Response of Posidonia oceanica and its associated N2 fixers to different iron and CO2 concentrations

Abstract

[eng] The Mediterranean endemic seagrass Posidonia oceanica is known to be threatened by climate change. Due to its several functions and services it is fundamental to protect, conserve and restore this vulnerable ecosystem in the presence of disturbing future climate change scenarios. Many CO2 enrichment studies on seagrasses have been performed. However, in some studies, elevated CO2 concentrations did not have an effect on seagrasses indicating other factors involved in the response of P. oceanica and its associated N2 fixers such as limiting nutrients. Here, we study the response of P. oceanica and its N2 fixers under the combination of Fe and CO2 concentrations. P. oceanica shoots were exposed to increases of Fe (7.5 µM and 12 µM in 2019 and 12 µM and 1850 µM in 2020) and CO2 concentrations (410 ppm and 1200 ppm). The measured responses were chlorophyll content, primary production and respiration, N2 fixation rates and alkaline phosphatase activity (the last two measured on different plant parts). Net primary production presented a significant higher response (p<0.01) under low Fe and ambient CO2 concentrations than under control Fe and elevated CO2 concentrations and high Fe and ambient CO2 concentrations. In 2019, APA rate in root was significantly higher (p<0.01) under low Fe and ambient CO2 concentrations than under control Fe and elevated CO2 concentrations, low Fe and high CO2 concentrations and high Fe and ambient CO2 concentrations. In 2020, high Fe and CO2 concentrations showed a significant higher APA rate (p<0.05) than control Fe and ambient CO2 concentrations in non-epiphytized leaves. ARA and APA from 2019 and 2020 show significant differences between plant parts (p<0.001). Our study suggests that the measured responses are affected by so many factors that can come to unexpected responses which suggest that further investigations are required to develop the future scenario of P. oceanica.