Isoprene emission from phytoplankton monocultures: the relationship with chlorophyll-a, cell volume and carbon content B. Bonsang, A,G V. Gros, A I. Peeken, B,D,E N. Yassaa, C,F K. Bluhm, B E. Zoellner, B R. Sarda-Esteve A and J. Williams C A Laboratoire des Sciences du Climat et de l’Environnement (LSCE), Institut Pierre Simon Laplace (IPSL), laboratoire CEA/CNRS/UVSQ, CE Saclay, Orme des Merisiers, F-91191 Gif-sur-Yvette, France. B IFM-GEOMAR Leibniz Institut for Marine Sciences, Marine Biogeochemie, Dienstgeba ¨ude Westufer, Duesternbrooker Weg 20, D-24105 Kiel, Germany. C Max Planck Institute for Chemistry, Atmospheric Chemistry Department, Johann Joachim Becher Weg 27, D-55128 Mainz, Germany. D Center for Marine Environmental Sciences (MARUM), Leobener Strasse, D-28359 Bremen, Germany. E Alfred Wegener Institute for Polar and Marine Research, Biological Oceanography, Am Handelshafen 12, D-27570 Bremerhaven, Germany. F Laboratoire d’Analyse Organique Fonctionnelle, Faculty of Chemistry, University of Sciences and Technology Houari Boumediene (USTHB), BP 32 El-Alia Bab-Ezzouar, 16111 Algiers, Algeria. G Corresponding author. Email: bernard.bonsang@lsce.ipsl.fr Environmental context. Isoprene, a natural product of both terrestrial vegetation and marine organisms, is rapidly oxidised in the atmosphere, and thereby plays a key role in the regional budget of oxidants. Although isoprene production from terrestrial plants has been extensively investigated, production processes and emission rates from marine species are still poorly understood. We present results from laboratory experiments showing that isoprene is emitted from living phytoplankton cells at variable rates depending on the light intensity, cell volume, and carbon content of the plankton cells. Abstract. We report here isoprene emission rates determined from various phytoplankton cultures incubated under PAR light which was varied so as to simulate a natural diel cycle. Phytoplankton species representative of different phytoplankton functional types (PFTs) namely: cyanobacteria, diatoms, coccolithophorides, and chlorophytes have been studied. Biomass normalised isoprene emission rates presented here relative to the chlorophyll-a (Chl-a) content of the cultures showed that the two cyanobacteria (Synechococcus and Trichodesmium) were the strongest emitters with emission rates in the range of 17 to 28 mgC 5 H 8 g 1 Chl-a h 1 . Diatoms produced isoprene in a significantly lower emission range: 3 to 7.5 mgC 5 H 8 g 1 Chl-a h 1 and Dunaliella tertiolecta was by far the lowest emitter of our investigated plankton cultures. Despite the group specific differences observed, a high emission rate variance was observed to occur within one phytoplankton group. However, a combination of literature and our own data showed a clear relationship between the actual cell volume and the isoprene emission rates. This relationship could be a valuable tool for future modelling approaches of global isoprene emissions. Additional keywords: ocean, sea–air exchanges. Introduction Isoprene (2-methyl-1,3-butadiene) is a hydrocarbon mainly produced by terrestrial plants. [1,2] It is known to have a sig- nificant impact on tropospheric photochemistry [3] and in parti- cular in the regional ozone budget [4] through complex reaction pathways leading to the formation of aldehydes, peroxides, carbon monoxide and other photochemically reactive species. Furthermore, isoprene has been shown to contribute to the for- mation of organic aerosols. [5] In comparison with emission from terrestrial ecosystems, the ocean is a weak source of isoprene with a magnitude of ,1 Tg year 1 or less. [6–8] However, it should be noted that due to the short atmospheric lifetime of isoprene (1–5 h), the large terrestrial source has no impact on the open ocean. Isoprene oceanic emissions result from ‘in situ’ biological production in the euphotic zone leading to a sig- nificant surface ocean supersaturation with respect to the atmosphere. The impact of oceanic isoprene emissions into the marine boundary layer (MBL), on both photochemistry and on the organic aerosol abundance, is probably minor considering the magnitude of this source on a global scale. [9,10] However, CSIRO PUBLISHING B. Bonsang et al., Environ. Chem. 2010, 7, 554–563. doi:10.1071/EN09156 www.publish.csiro.au/journals/env Ó CSIRO 2010 1448-2517/10/060554 554