Rising sea water temperature will play a significant role in responses of the world's seagrass meadows to climate change. In this study, we investigated seasonal and latitudinal variation (spanning more than 1,500 km) in seagrass productivity, and the optimum temperatures at which maximum photosynthesis and net productivity (for the leaf and the whole plant) occurs, for three seagrass species (Cymodocea serrulata, Halodule uninervis, and Zostera muelleri). To obtain whole plant net production, photosynthesis, and respiration rates of leaves and the root/rhizome complex were measured using oxygen-sensitive optodes in closed incubation chambers at temperatures ranging from 15 to 43°C. The temperature-dependence of photosynthesis and respiration was fitted to empirical models to obtain maximum metabolic rates and thermal optima. The thermal optimum (Topt) for gross photosynthesis of Z. muelleri, which is more commonly distributed in sub-tropical to temperate regions, was 31°C. The Topt for photosynthesis of the tropical species, H. uninervis and C. serrulata, was considerably higher (35°C on average). This suggests that seagrass species are adapted to water temperature within their distributional range; however, when comparing among latitudes and seasons, thermal optima within a species showed limited acclimation to ambient water temperature (Topt varied by 1°C in C. serrulata and 2°C in H. uninervis, and the variation did not follow changes in ambient water temperature). The Topt for gross photosynthesis were higher than Topt calculated from plant net productivity, which includes above- and below-ground respiration for Z. muelleri (24°C) and H. uninervis (33°C), but remained unchanged at 35°C in C. serrulata. Both estimated plant net productivity and Topt are sensitive to the proportion of below-ground biomass, highlighting the need for consideration of below- to above-ground biomass ratios when applying thermal optima to other meadows. The thermal optimum for plant net productivity was lower than ambient summer water temperature in Z. muelleri, indicating likely contemporary heat stress. In contrast, thermal optima of H. uninervis and C. serrulata exceeded ambient water temperature. This study found limited capacity to acclimate: thus the thermal optima can forewarn of both the present and future vulnerability to ocean warming during periods of elevated water temperature.

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Isoprene is a biogenic volatile organic compound (BVOC), which is predominantly emitted by terrestrial plants but also from marine systems. However, the marine contribution to isoprene emissions is less understood due to the difficulty of measuring trace concentrations in seawater. Previous methods using “purge and trap” coupled with flame ionization detection and/or chemiluminescence have been developed, although these methods have limitations including large sample sizes, excessive manipulation, and potential interference from other BVOCs. These limitations could lead to overestimations or sample loss due to the high volatility and reactivity of isoprene. Here, we present an improved method for measuring isoprene in coastal marine ecosystems with elevated isoprene concentrations using cumulative headspace injection coupled with gas chromatography–mass spectrometry (GC‐MS). Sampling technique, preservation, and analytical conditions were tested and optimized using an analytical cycle time of 13 min. The analytical precision was 2.6% (n = 7), limit of detection was 15 pM, limit of quantification was 285 pM, accuracy based on recovery was 79% ± 2.8%, and reproducibility yielded a coefficient of variation of 15%. This method provides an improved way to measure isoprene from seawater using a simple sampling technique and fast, automated analysis, reducing isoprene loss through sample manipulation, and human error. This method meets analytical requirements to accurately measure isoprene at pM concentrations, which are typical of tropical coastal marine habitats. This method can also be adapted to measure other BVOCs in seawater such as dimethyl sulfide, providing a versatile method for marine research.

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Terrestrial ecosystems emit large quantities of biogenic volatile organic compounds (BVOCs), many of which play important roles in abiotic stress responses, pathogen and grazing defences, inter‐ and intra‐species communications, and climate regulation. Conversely, comparatively little is known about the diversity and functional potential of BVOCs produced in the marine environment, especially in highly productive coral reefs. Here we describe the first ‘volatilomes’ of two common reef‐building corals, Acropora intermedia and Pocillopora damicornis, and how the functional potential of their gaseous emissions is altered by heat stress events that are driving rapid deterioration of coral reef ecosystems worldwide. A total of 87 BVOCs were detected from the two species and the chemical richness of both coral volatilomes—particularly the chemical classes of alkanes and carboxylic acids—decreased during heat stress by 41% and 62% in A. intermedia and P. damicornis, respectively. Across both coral species, the abundance of individual compounds changed significantly during heat stress, with the majority (>86%) significantly decreasing compared to control conditions. Additionally, almost 60% of the coral volatilome (or 52 BVOCs) could be assigned to four key functional groups based on their activities in other species or systems, including stress response, chemical signalling, climate regulation and antimicrobial activity. The total number of compounds assigned to these functions decreased significantly under heat stress for both A. intermedia (by 35%) and P. damicornis (by 64%), with most dramatic losses found for climatically active BVOCs in P. damicornis and antimicrobial BVOCs in A. intermedia. Together, our observations suggest that future heat stress events predicted for coral reefs will reduce the diversity, quantity and functional potential of BVOCs emitted by reef‐building corals, potentially further compromising the healthy functioning of these ecosystems.