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Impact of temperature on the vulnerability of Mediterranean seagrasses to phatogens

Final Report Summary - TEMSPATH (Impact of temperature on the vulnerability of Mediterranean seagrasses to phatogens)

Mediterranean seagrass meadows are ecologically and economically important habitats that have been identified as some of the most threatened marine ecosystems on the planet. Climate change, including warming and ocean acidification, is predicted to have a significant impact on seagrasses and warming has been identified as a major driver for seagrass loss in the Mediterranean. Another potential, but as of yet largely unknown threat, is posed by pathogens. Pathogens are present in seagrasses around the world and have been responsible for large-scale seagrass losses in the past. In the Mediterranean, the presence of pathogenic organisms in seagrass tissue has not yet been shown to increase seagrass mortality, but it is thought that pathogens pose a significant risk of seagrass decline under deteriorating environmental conditions. This project aimed to examine the impacts of warming and ocean acidification on Mediterranean seagrasses, to examine whether they might make seagrasses more vulnerable to infection by pathogens and to assess the combined effects of these stressors and pathogenic infection on plant health.

First, we experimentally examined the effects of increased temperature on growth and demography of two Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa by incubating shoots and seedlings at 25 °C - 32 °C, which encompasses the range of maximum summer seawater temperatures projected for the Mediterranean Sea during the 21st century. Several health indicators of P. oceanica were negatively affected by warming with seedlings being particularly sensitive. C. nodosa appeared to be stimulated by increased temperature to a threshold of around 29 - 30 °C, beyond which their health declined. Mediterranean seagrasses are therefore likely to be negatively impacted by the effects of global warming over the next century.

Second, the effect of long-term exposure to high pCO2 (low pH) on Mediterranean seagrasses was examined at two volcanic carbon dioxide (CO2) vents in Italy where P. oceanica (Ischia) and C. nodosa (Vulcano) are exposed to naturally elevated pCO2. These sites have shallow-water gradients of pH across tens of meters, reaching values as low as 6.6-6.8 nearest the most active vents, to a pH of 8.1 where the signal has been diluted by mixing with ambient seawater. To investigate how photosynthetic performance was affected by low pH, we took measurements along the gradient of pH using a PAM Fluorometer. No significant differences were observed in quantum yield or electric transport rate (ETR) for P. oceanica, whereas ETR was slightly elevated for C. nodosa at the low pH. Mediterranean seagrasses therefore appear not to be stressed under low pH conditions and may even benefit from the increased carbon availability at elevated pCO2.

Third, to investigate whether vulnerability of seagrasses to pathogens is greater under climate change stress, we examined the effect of temperature and acidification on infection by the protist Labyrinthula in a series of laboratory experiments. In a first experiment, P. oceanica and C. nodosa shoots were inoculated with Labyrinthula sp. at 24-32 °C. We used lesion area as a measure of degree of infection and examined the photosynthetic response of the plant using a PAM fluorometer. Lesions in P. oceanica were generally small compared to those in C. nodosa, but lesion area decreased with increasing temperature in both species. Temperature did not significantly affect photosynthetic performance in P. oceanica. The response of quantum yield to temperature in C. nodosa was bell-shaped, suggesting optimum performance at 30 °C, and was significantly reduced in Labyrinthula-infected shoots. In a second experiment we tested the combined effect of temperature and CO2 on Labyrinthula infection by exposing P. oceanica to two temperatures (24 and 30 °C) and two CO2 concentrations (360 and 1000 ppm). CO2 did not increase lesion area or alter photosynthetic performance of the plants. These results suggest that Labyrinthula infection is unlikely to increase in Mediterranean seagrasses under future scenarios of climate change.

The results of this project add to our growing understanding of climate change effects on Mediterranean ecosystems. They also highlight that there may be unexpected interactions between climate change and ecosystem health that are difficult to predict. These kinds of studies are extremely valuable to design effective regional adaptation strategies to protect and preserve key Mediterranean ecosystems faced with the challenges of global change.