| Summary: | Ocean acidification (OA) influences the production of volatile organic compounds (VOCs) by seagrass leaves and their associated epiphytes. We hypothesize that the perception of odour produced by seagrass leaf epiphytes will change with seawater acidification, affecting the behaviour of seagrass-associated invertebrates. To test this hypothesis, we collected epiphytes from leaves of Posidonia oceanica growing at two pH conditions (7.7 and 8.1) and identified the most abundant genera of diatoms. We tested the VOCs produced at pH 8.1 by the epiphytic communities in toto, as well as those produced by selected diatoms, on various invertebrates. A complex set of species-specific and concentration-dependent chemotactic reactions was recorded, according to the pH of seawater. In particular, VOCs produced by individual diatoms triggered contrasting reactions in invertebrates, depending on the pH. The perception of epiphyte VOCs is likely to vary due to alteration of species ability to perceive and/or interpret chemical cues as infochemicals or due to changes in the structure of VOCs themselves. Thus, OA alters the fine-tuned chemical cross-talks between seagrass epiphytes and associated invertebrates, with potential consequences for the structure of communities and food webs of seagrass ecosystems.
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