Chiral monoterpenes reveal forest emission mechanisms and drought responses.

Monoterpenes (C10H16) are emitted in large quantities by vegetation to the atmosphere (>100 TgC year-1), where they readily react with hydroxyl radicals and ozone to form new particles and, hence, clouds, affecting the Earth's radiative budget and, thereby, climate change1-3. Although most monoterpenes exist in two chiral mirror-image forms termed enantiomers, these (+) and (-) forms are rarely distinguished in measurement or modelling studies4-6. Therefore, the individual formation pathways of monoterpene enantiomers in plants and their ecological functions are poorly understood. Here we present enantiomerically separated atmospheric monoterpene and isoprene data from an enclosed tropical rainforest ecosystem in the absence of ultraviolet light and atmospheric oxidation chemistry, during a four-month controlled drought and rewetting experiment7. Surprisingly, the emitted enantiomers showed distinct diel emission peaks, which responded differently to progressive drying. Isotopic labelling established that vegetation emitted mainly de novo-synthesized (-)-α-pinene, whereas (+)-α-pinene was emitted from storage pools. As drought progressed, the source of (-)-α-pinene emissions shifted to storage pools, favouring cloud formation. Pre-drought mixing ratios of both α-pinene enantiomers correlated better with other monoterpenes than with each other, indicating different enzymatic controls. These results show that enantiomeric distribution is key to understanding the underlying processes driving monoterpene emissions from forest ecosystems and predicting atmospheric feedbacks in response to climate change.

Heat stress increases the use of cytosolic pyruvate for isoprene biosynthesis.

The increasing occurrence of heatwaves has intensified temperature stress on terrestrial vegetation. Here, we investigate how two contrasting isoprene-emitting tropical species, Ficus benjamina and Pachira aquatica, cope with heat stress and assess the role of internal plant carbon sources for isoprene biosynthesis in relation to thermotolerance. To our knowledge, this is the first study to report isoprene emissions from P. aquatica. We exposed plants to two levels of heat stress and determined the temperature response curves for isoprene and photosynthesis. To assess the use of internal C sources in isoprene biosynthesis, plants were fed with 13C position-labelled pyruvate. F. benjamina was more heat tolerant with higher constitutive isoprene emissions and stronger acclimation to higher temperatures than P. aquatica, which showed higher induced isoprene emissions at elevated temperatures. Under heat stress, both isoprene emissions and the proportion of cytosolic pyruvate allocated into isoprene synthesis increased. This represents a mechanism that P. aquatica, and to a lesser extent F. benjamina, has adopted as an immediate response to sudden increase in heat stress. However, in the long run under prolonged heat, the species with constitutive emissions (F. benjamina) was better adapted, indicating that plants that invest more carbon into protective emissions of biogenic volatile organic compounds tend to suffer less from heat stress.

Effects of drought and recovery on soil volatile organic compound fluxes in an experimental rainforest.

Drought can affect the capacity of soils to emit and consume biogenic volatile organic compounds (VOCs). Here we show the impact of prolonged drought followed by rewetting and recovery on soil VOC fluxes in an experimental rainforest. Under wet conditions the rainforest soil acts as a net VOC sink, in particular for isoprenoids, carbonyls and alcohols. The sink capacity progressively decreases during drought, and at soil moistures below ~19%, the soil becomes a source of several VOCs. Position specific 13C-pyruvate labeling experiments reveal that soil microbes are responsible for the emissions and that the VOC production is higher during drought. Soil rewetting induces a rapid and short abiotic emission peak of carbonyl compounds, and a slow and long biotic emission peak of sulfur-containing compounds. Results show that, the extended drought periods predicted for tropical rainforest regions will strongly affect soil VOC fluxes thereby impacting atmospheric chemistry and climate.

Absolute optical chiral analysis using cavity-enhanced polarimetry.

Chiral analysis is central for scientific advancement in the fields of chemistry, biology, and medicine. It is also indispensable in the development and quality control of chiral compounds in the chemical and pharmaceutical industries. Here, we present the concept of absolute optical chiral analysis, as enabled by cavity-enhanced polarimetry, which allows for accurate unambiguous enantiomeric characterization and enantiomeric excess determination of chiral compounds within complex mixtures at trace levels, without the need for calibration, even in the gas phase. Our approach and technology enable the absolute postchromatographic chiral analysis of complex gaseous mixtures, the rapid quality control of complex mixtures containing chiral volatile compounds, and the online in situ observation of chiral volatile emissions from a plant under stress.

Ecosystem fluxes during drought and recovery in an experimental forest.

Severe droughts endanger ecosystem functioning worldwide. We investigated how drought affects carbon and water fluxes as well as soil-plant-atmosphere interactions by tracing 13CO2 and deep water 2H2O label pulses and volatile organic compounds (VOCs) in an enclosed experimental rainforest. Ecosystem dynamics were driven by different plant functional group responses to drought. Drought-sensitive canopy trees dominated total fluxes but also exhibited the strongest response to topsoil drying. Although all canopy-forming trees had access to deep water, these reserves were spared until late in the drought. Belowground carbon transport was slowed, yet allocation of fresh carbon to VOCs remained high. Atmospheric VOC composition reflected increasing stress responses and dynamic soil-plant-atmosphere interactions, potentially affecting atmospheric chemistry and climate feedbacks. These interactions and distinct functional group strategies thus modulate drought impacts and ecosystem susceptibility to climate change.

Compartment specific chiral pinene emissions identified in a Maritime pine forest.

To track unknown sources and sinks of volatile organic compounds (VOCs) inside forest canopies we measured diel cycles of VOC exchanges in a temperate maritime forest at the branch, stem and ground level with special focus on the chiral signatures of pinenes. All compartments released day and night α- and ß-pinene as major compounds. In addition, strong light dependent emissions of ocimene and linalool from branches occurred during hot summer days. In all compartments the overall emission strength of pinenes varied from day to day spanning 1 to 2 orders of magnitude. The highest pinene emissions from ground and stem were observed during high moisture conditions. Despite this variability stem emissions consistently expressed a different chiral composition than branch emissions, the former containing a much larger fraction of (-)-enantiomers than the latter. Pinene emissions from dead needle litter and soil were mostly enriched in (-)-enantiomers, while the chiral signatures of the ambient air inside the forest showed mostly intermediate levels compared to the emission signatures. These findings suggest that different organ-specific pinene producing enzymes exist in Maritime pine, and indicate that emissions from ground and stem compartments essentially contribute to the canopy VOC flux. Overall the results open new perspectives to explore chirality as a possible marker to recognize shifts in the contributions of different VOC sources present within forest ecosystems and to explain observed temporal changes in the chiral signature of pinenes in the atmosphere.