Combined exposure to CO2 and H2S significantly reduces the performance of the Mediterranean seagrass Posidonia oceanica: Evidence from a volcanic vent

Although seagrasses are expected to thrive in future acidified oceans by overcoming low CO₂ diffusion into plant tissues, the co-occurrence of environmental stressors may affect their growth. Volcanic CO₂ vents are often associated with toxic gases and metal-rich fluids representing ideal sites to assess the effects of multiple stressors. We evaluated the response of Posidonia oceanica growing near shallow CO₂ vents characterized by H₂S spill-out by comparing meadow structure and phenology to an area with no gas emissions. Seagrass descriptors at meadow, shoot and leaf level indicated that P. oceanica experienced stressful conditions at the vent area, in clear contrast to the flourishing features of P. oceanica previously described at CO₂ vents with no evidence of toxic inputs. Furthermore, the reduction in both leaf δ³⁴S and growth at the vent area indicates that sulfide intrusion occurs and affects seagrass growth performance, dampening the expected beneficial effects of high CO₂ levels.