Benthic Isoprene Flux from Permeable Carbonate Sediments on the Great Barrier Reef

Isoprene is the most abundant biogenic volatile organic compound (BVOC) on the planet, with annual emissions accounting for up to about 50% of all BVOC emissions. Isoprene has been measured extensively in terrestrial habitats where it is highly abundant; however, there is limited knowledge on marine systems where it is found at lower concentrations. Here, we report for the first time isoprene fluxes from permeable carbonate sediments covered by benthic microalgae from Heron Island and Lizard Island, representing the southern and northern ends of the Great Barrier Reef. Using advective and diffusive benthic chambers, we measured isoprene fluxes over full diel cycles on both islands. Dark, light and net effluxes of isoprene from the sediment to the water column were observed at both Heron Island (net: 9.96 ± 1.09 nmol m² h^-1) and Lizard Island (net: 3.90 ± 0.90 nmol m² h^-1). Overall, isoprene fluxes in the light were much higher on Heron Island (mean: 15.73 ± 1.70 nmol m² h^-1) than on Lizard Island (mean: 5.86 ± 1.27 nmol m² h^-1) and both islands exhibited a decrease in isoprene production at night (mean: 1.88 ± 0.92 nmol m² h^-1 and 1.93 ± 1.60 nmol m² h^-1 respectively). The greater isoprene production measured during the day suggests an intimate link between isoprene production and photosynthesis. Furthermore, the lower isoprene flux at night either suggests isoprene consumption by microbial activity or the absence of photosynthesis. It is estimated that Heron Island carbonate sediments contributes to about 6% of benthic isoprene production from the reef flat while Lizard Island carbonate sediments contribute to about 3% of isoprene production. Therefore, permeable carbonate sediments should be considered as a source of benthic isoprene to the water column for global flux estimates.