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.