings extremely low rainfall and high temperatures to the Amazon basin.

Measurements deep in the rainforest

At the measuring station of the Amazon Tall Tower Observatory (ATTO), 150 kilometers northeast of Manaus, the researchers collected air samples at a height of 24 meters directly in the forest canopy. In the laboratory in Mainz, they later determined the ratio of the two α-pinene forms using chiral gas chromatography-time-of-flight-mass spectrometry. 

“First, we determined the ratio in which the two variants occur under normal conditions,” explains Joseph Byron, researcher at the Max Planck Institute for Chemistry and first author of the study. “We then observed how this ratio shifted during the El Niño-impacted dry season and slowly returned to normal afterwards.” 

Plants in survival mode

Project leader Jonathan Williams is impressed by these vegetation responses and explains further, “It is amazing that we can read directly from the air how the rainforest is reacting to current conditions. During the worst part of the drought, when the ratio flipped at midday, we knew that the vegetation had had enough, it had stopped photosynthesizing and closed up its pores to stop losing precious ground water ”.  This work builds on an earlier experimental drought study conducted in an enclosed forest grown within a greenhouse. (See https://www.mpic.de/5265797/spiegelmolekuele-trockenstress). There the Max Planck Research team  then showed that the two mirror-image molecules are released via different processes in the plant: while one form of α-pinene is released immediately after photosynthesis , the mirror molecule comes from storage pools within the plant.  The indoor experiment revealed this relationship and now this behavior has been recorded in the real-world extreme drought situation in the Amazon rainforest.

Significance for climate models

The Amazon rainforest is the world's largest source of biogenic volatile compounds. Using the ratio of α-pinene molecules, these emissions and their changes under drought conditions can now be represented more realistically in climate models. This is crucial because researchers expect more frequent and severe El Niño-related droughts in future.

Background ATTO

ATTO is a German-Brazilian joint project which was launched in 2009. It is managed by the Max Planck Institutes for Biogeochemistry in Jena and for Chemistry in Mainz, as well as by the Brazilian INPA and the Amazon State University (UEA) in Manaus. The project is funded by the German Federal Ministry of Education and Research (BMBF), the Ministério da Ciência, Tecnologia e Inovações (MCTI), the Max Planck Society and the Brazilian organizations including FAPEAM and individual researchers bring funding from other scientific funding agencies. 

The tower aims to deliver groundbreaking findings which will be the basis for improved climate models. With a height of 325 meters, the tower extends the ground-level boundary layer, and provides information from approximately 100 square kilometers of the world´s largest forest area. 

More on ATTO: https://www.mpic.de/3538403/ATTO