Abstract
Isoprene (2-methyl-1,3-butadiene) is emitted in vast quantities (>500 Tg C yr-1). Most isoprene is emitted by trees, but there is still incomplete understanding of the diversity of isoprene sources. The reactivity of isoprene in the atmosphere has potential implications for both global warming and global cooling, with human health implications also arising from isoprene-induced ozone formation in urban areas. Isoprene emissions from terrestrial environments have been studied for many years, but our understanding of aquatic isoprene emissions is less complete. Several abundant phytoplankton taxa produced isoprene in the laboratory, and the relationship between chlorophyll a and isoprene production has been used to estimate emissions from marine environments. The aims of this review are to highlight the role of aquatic environments in the biological cycling of isoprene and to stimulate further study of isoprene metabolism in marine and freshwater environments. From a microbial ecology perspective, the isoprene metabolic gene cluster, first identified in Rhodococcus sp. AD45 (isoGHIJABCDEF) and subsequently found in every genome-sequenced isoprene-degrader, provides the ideal basis for molecular studies on the distribution and diversity of isoprene-degrading communities. Further investigations of isoprene-emitting microbes, such as the influence of environmental factors and geographical location, must also be considered when attempting to constrain estimates of the flux of isoprene in aquatic ecosystems. We also report isoprene emission by the scleractinian coral Acropora horrida and the degradation of isoprene by the same coral holobiont, which highlights the importance of better understanding the cycling of isoprene in marine environments.
Highlights
Isoprene is one of the most abundant atmospheric trace gases, and yet, compared to methane, the biogeochemical cycle for isoprene is relatively poorly understood
Our understanding of the physiology, biochemistry, molecular biology and ecology of isoprene-degrading bacteria has focussed mostly on terrestrial environments, and so we summarise what is known about the biology of isoprene consumption using examples mainly from the terrestrial environment and focus on aquatic environments
Comparative analysis of genes encoding isoprene monooxygenase (IsoMO) has enabled us to differentiate this key enzyme, which initiates the metabolism of isoprene by bacteria, from other soluble diironcentre monooxygenases (SDIMOs) such as soluble methane monooxygenase (sMMO), alkene monooxygenases and toluene monooxygenases (Carrión et al 2018)
Summary
Isoprene is one of the most abundant atmospheric trace gases, and yet, compared to methane, the biogeochemical cycle for isoprene is relatively poorly understood. Comparatively little is known about the biological consumption of isoprene in the biosphere before it enters the atmosphere. Isoprene is released into the biosphere at around 500 Tg C yr−1, a flux approximately equal in magnitude to that of methane in terms of global emissions (Fig. 1) (Atkinson & Arey 2003, Guenther et al 2012). Isoprene is highly volatile and reactive, and it has a variety of effects on the Earth’s climate depending on the levels of other compounds, including nitrogen oxides, in the atmosphere (Atkinson & Arey 2003). Isoprene increases the longevity of methane in the atmosphere by reacting with hydroxyl radicals and indirectly, acts as a global warming gas (Collins et al 2002, Folberth et al 2006). Increased planting of major isoprene-producing crop plants such as oil palm, poplar and willow has stimulated considerable interest in the effects of isoprene on air quality (Sharkey et al 2008, Hewitt et al 2009, Karlsson et al 2020, Monson et al 2020)
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