93. Seagrass fitness under ocean warming and acidification

Tiago Repolho (1)*, Gisela Dionísio (1, 2), Ana R. Lopes (1), Tiago F. Grilo (1), José R. Paula (1), Ricardo Calado (2), Bernardo Duarte (3), Isabel Caçador (3), Rui Rosa (1)

1 MARE – Marine and Environmental Sciences Centre, Laboratório Marítimo da Guia, Faculdade de Ciências da Universidade de Lisboa, Av. Nossa Senhora do Cabo, 939, 2750-374 Cascais, Portugal
2 Departamento de Biologia & CESAM, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
3 MARE – Marine and Environmental Sciences Centre, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Campo Grande, Portugal

Background
Within marine coastal environments, seagrasses are experiencing a global decline at unprecedented levels never perceived before. Their response to combined climate change stressors is still elusive or even slightly understood and therefore, it is not known if they will be able to cope in a future changing ocean.
The aim of our study is to unravel, the combined effects of ocean warming and acidification on survival and several photobiological processes of dwarf seagrass (Zostera noltii Hornem). Moreover, it is worth noting that Z. noltii meadows are among the most biodiverse temperate marine ecosystems and hold significant ecological importance in many marine coastal habitats.

Methods
Zostera noltii was acclimated at rising pCO2 (ΔpH=0.4, pH 7.6/8.0) and ocean warming (+4°C). Seagrass survival was determined. Pulse amplitude modulation (PAM) fluorometry was used to monitor photosynthetic activity. Photosynthetic pigment quantification was performed spectrophotometrically.

Findings
Zostera noltii shoot density was significantly reduced under warming conditions. Photosynthetic electron transport rate (ETR) and maximum photosynthetic quantum yield (Fv/Fm) levels were significantly higher under control conditions and lower under warming/hypercapnic scenario. Chl_a, Chl_b and Chl_a/Chl_b ratio were significantly higher (control) and lower under the warming/hypercapnic scenario. Pheophytin a, Pheophytin b, Auroxanthin, Antheraxanthin, β-carotene, Lutein and total carotenoid concentrations showed significantly higher concentrations under the warming/normocapnic scenario. Positive correlations between quantified photosynthetic pigments and leaf coloration were found. De-Epoxidation State (DES) showed significantly lower values under control conditions. A negative correlation between DES and Chl_a, Total-Chl and Total carotenoids was also found.

Conclusions
Zostera noltii survival will be severely affected under a future warming scenario. Ocean acidification seems to be beneficial and improved seagrass resilience within a warming ocean scenario. Moreover, light harvesting and photo-protection mechanisms will be adversely affected by future warming scenario but not acidification.