The Science

Tropical forests are acknowledged to be the largest global source of emissions of the biogenic volatile organic compounds (BVOCs) isoprene (C₅H₈) and monoterpenes (C₁₀H₁₆). Current synthesis studies suggesting that few tropical species emit isoprenoids (20% to 38%) and that those that do, do so with highly variable emission capacities, including species within the same tree genera. This apparent lack of a clear phylogenetic thread has created difficulties both in linking isoprenoid function with evolution and in developing accurate biosphere-atmosphere models. In this study, a field-portable system was developed to identify and quantify isoprene and monoterpene emissions from leaves in parallel with measuring leaf physiologies including photosynthesis and transpiration. The system will enable the characterization of carbon and energy allocation to the biosynthesis and emission of isoprenoids linked with photosynthesis and their biological and environmental sensitivities (e.g., light, temperature, and carbon dioxide). Using this system, scientists from Lawrence Berkeley National Laboratory (LBNL) conducted a systematic isoprenoid emission study across the five most abundant tree genera in the Amazon basin.

The Impact

The hyperdominant species (numbering 69) across the top five most abundant genera, which make up about 50% of all individuals in the Amazon basin, showed a high abundance of isoprenoid emitters (isoprene, 63.8%; monoterpenes, 17.4%; both isoprene and monoterpenes, 11.6%). Among the abundant genera, only Pouteria had a low frequency of isoprene-emitting species (15.8% of 19 species). In contrast, Protium, Licania, Inga, and Eschweilera were rich in isoprene-emitting species (i.e., 83.3% of 12 species, 61.1% of 18 species, 100% of 8 species, and 100% of 12 species, respectively). In every genus, species were observed with light-dependent isoprene emissions together with β-ocimene emissions. These observations demonstrate that isoprene biological function and phylogenetic relationship studies cannot be conducted without including monoterpenes. These findings support the emerging view of the evolution of isoprene synthases from ocimene synthases. The finding (i.e., 64% of species observed versus 20% suggested in the literature) improves understanding of isoprenoid function-evolution relationships and represents a base for developing more accurate Earth System Models (ESMs).

Summary

Tropical forests are acknowledged to be the largest global source of emissions from isoprene (C₅H₈) and monoterpenes (C₁₀H₁₆), with current synthesis studies suggesting that few tropical species emit isoprenoids (20% to 38%) and that those do so with highly variable emission capacities, including species within the same genera. This apparent lack of a clear phylogenetic thread has created difficulties both in linking isoprenoid function with evolution and for developing accurate biosphere-atmosphere models. In this study, LBNL scientists present a systematic emission study of “hyperdominant” tree species in the Amazon basin. Across 162 individuals distributed among 25 botanical families and 113 species, isoprenoid emissions were widespread among both early and late successional species (isoprene, 61.9% of the species; monoterpenes, 15.0%; both isoprene and monoterpenes, 9.7%). The hyperdominant species (69) across the top five most abundant genera, which make up about 50% of all individuals in the basin, had a similar abundance of isoprenoid emitters (isoprene, 63.8%; monoterpenes, 17.4%; both, 11.6%). Among the abundant genera, only Pouteria had a low frequency of isoprene-emitting species (i.e., 15.8% of 19 species). In contrast, Protium, Licania, Inga, and Eschweilera were rich in isoprene-emitting species (i.e., 83.3% of 12 species, 61.1% of 18 species, 100% of 8 species, and 100% of 12 species, respectively). Light-response curves of individuals in each of the five genera showed light-dependent, photosynthesis-linked emission rates of isoprene and monoterpenes. Importantly, in every genus, the scientists observed species with light-dependent isoprene emissions together with monoterpenes including β-ocimene. These observations support the emerging view of the evolution of isoprene synthases from β-ocimene synthases. Study results have important implications for understanding isoprenoid function-evolution relationships and the development of more accurate ESMs.

Principal Investigator

Kolby Jardine
Lawrence Berkeley National Laboratory
kjjardine@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This material is based on work supported as part of the Next-Generation Ecosystem Experiments (NGEE)–Tropics project funded by the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy Office of Science, through Contract No. DE-AC02-05CH11231 to Lawrence Berkeley National Laboratory (LBNL) as part of DOE’s Terrestrial Ecosystem Science Program. Funding for the data analysis and manuscript preparation was provided by the DOE Office of Science Early Career Research Program under award no. FP00007421 to LBNL. Graduate student support was supported by the National Council for Scientific and Technological Development (CNPq) in Brazil. Logistical and scientific support is acknowledged by the Forest Management (MF), Climate and Environment (CLIAMB), and Large-Scale Biosphere-Atmosphere (LBA) programs at the National Institute for Amazon Research (INPA) in Brazil.

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References